Safari Ecology

Distribution of Ethiopian Bush-crow and the nature of explanations

2012-03-19T11:04:00.000-07:00

Yesterday I was sent a link to a press release from the excellent BirdLife International (read it here). It's talking about some research by an international team to try and explain the remarkably restricted range of the Ethiopian Bush-crow (cute picture here, since I've never actually been there to take my own), and in it, Paul Donald the lead author makes some interesting comments:

“The mystery surrounding this bird and its odd behaviour has stumped scientists for decades – many have looked and failed to find an answer. But the reason they failed, we now believe, is that they were looking for a barrier invisible to the human eye, like a glass wall. Inside the ‘climate bubble’, where the average temperature is less than 20°C, the bush-crow is almost everywhere. Outside, where the average temperature hits 20°C or more, there are no bush-crows at all. A cool bird, that appears to like staying that way.”

The reason this species is so completely trapped inside its little bubble is as yet unknown, but it seems likely that it is physically limited by temperature – either the adults, or more likely its chicks, simply cannot survive outside the bubble, even though there are thousands of square miles of identical habitat all around.

BirdLife International’s Dr Nigel Collar is co-author of the study. He added “Whatever the reason this bird is confined to a bubble, alarm bells are now ringing loudly. The storm of climate change threatens to swamp the bush-crow’s little climatic lifeboat – and once it’s gone, it’s gone for good.”

Two things jump out from this to me (1) the nature and validity of the explanation itself: we're told the authors think there's probably a direct physical limitation, not an indirect effect on, say, a essential food source. (2) The concern about climate change removing the suitable bubble of cool in the highlands. (I should point out that the paper is available here and I'm going to base the rest of the discussion on the actual paper, not the press release - both these points are made in the discussion of the paper, however.)

The first thing that interests me is the statement about other scientists failing to find an explanation for this distribution, and I wonder whether actually this point comes down to a change in what is considered a sufficient explanation for ecologists. There's also been another recent paper on the way biologists have changed how they think about explanations over the last few decades. This paper, by two philosophers of science, charts the first discussions of different levels of explanation by Ernst Mayr back in 1961 who defined proximate explanations (e.g. it got too hot and the bird's died of sheatstroke) and ultimate causes (people burnt fossil fuels, releasing carbon dioxide into the atmosphere leading to global climate change, and that was the end of the species). They're both valid descriptions, but they answer slightly different questions. Not long after then Niko Tinbergen (who my little brother did his PhD about, by the way - rather strange having a brother who studied biologists..) also refined these further saying that before something could really be explained you needed to answer four types of question what's the physiological mechanism? What's the development pathway that gets you there? What's the ecological function? And what's the evolutionary history? Now, the latest paper suggests the whole cause and effect idea might be more complex than we originally thought, with evolution acting in feedback sometimes - but that's a whole different set of ideas. Still, ecologists looking for an explanation of distribution over the last few decades may have been looking to explain distribution in these ways. Recently, I'm wondering if we settle too fast on a rather superficial explanation that doesn't really meet these criteria.

Certainly, no-one could call the work on the Bush-crow an explanation for the distribution according to any of the four criteria Tinbergen was looking for. And of Mayr's original two it might, perhaps, be an ultimate explanation. But let's look at that - in this study, as many others of the type, the authors started by looking at five different climatic variables (we don't know why they chose these from the 19 they had available): mean annual temperature, mean annual precipitation, temperature seasonality, mean diurnal temperature range and annual temperature range. Now, let's ignore the results for a moment and ask what sort of explanation we could possibly derive from these chosen variables. Could any come close to identifying a physiological limit? As they're all means (IE averages) I really don't think so: by definition the actual temperatue on any one day is pretty much certain to be above or below the mean. Similarly, it seems likely that a physiological limit to mean diurnal termperature variation is unlikely - though if the variation is too high on any one day I could see a real limit. Unfortunately, there's no way to tell from an average alone whether the sort of extremes that could potentially cause direct physiological problems are present as well. So instead of a direct (proximate) limit, using means is more likely to identify an indirect effect: one mediated and perhaps integrated over time and space by something else, or maybe the mean is simple pointing to some associated measure that is correlated with it. That correlated variable could be the extremes that might ahve direct effects, or it could be the demographic of some slow-growing plant that's essential for nesting, for example. In fact, the author's rule out any indirect effect by noting that the species is a generalist feeder that likes degraded habitats - unlikely to suffer any shortage of food or nesting spaces in any habitat. So we're left with the explanation that mean temperature (which the authors show has the highest association with the observed range) might be acting as a proxy for some as yet unknown temperature-related variable that could have a direct effect.

Now, even without bringing up my general concerns with this sort of analysis (e.g. that the statistics used are inappropriate for spatial data and will 'explain' any distribution you care to throw at them), I have to wonder if this 'explanation' for the bird's distribution has never before been suggested for the simple reason that it's never before been considered adequate? Indeed, is it, as a biological explanation, any more satisfactory that simply saying these birds live on hills in southern Ethiopia? Looking at one of the main figures (right) from the paper I can see two things that would seems surprising to me: firstly both preferred temperature and preferred rainfall at incredibly narrow. If this is true then, as the authors rightly point out, it would be of extreme concern given the likelihood of future change in these areas. But it seems implausible to me: annual rainfall in the 600mm range of eastern Africa is so variable year on year that the average itself is essentially meaningless to anything except a long-lived plant like a tree or slow-growing bush. I can't imagine a plausible ecological mechanism that would allow such a restricted range on a map of mean rainfall, when the annual rainfall is such that it rarely falls within this optimal range anyway! Secondly, I see in the seasonality plot two peaks - the birds apparently like moderate seasonality and quite high seasonality, but don't 'moderate-to-high' so much. Such a dislike of intermediate seasonality immediately rings alarm-bells for me: it's physiologically not plausible and, together with the implausibly narrow limits identified for rainfall suggests the model is 'over-fitted' - in other words, it's not going to be reliable for prediction of distribution under changed climate conditions.

Unfortunately, therefore, I'm not convinced we've really learnt much here. Certainly, we have no explanation of distribution that could meet any of the older types of biological explanation (nor the newly suggested one), but we do have a fairly typical example of what seems to be a lowering of the barrier in ecological explanations: to me, a suspect correlation with little critical ecological discussion cannot be considered an explanation. Come on ecologists, we're better than this!

Main Reference:

Donald, P., Gedeon, K., Collar, N., Spottiswoode, C., Wondafrash, M., & Buchanan, G. (2012). The restricted range of the Ethiopian Bush-crow Zavattariornis stresemanni is a consequence of high reliance on modified habitats within narrow climatic limits Journal of Ornithology DOI: 10.1007/s10336-012-0832-4

How do Kopjes form?

2012-03-13T11:51:00.003-07:00

It's a question I regularly get asked by guides and also one that seems to bring a lot of google-searching visitors to the site, but I've not actually posted much of an answer yet although we have covered it briefly here, so here goes...

Cross-section through a kopje in the process of formation
from smooth, uninterrupted landscape at the top to
typical kopje at bottom, following millions upon
millions of years of erosion.

We start by remembering that Africa is old - most of the surface rocks are pretty ancient (and consequently washed clean of most nutrients - an issue we've talked about repeatedly). During these millenia, mountains have been formed and then worn down to small hills, whilst the valleys, plains lakes and seas have been buried in the sands and gravels of this erosion process. Over time and with immense pressure these sands and muds too have sometimes been 'recycled' into sandstones and mudstones in someplaces. It's not just been static though: later volcanic events sometimes push magma (un-errupted lava) through the layers of rock towards the surface where it cooled and formed an intrusion of new rock within a mass of older layers. (As shown in the diagram!)

As more time goes by, the overlying layers are gradually worn away, until eventually the lump of volcanic rock becomes exposed. Granite is a typical volcanic rock and is known for its hardness - the surrounding rocks (especially if they're sedimentary rocks like sandstones or mudstones) may be significantly softer and therefore as the erosion processes continue, they'll get worn down faster than the hard granites - they do erode, but not at the same rate as the softer rocks within which they are embeded. Eventually the hard granites too start to split, and in time may even form beautiful piles of stacked rocks along points of weakness in the harder rocks. This is all illustrated in the diagram on the right: at the top we start millions and millions of years ago with lots of layers of relatively soft rocks forming a smooth, uninterupted landscape. Below that an orange volcanic intrusion pushes up through the softer rocks, but cools underground into hard granite. As more time passes (moving down the sequence of figures) the top of the intrusion s eventually revealed at the surface (fig 4), still further erosion of both the soft rocks around the granite intrusion and the granite itself (but at a much slower rate) continues and a kopje forms. Eventually erosion of the granite may form the typical piled rocks (called tors) of the most impressive kopjes.

Moru Kopjes in Serengeti have some impressive tor features (piles of rock)

As I say, all of this has happened over millions and millions of years: the kopjes of western Serengeti are themselves over 550million years old, and the surrunding rock through which they protrude far, far older still - over 1200 million years in much of Serengeti (these figures are all coming from here). The kopjes also don't need to be granite - many Serengeti kopjes are Gneiss, which is a metamophic rock (actually, often metamophosed granite!), the only important thing is that the kopje be derived from a rock that is substantially harder than the rocks around it.

And that's it really - nothing too complex at all!

Why do birds sing in the morning?

2012-03-12T05:20:00.001-07:00

Ruppell's Robin-chat: an impressive mimic. Lake Duluti

I enjoyed a walk around Lake Duluti yesterday morning and came across a couple of wonderfully singing Ruppell's Robin-chats. These are great birds, with an amazingly varied song hat's gull of mimicry (of you want to hear one, listen here!). For me, one of the best things about camping in the bush is being able to lie in bed and listen to the birds waking up while it's still too dim to see them properly. The dawn chorus is a worldwide phenomenon and I'm often asked about bird song, so I thought it would be worth exploring some of the theories behind bird song, and - particularly - why birds sing in the morning. It's something that's interested me since I was introduced to the question by a friend of mine who did a PhD on the subject some years ago, and I know he reads the blog so I'm hoping he'll make sure I get the answers right!

There are two parts to the question, of course: why do birds sing? And why do they sing in the morning more than at other times of day? The first I think we've had a pretty good idea about for a long time, and there are two main reasons: to attract mates, and to claim their territory. In general the two actions aren't mutually exclusive - as a male bird you can sing to both let the other males around know that you're still in your territory, and you can at the same time let females know that you're around and looking for a mate. For some species by listening to the song you can tell what the bird is actually trying to say - Nightingales are well known in Europe for their beautiful song, but it's not uncommon to hear them singing down here during the non-breeding season too (they seem to be more territorial than many migrant species). The difference is that here, I've almost never hear the typical long whistle notes that are so frequent in the breeding areas during the early breeding season (listen to them here). They also use even more of these whistles during the middle of the night before they find a mate, and it seems that these whistles are particularly for attracting females, which obviously isn't relevant when not breeding. As females might be flying over at night, singing in the middle of the night and whistling a lot, might be a very good way to attract a female to your territory.

Yellow-vented Bulbul - nesting all year round in my garden (March 2012),
both sexes sing far too early in the morning for comfort...

For some species song is probably more important for attracting females than in other species - particularly, I would suggest, for species where mimicry is very important. Like my Ruppell's Robin-chat, many birds are accomplished mimics, and observations suggest that the more varied an individual male's repertoire, the more attractive the male. The most impressive example of this that I'm aware of is the Marsh Warbler, a species that we mainly see on passage here in Tanzania from time to time - though it is another than sometimes sings during the non-breeding season. In what's become one of the classic studies of song mimicry, 30 individual males were recorded during the breeding season and the songs then analysed and mimicry identified. The total diversity of song mimicked by these 30 males was 212 species, 113 of which were African species! On average, each male knew the songs of 76 different species, mostly African - one male included identifiable fragments of 84 species - an astonishing variety of songs and calls! Why do they do this? Well, it seems most likely that females prefer the males that have the most varied songs - males that can learn lots of different songs have clearly got some good mental abilities, whilst - the hypothesis goes - must be tied to their ability to find food, avoid predators or otherwise help around the nest. So there's not only a chance to let a female know that you're around like the nightingales do, but there's a real competition going on to be the best singer of all.

As I said before, there's also plenty of evidence that it's not only about attracting a female: even after finding a female and her sitting on the nest many species keep singing for a while - this is clearly about territory defence. And the first example of how effective this is comes from a nice experiment where birds were removed from their territories and the time it took other birds to come and occupy the patches was recorded: it's pretty quick, but if you play a tape recording of the song of the resident male even though he's been removed, it takes a lot longer. Clearly birds looking for a territory recognise individual songs and as long as the territory holder is still around and still singing they won't bother trying to take over - an efficient system of defence if ever there was one.

Usambiro Barbet, a typical African species with duets. Serengeti, Dec 2011.

Now, that's the general answer to why birds sing. But here in Africa there's an additional complication we should consider: here, many female birds sing too. This isn't unknown in the north, but it's fairly unusual. And we've already seen that a whole load of African birds engage in complex duets too, with both male and female taking part. Why is it that many more females take part in song here in Africa and the rest of the tropics than they do elsewhere? There's no good answer to this one yet, but a couple of interesting theories are being developed that are worth mentioning. Firstly, as we've already talked about, birds in the tropics tend to be rather long lived - they do things slowly, and they have a rather tougher time raising young in many ways than in more temperate regions. So here they're often also with their partner longer, and often holding territories year-round - lots more reason for females to be involved in territory defence as well as males, so that's one possible reason. The other theory is that here in Africa the seasons might be harder to define (at least in the forests), with breeding possible much of the year (certainly the bulbuls in my garden think it's fine to nest all year around). But birds don't tend to keep their ovaries or other bits of breeding equipment large and ready to breed year round - it would be a waste energy to carry around fully developed genitalia if you're not going to use them, and flight is tricky. So in a less seasonal environment, how do both male and female make sure they're in breeding condition at the same time? Well, perhaps, by singing to each other - there's certainly some evidence that song can help bring on breeding condition in some birds. Interesting thoughts though, and (yet) another area of tropical ornithology where we're well behind on theory and experiment.

So. Having tried to answer why birds sing, let's tackle why we think they sing most in the morning. There are a number of (not mutually exclusive) theories here too. They all start with the assumption that singing is good (for the reasons we've covered above) - but it's costly in terms of time and energy (another reason why females might like males that sing more: they're showing honestly how strong they area). So the challenge birds face is in deciding when and how much to sing. One of the oldest ideas is that they sing in the morning because it's still too dark to be out and about finding food, so you might as well sing. That's a pretty solid idea - but it doesn't really explain why they sing more in the morning than in the evening, when the light also fades - or even in the middle of the night. Another theory is that the conditions early in the morning - often cool and with lower humidity than later in the day - might be particularly good for letting the sounds of the song carry further, though recent experiments suggest that actually the middle of the day might be the best time to sing if acoustic conditions are what's important so I don't think that's a winning idea. The third and most interesting theory is that birds sing most in the morning because that's when, most days, they've got spare energy to use up [Rob, surely there's a short snappy label for this theory? The McNamara/Mace/Houston hypothesis seems a bit of a mouthful? The stochastic environment hypothesis?].

Many species, like this Cisticola are best identified by song.
More drinks for the first correct ID! Nr Arusha, March 2012.

This is a much more complex argument that's absolutely fascinating in it's detail and worth looking into in some detail. It starts with some basic assumptions - like the others it says singing is good, but other activities are also important - feeding, sleeping, etc. And most important of all, it requires that the world is variable: the conditions you find today such as how easy it is to find food, what the temperature is, how long you have to sit out a rain storm, are all variable day to day. When you go to bed at night, you don't know how cold the night will be, and when you wake in the morning, you don't know how long you'll have to spend finding the food you need. Given these initial conditions, this theory says that birds are essentially playing a game of survival. Each day they need to find enough food to survive the following night (when they can't feed because it's dark) and they'll fit singing around these activities when they have energy to spare. Now, birds live very close to the edge much of the time and each night they loose a significant amount of weight just keeping warm: if they don't start the night with enough extra fat to burn the could easily die overnight. So because they don't know exactly how cold each night might be, the only way to be sure of surviving is to have enough fat to survive the coldest night they can imagine might realistically happen. Most times, of course, it turns out not to be so cold and when they wake in the morning, they've got some spare energy to use before they need to start feeding again: the perfect time to sing. During the day too they don't know how hard it will be to find enough food to have the required amount of fat by the evening, so once they start feeding they're wise to feed like mad for a bit, just in case it's tougher to find food than they thought, but then if they keep finding food all day, by the evening they might have actually got fatter than they really need to be (it's unwise to have too much extra fat, as it makes you less manoeuvrable and more vulnerable to predation), so again you might want to sing a bit in the evening too - which is exactly what we see. These predictions and assumptions were tested in a nice series of experiments by Rob Thomas (for example, in one experiment he fed individual robins the same amount of food during the day, but some were persuaded that there was a regular and constant supply of food throughout the day, others thought it much more variable getting lots of food at one point, but much less later in the day, for example, and then listening to how long they sang that evening or the following morning: more variable gives more song), with the general conclusion that the theory is fairly robust. Which also explains why the dawn chorus here in Tanzania, where the weather from day to day is fairly predictable, is much shorter and less impressive than, say, in a British woodland where it's quite possible one day will be warm and sunny and the next day cold and snowy! Brief as it is, the dawn chorus here is still impressive in many parts of Tanzania and well worth listening to when you're out in the bush - the best way to start the day I think!

Main References:
Slater, P., & Mann, N. (2004). Why do the females of many bird species sing in the tropics? Journal of Avian Biology, 35 (4), 289-294 DOI: 10.1111/j.0908-8857.2004.03392.x

Thomas, R., & Cuthill, I. (2002). Body mass regulation and the daily singing routines of European robins Animal Behaviour, 63 (2), 285-295 DOI: 10.1006/anbe.2001.1926

Lewa Downs wildlife corridor really works!

2012-03-08T11:14:00.000-08:00

As regular readers will have realised, I'm something of a sceptic about most things, and one of the things that I've been pretty sceptical about in the past is wildlife corridors. They sound like a great idea: wild spaces are increasingly fragmented (even here in East Africa), and as that process continues populations of plants and animals within these areas will become increasingly isolated from one another. Isolated and small populations are more likely to go extinct than large, well connected populations for a number of reasons ranging from inbreeding - in small populations you're rather more likely to have to mate with a brother or sister than in a large population, which can have serious genetic costs, to simply the risk of extreme events wiping everything out. So connecting those fragments with corridors along which animals can pass seems like a really good idea. Tiny experiments using micro-ecosystems where no-one cares if you isolate populations or connect them seemed to suggest that there might be something in this idea, and all of a sudden conservation corridors were high on the agenda.

The problem with this idea, is that like rather a lot of conservation, it's a good idea with some theory to support it and even some small-scale experiments, but there's rather little experimental evidence that it works in practice. Sure, if you increase the amount of available habitat whilst connecting one patch to another it's good news - but that's not really the point of a corridor and you might have been better able to spend the money buying the narrow strip of expensive land you want a corridor on purchasing a much larger area of cheaper land elsewhere if the benefit is only from the increased habitat area. What's more, once you've identified an area as a corridor it often means that the surrounding area is implicitly identified as unimportant, and may even exacerbate the problem. A quick google scholar search gives a fairly clear view of the debate: Do Habitat Corridors provide connectivity? Biological Corridors: Form Function and Efficacy. Movement Corridors: Conservation Bargains or Poor Investments? Etc. etc. On the one side there are the sceptics who see huge sums of money being committed to conservation corridors with little evidence to suggest they'll meet the stated objectives. And on the other side there are those who say it's sure to work, and can't be bad even if it doesn't.

So, when I heard from a friend recently that he'd been up on Lewa Downs (Kenya) and had seen their elephant corridor functioning, I was really impressed. This corridor connects My Kenya with Lewa Downs and from there to the rest of the Laikipia plateau. What would have made me extremely sceptical about the project's success was that it involves an underpass beneath a main road that's so small elephants have to pass one by one and large animals have to duck to get through! But, despite all this, they do it: the first animals made the crossing last year only four days after the corridor was completed (and the next day was darted to fit a radio collar - bet he regretted that move!). I'm told now that buffalo and smaller animals are also all making the crossing (though not rhino because they've fitted gates that elephants walk over [how about babies?], buffalo can fit through, but rhino can't - quite a good idea if you want to know where your rhinos are. And at the moment I think that's essential...) That's pretty impressive to me (I'm told there are similar - and even smaller - underpasses in South Africa too), and I'm pleased to hear about it's success. I wonder how much is possible because elephants use such traditional routes all the time - would similar underpasses be possible for the wildebeest of Serengeti under a main road if it ever gets built? I wonder... (I do know it wouldn't be such a lovely place to visit!)

I'll leave it to others to decide if it was the best use of the funds in the first place - I can't find an exact figure anywhere, but the rumours are that the whole corridor cost over $1million. Was a real cost benefit analysis done? I'd love to see it if so!

Nairobi bugs: WMD or Cancer cure?!

2012-03-06T08:50:00.000-08:00

15 times more toxic than cobra venom, you really shouldn't eat a Nairobi beetle!

Nairobi bugs (also known around East Africa as Nairobi Eye, Nairobi Fly, Nairobi beetles, Blister Beetles and a whole range of other names) are not the best loved creatures out here. This year they've come out in greater number than the last few years, presumably thanks to some relatively good rains, and whilst they're not loved, they're certainly fascinating wee beasties. But before we go into the details, let's start with some identification preliminaries.

There are actually at least two species of beetle known as Nairobi bugs around here, but they're so similar that most people won't notice them. Similarly marked relatives of these two are pretty widely distributed across the world, mainly in the tropics, and for now I don't think we need to bother about the precise identification. They're all small (7mm-1cm ish) and well marked with typical warning (aposematic) colours of black and red. In fact, despite the variety of names these are beetles (Coleopterans) of the family Staphylinidae, the rove beetles. If you don't know the Nairobi beetle, you might well know the Devil's Coach-horse and similar species - much larger and all black, but of a similar basic structure. The beetles we're interested in are of the genus Paederus and are carnivorous beetles that live mostly in long grass and anywhere with rotting leaves. And the most interesting things about them, as anyone will tell you, is that whilst they neither bite nor sting, they're still seriously nasty.

All you have to do is brush against or slightly squash a Nairobi but, and you'll likely end up with chemical burns wherever you came in contact with the beast. It's a pretty remarkable defence (and fairly effective at dissuading the main predator - wolf spiders - from eating them too, though not other insects) and whilst looking things up for this post I was amazed to discover the the British, Canadian and US armed forces all take the beetles very seriously: I particularly like the US paper on "Entomological terrorism", suggesting that Paederus beetles could be used by an enemy in a direct attack. And the Indian Armed Forces investigated the use of the beetles in chemical warfare back in 2002! Indeed, there's even a suggestion that two of the 10 plagues that the Ancient Egyptian suffered we caused by mass emergence of these beetles that was brough on by the first two (Nile and frogs): Number four (flies), followed a few days later by number 6 (incurable boils).

