Why do savanna trees have flat tops?

Umbrella Thorn, Serengeti: An icon of the savanna?

From sunsets behind a silhouetted acacia (properly Vachellia), to photos of rolling grasslands studded with isolated trees, a savanna landscape is immediately identifiable thanks to the flat-topped tree. But why is this? Why do so many Vachellia and other savanna trees have such a distinctive structure that they have become a virtual icon of the African savanna?

It's an interesting question that was given some answers in a nice paper by Sally Archibald and William Bond who studied one species called the Sweet Thorn (Vachellia karroo) that, rather like some of our Vachellia species in East Africa exhibits a range of different growth forms in different habitats. In the semi-desert of the Karroo, it grows as a medium-sized ball of thorns, whereas in the savanna it has a fairly typical medium-tall  flat-topped acacia look to it and in a forest it's a tall, thin tree. These differences are meditated mainly by genetic differences within the species, but equally could be caused in other species by a variable response to the environment - it's not really important to this discussion and, in fact, much of our discussion could focus on different species if we wanted. As always when we're thinking about what makes the savanna species, we'd be well advised to start with the savanna big four: nutrients, water availability, fire and herbivory.Now, the first two processes have impacts in all biomes, whereas it's the second two that are most distinctive about savanna and where we'll start our discussion.

Nitrogen in the savanna biome

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.

What is the savanna biome?

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?

Ecology of broad-leaved woodlands in the savannah

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.

The Serengeti Story 2: the great migration

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

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!

Phenology – the timing of biological events.

First rains arriving over Manyara Ranch, Nov 2011

This is one of my favourite times to be in the bush, as the rains arrive and the savannah turns green. I love the excitement of the birds as they greet the rain, and the miracle of new grass appearing in just a few days and I probably get as excited by the first thunderstorms as my children! But as we all know, the timing of these events can change year to year. In fact, never more so that recently – one of the first impacts of global climate change that we see here in East Africa. Despite the changing season being such a profound event in the savannah, there's a surprising amount that we don't know about the patterns of seasonal change that we see.

For example, it's obvious that grass growth responds directly to rainfall – or at least to soilmoisture. If the rains are late, the grass stays dry, if the rains are early, it turns green early. But how does it know? To all intents and purposes the grass (or the seed) seems completely dead until something tells it the soil is moist and it's time to start growing again. In this case, actually, it's fairly straightforward – the moisture in the soil is in direct contact with the grass roots (or seed) and as that moisture is absorbed the cell cycles are started up again.

Pre-rains green flush in Brachystigia woodland, Kafue NP, Oct 2011 (pic. H. Frederick)

Other patterns are harder to understand, and the one that fascinates me most is the green flush that we see in miombo woodlands (and on Commiphora and several Combretum species too) before the rains. Not just immediately before the rains either, but some weeks before. How, and why, do they do that?

Let's remember first that savannah woodlands are deciduous (the trees loose their leaves) because during the dry season their leaves would loose too much water to allow the tree to survive. Add water, and there's no problem, so you'll see evergreens in the savannah only in riverine and kopjie habitats. So why, just when water is in shortest supply, do some species 'choose' to use some of their remaining stores of water and put out leaves before the rains come – and not just before, but a long time before? One of the things that we do know that might help us understand this is that once the leaves are out, the plants once again 'switch off' until the rains arrive. They've got leaves out, but they're not photosynthesising and respiration (plants respiretoo, of course) is pretty much dormant too. But then, once the rains do come, they're active within 24hrs. And another clue might come from the fact that we know there's a flush of nutrients (particularlynitrogen) associated with the first rains, that rapidly declines after the first few days of rain. So there's obviously a strong advantage if you can be ready and waiting for the rain – other trees that aren't ready will spend those first few nutrient rich days busy growing leaves and not be able to take advantage of the nutrient flush. So as long as you can minimise the costs of having leaves before the rains come (by essentially shutting down as much as possible), it seems plausible that the benefits could outweigh the costs (and clearly, for some species they do, or they wouldn't survive!). One thing that suggests this idea might be right is the fact that legumes – like Vachellia and Senegalia (I will get you toforget about Acacias!) - don't do it, they respond to soil moisture and, as we know, being legumes hey have no shortage of nitrogen, unlike other savannah species.