Paederus crebinpunctatus or P. sabaeus? Anyone know? Arusha, March 2012

That actually leads nicely into what started me puzzling over these beasts in the first place: a beetle crawls on you, you damage it as you brush it away and nothing happens. Huh? Then a day or two later, your skin turns red and soon it starts to blister nastily, the sores lasting (I can say from personal experience!) for 10 days to two weeks. A pretty seriously long time for such a nasty experience. So I set out to research what makes these beetles quite so nasty. As ever, the more I learnt, the more fascinating it becomes. It turns out that the toxin involved is highly toxic - about 15 times more toxic than Cobra venom, (which means that the dose you have to give a group of 100 rats to kill 50 of them is about 1/15th that of cobra venom) so it's just as well they don't bite! And trust me, you really, really don't want to eat one of these beetles. But it's action is far from normal - this isn't an acid burn as some would have you think (in fact, the active ingredient is an amide, so probably slightly alkaline), but something far more interesting. It turns out that whilst we still don't know exactly how it does it, the active ingredient (called pedarin, as it comes from Paederus beetles) has the almost instant effect of stopping the cells making protein and DNA. That might not sound too bad, but over a day or two it's pretty serious (hence the interesting delay in the action) as cells that stop doing these things die (in fact, the cells kill themselves, in a process known as apoptosis).

Now, stopping protein and DNA synthesis (but, curiously, not RNA synthesis) is a pretty remarkable activity for a biological toxin, and like a lot of defence chemicals it's attracted a bit of attention from the pharmaceutical world. One property in particular is very interesting: it stops cell division (mitosis). Now, if you know anything about cancer, you'll know that a cancer is simply a bunch of cells that have forgotten to stop dividing. So a chemical that stops cell division, and can even induce cells to kill themselves is going to attract a lot of interest. And sure enough, pedarin has been shown to slow the growth of a cancerous tumour in mice. I've not found any research yet that takes this further into humans, but watch this space...

Interestingly too, it turns out that pedarin isn't actually produced by the beetles themselves. Only females can produce it themselves (though males can have some from the egg), and only some females at that. In fact, it's produced by symbiotic bacteria living within the insect, a species of bacteria that is fairly closely related to Pseudomonas aeruginosa, a fairly well-known disease causing bug. It turns out that females with the bacteria lay eggs that are also infected with the bacteria, allowing their offspring to be pre-infected with it (though males can't keep it and must make use of the tiny quantities of chemical in the egg). Females without the bacteria can't do this, but they can be infected with it if they eat the eggs or larvae of a female that is infected, and then they, too, can lay eggs pre-infected with the bacteria. Neat! And there's even a theory that squashing the beetle releases not only the beetle juices (technically called haemolymph), but also the bacteria too, which can keep living in the skin for a while, producing yet more nasty pedarin - and explaining quite why it might take so long to heal. As I say, they're really, really nasty bugs!

Main references:

Kellner, R. (2002). Molecular identification of an endosymbiotic bacterium associated with pederin biosynthesis in Paederus sabaeus (Coleoptera: Staphylinidae) Insect Biochemistry and Molecular Biology, 32 (4), 389-395 DOI: 10.1016/S0965-1748(01)00115-1

Jewett, J., & Rawal, V. (2007). Total Synthesis of Pederin Angewandte Chemie, 119 (34), 6622-6624 DOI: 10.1002/ange.200701677

Brega, A. (1968). STUDIES ON THE MECHANISM OF ACTION OF PEDERINE The Journal of Cell Biology, 36 (3), 485-496 DOI: 10.1083/jcb.36.3.485

Migrant bird population declines, an African perspective

2012-03-04T22:00:00.000-08:00

Willow warbler singing in Africa - 10g but probably headed to eastern Siberia...

March is the month when northward migration of songbirds gets underway in East Africa, so this weekend I was excited to be out west of Arusha with friends and to find stacks of migrants already on the move. Driving in I noticed some really smart looking wheatears (both pied, and the very impressive northern wheatear, though many of them have already set off on their mammoth treck - perhaps as far as Alaska). But the highlight for me was the bushes alive with warblers on Saturday morning. I saw flocks of Willow Warblers, Olivaceous Warblers, Common Whitethroats and even little groups of Barred Warblers, usually a very scarce migrant around here. Some of them were even singing, in anticipation of starting breeding in a few more weeks when they get back to Europe! Having a managed a few photos I thought it the ideal opportunity to talk about bird migration.

Barred warblers are always a treat to see: headed to eastern Europe.

All these birds have been rather scarce until now, this season, and many of us have been wondering where they've got to - usually Willow Warblers and Olivaceous Warblers are one of the commonest birds in the bush from November to March, this year there have hardly been any. It's a question that will be familiar to many readers from Europe - where have the migrant birds gone? Research has suggested over the last few years that in Europe at least, migrant birds are declining faster than resident species, a change that has been attributed mainly to climate change. A number of theories have been put forward to explain why migrant birds may fare worse than resident species from the impacts of climate change - from them simply missing the peak spring food availability by arriving to late in Europe as springs get warmer (and therefore earlier), to direct effects of drought or land-use change in Africa. A recent paper (sorry, not free) has attempted to look into some of these likely causes using data on breeding population changes in the UK, and it serves as a nice bit of background to some of the remarkable things that birds do when they set off on their amazing migrations.

Spotted Flycatcher, headed to the forests of Eurasia

Obviously any study seeking to understand population changes in migrant birds from a purely European basis (let alone a purely UK perspective!) is going to have some limitations - this study is no exception. Essentially, the study is a series of correlation analysis trying to match population trends with some of the things known about the ecology of the species and look for common patterns. If they find common patterns there's no guarantee that the patterns are really causal (IE we can't say for certain that populations are declining because of factor X), but it can give us some good pointers for further and more detailed research. That's especially true of this analysis, because data on many of the direct factors people are interested in simply aren't available: instead of having detailed information on how the climate has changed in each species African distribution, they have to use large-scale region the occupy (such as the arid Sahel, the moister Forest/Savannah mosaics, etc.) as a 'proxy'. But that same large-scale region, is also going to enjoy similar variation in any number of other factors, not least land-use pressures, so just because consistent patterns might be found between the regions each bird species spends the winter in, doesn't mean we can say hat's a consequence of climate change, or land-use change. Other factors they used in the study include the habitat used during the time in Africa - forest, wetland, open or generalist, and a variety of factors detailing arrival times in Europe in the Spring and the changes in these arrival times (this is potentially interesting because, say, birds that always arrive late in the spring might be more, or less, affected by changes to early spring temperatures than those that normally arrive early. Similarly, those that have changes their arrival times fast, might be less impacted by further climate change than those that have, so far, not changed at all).

Eastern Olivaceous Warbler - headed to central Asia or south-eastern Europe.

To cut a long story short Okenden and colleagues conclude that the most important factor, by a long way, in determining rate of decline is the large region within which each species winters: if a species winters in the arid Sahel, it's probably not declined, but if it prefers the more humid regions then the chances are high that a decline has occurred. Next most important was the habitat occupied during the winter: those generalist species showing few declines compared to those specialists of open or woodland habitats. They also find some evidence that the mean data of arrival in the UK is important - species arriving later having steeper population declines whereas those birds that are able to change their arrival data rapidly have slower declines (or even increases).

Now, it's pretty hard to interpret the main effects of region too much - it's quite possible that regional land use pressures are a factor (which is supported to a degree by the additional importance of habitat), but equally regional patterns in climate change also tend to vary together. So let's not worry so much about that for now (though it is far and away the biggest factor correlating with the declines - my guess from knowing the situation on the ground is that to date land degredation is far and away the biggest cause down here, not climate change yet), and focus on those factors to do with timing, because it's here that things get interesting in our part of the world- by watching when our birds leave here, we can help untangle some of the questions around departure / arrival dates. It has been suggested, for example, that it might just be that the birds don't know when to leave here: in most of the world we know that birds can monitor precisely changes in daylength to determine dates (17mins difference 'summer' to 'winter' is the shortest known detected difference by a bird). If the date of migration is 'hard-wired' into the bird (ie if it hatches with a programmed departure date), it takes evolution to vary that feature. That's probably even harder here on the equator, where there is essentially no meaningful day length variation. Here, we believe the only way birds can leave the area at the right time is by, somehow, maintaining a 'clock' throughout the year that counts the days and tells them when to leave (experiments keeping birds in constant environmental conditions demonstrate that this really does happen). What the mechanism of this clock might be, however, we have very little idea, but to maintain such a clock and then change the settings for a different departure date could be rather complicated, meaning these migrants are simply unable to change fast.

Poor picture of a smart male Pied Wheatear, heading to eastern Europe

Alternatively, there's a theory that suggests many long-distance migrants might be constrained in their departure date by having to wait for the start of the rains to encourage lots of insects out, so the birds can feed fast enough to fuel for their migration journey. [If that's the case, our early rains this year should be keeping everything very happy and maybe we'll see early arrivals in Europe this year too!] In other words, perhaps birds can happily survive here through the dry season, but they can't get the extra food they need to power a migration until the rains arrive and their departure is limited by when that happens - which doesn't vary as much, or in the same direction as changes have occurred to spring temperatures in Europe. Migration takes a huge amount of effort and energy (we covered some of it in the Wheatear Migration story) and the adaptations that some birds have evolved to undertake these journeys are extraordinary. Not only can they double their body mass with stored fat with no apparent heart-attack risk, but the physiological changes involved in migration are just extraordinary: birds feed like crazy to accumulate fat, but so costly is having excess weight, and so large the costs of migration that as they set off they metabolise some of their digestive system itself, an extraordinary adaptation that means on arrival or during stopover before they can feed effectively they must first rebuild their digestive system. It's not hard to believe, therefore, that they might ave to wait until feeding conditions are optimal before being able to start a migration.

If, like me, you find these adaptations extraordinary, even before you look at how the birds use magnetism, stars, sun and even sound cues to navigate, it's easy to forget how recent all of this has evolved. Although there might well be challenges involved in evolving different migration route or different migration times, evolution is an immensely powerful process. The whole of the current European/African migration system is, for example, a very recent affair: only 12,000 years ago most of northern and central Europe was covered in ice. So the current migrations have all evolved since then, whilst the average evolutionary age of birds involved in migrations (IE, when they last shared a common ancestor) is around a million years. Indeed, one of the most remarkable ideas coming from recent theory about bird migrations is that, once the basic ability to do partial migrations had evolved in birds (probably very soon after birds themselves appeared), it only takes around 25 generations (40 years) for a species to evolve complete migratory behaviour, or complete sedentary behaviour. So migration is actually a highly dynamic capacity of birds, and certainly far from fixed. Indeed, we're already seeing some interesting changes in migratory behaviour in Europe, the best example being Blackcaps which used to spend the winter around southern Europe or into north Africa (some make here here to Tanzania, usually in the forests), but a population has now evolved a completely different strategy involving heading north west from Germany to UK, where winters are getting warmer and there are lots of people who feed the birds. So effective has this migration been, that the population making this movement is rapidly increasing.

Not all our warblers are migrants - ID this one and I'll buy you a drink...
(Desperate attempt to get more people to comment, I know!)

So it's easy to look at current population declines of migrant birds and despair: the road seems set to zero. But a historical perspective suggests that birds still have one or two tricks up their wings. For sure we need to do what we can to help migrant birds, and if we continue to lose or degrade habitat across Africa it's bad news not only for these migrant species. But evolution is on our side, so let's hope what between us we can do something to keep this amazing migratory phenomenon going for future generations to enjoy! And in the mean time, if you're in the bush in the next few weeks, do keep your eyes open and enjoy all the migrants!

Main reference:

Ockendon, N., Hewson, C., Johnston, A., & Atkinson, P. (2012). Declines in British-breeding populations of Afro-Palaearctic migrant birds are linked to bioclimatic wintering zone in Africa, possibly via constraints on arrival time advancement Bird Study, 1-15 DOI: 10.1080/00063657.2011.645798

The role of termites in the savanna biome

2012-03-01T11:58:00.003-08:00

The ground is crawling with termites! Nr. Tarangire, Now 2011.

Termites are hugely important to the ecology of the savanna biome. We've covered some of their roles here before when we talked about termite mounds and when we covered nutrients and nitrogen in the savanna biome. The numbers of termites in savanna habitats can be quite extraordinary: with over 400/m² of soil, their biomass can exceed that of mammals in the ecosystem. Such a huge abundance of animals mean that termites, by weight of numbers alone, must have a massive impact on the ecosystem. We've seen how they are crucial for keeping nutrients cycling rapidly in the savanna, how their excavations can change the texture of the soil and how these impacts change the plants and, ultimately, the behaviour of animals within the savanna. Despite this obvious importance, however, there's surprisingly little research on what they actually get up to and where they really are - I guess researchers are generally too busy tracking lions sleeping under a bush than worrying about termites under their feet... It's important though, as processes that cause spatial variation in patterns of nutrients and such-like are increasingly being perceived as vital to the ecosystem as a whole, and if we don't understand the processes that cause variation, it will be much harder to understand what's going on at larger levels. Still, some work is coming out now, and a paper last year caught my eye.

Dead wood too is covered with the things!

This paper (hidden here, I'm afraid) is by Buitenwerf and colleagues and reports on an experiment they did to measure termite activity across patches of Hluhluwe–iMfolozi Park, South Africa. In some ways it's a direct follow up to a similar study carried out here in Serengeti and in fact Han Olff is an author on both studies. The idea is simple. It's possible to count termite mounds and get some ideas of the distribution of termite that way, but (a) most termite species don't actually make termite mounds and (b) even if you know where the mounds are, you don't necessarily know exactly where the termites are. So as well as doing that the authors of both studies set out a network of termite baits. In both places these consisted of cut, dried grass tucked up in a mesh bag that was big enough to let termite in and not much more. In Serengeti they also filled some bags with wildebeest dung. Each bag they then took to the field and nailed to the ground, around which they recorded a few things that might affect termite activity (like the amount of grass and bushy cover present). They left the bags out in the field for a month and then went back and brought them all in again. Since they knew how much grass or dung they put in the mesh bags at the start, all they had to do to get an idea of termite activity at each spot was to sift the soil out (termite often forage in soil tunnels they create) and weigh the remaining matter. Pretty simple set of experiments.

Females emerge with the rains and are food for everything!

Termite study area in Serengeti and shaded map of termite activity (B)
- dark is more active. From Freyman et al 2010

In the Serengeti they did the experiment twice - once in the dry and once in the wet season, in Kruger the experiment only happened once (and we're not told when). Whilst the Serengeti experiment is clearly a much better job than the South African one, we actually know that termite activity is far from simple to predict and changes dramatically month to month so I think there should be a serious health warning on both these studies. Never-the-less, there are some interesting results - in Serengeti (the first study) they discovered that during the dry season activity was much higher on the top of the ridges (where the soils are sandier, and you'll remember there are often more termite mounds) than on the bottom (where the soils are deeper, but more full of clay). In fact, there was almost no activity at the bottom of the ridges at all during the dry season. That makes a certain amount of sense - especially as the study are was the long grass plains south and south-east of Seronera where the Thomson's Gazelle's hang out in the dry season, and they prefer to be on the ridges with a good view of cheetahs: if you like nicely packaged dung to eat, with a bit more moisture than simply dry grass, you're better off concentrating on the ridges during the dry season. During the wet season the gazelles aren't in the long grass plains and the story is rather different, with activity both top and bottom of the ridges, but - it seems - a lot more variation within these areas.

Major differences in activity in relation to rainfall and large mammal presence.
Note each dot only represents two plots(!). From Buitenwerf et al 2011

With this as background the second experiment in South Africa set out to study that fine-scale variation in much greater detail, whilst also aiming to study directly the impacts of grazing animals and rainfall gradients on termite activity. Unfortunately, the experiment seems much less fully thought through than the Serengeti one - using only four pairs of plots (in each pair one was in an area fenced to exclude large mammals, and one beside the area where mammals are present) across the landscape to investigate two main variables: rainfall and mammal activity. Within each plot they had 100 bags of grass, but that still only leaves eight plots to explore the larger-scale differences that they were most interested in. Not a good survey design at all (not to mention the unknown season, when they already knew season makes a huge difference)... Still, the results are worth noting as a start, if nothing else, especially as there might be something interesting here. Unexpectedly, the total activity is higher in the lower rainfall areas than in the higher rainfall areas (though the difference is only from 600 to 850mm, so nothing like the gradients available in TZ parks). We'd probably have expected the opposite, as high rainfall areas have more grass that can provide more food to termites. So there's the first surprise. Secondly, the effect of mammals seemed to differ depending on whether the plot was in the wetter or drier area. In the wetter areas (where activity was generally low anyway), activity was lower in the grazed areas than in the grazing exclosure. In the drier area it was the other way around. Now, bearing in mind the (huge!) caveat that these samples sizes are so small as to be nearly useless, and the fact that activity can vary seasonally, but that the degree of seasonal variation might be related to overall aridity (less seasonal variation in drier areas), let's think about what this might mean if, indeed, it is real. The authors suggest that in wetter areas it might be a direct competition effect: grazers eat the grass, so there's not much for the termites. That might be true, but if so, why isn't it also true in the drier areas? They suggest that in the drier areas the activities of herbivores actually provide extra food for the termites - through dung and by dropping bits of grass, but have no ideas why it might be other way around in the wet areas.

Matabele ants on a raid - everything likes a termite... Mara River, Sep 2011.

My suspicion is that this is probably an artifact of a too-small experiment, perhaps being carried out at an inappropriate (or at least not complete) time of the year. But if not, it's something to puzzle about next time you come across a bunch of termites busy foraging... And if you don't fancy that, at least remember how hugely important termites are in the savanna - their activity certainly is patchy, which must be generating a lot of variation in grazing quality at both fine and large scales. Certainly worth some more study! [And that's before we remember just how many things eat them! There are specialist birds, mammals, invertebrates and all the rest - it's tough being a termite!]

Main references:
Buitenwerf, R., Stevens, N., Gosling, C., Anderson, T., & Olff, H. (2011). Interactions between large herbivores and litter removal by termites across a rainfall gradient in a South African savanna Journal of Tropical Ecology, 27 (04), 375-382 DOI: 10.1017/S0266467411000125

Freymann, B., de Visser, S., & Olff, H. (2010). Spatial and temporal hotspots of termite-driven decomposition in the Serengeti Ecography DOI: 10.1111/j.1600-0587.2009.05960.x

Exercise like a lion!

2012-02-28T11:58:00.001-08:00

Wildebeest wrestling - the ultimate fitness regime? Selous GR, June 2010.

I came across a paper this last week - I can't remember how, because it's certainly not my usual reading material (though my wife has just pointed out a report on the BBC today) - but it suggested an answer to one of the things that puzzle me about lions. Like most cats, lions like to sleep. A lot, in fact - they're perfectly content sleeping for 21hrs a day, so it's no wonder tourists don't normally see them doing very much. As a consequence, I think lions are rather boring: I'd rather be birding. Still, on the occasions when I've got visitors staying who need to see lions I do go and look at them, sleeping away, and I wonder. How is it that a lion, sleeping 21hrs per day, can still be so fit and healthy? On the rare occasions when they do shift themselves, wild lions are certainly lean, mean killing machines. But how do they remain in such good condition, when they sleep nearly all the time, and even when hunting tend to walk as slowly as possible, or sit motionless in ambush?

Lions certainly can get fat - zoo keepers have to be careful to maintain the weight of captive lions within normal bounds by restricting their access to food and trying to get them to be more active through 'environmental enrichment' activities. So it's not as though they are somehow immune to such fitness concerns. So what's special about lions that helps them through this? The answer, of course, is that they do sufficient exercise: although they are pretty inactive most of the time, the effort it takes to actually catch and kill an animal is huge, even if the action only occurs for a few short seconds. During those seconds they're giving it everything they have. And if there's anything similar about their metabolism to ours, these short but extremely intense bursts of exercise are more than enough to keep a lion - or us - in good shape. In fact, the paper I read (it's free to view here) is a review of numerous studies of different fitness training regimes, focusing on comparing 'High-intensity Interval Training' (HIT) to more traditional fitness regimes. In essence, Gibala and colleagues conclude that HIT - involving just a few minutes of absolute 100% capacity exercise per week - offers comparable fitness and health benefits to much more time intensive traditional fitness regimes that raise metabolic rates (or at least Oxygen consumption) to 60 - 75% of maximum. In other words, chasing down and wrestling a wildebeest to the ground for a minute or two each day, can give you the same fitness benefit as doing an hour or so of moderate exercise each day.

Lions do sleep. A lot!

In fact for the lion and other large carnivores it's well known that it's not an easy life at all - obviously lions in the wild don't worry about keeping fit, they worry about meeting their energy requirements. And there's another paper that explores some of the energetic consequences of carnivory that's really rather interesting too (free to view here). This is a theoretical study exploring body size relationships and energetic costs among carnivores. This is interesting, because not only do energetic requirements scale with body size in a non-linear way (if you're bigger you need more energy than small animals, but proportionately you need less: a doubling of body mass corresponds, more or less, to an increase in energy requirements of only 1.7 times), but there's also a distinct and very, very important difference between big and small carnivores (we'll define big as anything over 20kg). Small carnivores like wild cats tend to eat lots of small things - mice, invertebrates, birds, etc - that tend to be smaller in body mass than the carnivore. But above about 20kg carnivores switch dramatically, preferring to prey on animals that are as big, or bigger than themselves. Hunting small and common things is relatively cheap - you can walk along and then do a sudden leap. Hunting bigger things is a whole lot harder - they run and you've got to catch them, then bring them to the ground and deal with a struggling animal. That is going to cost a lot more energy than hunting several small things, so if you are going for this strategy it obviously makes sense (a) to only attack if you think you've a reasonable chance of success (which is why most hunts are called off well before anything interesting happens), and (b) to target something that's going to give a really good return on your investment. In other words, once you start hunting largeish prey, it's worth going for the biggest you can possibly handle.

Carnivore Body Mass and Energy Expenditure. Step occurs at 14.5kg.
From Carbone et al, 2007

Now, consider that the cost of hunting small things is a simply, linear function of number of little things you catch, and the reward (in terms of energy intake) is a similar straight line. For small carnivores, that's fine, as you get bigger, your energetic demands increase, but a linear increase in hunting time will result in a linear increase in energy intake too, and everything can balance out. However, as you get even larger, at some point you'll run out of time to keep hunting like this and still meet your growing energetic costs. At this point, you've got to either stop being a carnivore, or change tactic - and that's where you start going for a few, high reward hunts and become a big carnivore - using data on real energetic costs for a range of carnivore species, Carbone and colleagues show that there really is such a switch - at about 14.5kg. (The graph here shows Daily Energy Expenditure for carnivores of different body mass, based on real field measurements). What's interesting is to then look at actual measured energy intake for the same species and compare with the requirements and another pattern occurs: at the very low range of both sections of the graph (that smallest carnivore on the graph is about 7g, then things just over 14.5kg) they have plenty of food for their requirements. But at just below the threshold - say 14kg - and near the top of the range (like a lion), they're really struggling to make ends meet. And in fact, it turns out that near both these limit, carnivores do a lot to limit their energy requirements - large bears hibernate, lions sleep all the time, so do those 14kg carnivores. And by sleeping so much, they really can cut down on their energetic needs.

They do hunt sometimes, and when they do they look mean!

And so we come full circle: lions sleep all the time, not because they're lazy, but because hunting is such a high energy activity, that they can't afford to waste energy any other way. They don't get fat, because actually they're close to their energetic limits. It's tough being a big carnivore. (Though I still think I'd prefer it to being a wildebeest always on the look out for big carnivores!) It's interesting though - we're certainly in the large carnivore body size step, but our DEE is about 8500kJ/day in normal weight people, well below the line. We're obviously not carnivores! Though what it would be like for, say, the Hadzabe, I'm not sure - my guess is it will still be well below the line. Which isn't, to say, of course, that we can't learn from the lion's exercise regime and forget endurance-training, in favour of wildebeest wrestling. Who's first?!

Main references:

Gibala, M., Little, J., MacDonald, M., & Hawley, J. (2012). Physiological adaptations to low-volume, high-intensity interval training in health and disease The Journal of Physiology DOI: 10.1113/jphysiol.2011.224725

Carbone, C., Teacher, A., & Rowcliffe, J. (2007). The Costs of Carnivory PLoS Biology, 5 (2) DOI: 10.1371/journal.pbio.0050022

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Commelina, the Maasai Reconciliation Grass

2012-02-25T09:01:00.000-08:00

Commelina sp, Mongo wa Mono, March 2011

It's surprisingly easy these days to find information on the medicinal use of plants (there's a great list for the Samuru people here, for example), such as the Commiphora uses we covered last week, but many plants have cultural significance beyond the simple medicinal uses and it's often much harder to find information about these uses. One of the 10 things I like to get people talking about when there are no lions is all to do with people, and talking about cultural uses of plants is often quite interesting. I was seriously impressed when one of the guides on our training course in November said that in 'circumcisim school' he'd had to learn to identify about 200 plant species and know their cultural and medicinal uses, so this knowledge is still very much alive out here - though he did confess to having forgotten many (before going on to hive an example of a rather harrowing use for one of the Euphorbia species that really isn't suitable for polite company...). So it's rather surprising how shy people can be about sharing the information, perhaps thinking it's not interesting, or somehow backward.

Different Commelina sp, from Ethan...

My favourite example is Commelina, a very common set of species that grow in a range of habitats from hedges in town to montane forests and dry savanna. This plant (there are around 170 species) has a very wide distribution from East Asia across Europe and Africa to North America, where they're commonly called dayflowers as the flowers don't last long. There are several species here in East Africa that I'm not too certain about identifying, so although I think the two illustrated here are probably different species I'll not go further than that - which is probably fine for cultural use. (If anyone reading can help, please do! I'd love to learn...) Like orchids and grasses, they're monocots (come from a seed with just one seed-leaf), and when not flowering they look rather grass-like. Hence their common name around here, of the Maasai Reconciliation Grass. The cultural significance for some branches of the Maasai is as an act of reconciliation.

The story, as told to me with one or two variations by a number of Maasai from around Arusha is simple. If two people (particularly male relatives) get into a fight or simply have an argument that develops into a feud, things can get nasty. Usually the community will intervene if things are looking serious and may try and help agree a settlement. If, however, nothing works but one of the party - it doesn't matter which one - decides enough is enough and wants to end things he can make use of Commelina. He picks a fresh flower and takes and presents it to his 'brother', without saying anything, in the presence of others (usually including a tribal elder). The other protagonist must take the plant, also without saying anything, and that's the end of the matter - it's not referred to again and the feud is over. If, for some reason, one of the parties breaks this tradition it's a serious matter and village elders will impose some form of punishment on whoever has not accepted true reconciliation. I've had Maasai share this story with members of other tribes who frequently have a similar story, albeit with different plants. And, I have to say, to me this is an extremely elegant piece of social engineering. Anyone who's spent time in small communities can attest to the ease with which petty irritants or minor misdemeanours can develop into all out feuds that split the community (I've seen it some some small British islands!). Some community enforced conflict resolution option that, in this case, leave everyone with their pride intact is an excellent idea that could usefully be learnt elsewhere!

So, Commelina might not look much more than a small weedy thing, but it's pretty valuable hereabouts. [And it's much easier to find out about some of it's medicinal uses too - as a cough treatment or using the sticky sap to clean wounds (even, I'm told, traditionally to help seal the wounds post circumcision). It's also eaten by Black Rhino, so has some potential conservation benefits too!] Given how difficult I found it to find any published accounts of this or any other story of similar cultural use of plants, it would be great to get a little collection together. Any other good ones people can point me towards?

Main reference:

Elliot Fratkin (1996). Traditional medicine and concepts of healing among Samburu Pastoralists of Kenya Journal of Ethnobotany, 16 (1), 63-9

Why the hornbill shuts its nest

2012-02-21T11:07:00.002-08:00

Von der Decken Hornbill nest cavity - it's tiny!

This weekend I enjoyed a camping trip out to Simanjiro with some friends, and as well as finding some very cute new-born wildebeest, we found a number of nests. One of these, just near our campsite, was a pair of Von der Decken's Hornbill. All the hornbills of the genus Tockus have two fascinating pieces of nesting behaviour that it's well worth thinking about: firstly, after choosing (and sometimes modifying) a suitable nest cavity, the female climb inside and then plugs the nest hole with mud, faeces and other grot, sealing herself in until there's only a tiny slit through which the male feeds her and, later, her chicks. Secondly, as she's sitting there she carries out a simultaneous moult, meaning she drops all her flight feathers at once and is unable to fly. Nearly all birds moult their feathers once per year (larger birds sometimes take longer), but most do a sequential moult, meaning they drop feathers one after the other and replace them as they go, thus retaining the ability to fly throughout. (There are exceptions, of course - notably ducks and geese who also drop all their flight feathers at once.) In the case of the female hornbill, however, she goes in the nest hole, shuts the door and a little while later drops all her flight feathers. Interestingly, if she doesn't breed, she moults sequentially, just like the male, so there's some suggestion that the simultaneous moult strategy is triggered as a hormonal response to the dark interior of the nest. Now, that's the story that you'll read in all the papers (e.g. here and here), but it can't be the whole story as I was trying to find some pictures to point you to of birds inside nests looking all naked, and I couldn't. In fact, here are a few pictures that show females in the nest - this one is an African Grey Hornbill with smallish chicks, but clearly well feathered mother with fairly worn plumage - she's not just completed a moult. And here's an Asian species also with young chicks who certainly doesn't look in moult to me. I'm sure it happens though, and may be the norm, but there are obviously exceptions that haven't yet made it into the literature.

Male Northern Red-billed Hornbill brings insect to nest

Still, when looked at in detail there are some remarkable things happening here. Firstly, note that it's the female who seals up the nest, not the male. This helps us understand what the evolutionary purpose is, ruling out the theory that it's a defence by the male seeking to ensure paternity. In fact, the adaptations required to manage this sort of breeding strategy would also lend themselves to reducing male paternity assurance, as the hornbill has evolved a mechanism to store sperm for a very long time. Typically, the female enters and seals the nest cavity a week or two before she starts laying eggs. She therefore has to be able to store sperm from the male to fertilise the eggs as she develops them - most birds can only do this for a few hours at most, but she managed for weeks - once she's shut in the nest she doesn't come out again. What's more, she can fertilise several eggs (hornbills often have up to five eggs, a remarkably large clutch for such large birds in Africa) from the same store of sperm. That ability would mean that in the week or two before she enters the nest she could easily find time to sneak of and find the top male in the area for a quick mating. But interestingly, and very unusually for birds, there's no evidence this happens when the DNA of the chicks is compared with the parents. There's no clear reason why the female doesn't take advantage of this opportunity, but from the fact she clearly doesn't in at least the studied species, if seems likely it's strongly in her interests too, not to seek extra-pair copulations. The best guess at the moment is that since the female and her chicks are completely dependant on the male for food a female should choose the father of her young who is best at finding food, rather than one that has particularly good genes. Though I have to say I can't really see how that would stop her seeking good genes too...

Male Northern Red-billed Hornbill feeding female

Still, why do hornbills seal themselves into the nest? The obvious answer is nest defence. Threats can come from two sources - from obvious predators, potentially including Harrier Hawks, and a variety of snake and mammal predators, but also from other hornbills and other species that like to live in cavities too. Certainly predation risk is extremely low for hornbills, especially when compared with the generally high levels experienced by other tropical birds. So defence against predators is a strong advantage of the sealed nest approach. but there's also evidence to suggest the biggest threat is having the nest cavity taken over by other hornbills, who would kill any young they find in the nest if given the chance. I suspect the combined effect is a complete winner!

Feeding takes place through the sealed cavity. Note the black eye.

There is a risk though - if the male should die whilst the female has moulted all her feathers and is unable to fly, that's it for her and the chicks. Yes, she can break out of the nest (usually female Tockus hornbills wait until they've completed their moult, then break out of the nest cavity sometimes resealing the chicks in, sometimes the chicks themselves do that), but if she can't fly that's no good for anything. Happily, it's extremely rare - monitoring of 600 nests of one species found only two cases where males vanished, perhaps due to predation. And if it were to happen early in the nesting attempt, or once the female's moult was complete she can break out early (it has certainly been observed before, contrary to what some suggest) and, although she will be unlikely to succeed in the breeding attempt, will at least live to see another day.

Ruaha Hornbill, Serengeti. Note the pale eye and dark skin.

So, all in all, a pretty remarkable breeding strategy. Hornbills are very interesting birds! And that's before we even mention their distinctive 'ballistic transport' method of feeding or their major ecological role as seed dispersers in tropical forests (and probably woodlands too). One last thing to mention, perhaps, is the fact that taxonomists are busy splitting a number of groups into newly recognised species. In East Africa the Red-billed Hornbill group is the most obvious of these, with two species now recognised in Tanzania, the Ruaha Hornbill, from Serengeti down to Ruaha and the south, but Northern Red-billed in the north - Tarangire to Mkomazi (check that link for the original description and maps). A further undescribed species might be lurking in Samburu area of Kenya, so if you're up there keep your eyes open (and take some photos too!). Here in the Tanzania, as you can see in the photos, Northern has a black eye in yellow skin (all the breeding birds are this species, taken in Tarangire), whilst Ruaha has a yellowish eye in black skin. And if you see Red-billed hornbills in northern Serengeti / Maasai Mara, look very closely and take a GPS record so we can discover where the distribution limit it. My furthest north observation of Ruaha Hornbill so far is in central Ikorongo...

[With many thanks to Ron Eggert and Daudi Peterson for use of their photos of nesting hornbills (and a fun camping weekend! Check their website for many amazing pictures of many Tanzanian birds.]

Main References:
KALINA, J. (2008). Nest intruders, nest defence and foraging behaviour in the Black-and-white Casqued Hornbill Bycanistes subcylindricus Ibis, 131 (4), 567-571 DOI: 10.1111/j.1474-919X.1989.tb04791.x

Stanback, M., Richardson, D., Boix-Hinzen, C., & Mendelsohn, J. (2002). Genetic monogamy in Monteiro's hornbill, Tockus monteiri Animal Behaviour, 63 (4), 787-793 DOI: 10.1006/anbe.2001.1975 MUVRGH6EAW78

MILLS, M., BOIX-HINZEN, C., & PLESSIS, M. (2004). Live or let live: life-history decisions of the breeding female Monteiro's Hornbill Tockus monteiri Ibis, 147 (1), 48-56 DOI: 10.1111/j.1474-919X.2004.00340.x

Threats and opportunities of the bushmeat trade

2012-02-19T08:46:00.000-08:00

Snared giraffe, Serengeti NP Jan 2011. A major target right now

Following my post a while back about Dennis Rentsch's work on the bushmeat trade around Serengeti, Matt asked me to cover this issue in a bit more depth. (Though if you want to see how it's possible to have a sustainable harvest of around 100,000 wildebeest per year from Serengeti, with a net value of $2.5-$8.5 Million per year check the original post here!) And there've been a number of interesting papers recently that have started to fill in some details. It's not a subject I've much experience of, so I'm skimming the surface a bit, but I think it might at least highlight some of the issues involved.

We'll start with the caveats. Understanding the bushmeat trade is tricky - in most places it's illegal, and people aren't always going to talk freely. And if they do talk, there's a good chance they won't tell you the truth either - they might either say they do less hunting than they really do to play down the impact, or they might go down the macho route and tell you they're excellent hunters and never come back without a pile of buffalo, etc... Dennis's work took an alternative strategy, instead of asking the poachers to tell him what they hunted, asking the villagers to tell him how much bushmeat they consumed and working back to the harvest that way. Others have worked on data using poacher arrests, viewing this as an index of poaching activity - though there's no way to tell what proportion of poachers get away with it (what poacher, when arrested, will really tell you how often they've been poaching before and not been caught?!). The only comparison between these three methods is Dennis' and that suggests that measuring consumption gives a poaching pressure that parallels that from arrest records, but neither of these fit with pressure as assessed by poacher interviews. That suggests to me that Dennis' work is probably the most accurate, but he's not yet published these studies, so for now you can only read about it here. The vast majority of other work is based on poacher surveys, and we also know that when you compare what poachers say the meat is with what the DNA tells you, you get remarkably little agreement too.All of this suggests to me that we need to take the research based on poacher surveys with a large pinch of salt. So, with that in mind, I'm going to focus more on the declines that are reported to be associated with bushmeat, rather than the more poacher-based surveys.

Let's start at the source. Bushmeat across Africa seems to be increasingly coming from inside protected areas. This is presumably increasing as the number of animals available outside protected areas is declining almost everywhere, presumably from a combination of illegal hunting and habitat destruction. Now, whilst a lot of bushmeat hunting uses fairly non-selective hunting methods - snares being very popular - that doesn't mean the hunters don't have target species in mind. In fact, surveys suggest they try to be quite selective,

Poacher, or traditional hunter-gatherer? Hadza nr Eyasi

but as their favoured targets decline they'll swap to other species and trends vary. Recently, for example, there's been a surge in giraffe snaring across Tanzania and southern Kenya - though no-one I've spoken to seems to know where the meat is heading as it's not much consumed locally. This focused effort can lead to very big declines in target species - giraffe are one of very few species currently on the decline in the Serengeti ecosystem. Tim Caro (of zebra stripes fame!) reports from Katavi that poachers target giraffe, hippos and warthog, all of which are in decline in the ecosystem. And the stories in West Africa are even worse - primates and duikers are the primary focus in many of the forests and outside of protected areas the forests are largely empty, even in remote areas. This is clearly unsustainable in many cases with demand far outstripping natural regeneration rates: bushmeat hunting is a major concern across much of Africa, particularly in the west. Overall, the numbers involved are pretty staggering. 100,000 wildebeest per year in Serengeti alone, 385,000 tonnes of bushmeat in Ghana each year, Cameroon 78,000 tonnes, CAR 13,000 tonnes, DRC over 1million tonnes, Equatorial Guinea 10,000 tonnes, Gabon 11,000 tonnes, Congo 16,000 tonnes, etc. (latter figures from here). We know rather less about the totals for southern and eastern Africa, but we do know it's growing in both areas (the comparative estimates are that only 0.1% of protein in the average diet comes from bushmeat in southern Africa, whereas it's closer to 8.5% in west Africa - though I really doubt these figures are accurate, I only trust the west being bigger than the south).

Close contact with raw bushmeat is inevitable.

Those figures are enormous and impossible to understand alone. So, a little bit of context: the same central/west African countries have a human population of just over 100 million. So 150 million tons of bushmeat gives around 15kg of bushmeat per person, per year. Not so much, when for many it will be their main source of meat. Compare that with the (legal) harvest of deer in one US state in 2010 - 418,000 animals killed, with a mean body mass of c.90kg, gives a harvest of 37,000 tonnes per year, in a state with a population of 9.8 million. (I couldn't find national figures - if anyone has some I'd be fascinated! I couldn't be bothered to trawl through all the individual state reports...) That's still nearly 4kg per person, in a country that consumed 41.5 million tonnes of domestic meat in 2006, an average of 106kg of meat per person.

Still, undeniably a huge conservation issue. But it's not just that - for me the really scaring things about the bushmeat trade is the way the contact with raw or poorly cooked animal meat gives a potential source for infections to jump from species to species. For example, there's very good evidence to suggest that HIV made it's way to humans from monkeys via the eating of undercooked bushmeat, probably more than once (giving a variety of different HIV strains). What's more outbreaks of the very nasty Ebola virus and some other related Hanta viruses probably also come from contact with animals as a result of the bushmeat trade. These are scary diseases indeed. The one you've not (yet) heard about it probably monkey pox, a close relative of the now extinct small pox that has so far only made limited jumps to humans (mostly from squirrels, often popular bushmeat), but if it evolves to pass human to human more rapidly I suspect you'll soon hear about it. So there's an even darker side to the bushmeat trade than you might have imagined. And in case you think you're safe because you live in Europe or North America, bear in mind that Estimates based on a 17day intensive stop and search in a Paris airport suggested perhaps as much as 270 tons of bushmeat is illegally imported into Europe from Africa each year, including several threatened species. Monkeys sold at about $5 each in Congo fetch over $100 in Europe. Chicago seems to be the import hub into the US, in case you where interested! And these American samples at least, did contain several types of nasty viruses.

Not much meat on a squirrel, but you might get Monkey pox!

So what are the opportunities here? Well, there are many countries where hunting is controlled and can provide a sensible source of both income and protein. (See the comparison above with the US deer harvest.) The key is, of course, sustainability. If the trade isadequately controlled, there's a real potential for income here –about $5million per year could come from sustainable harvest of the Serengeti wildebeest, without even seeing a population dip. That'ssurely something to think about. What's more, there are culturalconsiderations here – many tribes traditionally hunt wild animals,some, like the Hadzabe in Tanzania, still rely mainly on bushmeat fortheir protein requirements. By calling all such hunting poaching, weeffectively make a way of life, indeed a whole culture, illegal(whilst simultaneously allowing rich (and usually foreign) hunters toshoot the same animals in game reserves next door). This is clearly aproblem and control of the bushmeat trade must be seen as the way togo. One option is to see protected areas as a 'no-take' refuge from which surplus animals are allowed to disperse into areas where they can be hunted legally, but for that to work these still needs to be effective control methods. And how to do that, in rural Africa? Well, there's another problem... I'm interested in ideas!

PS for the record, I'm a vegetarian...

References:
Macdonald, D., Johnson, P., Albrechtsen, L., Seymour, S., Dupain, J., Hall, A., & Fa, J. (2012). Bushmeat trade in the Cross–Sanaga rivers region: Evidence for the importance of protected areas Biological Conservation DOI: 10.1016/j.biocon.2011.12.018

Knapp, E., Rentsch, D., Schmitt, J., Lewis, C., & Polasky, S. (2010). A tale of three villages: choosing an effective method for assessing poaching levels in western Serengeti, Tanzania Oryx, 44 (02) DOI: 10.1017/S0030605309990895

Chaber, A., Allebone-Webb, S., Lignereux, Y., Cunningham, A., & Marcus Rowcliffe, J. (2010). The scale of illegal meat importation from Africa to Europe via Paris Conservation Letters, 3 (5), 317-321 DOI: 10.1111/j.1755-263X.2010.00121.x

Wilkie, D., & Carpenter, J. (1999). Bushmeat hunting in the Congo Basin: an assessment of impacts and options for mitigation Biodiversity and Conservation, 8 (7), 927-955 DOI: 10.1023/A:1008877309871

Smith, K., Anthony, S., Switzer, W., Epstein, J., Seimon, T., Jia, H., Sanchez, M., Huynh, T., Galland, G., Shapiro, S., Sleeman, J., McAloose, D., Stuchin, M., Amato, G., Kolokotronis, S., Lipkin, W., Karesh, W., Daszak, P., & Marano, N. (2012). Zoonotic Viruses Associated with Illegally Imported Wildlife Products PLoS ONE, 7 (1) DOI: 10.1371/journal.pone.0029505

Myrrh trees (Commiphora) are useful things...

2012-02-16T23:33:00.000-08:00

Most Commiphora have distinctive peeling bark, Eyasi Aug 2011.

Having last week given you the bad news about the biological warfare that plants with thorns are engaging in, I thought it only fair to share some tips that may help you stave off those tropical nasties threatening to kill you... So the good news is that some of those very same thorny trees that are out to get you also hold the cure in their sap. Traditional healers and many folk still living in the country have long known about the beneficial effects of the sap of Commiphora trees. Indeed, the earliest recorded use of Commiphora to treat infections goes back to 1100BC where Sumerians were recorded using myrrh (for that is what Commiphora is) to treat tooth infections and intestinal worms. It's use is also clear in the Bible, as one of the three famous gifts presented to the infant Jesus. Today it's still commonly used in village communities to treat an endless list of infections and maladies, and it's also pretty good as a mosquito repellant if you need emergency cover!

Commiphora sap flows freely from wounds and is extremely useful!

Until recently, however, there'd been very few scientific studies of the efficacy of Commiphora in treating various diseases. Making up for this, a whole suite of papers have been published in the last month or two testing various anti-bacterial and anti-viral properties of a range of Commiphora species, and the good news is that they really do work (that's not always guaranteed with traditional medicines...)! The study I like best is actually on anti-viral activity and was carried out by scientists here in Tanzania - it's my favourite because anti-viral activity (rather than anti-bacterial) is really rather rarely studied. Anti-bacterial activity is easy to measure - you grow your bacteria on something they like, then drop your plant juices in place and watch whether the bacterial colonies die or not (and they do - even more so if you mix it with frankensense oils, it seems). Anti-viral properties are much harder to measure and Bakari and colleagues tested this using chicken eggs infected with Newcastle disease. Newcastle disease is a viral disease of birds and the authors injected it into developing eggs. This would normally lead to almost 100% death of the developing embryo (as observed within two days in the control groups in this study), but they also injected various Commiphora juices (from leaves, sap, etc.) into some of the eggs at various concentrations. And in many of these eggs, the chicks lived and hatched. In fact, for the resin and bark treatments at intermediate concentration, none of the developing chicks died, and when many of them hatched they showed no antibody response to the virus, suggesting the Commiphora juices had killed the virus before the embryo mounted any immune response. At the highest concentrations some embryos did die, presumably as the Commiphora juice became toxic to the animal itself. Still, a very impressive demonstration of how good raw Commiphora is as an anti-viral agent.

Commiphora flower August 2011, Mwiba Ranch

The fruits are often in the dry season - August 2011, Mwiba Ranch

Commiphora is most abundant in the drier, more arid savanna, where thickets can develop mixed with various Vachellia and Senegalia species (are we really going to have to change the name of this habitat from Acacia-Commiphora woodland now?!) and form an interesting habitat for a number of species not often seen on safari - Lesser Kudu like these thickets, so too do Gerenuk and Fringe-eared Oryx. I have to say, I don't think any of these animals feed on Commiphora particularly (though I'm sure they'll eat the fruits), because the reason the sap is toxic to bacteria and viruses is simple: it's yet another plant defence. And a very effective one too (though they still have to combine it with thorns!) - most things that do eat them eat the young leaves that come just before the rainy season when they're still poorly defended. Indeed, elephants seem to dislike them so much that they'll selectively weed them out of the landscape, favouring Vachellia , pushing over Commiphora trees but not eating them at all. The fruits, however, are an important food item for lots of species, particularly of birds. Most Commiphora fruit (and flower) in the dry season when there's relatively little food on offer in the semi-arid savanna, so you can sometimes find busy flocks of birds concentrated on fruiting Commiphora trees - barbets are very fond of them and if there are Vulturine Guineafowl in the area you're bound to see them feeding under the tree. Interesting habitats, for sure.