More pre-rain greening, Kafue NP (H. Frederick)

But why, then, be so early – why not just wait until the week before the rain before growing leaves and further minimise your costs that way? And here is where we really run out of hard facts and enter the realms of interesting scientific speculation – my guess is that because the date when the rains start is variable, you can't predict it that accurately. If you want to take advantage of that first nutrient flush, you've got to be ready for the earliest possible date the rains might fall – which (like this year) might be several weeks before the rains begin in normal years. I'm far from certain this is right – among other things, it requires that the benefits of being ready for that first flush are extremely strong, such that plants that catch it every year have a meaningful evolutionary advantage over plants that only catch it most years, which is testable but not guaranteed. But it's a good theory to work on for now.

The next part of the story that I'm interested in, of course, is how they do it? How do these plants 'know' when it's October and the rain is coming in a few weeks time? Unlike the grasses that simply detect water, these plants must keep track of the changing date directly. In the north where these processes have been studied in extraordinary detail, plants (andanimals) use changes in day length to keep track of the seasons – in spring and autumn in Aberdeen where I used to live from one day to the next day length could change by as much as five or ten minutes. But I find it hard to conceive that the same process is possible here where day length changes only by two minutes across the entire year – the difference from one day to the next can only be measured in seconds or fractions of seconds, and I find it hard to believe this can actually be the cue. But, amazingly, no-one's studied it so we just don't know.

There are other biological events that depend on precise seasonal timing, of course – like the millions of birds that spend months here until April, then head north to breed, but even here we don't always know the signals that the birds are using and why, for example, so many species seem to have been rather late arriving this year. But this has already been a long enough blog for one day, so that will have to wait for another time...

Nutrients in the savannah biome

Of all the 'big four' processes that drive the ecology of the savannah, I think I've probably spent least time talking about nutrients. That might be surprising, because in some ways the cycling of nutrients is what helps us define an ecosystem. (Ecosystem is, in fact, a rather poorly defined term, but if there's anything that separates a habitat from an ecosystem, it's the fact that most nutrients and energy are well recycled within an ecosystem, whist habitats, once you ignore solar inputs, often have large in and out flows of nutrients and energy. So an ecosystem, such as Serengeti, can have lots of different habitats. On the other hand, a biome is a globally occuring set of similarly functioning ecosystems - savannah ecosystems around the world form the savannah biome. But let's get back to the point...) Nutrients are, however, extremely importand in shaping savannah habitats, both at large scales, and fine scale.

Termite mound, Mwiba Ranch, August 2011

The large scale patterns of nutrient availablity lead to different vegetation types in different areas, and drive large-scale migrations, both issues we've touched on already in this blog. So today I'm going to concentrate on the smaller-scale processes that act over just a few metres, but still have important roles to play in the ecology of the savannah. Let's start with what might well be the most important nutrient cyclers of the savannah - the termites. On the right is a typical termite mound in the middle of the dry season - note there's lots of uneaten grass in the foreground and background, but on and around the mound itself, there's nothing left but nibbled grass stems. Why? Because the termites have been busy working in the surrounding area to gather up bits of vegetation, and bring them to their mound. By gathering vegetation, then processing it in one spot, they concentrate nutrients at this spot, and the grass growing there is richer and better food than grass growing further away. The termite mounds become a nutrient hotspot, and animals know how nutrient rich their food is. Particularly in the dry season, when they only have dead matter to graze, small differences in nutrient content are very important. And even in the wet season these are preferred foraging areas and are often kept permanently short, as a grazing lawn. Once the process starts, in fact, it becomes self-perpetuating, as regularly grazed grass keeps growing new shoots and new shoots are always tastier (full of nutrients and low in the silica (a natural glass) grasses use as defence against grazing.), prompting more grazing and keeping the grass short, perhaps even spreading beyond the original termite mound as the additional benefits accrue - precicely the definition of a grazing lawn, and a very obvious example of how important the nutrient cycling carried out by termite really is at the large, observable scale we work at.

Impala Midden, Manyara Ranch, Nov 2010

Dikdik midden - what a lot of poo! Manyara Ranch, Nov 2010

The next process that's concentrating nutrients in the savannah is also so obvious that we often miss it - animals that use middens concentrate nutrients over several years in one spot. This nice impala midden shows another feature you often see about middens - again, the grass all around has been grazed to nearly nothing. And for exactly the same reason as before - the nutrient hotspots great lush grass that is heavily grazed, neighbouring grasses are also grazed and the impact spreads out to create a grazing lawn. (In fact, this one has suffered rather from cattle grazing too, but they respond to the same processes as the wildlife.) As you can see, the grazing is much more widespread than simply the focal nutrient spot, but it's quite possible that this midden and the others you can see around are the original cause of the heavy grazing over the whole of this little stretch. Always good things to point out when you're on a walk, especially if there are children about like my two...