Hadzabe near Lake Eyasi use Commiphora to light fires

You can also make some money from Commiphora - myrrh (from Commiphora myrrha) is fetching £10 per 100g in the UK, other species are also useful, and if you're caught without a match it's the preferred wood of many for the base when lighting a fire by friction, being nice and soft. All in all, a very handy plant to have around. So, don't be like Hemingway's Harry and allow yourself to die of an infected thorn wound, get out there, find some Commiphora sap and disinfect!

Main references:

Bakari, G., Max, R., Mdegela, R., Phiri, E., & Mtambo, M. (2012). Antiviral activity of crude extracts from Commiphora swynnertonii against Newcastle disease virus in ovo Tropical Animal Health and Production DOI: 10.1007/s11250-012-0076-6

The wheatear's remarkable migration: Alaska to East Africa

2012-02-15T05:46:00.000-08:00

A great piece of research came my way today, detailing the migration of the individual wheatears from their Alaskan breeding areas to winter territories in East Africa. We've long known this must happen, as pretty much all the world's Northern Wheatears Oenanthe oenanthe spend the winter in Africa, but now technology has allowed us to follow individual birds on their 14,600km long migration from Alaska to East Africa and back. It's a remarkable story, not least that a 20g songbird can repeatedly do this sort of movement, but that we now have devices that can be attached to such small birds and record their journey. Wheatears are also a favourite of mine, and their migration has been the subject of one of my student's research so I almost feel qualified to make a few comments!

Migration routes and wintering grounds of three northern wheatears breeding in Alaskan (AK) and one in the eastern Canadian Arctic (CN; grey dot, breeding area, blue, autumn migration, orange, spring migration, dashed lines indicate uncertainty in migration routes close to equinoxes). Fifty per cent kernel densities of winter fixes (beginning of December 2009-end of February; purple, bird AK-1; green, bird AK-2; orange, bird AK-3; blue, bird CN-1) are given depending on the sun elevation selected (with 228 for most southern and with 24.58 for most northern densities). Pie charts indicate the proportion of individuals (AK: n ¹/₄ 9, CN: n ¹/₄ 4) originating from one of the three pre-defined wintering regions (red, western; orange, central; yellow, eastern) [8] based on stable-hydrogen isotope (dD) values in winter grown feathers and the dD values within each wintering region (mean+s.d. shown); Credit: F. Bairlein et al. 'Global migration of wheatears' (doi:10.1098/rsbl.2011.1223) in Biology Letters

Female wheatear by Adam Seward. Who couldn't love these birds?!

It's a simple enough study - they caught 30 wheatears in Alaska (and another 16 in NE coastal Canada) and fitted them with harnesses that had geolocators attached. These devices aren't GPS devices, nor are they satellite loggers, rather they record simply the time and whether or not it is light or dark. From that information it's easy to calculate day length, and if you know the day length and the time of sunrise for any given date you can work out more or less where you are. It's not the meter level precision of a GPS device, but when you're tracking a bird from one continent, across a second and into a third it's precise enough! The problem is, you've then got to catch the birds when they come back to breed the next year. And from our studies I can assure that survival between is only about 60% for an adult wheatear, lower for a juvenile. Plus the bird has to come back to the same place. And what's more, once you've trapped a bird once it becomes much harder to trap it a second time! Unsurprisingly, not so many came back (though 5 of 30 seems rather low), and one had lost it's geolocator, whilst the another outwitted the scientists. [Of course, in the good old days a shot-gun would have been required...] So they only got three loggers back from the Alaskan birds, and one from the Canadian birds. But even these results are amazing!

Wheatear pair by Adam Seward. It's cold up north!

They backed up these results with an additional study of the chemicals in the bird's feathers. Many wheatears moult some of their feathers whilst they're spending the winter in Africa. As they grow new feathers they will be making use of chemicals picked up in their food. Now, just like when we discussed carbon dating we mentioned chemical isotopes, the same principal is used here. In this case it's not Carbon we're interested in, but the hydrogen in water: the same hydrogen element can be found in slightly different forms, and the frequency of the two forms varies spatially. People have mapped this spatial variation, and once ingested by an animal and fixed into biological materials (in this case feathers) the ratio of the two is fixed and by comparison with global maps it's possible to work out where the feather was grown. Neat! So the authors also did this - the results of that are the little coloured wheels in the breeding ground with the source of the material the same colour mapped across the African wintering ground: it's clear that c.50% of Alaskan birds wintered in East Africa, whilst a similar proportion of Canadian birds wintered in West Africa, exactly as we'd always expected. All in all a very nice study.

This isn't the longest migration of any bird (Arctic Terns must hold that record), but it probably is for any passerine. And there are a few more interesting details we can fit in too, such as the flight speed. That bird crossing the Atlantic toBritain averaged 850km per day, for four days (perhaps stopping inGreenland), which is pretty remarkable. (Actually, I happen to knowthat the flight speed of a wheatear is 47kmh, so that's about 18hrsflying time per day, if there's no tail-wind. And I also know theylike to make this hop when the wind is in their favour, so if itstopped in Greenland it will have bee a short stop unless there was astrong tailwind). The movement across Asia is also nice to study,showing typical travel times of migrations, being 91 days on the waysouth (160km / day), and a faster 55 days (250km / day) when the rushis on to get to the breeding grounds in the spring. (Curiously, theCanadian bird was faster in autumn). Knowing that the lean weight ofa wheatear is about 20g (not too different from a house sparrow), andwhat the fuel efficiency rate is, if a wheatear were to do the14,600km trip in a single flight it would need about 20 times it's own weight in fat. It's unusual to see a wheatear with this much fat(!), with 50-60% of lean body mass the typical departure fuelload for long flights in this species, and much less when there's good feeding to be found enroute. In fact, our birds on autumn migration carried loads of fatthat would carry them on average about 2000km before needing to stopand refuel, so if we assume this is much more reasonable, 2000km requires about 10g of fat (50% body mass) the total trip might well have been carried out in 7or 8 stages, interrupted by feeding stops in nice areas. We've found that this race of wheatear can, if the conditions area really good, deposit an average of about 5% of their body mass (1g) of fat per day (exceptionally the Canadian race can get up to 20-25%). There's at least 300hrs of flying time required to do the distance (~13 days). So each stop over to reach 10g and the next flight might be about 10 days, giving about 73 days of travel. Obviously to make the trip in 55 days they've got to be faster, making sure they pick the best places to refuel and average stop overs of only 5 or six days. Pretty had work though, with no time to rest! And imagine repeatedly gaining and losing 50% of your body weight - I think it would kill us!

How we know the extra stuff! Colour-ringed wheatear weighing itself
and snacking on mealworks. Big brother is watching... Thanks Adam!

Anyway, what surprises me most about these results is the fact that some wheatears breeding in Alaska seem to have wintered in west Africa (having completed an even loinger migration!) and some Canadian birds here in east Africa. That's particularly interesting to me as it means that some of 'our' colour-ringed birds from Europe and Greenland might have be wintering on the plains near me! I'm going to have keep my eyes open...

PS, thanks to the very nearly Dr Adam for providing the photos of his wheatears! I'm looking forward to cute squirrels next...

Main reference:

Bairlein, F., Norris, D., Nagel, R., Bulte, M., Voigt, C., Fox, J., Hussell, D., & Schmaljohann, H. (2012). Cross-hemisphere migration of a 25 g songbird Biology Letters DOI: 10.1098/rsbl.2011.1223

Tarangire wildebeest migration

2012-02-14T00:04:00.000-08:00

Tarangire wildebeest on the move, Sep 2011.

Following the ATBC / SCB conference in June I mentioned a talk by Thomas Morrison on the movements of the wildebeest in Tarangire. The Tarangire migrations is, of course, tiny in comparison to the better known Serengeti migration and involves a different race of wildebeest (C. t. mearnsi in Serengeti, C. t. albojubatus in Tarangire) , but it's just as interesting to understand, and Tom and his supervisor Doug have recently published some work describing the movement that was covered in the conference talk. Until fairly recently, Tarangire was home to a large wildebeest population, though only around 6000 remain today. It's still one of my favourite places to visit though... These animals move into Tarangire in the main dry season (arriving in June) and then move out to one of two main areas for the wet season either east onto the Simanjiro plains, or north-west towards lake Natron. As with the Serengeti migration, these wet season movements are onto grasslands growing on recent volacanic soils with high nutrient content and just what is needed during late pregnancy, then when lactating after calving in February. One of the mysteries, however, is whether the population that moves to Simanjiro is the same as that moving to Natron - do the animals go one direction one year, and the other the next? And as those moving to Natron pass close to another population in Manyara, do those Manyara animals also join the movement? It's important to know the answers to these questions if we're to try and protect the animals, given that they spend around six months of the year outside the National Park system.

Wildebeest flank stripes are unique

Instead of using expensive GPS collars and tagging a few animals, Tom set out to use the individual stripes on wildebeest flanks as identification features. By photographing animals he was able to use computer software to match the pictures up and try and locate the animals at different times of year and after taking over 5600 photos he'd managed to identify some 2557 individual wildebeest (nearly 50% of the population, so a very good effort!), 150 of which he managed to photograph is successive wet seasons enabling him to identify whether individuals were returning to the same site or changing between year. And his answer is fairly simple - most animals don't change. But enough do, sometimes, that this can certainly be considered a single population from a management perspective: between 2005 and 2006 no shifts occurred, whereas between 2006 and 2007 some 20% of animals changed breeding season ranges. There was no consistent direction to these range changes - as many animals from Simanjiro in 2006 moved to Natron in 2007 as the other way around. They also found a small amount of movement between Lake Manyara populations and elsewhere (Natron, Tarangire and Simanjiro), but essentially confirmed this population as resident. More interestingly, they found that dry-season herds after a month or so in Tarangire contain an apparently random mix of animals that had spent the wet season near Natron or in Simanjiro, suggesting social ties aren't particularly important in determining the movement patterns. But the most surprising result of all, was that when considering only females, animals that had succeeded in raising a calf to at least two months old were more likely to shift the following year than those that hadn't. And unfortunately it seems as though they still haven't come up with any reasons why this might be. I'm sure Tom will be interested to hear any answers for what seems to be a fairly strong effect... (Any chance it's just are artefact of more animals shifting in 2006-7 and, perhaps, more animals not breeding in 2005-6? Not sure...)

Serengeti wildebeest are a different race to the Tarangire animals

Hopefully Tom will have more to say about this study in due course - there are plenty of other questions about populations we could use them for and I know he wants to get more funding to continue. We'll see. Meanwhile I'll be happy with the hints I'm hearing that the population in Tarangire has at last stabilised and may even be increasing slowly again, which would be good news indeed! I'll see if I can prod Tom into making some comments here too...

Reference:
Morrison, T., & Bolger, D. (2012). Wet season range fidelity in a tropical migratory ungulate Journal of Animal Ecology DOI: 10.1111/j.1365-2656.2011.01941.x

On introducing elephants to Australia...

2012-02-12T10:03:00.000-08:00

Sometimes scientists suggest the mostabsurd things. In the news last week (with thanks to an Australianfriend for tipping me off) was a paper published in the prestigiousjournal Nature that suggested in the text and headline that Australiashould introduce elephants to control an invasive grass thatoriginally came from Africa: Gamba grass, Andropogon guyanus. Theauthor made a number of sound observations: Australia (like too muchof the world) is riddled with invasive species, has suffered amassive extinction of it's native mammal population and has had somepretty nasty wildfires in the last few years. But how you get fromthose observations to suggesting elephants (and even rhinos) shouldbe introduced to the savannas of Australia is a story worth lookinginto.

Invasive species can be extremelydamaging to biodiversity, farmers and even human health. Here inAfrica we have our own set of problem species, highest priority inEast Africa at the moment is surely Parthenium hysterophorus, a weednative to the American tropics that is currently invading savannah habitats acrossEast Africa, after introduction for the cut flower trade. InEthiopia, where the species is already established, the grazingpotential of rangelands has already been reduced by 30% as the weed is toxic tomost mammals. It's also causing unknown damage to human health, asmany (most?) people coming into regular contact with the plantdevelop allergic reactions to the pollen and sap. In South Africa it is estimated that invasiveweeks cost the economy around 6.5billion Rand ($800Million) per year. Soinvasive weeds can certainly be a major problem. Most scientistsrecognize that these plants, which may be perfectly innocuous intheir own environment, become serious pests in the areas they're areintroduced, because they are freed from their natural predators thathelp keep the populations in check in the native range. One of thebest solutions, therefore, is the introduction of a suitable plantpredator, to keep the population under control. Some great examplesof this exist: the Prickly Pear (Opuntia) cactus, for example, is native in theAmericas but a weed across Africa and Australia. A moth was identified that is a specific herbivore of the prickly pearand has been introduced in Australia and parts of Africa with oftendramatic success. After introduction to Australia, the moth (Cactoblastis - the perfect name for an effective cactus eradication agent!) rapidlyinfested many of the cactus plants with the effect that today the infestation has beeneliminated. Prickly pear is no-longer considered a serious threat inAustralia.

White rhino seem to shun gamba grass...

Biological control, however, is not astraightforward option – to deliberately introduce one species tocontrol the population of another species is a rather risky option.You don't want to end up like the old woman who swallowed a fly, thena spider to catch the fly, then a bird, then a cat, etc., etc. Manyexamples of apparently good ideas have gone horribly wrong, a favorite example of mine being the introduction of predatory snailsto eat the introduced giant land snail on Tahiti where, onceintroduced, the predatory snail decided the native snail populationswere far tastier than than invasive species, and rapidly drove thenative species to extinction in the wild (happily a few were savedand are bred in captivity to be, hopefully, reintroduced once thepredator has been dealt with. Somehow...). Recognising this,conservation organisations got together to come up with a list ofguidelines for the introduction of species under the umbrella ofIUCN, the World Conservation Union. These guys propose a number ofquestions and guidelines that governments should consider beforemaking introductions: “What is the probability that the species tobe introduced will threaten

the continued existence or stability ofpopulations of native species, whether as a predator, competitor torfood, cover, breeding sites or in any other way? If the introducedspecies is a carnivore, parasite or specialised herbivore, it shouldnot be introduced if its food includes rare native species that could beadversely affected.” Etc. (Note the implication that you'd only beconsidering introducing a specialised herbivore!)

They note that “No introductionshould be made for which a control does not exist or is not possible.A risk-and-threat analysis should be undertaken includinginvestigation of the availability of methods for the control of theintroduction should it expand in a way not predicted or haveunpredicted undesirable effects, and the methods of control should besocially acceptable, efficient, should not damage vegetation and fauna,man, his domestic animals or cultivars.”

In this paper David Bowman suggeststhat the ideal control method for gamba grass is the introduction ofelephants. (He further suggests that introduced grasses such as thisspecies, by leading to a build-up of fuel, are responsible for thefires that have killed so many in Australia. This notion is so absurdI don't think it's worth going into: the grasses in question are inthe savanna zone of northern Australia, the worst fires in the south.As with all savannas globally, there are and always have been firesin the savanna belt – to remove them would be to cause untilddamage to these savanna habitats, etc., etc.) He suggests that othermammals in Australia, such as Asian water buffalo Bubalus bubalis, cattleand the rest are too small to eat the exotic grass, but states that“gamba grass is a great meal for elephants or rhinoceroses”. So,how would this proposal fit the IUCN guidelines? Well, the firstthing to do would be to find out if elephants and rhinos really doeat gamba grass. It took me about 10 minutes online to discover thatelephants do eat gamba grass, but they certainly don't select it overother species but rather eat it in proportion to it's abundanceduring the wet season (and not at all during the dry season). RhinosI could find less details of: black rhinos are browsers, so we canignore them for now and look at white rhinos. Contrary to Bowman'sassertion, I certainly couldn't find any evidence suggesting gambagrass is a great meal for them, but I did find evidence that theydon't like it, with areas covered in gamba grass consideredunsuitable habitat for white rhino reintroductions.

What's more, as anyone who's studiedelephant diets would have been able to tell the author, elephantshave a mixed feeding strategy – being predominantly grazers in thewet season, and predominantly browsers in the dry season. Evenassuming they could be persuaded to eat gamba grass in the Australianwet season, what will they eat in the dry season? If they findsomething they like, their impacts can be very serious indeed (and ifthey don't what hope can their introduction have of success?). What'smore, if they start having unintended consequences and people wantedto wind the introduction back, I can't see a cost effective method ofcontrol ever fitting the “socially acceptable” criterion. Livetrapping would be incredibly expensive, and even if possible, wherecould you put several thousand elephants to live out their lives?!

So, it seems to me that introduction ofelephants to Australia could never reduce the fuel loads in southernAustralia (particularly not around habitation, where fires are mostdangerous). Nor is it likely the animals would even do the jobthey're being promoted for. There's no way the introductions couldever come close to meeting the IUCN guidelines. And it would be atotally crazy thing to do, as anyone with a bit of knowledge ofelephants and rhinos could tell you. So why did this get written? Andthen why did it get published? Sometimes I wonder about mycolleagues...

Main reference:

Bowman, D. (2012). Conservation: Bring elephants to Australia? Nature, 482 (7383), 30-30 DOI: 10.1038/482030a

Butterflies moving again...

2012-02-10T04:09:00.000-08:00

Well, the initial movement seems to have petered out, with nothing major in Arusha and most other northern Tanzania areas since Sunday. But starting last weekend I started hearing word of movement down on the coast, and then in Kenya, around Nairobi and Mt Kenya areas, all of butterflies headed south. The last couple of days I've been hearing about massive movements - at least as impressive as the original movements over the Pare Mts (around 400/min over a 20m line!) and in much of Kenya (but the Kenyan's haven't yet given me anything specific enough to actually map - come on!). These animals now seem to have arrived more widely in Tanzania, with arrivals in northern Serengeti reported for the first time now, as well as continued movement on the coast. And there are even a few trickling over Ilboru again now - though we're clouding up here and I'm not expecting much. But if you've been following the story, please keep your eyes open and keep reporting. Here's the latest map. Please do encourage your friends and contacts to get involved. (And I'd still love to know what happened around Singida to the original movement!)

View Butterfly eruption 2012 in a larger map

Why is the African Savanna so full of thorns?

2012-02-09T08:19:00.000-08:00

Giraffe lick leaves between thorns. Note how obvious the white thorns are.

Spinescence. Now there's a word! It simply means having spines and one of the first things many visitorsto the African savannah notice is that everything is covered inthorns. Or, in other words, Africa is spinescent. It's not a wise idea to brush past a bush when you'rewalking, and you certainly want to keep arms and legs inside a carthrough narrow tracks. These are thorns that puncture heavy-duty cartyres, let alone delicate skin. But why is the savanna so muchthornier than many of the places visitors come from? Or even than otherbiomes within Africa, such as the forests?

This post I've just written as a guest blog over at "Nothing in Biology Makes Sense". I'm incuding it here too, but do go and check that blog out if you're interested in evolution! You can read the rest here, so skip to the story there if you want...

At one level the answer is obvious –there are an awful lot of animals that like to eat bushes and treesin the savanna. Any tree that wants to avoid this would probably bewell advised to grow thorns or have some other type of defencemechanism to protect itself. But then again, perhaps the answer isn'tso obvious: all those animals that like to eat bushes seem to beeating the bushes perfectly happily despite the thorns. So why botherhaving thorns in the first place? There's certainly a serious cost to having thorns: plants that don't need to grow them have been shown in experiments to produce more fruits. So if animals eat the plants with thorns anyway, why pay this cost?

Impala are often the major herbivore in the savanna

Look a bit closer at an animal browsinga thorn tree and you'll see it has one of two strategies – it bites the end of a branch off, wood, leaves, thorns and all (anactivity we could call 'pruning'), or it can nibble carefully amongthe thorns, picking the leaves out with care. And look closer stilland you'll see that most animals 'pruning' trees only eat the softnew tips, where the thorns haven't hardened yet, which suggests that thorns do at least make it difficult for these animals, even if they still make their living from eating thorny bushes (I don't think I've ever seen an impala eat anything that isn't thorny!). So whilst thorns might slow animals down, it would still seem that thorny bushes are paying two prices - first they pay the cost of growing thorns in the first place, and secondly they still experience herbivory. Why, then, do they do it? The only way growing thorns makes sense (beyond the Genesis explanation that the land was cursed following the fall, of course), is in terms of an evolutionary arms race.

Browsing pressure can be extremely high!

Just as in the Cold War 'the West' and 'the East' were busy building costly weapons stockpiles with no obvious benefits beyond "they did it, so we have to as well", we can think of plants and herbivores as being in a constant war. Plants trying not to get eaten, herbivores trying to eat plants. For every adaptation that a plant evolves in defence, before long the herbivores are likely to evolve a way around it (behavioural or morphological). Grow prickles and you'll be well defended for a while, but then some animal will learn that eating new shoots whilst the prickles are soft is easy, grow thorns and before long something will evolve a long tongue that can lick the leaves out between the thorns, or a very narrow muzzle to squeeze between them. Once the process has started, there's little obvious way to stop - nuclear disarmament treaties require both sides to sign up and trust one another, not something that's common in nature. We can also see how it happens within the species: consider the first bush to evolve a few stiff stipules (the little bits where leaves join the plant stem) to make tiny thorns: there'd be one bush among many that was a bit prickly, and all the herbivores would avoid it, with many undefended plants to eat instead. So before long the genes that led to this mutation would spread and the whole population would have small prickles. Herbivores still need to eat, so with no choice now they'll evolve a way to eat the prickly plant, or they'll die. Then another bush has a mutation that makes those little prickles longer and now there's one bush with spines among many with prickles. Again, the herbivores will eat the ones with little prickles, and the longer one has an evolutionary advantage. It's easy to see how this process will rapidly run away until all plants have long, nasty thorns that cost them lots to grow, but still get eaten. This competition between the plants themselves can be seen as another evolutionary arms race, one we often call the Red Queen effect after a character in Lewis Carroll's Through the Looking Glass who said "it takes all the running you can do, to keep in the same place". It follows that if some evil scientist went out with a pair of nail clippers and removed all the thorns from a thorn tree, it should suffer dramatically from being less well armed than it's neighbours. And you might not be surprised to lean that someone's actually done the experiment (never let it be said that science is about proving the obvious...)! Wilson and colleagues removed spines from a whole lot of thorny African trees and bushes and watched what happened when goats and bushbuck came along. I doubt they were too surprised to discover that the animals took bigger bites and consequently fed faster on the branches they'd removed spines from than those they hadn't...

Klipspringer are just one of many small browsers

In fact, the defences provided by thorns are pretty sophisticated. As there's a cost to being spinescent, it's only sensible to grow thorns if the benefits outweigh the costs and where that happens depends on a number of things above and beyond simply the density of herbivores: it's the actual cost of that herbivory that matters. Herbivory is more costly in places where you can't grow much to replace what's eaten, so it makes sense that in the driest environments thorns are more valuable than in wetter places where new growth can rapidly replace lost material. Exactly what has been found for Vachellia tortilis in experiments in Israel. Using the same logic, you might expect that if you give fertiliser to a growing tree it will also be able to grow faster so will invest less in thorns. But sadly, when that experiment was done on the same species, it didn't hold out - more fertiliser meant more thorns, which the authors of that study took to mean that the ability to grow thorns is nutrient limited. Now I suspect their side note that trees (even of the same species) growing on nutrient rich soils often have more thorns than those on poorer soils is more relevant here - if you're packed full of nutrients you're probably a much better target for herbivory than if you're not, so although fertiliser means you can grow faster, it also means you'll face higher herbivory rates. Which in turn means if you grown on rich soils you'll face higher costs and would be wise to invest more in defence.What's more, it makes sense for plants to be able to sense the amount of herbivory they're facing and only grow thorns when herbivory is high: again, exactly what's seen in experiments.

Despite masses of white thorns this Yellow-barked Acacia is still stunted

Similarly, if you're going to pay the costs of being thorny, it's worth making that immediately obvious to potential herbivores to ensure that they avoid you rather than taking a few bites before making the discovery. So instead of hiding your thorns, why not make them obviously white or even red as a warning? And just as well defended insects are often bright and obvious (we say they're aposematic), many thorns are also aposematic. But where it gets really scary is recent work suggesting that thorns not only provide direct defence, but are actually used as needles to inject bacteria and fungi into whatever brushes against them. There's evidence to suggest the plants have evolved such that the thorns are particularly good at making homes to some pretty nasty beasties: Clostriduim botulinum, Bacillus anthracis (I'm sure you can guess what those two give you) and many other nasties are reported to be happy living on thorns. What's more, those nasties are happier and therefore in higher densities on the thorns than the photosynthetic green parts of the plants, suggesting the plants really have evolved thorns that are really hypodermic needles. Truly plant biological warfare! No wonder that tiny thorn scratch can go nasty on you.

So, to summarise, African savanna is thorny because of all the animals, just as we first thought, but hopefully we've learnt something interesting in the longer answer: even if it is only to pack the disinfectant when going on a walking safari!

Main References:

Halpern, M., Raats, D., & Lev-Yadun, S. (2007). Plant biological warfare: thorns inject pathogenic bacteria into herbivores Environmental Microbiology, 9 (3), 584-592 DOI: 10.1111/j.1462-2920.2006.01174.x

Wilson, S., & Kerley, G. (2003). The effect of plant spinescence on the foraging efficiency of bushbuck and boergoats: browsers of similar body size Journal of Arid Environments, 55 (1), 150-158 DOI: 10.1016/S0140-1963(02)00254-9

How the zebra got his stripes?

2012-02-09T00:22:00.000-08:00

Most animals in the savanna come in one shade of brown or another, except for zebra. Zebra, as everyone knows, are stripey. Black with white stripes, at that; or are they white with black stripes? Anyway, why they're stripey has puzzled many people for a very long time: even Wallace and Darwin debated whether zebra stripes make them conspicuous or not! For stripes to have evolved there must be some evolutionary advantage, but what, exactly is it? There are a huge number of theories out there (many reviewed here), from the rather obvious to the some more ingenious ideas too:

For many (and despite Darwin), it's obviously camouflage - just as the stripes on a tiger disguise the outline of this animal and patches on army vehicles hide them, so too, do zebra stripes hide them from lion, at least at a distance.
It's a temperature regulation thing - black areas warm up fast and create tiny areas of hot rising air that move cool air over the white bits, to create a series of tiny eddies to cool the animal.
It's simply so zebra can see their conspecifics easily in the distance, an advantage as if attached they'll be able to get back into a herd quickly.
Whilst they might be conspicuous by day, in low light, and importantly, through a lion's eyes (don't forget they have different visual systems to us) zebras are rendered invisible at night.
It makes then invisible to tsetse flies
Moving stripes dazzle and confuse predators
Etc., Etc.

Lion's vision of zebra?

In fact I've been told by Tim Caro, who's thought a lot about this problem (here and here for good examples), that there are at least 17 different hypotheses going around, but none of them really have good experimental evidence. The one in the news today is the fly story, thanks to a new paper that describes experiments involving different coloured model animals. In the paper (available here, but probably not free I'm afraid) Ádám Egri and colleagues set up a series of experiments in Hungary (not the first place you might think of studying zebra colours, but why not!). They started with sticky trays with different degrees of a cross pattern and set them out to see how many tabanid flies (commonly known as horseflies) were attracted (and stuck) to each tray (click here for photos). Not immediately related to zebra stripes, I know, but they gradually increased the complexity of the experiment, making stripes and comparing black to white alone, etc., until they ended up with sticky model 'horses' of white, black, brown and zebra stripes (there's a nice picture on the BBC site!). And they counted the horseflies that got stuck to each one.

Results, well, black 'horses' attracted 562 horseflies, the brown one 334 the white model 22 and the zebra striped one only 8. So stripes certainly do protect these animals against horseflies. Why this is was the subject of many of the other experiments in the paper, and the authors suggest it comes down to the way light is reflected or not off the animals, resulting in polarisation (i.e. all the waves of light are parallel) which the flies are rather good at detecting (horsefly larvae are dependant on water, which is a good source of polarised light: if you want to find water, looking for polarised light is a good technique). Now white animals do not polarise any light, but striped animals do (from the black bits), so why the striped animals should still be less preferred than white ones isn't clear. The authors suggest it might be something to do with the resolution of the compound eye in the flies. Essentially, they argue that thanks to the width of zebra stripes combined with the resolution of the compound eyes in the horseflies, zebra are invisible to the flies until they're almost on the animal anyway.

Zebras are harder to see than wildebeest? Seronera, Nov 2010

Now, is this the answer to why zebras are stripey? Well, obviously the authors haven't tested any of the other theories, so can't rule those out. What's more, I need convincing that horseflies are equivalent to tsetse. Horseflies are dependent on water to complete their life cycle, and are consequently rather scarce in many savannahs. Does such a rare species really have a large enough influence to overcome the risks associated with stripes, if stripes make them conspicuous to predators? Tsetse, on the other hand, might be a serious selective force - they can certainly appear in such numbers that they kill animals, and are vectors for Trypanosoma (the animal causing sleeping sickness) that can be fatal to horses. So are the results for Hungarian horseflies going to be the same here with tsetse flies? If you've been in the bush here you'll certainly know that wearing dark colour clothes will give you lots more tsetse bites than wearing light ones (and you'll probably have seen the black and blue tsetse traps in many places - coloured for maximum attraction to the flies). But I still think the work needs to be done - movement alone seems very important for attracting tsetse, and stripes don't seem to hide movement. Similarly, if the resolution of tsetse compound eyes is different to that of horseflies, the hiding won't work. And, of course, tsetse have a very unusual breeding strategy, which, thanks to internal development of single young at a time in the female, means they're not reliant on water - so do they really have the same attraction to polarised light? I think we need a few more answers before I'm quite convinced yet.

And in fact, I spoke to Tim Caro back in December at the TAWIRI conference about his research in this area, as he's been busy doing experiments down in Katavi to test all the hypotheses he could find (involving such crazy things as dressing up in zebra skins and walking about in the savanna! That is seriously brave science - it's just asking for trouble!), including the biting flies option, and his answer so far is that he still doesn't know. Nothing conclusive. So until he's finished and written up all these experiments I'm not going to be convinced by a single study that only tested one hypothesis. I wouldn't be surprised if it was part of the story though - nor would I be surprised if there are multiple processes at work here, but I'm not going to just jump on one theory just yet.

Whatever makes they stripey, zebras are cool!

There's a more solid answer to that other zebra puzzle, by the way: they're black animals that evolved white stripes. Or at least that's the way the mechanism in the development pathways works - the melanism producing pathways have a default that is switched on, a pathway that is inhibited in the white stripes and not the other way around. Moreover, most other Equuids are grey / brown or blackish, suggesting that the common ancestor of zebras and other horses was probably dark coloured too - colouration that would need switching off, not switching on. I'll leave you to ponder that whilst I go out to paint my landrover stripey...

Reference:

Egri, A., Blaho, M., Kriska, G., Farkas, R., Gyurkovszky, M., Akesson, S., & Horvath, G. (2012). Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes Journal of Experimental Biology, 215 (5), 736-745 DOI: 10.1242/jeb.065540

Climate change and African vertebrates

2012-02-08T04:44:00.000-08:00

Last year I spent a very happy evening in Cape Town enjoying some of the local specialities with a colleague and a visiting student. Or at least, that's what I thought - poor Raquel now tells me I was giving her a hard time... Still, good practice for her eventual defence of her thesis I hope. Anyway, she pointed me in the direction of the paper she was writing at the time that's now out and attempts to describe what's going to happen to some 2723 species of African vertebrates as the climate changes over the next several decades. Now, despite climate change being a huge conservation issue and one of my main research interests (and climate/weather being one of my 10 things to talk about), we've not talked much about it here on the blog before, so the chance to discuss what might happen to 2723 species across the continent as a whole is an ideal opportunity to start!

First, let me explain what Raquel and her colleagues have done. Essentially, they've said that we know that climate is an important driver of species distributions - coldness, wetness, hotness and everything all act to limit distributions, a concept that will be familiar to all. From that understanding Raquel set out to identify, for each one of the species, which climate variables are important and in what way - what the upper limit for temperature is for a given species, say. She made use of a suite of methods called bioclimate envelope models (and a whole range of other names too) that seek to identify the limits in a whole range of ways, using the observed distribution as the basis. So if a species is only found in areas where the temperature is never below 0^oC or above 20^oC, they assume these are the thermal limits in this species. Lots of different methods have been developed to do this, and once you've got a good idea of the limits using these methods (that idea of the limits we'll call a 'model') you can combine it with output from global climate models that predict what the climate will be like in the future to project where suitable climate will be for that organism (assuming the climate preferences don't change). Now, there are lots of methods to make these models, and also lots of different global climate models (not to mention alternative scenarios about how the world might respond to climate change), so one of the things Raquel was interested in knowing is how much variation is there between different model choices and, in particular, whether some sort of methods for combining the projections from each models into a since 'ensemble' model can be useful.

In doing so, of course, she mapped the changes in distributions of each one of the 2723 species involved which, if you're like me, is the first thing you'll want to know about. In fact, the authors don't give us all the individual predictions, but combine them to give us measures of 'species turnover' across the continent. It's not quite clear from the paper exactly how this is calculated (which is a shame, because it's another subject I'm interested in, and some methods are much better than others), but they say it's some combination of both the species that are predicted to colonise an area and the species that are predicted to go extinct in an area (presumably with those also not expected to change). So, what's the rub? Well, a subset of the results (from the more moderate climate change scenario) are pasted on the left here. Red areas are where there's high turnover (100%, whatever that means), blue where there's little or none. Rows are different species groups, columns (with the heading "Cluster 1, 2, etc.") are from different groupings of the climate change model predictions (those in cluster 1 and 2 seem more moderate than those in cluster 3). So you can see there's lots of turnover predicted for Namibia and the Sahel regions - both very arid areas. All the taxonomic groups show broadly similar patterns (a set of patterns that, sadly, weren't investigated, and something I'll come back to). And most of the variation shown here is between climate models.

Now, perhaps the most important bit of climate change isn't the movements per se, but the local extinctions. So Raquel and co also plot just the proportion of species that remain in an area - i.e. they're species that are there are the moment and are predicted to remain there in the future (right). This time the columns are the different climate change scenarios and, rather counter-intuitively I think, the redder the area the lower the local extinction rate. IE, in the dark blue areas, everything currently present int hat area is expected to be lost by 2080, whilst in the red areas only just over 50% of species are expected to be lost. (Don't forget, other species might come in to replace those lost.). So again we see massive extinction in Namibia and the Sahel in all groups, and greater stability in places like Tanzania and, perhaps, the Ethiopian highlands. But even here Raquel and co are saying more than 50% of our current species will have gone by 2080.

With thanks to xkcd

Now, before I tell you what I make of all this, I should point out that Raquel and colleagues frame their paper in terms of understanding how to generate 'ensemble' models from sets of predictions. It's not something I think is necessarily a wise idea (I've published a bit about it here if you're really interested) and what I like most about this paper is that they've shown both that different ways of generating ensembles can make very large differences to your results, but also that they've not always reduced the complexity to single combinations. I don't think any of them really cover my main objection though, which is rather nicely illustrated by the problem with any averaging method in this area and shown simply on the left! I should probably also confess that whilst my wife complains I'm generally very critical of anything, I'm know to be particularly critical of these methods, having published some stuff on their failings here and here.

Usambara Pygmy-chameleon. History, not climate, must explain this distirbution.

So, what do I think overall? Well, I think there are a few minor things I'd quibble with - I'm not sure the choice of climate variables are the most meaningful options out for Africa (there's nothing about dry season length in there, for example, which I think is crucial). I also really disliked reading things like "method X gave very accurate predictions". Now I think I know what they mean - accurate in the sense that the variability associated with the predictions was relatively small - doesn't mean it's right, just that the predicted variation was small (precise predictions can be very wrong!). But not in the sense that any normal person who reads it might think - where accurate prediction means, well, it's telling you what really will happen. It's a technical issue I got a bit heated about here. And, of course, I have to confess I still don't think most species distributions are mainly driven by climate anyway - I don't think these methods are particularly good at determining when a distribution is associated with climate, and when it's not. They start from the premise that all species are, ultimately, climate limited and, when tested, that doesn't seem to be the case (or at least, that's what I suggested in that paper). And if there's anywhere that's going to be wrong, it's going to be Africa because, as regular readers know, the savanna biome is very special and there's a lot of savanna in Africa! What's special about the savanna biome? Well, importantly here is that it's distribution isn't mainly climate driven. Climate is important, but so too (as much, or more) is fire. And so too, is history - many of the species that differ from mountain to mountain are there simply as isolated forms no longer connected, but probably perfectly happy to live in any mountain and certainly not limited to that one mountain along by climate. Not to mention a whole suite of other issues.

Burchall's Coucal: limited by competiton with White-browed?

Put all of these things together, and add a little gut feeling (because I think science at times should be tempered with common sense) and I have to say I find it extraordinarily unlikely that there's going to be nowhere in the whole of Africa where more than half the current vertebrate species have died off. Maybe I'm an eternal optimist, but I really can't see that degree of change happening over the next 70 years. That's not to say that I don't think human driven climate change is one of the biggest threats to biodiversity we currently face, but I really don't think the estimates here are plausible. Because, ultimately, I don't believe the assumptions that underpin these models. There are important points in this paper, but I don't think the overall view of impacts will stand up to scrutiny.

Reference:

Garcia, R., Burgess, N., Cabeza, M., Rahbek, C., & Araújo, M. (2011). Exploring consensus in 21st century projections of climatically suitable areas for African vertebrates Global Change Biology DOI: 10.1111/j.1365-2486.2011.02605.x

Notes on a butterfly eruption: a billion and counting!

2012-02-05T09:55:00.000-08:00

Just a brief post with the latest news, as it's coming in. The bulk of the movement seems to have passed Arusha Moshi now - there are still large numbers going through but (a) they've turned South West in most areas, and (b) they're coming through in groups now, not as a continuous stream. By contrast, I'm hearing of increasing numbers now in and around the Crater, with the first being reported (headed south and south west) from the broader Ndutu area. I've also confirmed that the movement hasn't been noted north of the Pare Mountains, suggesting the idea that they originated in the Maasai Steppe and headed north from there might well fit. The other very interesting observation comes from Tent with a View (I think that's their website!), who report 1000s appearing just recently headed south down the coast in Saadani NP. So, if you're in Dar, do look out over the next day or two. How this fits into the pattern, I'm not sure! But keep the records coming and we'll find out.

Meanwhile, I've been reading more papers on butterfly movements in Africa!

One thing that I have discovered is a fascinating account of a mass emergence in Sudan in 1928! What is most interesting about that account is what formed the original synchrony in the emergence (in this case it didn't result in a particular eruption): a few showers at the end of a drought resulted in a large growth of Maerua oblongifolia, all of which got laid on by a few butterflies. The next generation, as is to be expected, rapidly ate these plants and the adults emerged en masse, having eaten everything as caterpillars. So a couple of rain storms might be enough to synchronise massive numbers of butterflies, who will rapidly exhaust the available food and then they set off in search. Why they don't head in all directions from there isn't clear (maybe they do, and maybe that's why we're finally seeing animals on the coast now?), but the other observation in that paper that is very relevant to the current situation is the observation that many eruptive individuals are smaller and paler than the usual forms of these butterflies, a typical result of overcrowding and low food resources for the caterpillars. The individuals we've been seeing in this movement are also slightly smaller and paler than average, suggesting they may well have started off in very crowded conditions where competiton for food was high.

The next thing I liked in that paper was some estimates of how many individuals may be involved in some of these movements. Unfortunately, the calculations weren't given. But it's worth one of those back of an envelope calculations I'm so fond of, so let's give it a go!

During the peak movement here in Arusha last week, of a 20 front I was consistently recording over 100 butterflies, all on the move. Numbers elsewhere sound similar. Other days since then I've been getting around 50 per minute, dropping to an average of only 20 the last couple of days. So, from the map everyone's been contributing to, the movement front is at least 200km wide. If the butterflies are moving at this density (average of 50 per minute for 5 days now) across the whole front, then each minute about 500,000 butterflies move across the whole 200km front. They've been moving for about 7 hours each day, five days now. Which gives us 1,050,000,000 butterflies on the move! Yes, that's more than 1 billion individuals - and they're still moving! (To put that in context, reported a movement of 3 billion Painted Lady butterfly in 1949. But it's still impressive!) Out of interest, I've weight a few butterflies and they come to about 0.2g each, which gives us about 315 tons of butterfly on the move. Or put it another way, with a wingspan of 5cm, put them all together and you can easily go around the earth! Certainly numbers that might just impress enough to look away from a wildebeest for a while...

Anyway, please keep the records coming, especially as they move west and south. If you know anyone in these areas, get them on the case too, please! Keep checking the interactive map too, it's getting new points all the time. And with much thanks to MANY people who've been contributing records from across the country.

Main Reference:

Shields, O. (1974). Toward a theory of butterfly migration Journal of Research on the Lepidoptera, 13 (4), 238-238

Butterfly migration update

2012-02-03T23:14:00.000-08:00

Thanks to all who've given me information so far! Numbers are definitly falling now around here, but some big waves are still coming through and that big wave of movement must be headed somewhere else... If you've been following you'll notice that new points have been appearing on the map, though more would be great, so if you've been hesitant, please do let me know. You can either add points yourself here, email or SMS me (if you know my number of course) or add a comment to the blog and I'll update things. I'm just as keen on negative data as positve ones if you're in the area - I'm getting hints that there's nothing moving in southern Tarangire, for instance, but only because I've been asking folk there and they've all been silent! Similarly, I think it's negative all over Serengeti, but I'm struggling for people to tell me that despite sending lots of messages!

Vachellia and Senegallia species in massed flower near Longido, Jan 2012

Some things I've been wondering about that might help us piece things together include information on average flight speeds for butterflies of other species - around 14kmh for Monarchs, butteflies that like a little tail wind. And between 10kmh and 6kmh for a range of other species in still air (higher speeds usually by migrant species, but not specifically recorded on migration). When our African Monarchs are on the move they seem to travel at about the same speed as the whites and they use a following breeze too, so let's assume a similar flight speed. Now I first noticed movements in Arusha on Tuesday, but it could have started sooner. Most movement has been between 10am and 5pm, giving 7 hrs of movement time. And most of the butterflies haven't been stopping much to feed. So let's say each individual has been moving for 7hrs on three days, they should have covered nearly 300km in that time. My most easterly records of the movement on the map (did I say to help me fill it in?!) are from Korogwe on Wednesday, when a notiable movement was headed NW - not as many as elsewhere at that time, but I don't know what it was like there on Tuesday or Monday. So those eastern butterflies are around 300km from me, if they followed the direction of movement we've been recording. If they flew each day, they'd be passing here about now. It's tempting to think the lower numbers at the moment represent the end of a continouos movement from Korogwe to here, whilst the peak that was here should no be about 300km further along, takingus beyond Eyasi and into uncharted territoriy. Anyone know folk in Singida or out that way who might tell us what they've seen?

The other interesting observation is that the only record I have of the bulk of the butterflies stopping to feed, rather than just a few, is in the bush north of Meru, where they're apparently completely covering the Vachellia and Senegallia species that are in flower over there at the moment. Intersting observation that - I was up in the Loliondo area a couple of weeks ago and the flowering event was truly stunning - V. tortilis and S. mellifera in amazing abundance. How widespread is that? Does that have anything to do with the moment? I'm not sure, if you're in the bush tell me how many flowers you've got around you!

The other obvious things that might explain movement patterns is weather - in particular how fast the land is drying out, and what the wind is doing. Now, whilst some have simply been flying downwind and most of the butterflies I've been told about are not flying into the wind, but they're not simply flying with the wind everywhere, there is some movement to one side or another, so it does seem like they're not just passively blowing along in a random search for food - there is some direction to it as well. Check the wind charts here and compare with the interactive map if you need convincing...

So what is going on? Well, my best guess at the moment is that conditions were pretty good in the Maasai Steppe over the last few months - we had plenty of rain in November / December, so lots of food. Now it's drying out very, very fast down there, and food plants on which to lay eggs are vanishing, so a movement began. What the precice trigger was, I have no idea - I doubt there's any coordinating event, but there might have been some initial condition that suddenly hit across a very wide area - perhaps air pressure combined with drying? I don't know but it might be worth havinga little look at some data. Then once moving they headed initially north out of the Steppe until they saw the massif of the Pare Mountains blocking the way which alsomay have served as a sort of collecting point. Then they headed along the ridge NW, and were also blown west by a prevailing north-easterly wind. As this wind turned easterly near Kili, so did the butterflies and on they went, etc. I'm sure wherever they find suitable food plants on their way they'll be laying eggs (so we'll probably have another emergence in about a month - generation time for Belenois is about 30 days), and my guess is that's they'll probably just keep moving and laying along the way until they die. Though it would be interesting if they arrived en mass somewhere as they are reported doing in parts of southern Africa. The only way to tell is if everyone sends their records to me - no rewards beyond that warm feeling associated with being part of something... I think we should be able to combine everything into a note for some journal East African or Lepidopteran focussed - I think we've already got more information on this current movement than I can find out about any previous ones in the region and please let's get more!

Thanks to everyone for contributing so far. Keep the records coming in - and if you're just on your way back from safari, tell me where you've been this last week and what you've seen!

Mapping the butterfly eruption

2012-02-01T23:57:00.000-08:00

OK, I got rather a lot of information thanks to the previous post and thought we'd cash in on it as fast as possible - the butterflies are moving again over Ilboru right now, not in large numbers yet, but it's still cool. I think I've created a google map that anyone can edit with their location and observations. It's not a polished item, but I've put what I've discovered in for far and it's already interesting. (If anyone tech-savvy can make it neater I'd love to have icons that reflected the direction and volume of movement, rather than just pins...) If we get lots of observations on this it will definitely make a note for some lepidoptera publication and we'll have pushed back the frontiers of science, which would be great!

View Butterfly eruption 2012 in a larger map

So, instructions....

I think all you have to do is click on the link below the embedded map (or HERE) above to open it in google maps. You'll need to log in to google I'm afraid (if you're not there already), but once you've done that it should just be a matter of clicking first on the red EDIT button on the left hand side of the map, then clicking on a location to add your information. If there are no butterflies where you are, chose a red cross (click first on the pin icon on the top left of the screen, then click where you want to put it in and then click on the pin to edit the data - click on the icon of the pin and chose My Icons to get a set of icons I've drawn and currently in use), if there are some butterflies use the green arrows, if none use the red cross, use a paler colour if there weren't many butterflies. Once clicked in place, click to edit the information and let me know direction of movement and numbers. Ideally I'd love everyone to be out in the hot part of today some time and count the number of butterflies that cross an imaginary 20m line in a 1 minute period. This would be nicely comparable data everywhere. If it's anything like yesterday you'll get hundreds in this period! If you can't give me a detailed count, or they're not moving in your area today, let me know roughly how many (10s, 100s, 1000s or millions you think there were when you did see them). Oh, forgot to say, when you've adden your point, press the "Done" button on the left (where the Edit one was originally) so submit the records. If it's all too technical just add your observations in the comments below, or send me a message and I'll do it for you!

Thanks very much! We'll learn together (and I'll learn to use google maps...)! And please do share this like with any of your facebook friends or elsewhere - the more the merrier! You can click on the facebook like icon below, if anyone twitters even better!

Butterfly migration out now!

2012-02-01T20:47:00.000-08:00

African Caper White Belenois aurota

Beleonis creona African Common White

If you're anywhere near me at the moment and it's not dark, get off your seat and look outside. There is a phenomenal butterfly migration happening in the Arusha area at the moment - from my garden in Ilboru I'm counting around 100-150 white butterflies headed west over a 20m wide patch of sky every minute. Truly spectacular! I'd love to know how widespread the migration goes, so if your on safari and getting this on your mobile, let me know what's happening where you are too. I first noticed the movement yesterday and expect it will keep going for a while yet. The butterflies involved all seem to be the African Caper White B. aurota, though there could easily be some African Common White, Belenois creona (subspecies severina) in there as well. Look carefully for a while and there are one or two of the African Migrant Catopsilia florella a much larger and yellow tinged butterfly. The main difference between the two Belenois is that the male in the Caper White has a bar on the forwing and the Common White just as a spot - but it's not always easy to decide if it's a spot or a proper bar as there's lots of individual variation, so I might have got a few pics confused here. Let me know if you're better at these butterflies than me!

Belenoisa aurota African Caper White on the move now!

Migrations such as this happen every year to a certain degree, but in varying numbers - last year there were no major movements, two years ago we had something similar to this. These Belenois species all feed as caterpillars on members of the Capparaceae, for which they're often given the name Caper butterflies. The most common food plants in that family here are Maerua and Boscia species, and there's lots of those in the Maasai Steppe. Migrations like this have been recorded at erratic intervals for a very long time -I found a couple of old papers describing migrations of these species from the 1930s at a variety of times of year and with animals heading in a variety of directions, but nothing that really tells us what's going on in any detail. It's hard, really, to call such erratic migrations - they're probably moreproperly called 'erruptions' - when the population reaches a particularly high level and there's not enouch food the population en masse heads off to find new foraging grounds.

Painted Lady Vanessa cardui, Nr. Tarangire, Nov 2011

Other species do proper distance migrations too, of course, and I'll have to tell you about the Painted Lady Vanessa cardui some time, populations of which migrate from Europe to Africa and back again (other populations do a North American migration). But that will have to wait for another occasion, today is the day of the caper whites and erruptions! When they arrive in huge numbers their larvae have a massive impact on the food plants - they can completely defoliate (remove all the leaves) from thickets of Maerua, and substantially change the vegetation dynamics of the areas they settle. Such events no doubt give other species the chance to establish in the gaps that form and promote diversity in the system as a whole, but clearly show how the notion of 'balance' in nature is a myth - ecological systems are a complicated and dynamic system constantly changing, sometimes rapidly and dramatically.

Belenois creona - note the spot on the forewing, not a bar.

PS Just heard there are huge numbers going past Kili too, also headed West. Eouldn't it be fun if everyone in the region who reads this would be able to count a 20m stretch for 1 minute and record the predominant direction, then stick a comment in below so we can plot the direction and numbers across the region! I know there are lots of you reading this around Tanzania, even a few in Kenya...

UPDATE: Check the next post for an interactive map to add your sightings to!

A few things you (probably) didn't know about weaver ants

2012-01-31T11:25:00.000-08:00

Ants aren't usually the first things people look at when on safari, but they are fascinating beasts when looked at up close. We briefly featured siafu here once before, but that's not enough for a really important group of invertebrates, and it's time to rectify that. Finding I had some nice pictures of Weaver Ants Oecophylla longinoda (right) I thought they might make a good start as they're not only fairly common in some areas (particularly near the coast), but they're pretty interesting too. In fact, on starting a bit of research I discovered they're even more interesting than I first thought! There are actually two species in this genus, the African species, and a closely related species that occurs across Asia and into Australia. There being (I suspect) rather more myrmecologists in Australia than Africa, a lot of the relevant research comes from there, but it seems highly likely 'our' species do the same, so here are a few things you might not have known about weaver ants before.

Firstly, weaver ants are are the first recorded organism ever to be used in 'biocontrol', their use being recorded back in 304AD by Chinese farmers to protect their fruit orchards. The descendants of these farmers, and other farmers across Asia and Africa still use the ants in exactly the same way today. Studies have found that these predatory, arboreal ants are much better at keeping fruit trees clean of important pests than other ground nesting ant species, and that farmers who look after their ant colonies (by leaving the undergrowth rough to discourage the ground-nesting species that are dominant in the shorter-canopied orchards, and by preventing pruning of nesting colonies) currently use on average half the chemical control needed by their neighbours who don't do this, and a significant proportion (20%) don't need any additional chemical control at all. So they're rather handy things to have around.

Weaver ant nest, Ushongo, July 2011

Next on the list is the remarkable way they go about building their nests. The nests consist of living leaves, joined together in a ball using specially produced silk. But how they sew the leaves together is a remarkable feat of cooperation. Typically, some tens of ants will need to form a chain to first bridge a gap between two leaves, then pull them together so another team can hold them in position whilst yet more sew the gap together with silk. But adult ants can't make silk, so they have to use larvae to do it, picking the larvae up and using them like little pots of glue to spin a mat of silk between the two leaves. This is a pretty remarkable piece of evolution on its own, but even more so when you consider how hundreds of ants can cooperate to achieve this, a far more complicated procedure than simply digging holes. Recent research has suggested that there really is no central control to this process. If given two exactly identical gaps to close, there's no central decision made over which to fill first, but still the nest 'decides' to do one first. How? The observations and models suggest simply by each ant doing his own thing with a very simple set of rules, most important of which is something like 'join the biggest chain of ants you see'. That simple rule alone means that a initial chance difference in the number of ants seeking to fill each gap is rapidly re-inforced until there's only one gap being worked on. Remarkable how such simple rules can result in apparent group decisions very quickly indeed.

There's money in that nest!

And finally (though there are lots of other fascinating things I could have thought of, but three points is alwaysa good number to remember!), I was amazed to learn that there's a major market for these ants for human consumption. Yes, it's true! On some Asian markets the price per kilo is twice that of beef! Must be very tasty... Apparently you can also sell them to Europeans as pet food, or in China and India for traditional medicine, but most of the markets across Asia and also in Cameoon and Congo are for human consumption. So valuable are these markets, and so easy to keep are the ants, that farmers who feed the ants on invertebrates atracted to kitchen scraps and the like stand to make over 4.5 times their costs on each brood sold. Now that's a good return rate - I'm thinking of changing job...

So, there you go, three pretty remarkable things about weaver ants. If you've not seen them before but have been in East Africa, believe me, it's only because you've not been looking hard enough. Now you know how interesting they are, hopefully you won't overlook them next time! And that's before we start talking about how important to the ecology of many ecosystems are ants in general. Stories for a future post, I think...

Main references:

Van Mele, P. (2007). A historical review of research on the weaver ant Oecophylla in biological control Agricultural and Forest Entomology DOI: 10.1111/j.1461-9563.2007.00350.x

Lioni, A., & Deneubourg, J. (2004). Collective decision through self-assembling Naturwissenschaften, 91 (5), 237-241 DOI: 10.1007/s00114-004-0519-7

Offenberg, J. (2011). Oecophylla smaragdina food conversion efficiency: prospects for ant farming Journal of Applied Entomology, 135 (8), 575-581 DOI: 10.1111/j.1439-0418.2010.01588.x

Why are female raptors usually bigger than males?

2012-01-29T09:58:00.000-08:00

A friend of ours foolishly sent me a message yesterday saying he'd got both spotted eagle owls and african wood owls in his garden at the moment. As I'm sure anyone sensible would have realised, that immediately resulted in my inviting the whole family over to his house for lunch today with the join aim of avoiding washing up and seeing some nice owls up close. I'm pleased to report success on both fronts! After the initial surprise of seeing the two species in practically neighbouring trees (eagle owls are well known predators of other owl species, making over 10% of the diet in one study), the thing that struck us most was the extraordinary degree of dimorphism exhibited by the pair of wood owls. In most birds of prey - the owls, hawks, eagles, etc - the female is the larger bird, the male being smaller which is exactly the opposite of what is normally the case in birds. Why this should be is a fairly interesting question which comes in two parts - firstly, when should there be such a large difference in body size in (particularly) birds of prey? And secondly, why should the females specifically be bigger than the males (why is it the opposite of most other species)?

The pair of wood owls that set me thinking. Arusha, Jan 12

For the first question there's a fairly well agreed upon reason for why raptors show such strong dimorphism (i.e. why there are such marked differences between the sexes): it's to avoid competition between members of a pair. The more potential competition there will be between the male and female for food, the stronger should be the dimorphism, so they can concentrate on different prey. Raptors, because they are higher up the food chain, have much lower food abundance than, say, a quelea, eating grass seeds. They also tend to be larger than birds feeding on other food types, and consequently need more food. So it makes sense firstly that they should be rarer than their prey, and secondly that there's going to be much stronger competition for the prey that is available. To reduce the competition between male and female who share a territory, it's very sensible for the two sexes to concentrate on different food types. In fact, it's rather hard to find evidence of this difference: peregrine falcons, for example, show no obvious differences in some circumstances. Nor do all Northern goshawks. But there is enough evidence from a variety of species to suggest perhaps this is fairly normal, though might depend on the specifics of breeding location. Let's accept it for now... One possibility that follows from this, is that the degree of dimorphism between raptors species should also vary with food availability/competition. And there's a degree of evidence that this, too, happens - dimorphism is minimal in vultures, intermediate in hawks and eagles catching terrestrial prey, and highest in those species feeding on birds, where foraging is really tricky. So that makes a certain amount of sense, even if it is a bit of a "Just-so" story, since we can never do real experiments to test the ideas.

Baby Spotted Eagle Owl also in our friend's garden

But why should the females be bigger? This is a harder issues where there is less to go on and even more Just-so type thinking - in fact, no fewer than 20 hypotheses have been proposed! I'm not going to review them all, but will focus on a few of the most popular - and to be honest there really isn't a definitive answer, we just don't know! Still, one of my favourite is that birds of prey need to be more agile than other species if they're going to manage to catch their prey. (It's pretty obvious that a bird that eats another birds really does have to be more agile to stand a chance of success!). How agile birds are depends to a large degree on their mass - heavy birds aren't so agile and can't corner as fast, etc. But when they're about to lay eggs, female birds have to put on mass, which makes them less agile. But a fixed increase in body mass has a lower effect on a larger bird than it does on a smaller bird - so females should be the bigger ones. What do you think? nice idea? Maybe... I'm still not 100% certain it doesn't also apply to other bird species too, where the male is still larger than the female.

So how about this? Raptors need to tear food up to feed young, which is hard work. The females do more tearing to feed the chicks than the males, so they should be bigger. Maybe, but what about raptors (and owls in particular) that mainly eat beetles? And where people have looked at it, there's little evidence of clear sex differences in ripping behaviour anyway.

Adult Spotted Eagle Owl. Male? It was smaller than the chick.

My favourite is to do with prey abundance again - it fits with the original dimorphism argument after all. Smaller prey are usually more abundant than larger prey - there are simple more small animals around thanks to the pyramid of numbers again. Females spend most time incubating and caring for small young, so males are going to spend more time foraging, and should therefore specialise in the species that are more abundant - the smaller prey items. But whilst I like the simplicity of this argument, there's no real evidence it's right, nor is there always evidence that males spend more time foraging!

There are plenty of other ideas out there (reviewed here), but you can try and make your own up too - I think there's plenty of scope for new ideas in this area, especially if they can actually generate hypotheses which might be testable. At least it will give you something to think about next time you see a pair of raptors...

Main references:

Wheeler, P., & Greenwood, P. (1983). The Evolution of Reversed Sexual Dimorphism in Birds of Prey Oikos, 40 (1) DOI: 10.2307/3544210

Helmut C. Mueller (1986). The Evolution of Reversed Sexual Dimorphism in Owls: An Empirical Analysis of Possible Selective Factors The Wilson Bulletin, 387-406

ANDERSSON, M., & NORBERG, R. (1981). Evolution of reversed sexual size dimorphism and role partitioning among predatory birds, with a size scaling of flight performance Biological Journal of the Linnean Society, 15 (2), 105-130 DOI: 10.1111/j.1095-8312.1981.tb00752.x

How to survive the Serengeti: predation, food and body size

2012-01-25T09:30:00.000-08:00

Serengeti Lions eat a diversity of mammal species.

Another paper describing one of my favourite talks from the TAWIRI conference back in December has just been published, this time in the Journal of Animal Ecology (available to some here). Grant Hopcraft and colleagues are interested in why different grazing animals use different parts of Serengeti in different ways when they all eat grass. Why, for example, do you usually find buffalo around riverine areas, whereas gazelles tend to be on the open plains? At some level it's obvious that where you find an animal is the place that it survives best and there are two aspects to survival that most ecologists would agree are crucially important: where will you eat? And where will you be eaten? Although both buffalo and gazelles eat grass, and both are eaten by large cats, it's quite possible that grazers of different size will be affected differently by these same two processes. And that's what Grant and colleagues set out to test.

Topi are definitely on the menu, but need enough food too. Moru, Dec. 2011

From a knowldge of physiology we can expect that smaller grazers have a smaller mouth and therefore lower intake rates than larger animals. What's more, smaller animals generally have a higher metabolic rate than larger ones, so need more food. They also have a smaller space for digestive organs and consequently what little they do eat can't stay inside very long, meaning they can only get the easily digested nutriment from what they do eat. By contrast, larger animals will have higher intake rates, relatively lower energetic demands and a longer digestive tract that allows for some fermentation of the food even among ruminants, meaning they can get more nutriment from the same food than smaller animals can. They are bigger though, so still do need more food overall. So you'd expect that small animals will be looking for higher quality grass (with more of the important nutrients like Nitrogen) than larger ones, which will just be looking for large amounts of grass. In essence, you can suggest with some certainly that both food quality and food quantity are going to be important to some degree, with perhaps animals favouring one over the other.

Everyone eats a Tomson's gazelle, but they don't eat much! Grumeti, Sep 2010

Predation risk similarly differs with body size. Very large animals like elephants and rhino at least as adults don't ahve to worry about predators at all, whilst smaller animals certainly do. If you're a large herd of buffalo you're not going to worry about much except, perhaps, a particularly hungry pride of lion - on the other hand the Thomson's Gazelle is pretty much the universal snack for all the large predators in Serengeti and is going to be very, very concerned about where predators might hide! So, what we've called the landscape of fear will have different influences on different animals too, in a way that might well be directly related to body size.

Buffalo are less troubled, but need lots of food, Seronera, Dec 11

Now Grant and his colleagues set about testing this through the known distribution of animals (from areal survey data of buffalo, topi, Coke's hartebeest, Thomson's and Grant's Gazelles) using statistal modelling methods. They (sensibly) decided there might be different factors that explain presence or absence in a particular census square to the factors that determine if there are any animals there at all. (For example, for some animals that drink a lot presence of water within, say, 2km might be essential to presence, but as long as there's a little bit of water, more water won't mean more animals, whilst more food might well.) So they set about modelling in a two stage way - one to predic presence or absence, then another to predict the relative abundance of each species. Having done that they set up a model that isn't often used by ecologists, called a Structural Equation Model, which allowed them to link things they could actually measure in the field (like bush cover, nitrogen content of grass, grass biomass, etc.) into the three main sets of processes that they considered likely to influence abundance: those factors to do with food quantity (like rainfall), those affecting food quality (like nitrogen content) and those affecting predation risk (like presence of bushes), and then set off to see if their predictions held: did smaller grazers really select high quality areas and avoid areas with high predatin risk? Did buffalo just focus on places with lots of grass, irrespective of predation risk and foraging? And where topi and hartebeest somewhere in between? And I guess they were pleased when they found the answers, broadly speaking, were yes.

Elephants have escaped most predation, but eat continuously. Moru, Dec 2011

In particular, buffalo don't care about anything but food abundance, the gazelles were the only things to chose the high nutrious, low biomass areas with low predation risk, and the hartebeest avoided both high biomass and high quality areas, perhaps searching for low quality grazing in areas with low predation risk, though they were the 'wrong' way round relative to topi, given their body mass. But still, body size alone and it's interaction with food quality, quantity and predation risk can explain some of the patterns of distribution in these species. The key here, of course, is that some - and it's a rather small amount of variation that the authors explain. That's not surprising to me - masses of things affect animal distributions, especially in herding species like these. The analysis can't really consider social effects and there's also bound to be some massive random variation in just where the animals are when the surveys happen - try again a day later and you'd have different patterns to explain. So not a bad effort at least! (Anyone who knows my gripes about distribution modelling, however, will know I've one or two issues with some of the statistics, but nothing that will make a huge influence to the overall results, I guess...)

And another question for Grant - why not include wildebeest? Beacuse you knew they wouldn't fit the pattern? Or what?!

Anyway, the summary is that, to survive in the Serengeti, you need to know how big you are. As for me, I'm somewhere between a Grant's Gazelle and a Topi - that means I should definitely be looking to avoid places of high predation risk, and I should probably be looking for my grass to be short and nutritious - if it doesn't taste good, I'd better not bother... Sticking to the short grass plains definitely sounds like a good survival strategy to me!

Main reference:

Hopcraft, J., Anderson, T., Pérez-Vila, S., Mayemba, E., & Olff, H. (2012). Body size and the division of niche space: food and predation differentially shape the distribution of Serengeti grazers Journal of Animal Ecology, 81 (1), 201-213 DOI: 10.1111/j.1365-2656.2011.01885.x

Nitrogen in the savanna biome

2012-01-19T23:12:00.000-08:00

Nutrients, along with fire, water and herbivory, are one of savanna ecology's big 4 and we've covered quite a bit about nutrients in the savanna biome in general already on the blog. So far we've mostly covered the issue in a general sense, not focussing on specific nutrients but a new review by Corlie Coetsee and others of the nitrogen cycling in Kruger National Park (available,but sadly not free to view here) made me think it was time we tackled the issue slightly more specifically.

Middens are amazingly rich patches!

Nitrogen, although the most abundant gas in the atmosphere (nearly 80% of air is nitrogen), is one of the three commonest elements limiting plant growth (the others are the P - phosphorus - and K - potassium - of traditional NPK fertilisers). It's vitally important to life, because it's a major component of protein, but most of that nitrogen int he world is in fact useless for plants or animals being what we call inorganic. Before plants or animals can make use of it a chemical transformation needs to occur from the inorganic form, to a form that is bound to hydrogen or oxygen atoms and can be used by plants. The cycling of nitrogen from inert inorganic forms, to useful organic forms is referred to as the nitrogen cycle, and the rate at which conversions happen are can be critical at determining the fertility of soils. The most important process that converts inorganic to organic nitrogen is driven by various types of bacteria, in particular there's a group called Archaea that we now think probably aren't bacteria at all that play an absolutely critical role in this process.

Termite mounds produce fine-scale variation in nitrogen, Serengeti, Aug 2011

Once in the soil, organic nitrogen can be picked up by plants, which may in turn be eaten by animals who defecate and eventually die allowing (mainly) bacteria to recycle the organic nitrogen to the soil. There's some loss in the process (other bacteria are called denitrifying bacteria because they return the organic nitrogen to inorganic forms) so there's no continual build-up of organic nitrogen in the soil and, as we know, it's often limiting to plant growth - add a nitrogen fertiliser to savanna and plants grow better. But it's distribution across the landscape isn't even and this is where it gets interesting - the full range of processes operating in the savannah affect both the fine and large-scale distribution of nitrogen which, in turn, affects the distribution of plants and animals across the landscape.

Grazing lawns are locally high in nutrient, Kruger, June 2011

We've talked a lot about various ways in which nutrients are moved about the landscape and result in different patterns, and I was pleased that I've not missed many of those Coetsee et al talk about. In fact, seeing them all together in one paper made me realise just how complicated the various processes are in savannahs, and it's worth running over them one more time here. Let's start by talking about large-scale patterns that are ultimately related to geology: in Kruger the higher lands regularly have nitrogen washed out of them with nitrogen accumulating at valley bottoms - exactly as we've talked about for broad-leaved woodlands (the same doesn't always hold in Serengeti as there are some particular fertile patches on a few ridges in the western corridor). And the underlying geology itself affects the soil type, which determines how long nitrogen stays in the soil, another process we've talked about at length. So there's the large-scale process they talk about (they might also have mentioned rainfall gradients being important, as rain can wash organic forms of nitrogen from the atmosphere to the ground, with the effect that wetter areas often have more nitrogen deposition than drier areas). But within these areas there are a large number of fine-scale processes that mean that even within low nutrient areas there can be particular hotspots that are, actually, richer and more fertile than the generally higher nutrient areas: termites are particularly important at concentrating nitrogen around termite mounds (we covered termites). Large animals also move them around a lot (especially if they make middens), and if they develop grazing lawns they can encourage grass with higher nitrogen content to keep growing. We've also mentioned the work of dung beetles here being a further process that creates fine-scale variations in nitrogen content in the soil. And we've talked about how nitrogen fixing legumes - all those trees that were formally known as Acacias - make a huge differece. In fact, Coetsee remind me that other trees too increase soil nitrogen levels by pumping the nitrogen they find deep down up to the surface where they can use it, and they also mention the nitrogen hotspots that develop around waterholes thanks to animal droppings, and issue we didn't mention at all when discussing waterholes in the savanna biome. They also talk about the role of fire and note that in general fire frequency doesn't decrease nutrient quantity in the savanna (except, perhaps, in areas with annual fires - which would actually include much of Serengeti), but that in fact through a complex set of interactions it might lead to increased fertility: when a fire happens it can actually mineralise some nitrogen (nearly making it available to plants) and it certainly stimulates grass growth. This grass is actually higher in nitrogen than the old grass, which encourages animals to come and graze on the new shoots. The animals, in turn, bring their dung and fertilise the soil further. So although it might seem that burning off vegetation will result in a loss of nutrient (all that ash blowing away), in fact the evidence from Kruger at least is that it's at worse neutral, at best beneficial.

This Vachellia tortilis will be creating a nitrogen hotspot. Serengeti NP Dec 11

Put together and you have a picture of variation in nutrient quality at a large scale, with fine scale variatoin superimposed to make an incredibly complex picture. I also like the way they show how the savanna big four influence one another - fire changes nutrients, which changes grazing, which further changes nutrients, etc. It's a wonderfully complicated set of processes, and I thoroughly encourage you to read this paper if you want a more detailed overview of nitrogen in the savanna biome!

Main reference:

Coetsee, C., Jacobs, S., & Govender, N. (2011). An Overview of Nitrogen Cycling in a Semiarid Savanna: Some Implications for Management and Conservation in a Large African Park Environmental Management, 49 (2), 387-402 DOI: 10.1007/s00267-011-9779-0

Life spans of tropical birds

2012-01-15T10:42:00.000-08:00

Admirng a White-browed Scrub Robin!

Pangani Longclaws are very impressive up close

This morning I took my children with me to go out and do some bird ringing (banding, if you're from the US or Australia!) with some friends just out of town. As is often the case when I'm ringing, I get asked lots of questions about why we ring birds, today's best was why we keep catching immature birds? There are lots of answers to this question and some of them focus on the fact that young birds are just less careful and are generally more stupid than their parents - but this bias aside it is a really good question and had me thinking again about the differences between bird lifestyles here in Africa, and those in higher latitude areas like Europe and North America. It just happened that the question came up after we'd caught two migrant birds (from Europe) and a very new baby White-browed Scrub Robin - all birds hatched within the last year. What I said at the time (and now I can look back over all the birds we caught this morning it's even clearer) was that whilst we often catch more immature than adult migrants, that's not true for the resident African birds we were catching. In fact, we only caught about 25 birds this morning, and only three of them were migrants (we were hoping for more, but this year has been surprisingly poor so far down here, the talk of all the local birds at the moment, as conditions seem good to us!), but all the migrants were young, whilst of the remaining resident African birds, only three were immature. This sample alone, of course, is of limited use: as a scientist I'd want much larger sample sizes - and I'd also want to know it wasn't just a seasonal thing - many of our local birds are only just begining to breed, so there simply aren't many babies around yet. But put our results together with many others, and start to look at the data gathered in more detail and it is indeed clear that African birds do things rather differently to their higher latitude counterparts - as described in a nice paper by Wiersma et al "Tropical birds have a slow pace of life" (get it free here!). Not unlike a lot of people living here either!

Lovebirds can live up to 20 years - they can also bite hard, so be careful extracting!

Each ring is uniquely numbered and has an address in Nairobi.

Measuring the tarsus...

and measuring a wing can help you sex and identify populations of your bird

We know from lots and lots of studies that survival of rates of birds in the tropics are generally higher than those of their counterparts in higher latitude areas (though there are also plenty of exceptions). We also know that clutch sizes (the numbers of eggs) are also usually lower in the tropics than elsewhere. And incubation periods (how long the birds sit on eggs) are also generally longer. This whole cluster of behaviours - long lives, slow reproduction, long parental involvement, etc. - we call a life-history, and all point towards birds in the tropics doing things sloooowwwwwly. But why? These observations go back rather a long time and have their roots in the observations of Moreau here in Tanzania way back in the 1940s, and ever since then people have been trying to explain it. As you might imagine, we've come up with some pretty interesting ideas in that time, from the fairly widely discussed idea that it all stems in the very high predation rates on bird's nests here in the tropics (all those snakes, plus masses of mammals). This argument runs that high predation rates mean you'd be better off planing to have several nesting attempts and not having too many eggs in each attempt or you might run out of energy to try again. But then if you do have a successful nest, you've only got a few young from it so (a) you'd better look after them well because you'll be unlikely to raise more again soon, and (b) you'll not suffer the costs of raising so many children, so you'l live longer (yes, it's true!).

Weighing a bird: 14.3g, similar to a big coin...

Less widely accepted theories involve the idea that local birds in the tropics have smaller clutches because of competition from migrant birds from the north, who arrive just as they're tring to breed. This idea doesn't seem to explain all the different aspects of the tropics bird's slow lives, mind (why should they also have longer incubation times? Or live longer in general?). Or even that it's driven by the comparative lack of seasonality in the tropics (though this was written by a bunch of folk who I doubt have ever actually visited a savanna in the dry season, and I have extreme difficulty thinking it's really valid!). But the paper by Wiersma has some interesting additional observations that might just help explain things - they suggest that tropical birds do things slowly, because their whole metabolism is slower. (What's more, they actually show this is true by direct measurement of metabolic rates in lots of birds). A slow metabolism could directly explain many of the observations we see - if they can escape being eaten they'll inevitable live longer, if they might going to live longer they should take fewer risks, so invest less in each nesting attempt and not attend the nest as often (which will result in the increased predation risk we see, and at the same time the longer incubation periods (though attentiveness doesn't explain all the difference, the lower metabilic rate of the chicks themselves could make up the difference)). Now that sounds rather neat, but the question remains - why should tropical birds have a lower metabilic rate. And this is where I think the ideas that Wiersma et al have are so neat, since they also showed that tropical birds can't increase their metabilic rate as much when placed in cold environments as birds from temperate climes. Which means, of course, that they can't generate as much heat to keep themselves warm in the cold - which is perfectly sensible, since they live in the tropics where cold is unusual. So the authors suggest that if you want to be able to keep warm - as you have to if you live in high latitude areas - you have to have a higher metabolic rate than your relatives who live in the tropics. And if you do that, you'll live shorter lives and must do things faster.

Male Yellow Bishop are rather fine just now!

Now I think this is a really neat theory - there's still lots of experiments we can do to test all the parts of it. But I also like it that a lot of these questions can only ever be answered if we have a good idea of how long birds actually live - and that means we need to keep doing regular ringing programmes and keep re-catching those birds uniquely marked throughout their lives - particularly here in the tropics where we really know so little.

Main reference:

Wiersma, P., Munoz-Garcia, A., Walker, A., & Williams, J. (2007). Tropical birds have a slow pace of life Proceedings of the National Academy of Sciences, 104 (22), 9340-9345 DOI: 10.1073/pnas.0702212104

What is the savanna biome?

2012-01-12T07:55:00.000-08:00

Savannas around the world are often open woodland like this Serengeti pic.

Remember a few months ago I reported on some talks I'd enjoyed at the ATBC/SCA-Africa meeting here in Arusha? Well, the research behind one the the talks I enjoyed most was published late last year and I think it really helps explain what makes the savannah biome (which is probably why it was published in Science - great job Carla!). However, before going into too much detail about this research, I think it's important to make sure we know exactly what I mean by a biome in the first place...

When I talk of the savanna biome (or, indeed, the savannah biome, since I am English) I'm not referring to a habitat like grassland, or Acacia woodland; nor am I referring to a specific ecosystem like the Serengeti Ecosystem. Rather, I'm refering to the set of habitats that make up the savannah biome globally - the collection of grassland and woodland types that all have the same main processes operating on and in them, the savannah big four are, of course, nutrients, water availablity, herivory and fire. They might (and often do) contain remarkably different species, but the same processes are at work and, to remarkable degree, they show largely similar vegetation forms and structures. A biome may therefore be thought of as a set of habitats that share similar ecological processes wherever they occur across the world. By contrast, I tend to define an ecosystem as a single geographical area (like the Serengeti Ecosystem), within which nutrients are cycled with relatively little input or output from neighbouring ecosystems. Hope that's clear...

Fire is crucial to maintain savanna, particularly in wetter areas. Tarangire July 11

So, if we're going to understand what the savanna biome is, we need to look at the processes that are the dominant forces within it. If we can understand these, then we can predict where savannahs will be found throughout the world - and we can also understand what might happen to savannahs if we humans mess around with these processes through, for example, the effects of climate change. And this is exactly what Carla Staver and her co-authors have done in their paper (which I'm afraid is probably hiding behing the paywall here). They were particularly interested in discovering what determines the boundary between forest and favanna biomes (the boundary with desert is a much more obviously rainfall driven boundary), and decided that the most likely factors were rainfall and fire. Whilst that might well be true, there's plenty of evidence that herbivory is a major player too (at least here in Africa) and nutrients have been proposed as major players too (though there's also plenty of evidence to suggest this really isn't the main thing). However, they didn't look at these - herbivory they mention but exclude from their analysis with the very reasonable excuse that we just don't have a good idea how much herbivory there is around the world, but I think it's a bit of a shame they didn't have a go at nutrients too. Still, enough of what wasn't looked at - what did they find?

Amani NR is forest - and WET!

They extracted data from satellite records from the whole of Africa, Australia/Southeast Asia and South America (not sure why India didn't feature, but there you go), looking at tree cover, fire frequency, rainfall, etc. As expected, they found that as you increase rainfall from the minimum within a savanna of around about 400mm you gradually get more and more trees, until at about 1000mm per year something strange happens and, instead of there being a continued gradual increase, two patterns emerge - one of continued intermediate tree cover (below 50%), typical of the savanna biome, and another of around 80% cover - typical of the forest biomes. And (at least in Africa and South America) this pattern continued between 1000mm and 2500mm, after which all areas were definitely forest. From this pattern they conclude that in the regions they studied at least, below 1000mm of rain savanna is the only possible biome, whilst forest is the only possibility above 2500mm - but in this intermediate area either forest or grassland can occur, and the overwhelming factor predicting whether you end up with forest or savanna was the presence of fire. What amazes me is just how much of the world (and Africa in particular) this intermediate zone covers - in the picture I've included from the paper below you can see that whilst most of East Africa is too dry for proper forest biomes and we're in a stable savanna area at the moment, most of the Congo forests are, in fact, potential savannah biomes too. And what's more, if climate change really does result in more rainfall for east africa it will move parts of us from the certain savanna regions to potential forest zones. Of course, the authors pointed out that it's not quite as simple as total rainfall, but the seasonal pattern is also very important - if the dry season was longer than 7 months it doesn't matter what the rainfall is, you'll have savannah, a feature that they suggest might explain why things seem to be a bit different in Australia. On the other hand, it seems you can have savanna in areas with remarkably short dry seasons (though in Africa, not less than 2 months is needed) - I wonder a bit how fuel ever gets dry enough to burn in such areas - maybe these are the locations we should be looking for the most serious herbivory impacts?

Look how little really wet forest there is in Africa! From Staver et al

Anyway, all in all a pretty convincing story showing once again that whilst climate is important, fire is also essential to many savannas (though we can still wonder about their necessity in those drier savannahs, below 1000mm) . And what's more it's another great example of how different stable states can exist in ecology largely, the authors argue, as a result of what's happened in the past: in the overlap zone where forests are currently found it's because these areas were once wetter, whilst in the current overlap zone where savannahs are found, it's because these areas were once drier.

Main Reference:

Staver, A., Archibald, S., & Levin, S. (2011). The Global Extent and Determinants of Savanna and Forest as Alternative Biome States Science, 334 (6053), 230-232 DOI: 10.1126/science.1210465

How colourful are birds?

2012-01-08T09:28:00.000-08:00

Lilac-breasted Roller, eveyone's safari favourite! Indigo & Violet

One of the best things about birding in the tropics is the sheer brilliance of many of our bird's plumage. After a day in the field you can easily think you've seen birds of every colour of the rainbow - and you're probably right! In a paper published last year by Mary Stoddard and Richard Prum (available free to all here) they demonstrate nicely that whilst they do cover the whole rainbow (and more besides), they still don't cover even the majority of the potential colours available to them - only about 26-30% of the potential options, it seems.

Red: Scarlet-chested Sunbirds use structural and pigmented colours

To understand how anyone can ever assess exactly how many colours are availalbe to birds, you need to start by understanding that many birds (though not all) actually see more colours than us - a lot can see ultraviolet light, as well as the usual combination or red, green and blue that we humans (and most other primates) see. (By contrast, most other mammals lack even our ability to see red, so they must live in a rather dull-looking world!) So by knowing the entire set of colours that can be generated from red, blue, green and ultraviolet the authors identified the potential range of colours available to birds, and they then sat down and looked at nearly 1000 (965 to be precise) sets of feathers and precicesly measured their colour, then plotted it in the red/green/blue/uv colour space. And they discovered that despite their efforts to find feathers covering as many different colour types as they could, they only found colours in about 1/3rd of the available space. In particular, birds seem to be missing a lot of the different options of green and purple. Now, there certainly are green and purple birds out there, but not all the possible forms of green, and not all the possible sorts of purple. (They're not particularly good at pure UV either - but that might in part reflect the author's inability to identify strongly UV feather groups in their initial search - we can't see it after all!)

Red-capped Robin-chats get orange from carotenoids in the diet

So why don't birds use all these possible colours? Well, there are probably two answers to this: on the one hand they can't really be expected to create every single colour going (especially given the pretty limited range of mechanisms they have available to create colours) and in particular getting close to pure colours of one extreme or the other is going to be extremely difficult to evolve, and on the other hand the birds might simply not like some other options, either they not attractive to mates, or they're particularly bad for avoiding predators for one reason or another (though the latter doesn't hold much water as an argument with me, as I'm sure many bright colours are essentially a deliberate handicap evolved by male birds to should how tough they are to females!).

Parrots, like these Yellow-collared Lovebirds have a unique yellow pigment

Yellow in a Speke's Weaver is from carotenoids in the diet

What's that about a limited number of options for how birds generate colours, I hear you ask? Well, yes, it's true. There are two basic mechanisms birds have for generating colours; they can use a pigment to reflect light, or they can change the structure of the feathers to generate refracted colours. The pigments are rather limited options, all birds can create melanin, which they can use to generate browns and blacks (it's the same pigment that makes black skin black, and my white skin a bit brown if I've been sitting in the sun). For many birds that evolved rather early on - like ostrichs, that it for pigments. But many other birds can also use another pigment group, the carotenoids to generate yellows, oranges, pinks and reds. Unlike melanin, the birds can't make these colours themselves, but must pick them up from their diet - it's well known that flamingos in zoos if they aren't fed correctly loose their pink colour and end up white (sad, but true). After that, there are only two other types of pigments that any birds use, and they're both really rather rare: the porphyrins that make Turacos green (and are also found in jacanas, bustards and pheasants), and the psittacofluvins that give parrots a yellow option. Four basic pigment sets, covering black, yellow, orange, green and red. Yes, you can mix them to produce some other options - but not many, and Stoddard and Prum's study showed they account for less than 1/3rd of all avian colours. Where birds really win (and despite the fact I'm trying to convince you they're not as colourful as they might be, they are more colourful that mammals or plants - even including flowers - so they do win!), is in the use of structural colours.

Blue on a woodland kingfisher - a structural colour

More structural blue, but the orange is from carotenoids - Superb Starling

Structrual colours are rather harder to understand than pigments - whilst pigments can be thought of as paints, there's nothing normal we have to generate structural coours. Essentially structural colours are formed by the birds leaving holes of precise shape in their growing feathers. These may be small, nearly perfectly round bubbles, or they can be long thin tunnels and we've recently learnt how they are formed in the growing feather - a simple property of chemical mixtures, it seems, though it involves mixtures of nanostructures showing once again that our highest technological advances with nanotechnology were developed by animals millions of years ago! Light passing through these bubbles and holes is split into different wavelengths in a similar way to light on the surface of soap bubbles can make rainbow colours. By being very careful about the size and shape of the bubbles and holes, specific colours can be chosen and they often have a metalic, glossy appearance (and look dark if light is coming from the wrong angle) - though further structural adjustment can ensure that the refracted light is completely scattered and appears similar from all angles and without the shiny/glssy appearance. Using these structural colours burds are able to create colours completely unavailable to plants and reach the colour spectrum other animals can't reach.

Irridescent green from structural colours: female Collared Sunbird

Again, a lot of birds mix structural colours and pigments to generate even more colours, but I still find it remarkable how many colours birds can produce given this rather restricted list of options. Why they bother is, of course, a whole other question that we'll have to think about another time! (PS this has really just be a great opportunity to post some of my favourite bird pics!)

Main reference:
Stoddard, M., & Prum, R. (2011). How colorful are birds? Evolution of the avian plumage color gamut Behavioral Ecology, 22 (5), 1042-1052 DOI: 10.1093/beheco/arr088

Ecology of broad-leaved woodlands in the savannah

2012-01-05T11:31:00.000-08:00

Terminalia Woodland, Sasakwa Hill, Grumeti Reserves, July 2009

We've covered a number of savannah habitats so far, but one we haven't touched on so far are the broad-leaved woodlands. On the one had that might seem surprising - broad-leaved woodlands of one type or another are the dominant vegetation of much of the savannah biome, but on the other hand and despite the area they cover, they're not the areas most safaris spend too much time in. The reasons for this are obvious if you do start driving around them - wildlife densities are much lower in the broad-leaved woodlands than in the grasslands and Acacia woodlands that form the other major habitats of the biome. Despite this overall pattern, however, some species are actually commoner in this type of habitat than elsewhere. So why is this, and what is special about the broad-leaved woodlands?

Our Nov training camp was in broad-leaved woodland nr Tarangire!

As usual, we'll answer these questions by reference to the savannah big four: water, nutrients, fire and herbivory. The most immediately obvious thing about the broad-leaved woodlands in savannahs is that they're usually found on the higher ridges of an ecosystem - as you move off a ridge you come through the broad-leaved woodlands (typically Combretum - Terminalia woodland in much of East Africa, though a lot of Brachystegia in the southern part of the region) and gradually enter a belt of Acacia woodlands and grasslands on the lower areas. As we should know by now, these ridges are likely to have very poor nutrient loads in their soils - the ridges are often of very old rocks, 550 Million years old or more and have been well and truly washed by rain for much of that time, with the nutrients that were once present now washed down hills to the lower areas where they get used by Vachellia/Senegalias and grasses. So life is pretty tough in these areas, whether your a plant or an animal that feeds on the plants. Nutrients are particular hard to come by, so growth rates tend to be lower and there's consequently less nutritious food around to browse, explaining the relative lack of wildlife in these areas. The lack of nutrients also explains why the leaves of broad-leaved woodland species turn yellow before falling, whilst those of Vachellia and Senegalia do not: plants with nutrient shortages will try and recover as many nutrients as possible from their leaves before they drop them, and as they withdraw nutrients, so the leaves change colour. By contrast Vachellia and Senegalia ('Acacias') are legumes and have a have an ample supply of nutrients so don't need to do this withdrawing so much before loosing their leaves at the start of the dry season. The comparative lack of browsing in these areas, of course, also explains why broad-leaved woodlands aren't as thorny as other woodland types - there's little nutrient, so there's little browsing, so there's little need to defend yourself from browsing in these areas.

Lesser Kudu are often in broad-leaved woodlands. Tarangire Aug 2011

Herbivory then, is reduced. It's far from absent, but it's definitely reduced - and mainly done by some of those animals that are less frequently seen in other areas: Greater and Lesser Kudu are fans of broad-leaved woodland, so too are eland, grey duikers and the like. Why is it that these animals actually seem to like spending time in the nutrient-poor broad-leaved woodlands? Well, on the one hand you could suggest that if they didn't eat there, nothing would and even though it's nutrient poor it would still be a wasted resource, and that's certainly true to a point. But also these animals are all mammals that aren't the best at dealing with predators - they'll usually run a bit, then freeze, which works well enough when predators are at low densities, but isn't going to be so effective on a plain, or where there are large numbers of predators. So it might be predator avoidance that drives these animals to the nutrient poor hillsides that othe animals avoid - though of course we've no way of saying if in fact it might be the other way around, that once you specialise on nutrient poor food you don't need to be so good at avoiding predation!

Tabora (long-tailed) Cisticola is often common in broad-leaved woodland

What of the other two processes, water and fire? Well, they don't tend to differ so much between broad-leaved woodlands and the 'Acacia' woodlands. Maybe a little less fire (the grass doesn't grow so well), and a little less water remaining on the shallower soils, but these differences are tiny compared to the major differences in nutrient availablity and herbivory. So, in the interests of simplicity, let's leave the broad-leaved woodlands there for today - interesting places to visit for specific animals and plants (and some nice birds too!) that specialise in these habitats, but not the main focus of many game drives.

How old is a baobab?

2012-01-03T04:35:00.000-08:00

Baobabs even make Giraffe look small! Tarangire, April 2010

I was lucky enough to see the New Year in, at Tarangire NP where, as always, I was impressed by the immensity of baobabs Adansonia digitata. When you see elephants dwarfed by a tree, you know it's impressive, and Tarangire is the place to spot both elephants and baobabs. But the two questions I'm often asked about baobabs are firstly, how old are they? And then, why don't we see baby baobabs? As they're perhaps both related issues, and because I found a nice paper that explains how we determine the age of baobabs (available, but not free I'm afraid, here), I thought it would make a nice post.

Room for one more? Take samples from the cavity...

The age of many trees is easy to estimate by simply counting annual rings: in temperate climates growth happens during the summer period, and slows during the winter, depositing a dark ring each year. In the tropics many trees have growth rings formed during the dry season. The problem with baobabs is that they have a succulent trunk that (a) gets stripped by elephants for water, (b) doesn't really have clear growth rings and (c) is often hollow. So a standard method of counting rings won't tell us how old the tree is. Instead we have to turn to radiocarbon dating. This is a method commonly used to age archaeological remains and replies on the fact that when a plant grows to 'fixes' CO2 from the atmosphere in its woody matter. Now, the carbon (C) in the CO2 of the atmosphere occurs in two forms which we call ¹⁴C and ¹²C. Now ¹⁴C is radioactive and changes ('decays') at a constant rate to Nitrogen, whilst ¹²C is stable. So if we compare the proportion of ¹⁴C and ¹²C in a sample with the proportion present in the atmosphere, we can calculate how much ¹⁴C has decayed, and therefore how old our sample is. So what Patrut and others have done, is to take samples of wood from with the hollow cavities within one particularly large baobab, and use radiocarbon dating methods to estimate the age of the tree. And they find some interesting results even for this one tree - the tree in question has two stems, one much larger than the other. We might suspect that the larger stem is, of course, the older one. But this isn't the case at all - the smaller trunk is estimated to be over 1060 years old, whilst the much fatter trunk is around 300 years newer. So once the tree is already pretty big, the size of the trunk is no useful guide to the age of the tree - and the authors also note several other similar studies they've done of other trees which confirm this pattern. They suggest that what matters is the initial conditions over the first 100 years of a stem's life - if it's particularly favourable the stem grows very quickly, and then keeps growing quickly for the rest of it's life (early life conditions are often very important for later growth rates in a range of organisms). So if a stem starts in a particular good time, its growth can easily overtake older trees who struggled to grow fast early on in life, so the biggest trees aren't necessarily the oldest. None the less, from this and other studies of baobabs it's still clear that very large baobabs are often over 1000 years old - the oldest known was over 1275 years old when it died.

You do see baby baobas - this nr Tarangire Nov 2011

Now, why is this longevity perhaps related to the lack of baobab recruitment (i.e. why don't we see baobab seedlings)? Well, firstly let's clarify - lots of people all over Africa have commented on the apparent lack of baobab recruitment (several cited in here), but it isn't actually true that we never see baobab seedlings, just that we don't see many - if you really get to know a park with baobabs well, you probably do know where there are one or two seedlings at least. And now think a little about population biology. All it takes for a population of a species to remain stable is that births equal deaths. If births exceed deaths it's obvious that we'll soon have a growing population, and if deaths exceed births we've got a population in decline. So how often does a baobab have to be successful in having a baby for the population of baobabs as a whole to remain stable? Only once in 1000 years, of course. But it may produce seeds (many!) every single year of that life - only for them all to die - let's say a mature tree produces 1000 pods per year, each with 100 seeds (guesses!) and it does that for 1000 years - that's 100 Million seeds, of which only 1 is expected to survive to maturity! So are we really likely to see lots of baby baobabs if the population is stable? No, I don't think so; especially as we've already got the suggestion that early conditions - for a baobab maybe periods as long as 10 or 100 years - are pretty important, and maybe those conditions only come around ever 4-500 years - nothing that we'd ever expect to experience in our lifetimes. So do I worry about baobabs? No, not really. But I do like to see them, and they're seriously important for the ecology of the areas where they occur, something I think we'll tackle in a later post...

Good for sunset photos too! Tarangire, Oct 2009

Main reference:Patrut, A., Reden, K., Pelt, R., Mayne, D., Lowy, D., & Margineanu, D. (2011). Age determination of large live trees with inner cavities: radiocarbon dating of Platland tree, a giant African baobab Annals of Forest Science, 68 (5), 993-1003 DOI: 10.1007/s13595-011-0107-x

More amazing honeyguide discoveries!

2011-12-28T14:00:00.000-08:00

Steel-blue Whydah, Seronera, Dec 2011.

We've featured honeyguides on the blog here before, and I wouldn't normally come back to the same species so soon, but another recent paper (available to all for free here) by Claire Spottiswoode and colleagues has grabbed my attention by demonstrating nicely some of the challenges that generalist brood parasites have to overcome. There are, of course, three groups of brood parasites (birds that lay eggs in the nests of other species to let them raise the young) in East Africa: the well known cuckoos and the less well known honeyguides and whydahs.

Greater Honeyguide, Tarangire, Sep 2011

Now most whydahs are extremely host specific - the Eastern Paradise Whydah will only lay in Green-winged Ptylia nests, whilst Broad-tailed Paradise Whydah nests in Orange-winged Ptylia. Similarly, Straw-tailed Whydah is pretty exclusive to Purple Grenadier, Purple Indigobird is perhaps best identified by listening to snatches of it's host, Jameson's Firefinch, etc. Others are slightly less specific - the Steel Blue Whydah lays in the nests of the very closely related Black-cheeked and Black-faced Waxbills and Pin-tailed will parasitise several waxbill species. Cuckoos and Honeyguides too tend to specialise somewhat, but not completely. And this is where it gets interesting. To some degree is obvious that in a species with a single host there are strong evolutionary pressures on the female to lay eggs of a similar size, colour and marking to that of the single host, a relatively simple problem. But it's less easy if you're trying to match several different species all at once as even closely related species often have differently marked eggs (perhaps as a mechanisim to make brood parasite's lives harder?). And so we find that in some of these groups some interesting evolution has taken place - in Cuckoos we've long known that females will (nearly) always lay their eggs in the nests of the same species as they were fostered in. If such differentiation happened over the long term, one might expect a new species to evolve - one that parasitises one species, another on another (which might well be what happened in the whydahs, or even among the other groups too). But the difference here is that males don't care - they'll mate with any female that looks right and is willing, so the species as a whole remains united, despite female 'races' developing. So then you have to ask whether females from one host lay eggs that differ to those of females from another host, and if so, how can they possibly have evolved such specific genes to colour and pattern the eggs in the face of complete mixing from the males? And this is (part of) the question that Claire and colleagues were interested in.

Male Greater Honeyguide, Tarangire, Sep 2011

They show very nicely that Greater Honeyguides have two main groups of host species - birds that nest in tree holes (African Hoopoe, Green Wood-hoopoe, etc.), and those that nest in earth holes (Little Bee-eater, Striped Kingfisher, etc.). The former have larger and longer eggs, the latter smaller, rounder ones. And so two forms of female greater honeyguides seem to have evolved - one specialising in the tree nesters, one in the ground nesters and as expected the females of each group lay appropriately shaped and sized eggs. So how do they do it? Well, one of the important theories that was developed as long ago as 1933 is based on another fundamental difference between birds and mammals that's important to know. In both mammals and birds the sex of a developing embryo is determined by chromosomes, the DNA containing structures that control inheritance. In mammals, everyone has one 'X' chromosome we inherit from our mothers, but from our fathers we can either inherit another 'X' chromosome (which would make us female), or - like our father - we could inherit a 'Y' chromosome, which would make us male. The 'Y' chromosome is therefore inhereted father to son, to grandson, etc., without ever finding itself in a female, and it's this pattern of inheritance that makes us male or female. Now what differs in birds is that instead of the X and Y combination making us male, it would make a bird female. Male birds have two of the same type of chromosomes, females are the ones with the different pair, and to make this distinction easier we don't use the X and Y terminology, but talk of W and Z chromosomes instead. So, unlike in mammals, it's the females of birds who have a unique chromosome that is passed one through mother to daughter to grand-daughter, without ever passing through a male. So if the information for how to colour your egg is stored on this chromosome, no information about it will ever come from a male. A neat solution to how the species as a whole can be unified by the males, but females can differ (possibly substantially) in the genes they have on their unique chromosome.Hope that's clear...

Now, Claire and her group went one step further and decided to look for differences in a special sort of DNA called mitochondrial DNA that is also only inherited from mother to daughter, and compare the degree of difference between the two groups of tree and ground parasitising females in the mitochondrial DNA with the difference in the DNA in the main part of the cell that comes from both male and females. They expected - and rather neatly demonstrated - that there might be substantial differentiation between the females in mitochondrial DNA, but that the males would mean there's little difference in the main 'nuclear' DNA. And the degree of difference in the mitochondrial DNA between the tree and ground nesters was so much that their ancestors started breeding in these two different way millions of years ago! That's pretty remarkable, and rather different from the more recent splits reported for cuckoos, probably brought on by relatively recent host changes. Why this difference? Well, they speculate that it's thanks to the greater staility of the African climate compared to the Northern one where most of the work on other brood parasites has been undertaken, but I'm not yet convinced - if we could compare similar patterns for a few local cuckoos too, that might be very interesting!

Anyway, all very impressive and a great lesson not only in the complexities of brood parasitism that is fascinating to me, but a bit on sex determination too - a subject we're sure to return to in the future...

Reference:

Spottiswoode, C., Stryjewski, K., Quader, S., Colebrook-Robjent, J., & Sorenson, M. (2011). Ancient host specificity within a single species of brood parasitic bird Proceedings of the National Academy of Sciences, 108 (43), 17738-17742 DOI: 10.1073/pnas.1109630108

TAWIRI conference discussions continued: Ruaha River

2011-12-26T05:06:00.000-08:00

Wetlands, like Silale Swamp in Tarangire, are vital for feeding rivers

Returning to the TAWIRI conference back in December that I posted a bit about already, the other talk that set me thinking about economics of conservation was a fascinating talk by Eric Wolanski about ecohydrology. About what, I hear you ask?! Ecohydrology, the study of the interactions between water (hydrology) and ecosystems. Now it occurred to me that we've not done a post specifically about wetlands yet, which is a bit of a major ommission, given their importance in savannah ecosystems. We'll have to rectify that in time, but for now we're going to plunge straight into some important stuff.

Flows in the Mara River have been disrupted by deforestation in the Mau Forest

As we all know, the life-blood of a savannah ecosystem is its permanent water source(s). As we've talked about in our Serengeti Story, the Mara River is the only important permanent water source in the Serengeti Mara ecosystem, and the animals move a long way to get there. Tarangire has the Tarangire River. Ruaha has the Great Ruaha River, etc. What makes these rivers permanent and other rivers in the savannah only seasonal is that they're fed by sources that capture the rain in the wet season and slowly release it during the dry season, whilst sand rivers tend to just be rain fed. The Mara is fed by the Mau Forest in Kenya, Tarangire River by Silale Swamp, and the Great Ruaha by a series of wetlands, including those of the Usangu Flats. Meddle with these 'sponges' and you can get in all sorts of trouble with your permanent water sources.

Birds and other wildlife also love wetlands like Silale! Sep 2011

Eric told a fairly simple story, but a fascinating one none the less (especially when you start doing the sums that I've been looking at). If you have vegetation covering a waterbody, he said, the water loss through evaporation and transpiration (plant breathing) is about 50% of the evaporation you have from open water. Somore water flows from vegetated wetlands into rivers than from ones taht have lost their vegetation through excessive grazing. Which is exactly what had happened during the 1990s and 2000s on the Usangu flats, above Ruaha. Much of the water that flows from Usangu at the end of the wet season is the water that subsequently fills Mtera Dam, so keeping the water flowing - as well as providing a vital resource for the wildlife of Ruaha National Park - is pretty important for electricity generation in Tanzania! Uncontrolled (but illegal) immigration had allowed hundreds of people with an estimated 300,000 cattle to occupy the Usangu Game Reserve (as it was), and the cattle ate all the vegetation over the wetland. As a consequence the evaporation rates increased and less water flowed from Usangu into the Great Ruaha. In 2006 the government decided to evict these people and incorporate the Usangu Game Reserve into Ruaha National Park (in so doing creating the largest National Park in Africa), hoping to restore water flows, though among serious concerns about human rights. And Eric was able to watch the consequences in the flow rates through the Usangu flats and into the Great Ruaha river. Amazingly, this operation alone has resulted in the Ruaha river flowing for an extra month. Now that result is a great success for conservation, but it's not the end of the story by any means and I did some quick back of an envelope calculations of my own that are pretty staggering, but before we go there let me post a few caveats - firstly, I'm not a hydrologist and I'm just reading a few things quickly, I'm not guaranteeing these figures in any way. Also, as I've not always found the exact figures I've tended to err on the side of caution - for example, I've found statements that show dry season flow is unimportant to Mtera water levels (as you'd imagine), but that the dam mostly fills when the Usangu wetlands are flowing full rate at the end of the wet season, but I've not got the relative figures for this so I've assumed that the flow operates continuously - which should underestimate the importance of Usangu. Still, here are some interesting numbers...

Early in 2011 the IMF downgraded it's forecasts for growth in the Tanzanian economy from 7.5% to 6%, due to the costs imposed by TANESCO power cuts (up to 16hrs per day in much of the country!). Tanzanian GDP in 2010 was abour $23 Billion, so the cost to Tanzania of the powercuts, according to the IMF, is $345M. [Interestingly, that GDP is a tiny bit more than George Soros and family have tucked away, is similarly a tiny bit more than the value of the treasure recently discovered in the vaults of an Indian temple and is not even half the annual turnover of GlaxoSmithKlein!] Most of these power cuts were caused by low water levels in dams preventing power generation - Mtera dam (fed by Usangu through the Great Ruaha) feeds two power stations, Mtera and Kidatu, producing between then 284MW of power, which is about 40% of Tanzania's total capacity of 769MW (Thanks TANESCO for these figures). I've not found how many months the stations were going for, but I did discover that for the last few years the Ruaha has flowed for only 9 months, so let's assume it's similar. Now if we assume that 40% of the cost of the blackouts is caused by Mtera not producing (actually, I'm sure it's much higher since it's mainly the Hydro part of the production that's failing, but that's the proportion of overall capacity sourced by Mtera and will give a conservative estimate), those thee months of non-flow each cost the Tanzanian economy $46M. So the government's action to remove cattle, by providing an extra month's flow from Usangu, might well ahve saved about $46M per year. Not a bad investment, I think, even if they had paid the going rate of $150 per cattle the $45M required would have been paid off in one year and as it was many of these cattle moved elsewhere where they caused less damage to sensitive wetland habitats.

Now the final question you'll be asking, I guess, is what about the remaining 3 months of no-flow? How can we get that back? Well Eric and colleagues estimated that if the rice farms that are the other major water user returned only 25% of their water, there's be no problem at all. And I've just done a quick check and found that the estimated cost of completely closing the rice farming industry in this area would cost the national economy 'only' $15.9M per year. If that's what it takes to keep Mtera flowing, it doesn't seem a particularly hard decision for me, and think of the wildlife benefits too!

Anyway, I hope those figures are of some interest - it just goes to show that conservation really can be a 'win, win' option, even when hard decisions need to be made. Let's just hope it doesn't take too long before someone sees sense here - good luck to those people and organisations trying to build awareness of these issues!

The Serengeti Story 2: the great migration

2011-12-24T04:38:00.000-08:00

Lion admiring the massed migration on the plains, near Naabi, Dec 2011

The second part of the Serengeti Story is the tale of the great migration, the defining heart of the Serengeti Ecosystem. At the broadest level, this is an easy enough thing to understand - thre are two very important environmental gradients across the ecosystem and the wildebeest (and zebra and eland and gazelles, etc.) are trying to maximise their access to important resources. So, let's start with the two important gradients: rainfall and nutrients, the remaining two of the big four we didn't cover in the first part of the story.

Average Serengeti Rainfall, adapted from here

Starting with rainfall, the broad pattern is for lots of rain in the north and west, and (much) less in the south and east. Perhaps more important still is the seasonal difference in rainfall patterns - most rain falls during the wet season, of course, and the wet season rainfall shows a similar pattern to the overall pattern. But dry season rainfall is the key - the far north and the far west have an average of 400mm of rain even during the dry season, and what's more that's fairly reliable rainfall - the rest of the ecosystem is either compeltely dry, or only ocassionally it by a shower every few years. There's also only one permanent river in the ecosystem - the Mara river in the north. So dry season rainfall means there's green grass to eat, and the Mara river means there's water to drink during the dry season in the far north - an obvious reason for migrant animals to be on the Kenyan / Tanzanian border during the dry season. (In fact the animals move around quite a lot at this time, following local patterns of rainfall and often crossing and recrossing the Mara river throughout their time up there.

A small crossing of the Mara: local movements, not migration, Sept 2011

As the rains become more widespread in November the animals quickly move south, heading away from the woodlands to the short grass plains of the Serengeti NP / Ngorongoro CA border. Why? Well, this is where the other important gradient comes into play, that of nutrients. And this is best understood by looking at the geology of the Serengeti ecosystem in the figure below. Orange areas are 540 - 1500 Million years old, grey areas are recent (within 65 Million years - most only 3 Million years old), Pink areas are over 2500 Million years old and tan coloured bits are also relative recent alluvial (flood) bits, derived from earlier shorelines of Lake Victoria.


Geology of Serengeti, detail from Ordanance Survey map, Saggerson 1961

Broadly speaking there are three geological areas in Serengeti - the southern areas with very recent soils formed on top of the ash deposits from the crater highlands (which form a hard pan that plants can't get their roots through, and only having shallow soil - as illustrated in this picture below froma cutting just east of Naabi gate), the western areas and the north-eastern areas. The north eastern areas are characterised by rocks formed over 2500 Million years ago, whilst the western areas have some more recent deposits from the rivers and different shores of lake Victroia. Unsurprisingly, the nutrients from the ancient rocks in the north have long-since washed away, leaving the north in particular extremely nutrient poor, whilst the short grass plains of the south are very, very rich. Particularly in phosphorus and calcium, both particularly important nutrients for pregnant and lactating wildebeest. The recent soils of the west are rich too, but mainly in Nitrogen, important, but not especially when pregnant. So here, immediately is a massive pull for animals away from those wet, but nutrient poor northern woodlands, down to the dry but nutrient rich grasslands of the south. Obviously they can only get here when it's wet, so timing their breeding to the rainy season on teh short grass plains is a great idea. What's more, predation down here is much lower too, as the hard pan and low rainfall prevents trees and lions have a much tougher time hunting away from the rivers and woodlands, which is great for baby animals.

Soak-away near Naabi showing the hard pan that limits tree growth, but makes grass very fertile

So, now we've got the important data we need for understanding the broad-scale movements of the migration. During the dry season, you've got to be near the Mara, in the far north. Once the rains come you want to move as fast as possible down to the nutrient rich grasslands of the south, where it's wise to give birth. But then once the rains stop, the bad news is that even though the grass stays green for a while, the standing water at Masek and Ndutu is so rich in nutrients that it's actually toxic - so even though the food is still there and still good you've got to start moving off as soon as the rain stops. But instead of heading straight back up the the north, it makes sense to move west, where there's still relatively rich grazing and water remains in the Grumeti and Mbalageti rivers. So come late May the migration moves away from the short grass of the south and heads into the Western Corridor, staying as long as the grass remains before gradually filtering north again as the good grazing is eaten in the west. (That date has got later in recent years, as there's now a lot more grass left in the Grumeti Reserves, thanks to a policy of burning only after the migration has been through - which explains why those northern camps have had some tough starts to the season in recent years!)

Movements of individual wildebeest caught near Seronera (blue circle) from here

And so you have the broad pattern - a triangular migration in a clockwise direction, covering between 500 and 1000kms, and one of the most amazing wildlife sights anywhere on earth. But, as always, the broad scale picture isn't all there is to it. Individual animals take some remarkably different routes around the ecosystem, as some data from gps collared indivudals shows - all these animals were caught near Seronera at the same time, but all have done different things - the dark blue one is particularly interesting, and none of these animals came down the eastern side of the NP at all. Why not? No-one knows - maybe simply because they were all passing Seronera instead. More recent work in the Masai Mara has made even more exciting discoveries, with animals I'd have assumed previously to be local migrants into and out of the Mara showing some extraordinary movements, even joining the main Serengeti migration in some years, but not others - look at these maps from here (they're updated very regularly, as the animals are still out there!)

The first of these spent a year in Kenya, migrating from wet season home in the west to the east and back, but then joined the main Serengeti migration this year and is somewhere in the NCAA today, whilst the other left Kenya last year and headed off to Loliondo for the wet season, before returning this year to wet season home in the north east! What made these animals change their routes from one year to the next? It will be fascinating to try and find out as more data on the movements of individual animals become available. Clearly, understanding the broad scale pattern is only a tiny fraction of the question as a whole and we've lots more to learn.

Anyway, I hope that's a pretty good introduction to some of the Serengeti Story. It's far from static, and there's still lots more to learn, so we're bound to return to the issue in subsequent posts, but I hope this is a good start at least. Meantime, Happy Christmas!

The Serengeti Story, part 1: history

2011-12-20T06:07:00.000-08:00

So I guess this is the post I've been putting off longest. Not because it's not interesting, but because I know I'm going to forget some crucial component. But I'm just back again from a fantastic trip (thanks to all the guys at Dunia!) and decided it's definitely time to bite the bullet. However, it's going to be a long story, and I'm going to split it in two sections so I don't spend all night here (and so I stand a chance of remembering what I've forgotten before I consider the story told!). If you want more details on any of these things the essential references are the excellent series of very technical books edited by Tony Sinclair and colleagues you can get from Amazon. I've cut and pasted a few of the graphs from 'Serengeti III' into this post, hopefully 'fair use' for education...

I always start telling the Serengeti story with a bit of history, since it helps us understand how scientists have uncovered some of these things. There's no really obvious beginning to the story, but let's start with something we've already discussed on Safari Ecology - the introduction of Rinderpest to Africa in 1887. As we saw in that post, this had a massive impact on wildlife throughout Africa, the disease reaching Cape Town by 1897. The Serengeti migration was decimated, and when it was finally erradicated from the wildebeest population in 1963, there were still only around 250,000 wildebeest (see the plot below).
As you can see, once rinderpest was erradicated the wildebeest population exploded, reaching it's current total of somewhere betwen 1.2 and 1.4 million in about 1977, and this is the huge change that has let us understand so much of what happens in Serengeti.

Now, by now we should all know the 'Big 4' of savannah ecology, so it shouldn't come as a surprise that such a huge change in herbivory had a massive impact on the ecology of Serengeti, perhaps most obviously on the amount of another of the big 4 - fire. The figure below shows very clearly how the rise in numbers of wildebeest reduced the amount of fire in those northern woodland areas (essentially the woods from Seronera north).

This is clearly down to the very simple fact that wildebeest eat grass and grass is what carries fire through the savannah - more wildebeest means less grass which means less fire. And a change in the fire regime, of course, will alter the ecology too. So introduction around 1890 and then erradication of rinderpest in 1963 led to a massive change in both grazing pressure and fire frequency. It's not surprising, therefore, that massive changes occurred in Serengeti during the 1900s, most obviously the change in woodland cover. If you dig through old photos of the Serengeti / Mara area you can find some fantastic images of change. Tony Sinclair did it and came up with this beauty from 1944, that he then returned to in 1983 and took the subsequent photo (I've borrowed them from his talk available online here).

It's pretty obvious that the woodlands vanished sometime between these two photos were taken and more detailed work suggested a rapid decline in woodland cover from about 1945 to 1980 - just the sort of delay you might expect from the increase in fire around the turn of the 1900th Century, given that fire doesn't kill savannah trees above 2m tall, so any established trees would gradually die of old age some time later.

Interestingly, as a direct consequenc of the decline in trees the national park authorities changed their fire management strategy in the 1970s from late burns at the end of the dry season and in anticipation of the rains, to one of early burns which tend to be cooler and rather less damaging to tree seedlings. At the same time, of course, the wildebeest population was recovering and the fire was declining in frequency as a consequence, so this change was probably less necessary than it seemed at the time (though everyone at TANAPA has since forgotten that the current fire strategy is a relatively new one, of course!). And as you might expect, more recently the trees have returned. Again, Tony Sinclair has some fantastic series of photos of these changes too, this from relatively close to Seronera:

(There's a whole lot more of these sorts of photos available on the web if you search for Tony's various talks.) And so the woodlands returned to Serengeti, as a consequence of the return of wildebeest and subsequent decline of fire. [It's interesting too, that savannahs globally are getting woodier, so there's a chance that this change is also related to global change too, not simply a local Serengeti effect - we might return to this in the future...]

But the story's not quite complete yet, as there's a neat twist at the end involving elephants. During the 1970s and 1980s there was massive and nearly uncontrolled poaching of elephants throughout Serengeti, ending abruptly with the band on ivory trading in 1989. It's had a massive impact on elephant numbers in Serengeti:

At the same time, however, across the border in Kenya poaching remained under tight control, with no such dramatic change in elephant numbers. Such large herbivores can have a massive impact on the vegetation and the story in Serengeti is a particularly interesting one - Elephants walking across grassy plains often 'weed' out the tree seedlings instead of eathing grass. In woodlands they tend to leave the seedlings and concentrate on adult trees. So if there are lots of elephants it can be rather hard to turn grasslands into woodlands, even if the fire frequency is reduced. The difference between Kenya, where elephant numbers remained high throughout the period, and Tanzania, where they crashed at just the same time the fires declined, is stark. And elephants being rather clever animals, they knew where the border was and they were safe. So here's one last picture of Tony's from northern Serengeti / Mara, where the international border is clearly defined by woodlands.

Amazing to see the impacts of elephants so clearly, but also amazing to see how two different habitats (grassland and woodland) within the savannah biome can be stable under exactly the same environmental conditions - these days elephants are common both sides of the border and yet the woodlands remain in Tanzania, thanks to the different way elephants behave in grasslands from woodlands. So the history lesson ends with an important lesson about how important the initial conditions are to how a savannah looks - to turn a grassland to a woodland you need to reduce fire frequency (which can be done by increasing herbivory), but you also need to at least temporarily exclude elephants. All very complicated...

So, that's the history lesson and the broad overview of some population changes as a whole. The next post will continue the Serengeti Story by, I hope, explaining what we know about the migration and the regional differences across the ecosystem today. Hopefully it won't take so long to create either!