Elephant diggings near Gibbs Farm, Dec 2010

See the tusk marks?! Elephant digging - there was also buffalo horn marks!

And for now I'll leave one final special case of nutrient hotspots impacting savannah ecology - the rare locations of mineral deposits utilised by a range of wildlife, but most famously by elephants. In some places, elephants have dug caves over 160m deep into mountainsides in search of nutrients (most famously on Mt Elgon where elephants have dug in search of calcium, sodium and magnesium). Other animals also come from far afield in search of the nutrient rich soil, leaving wide paths through the forest. These photos are from Ngorongoro, behind Gibbs Farm, where elephants are primarily searching for Molybdenum, selenium and cobalt. These micronutrients (nutrients required only in tiny amounts) are important for animal health and animals with deficiencies are generally rather unenergetic and not alert to the dangers of the world. So, rather a useful thing. And, of course, you don't have to head specially out to these caves (though it's worth a walk, and the birding around there is great!) to show this sort of activity to folk - several cuttings ont he main ascent road from Laodare gate to the crater viewpoint show obvious evidence of nocturnal mining by elephants, and the rock here is just as nutrient rich as in the caves at Gibbs.

So, next time your out and about, have a look for signs of nutrient concentrations, and see what's responding to it. And then remember the large-scale variations in nutrients too, that are so important for other processes!

Echos of extinction...

Black Rhino browsing (?!) Ngorongoro Crater, Feb 2011

Driving around places like Tarangire I often find myself wondering what impact the loss of black rhinos have had on the ecosystem. Sadly, these days there's nowhere in Tanzania with a significant rhino population (though you've got a good chance of seeing them in the Crater still) and it's easy to forget just how common they were, not so very long ago - more than 700 in Serengeti alone in 1974. Friends of mine (rather older than me!) talk of driving from the Tarangire gate to Silale and seeing over 20 rhino in a drive, and being areas they'd avoid walking in because the rhino densities were just too high to make it safe. They're big animals, at about 1500kg for a bull, and they eat a lot of food. Such large populations of a large mammal crashing about in the bush like a tank must have had massive impacts on the landscape - but there's nowhere we can go now to see what an East African savannah with a decent rhino population looks like.

Close enough to this Black Rhino! Kruger NP May 2011

Black rhino may have been important dispersers of Sausage tree seeds.

Fallen Kigelia africana flowers are a valuable food source for many species

We can speculate that perhaps some of the large, thick patches of bush of a single age now found in central Tarangire might well have been more open and varied habitats when there were rhinos there - but we really can't say for certain, because no-one's been monitoring the changes and we've nowhere to cross-check against anyway. There is one species that might, in time, show some changes - that essential savannah tree, the Sausage Tree Kigelia africana. Ever wondered what disperses Kigelia seeds? Not much eats them - baboons do when they're desperate, insects have a go and elephants will when pressed, but did you know they're a favourite of Black Rhino, and considered important seed dispersers for this species? In fact, Kigelia seeds don't germinate unless they've been eaten by something like a rhino, so mayube much of the pattern we see currently in this species related to ghosts of past rhino distribution? Who knows.

Interestingly, following the recovery of rhino populations in South Africa before the latest spate of poaching, we did discover that White Rhino are massively important in shaping landscapes - as massive grazers they create grazing lawns that are then colonised by a large number of other grazers and, impresively, but some rather rare grassland bird species that specialise on these grazing lawns. Before rhino number built up to their current levels, this sort of lawn was rather rare, and the increase in rhino has greately increased the variability of the landscape of Kruger and Hluhluwe-iUmfolozi parks, with important benefits for the populations of other species - this species, if not necessarily a keystone species, is certainly an 'ecosystem engineer'. (Why Tanzania and Kenya - despite what many will tell you - have never formed part of the range of the critically endangered Northern White Rhino is a mystery to me - so much grass here, they'd surely love it (and when not being poached they do rather well in Kenya, it seems).) Maybe they were driven to extinction by our ancestors long before modern times?

Southern White Rhino trying hard to make a grazing lawn, Kruger NP, May 2011

Anyway, all interesting things to think about when you're lucky enough to come acros a rhino, or even if just enjoying the shade of a sausage tree. Let's hope the current spate of poaching can be controlled and the increases seen earlier will continue again. (That's twice in a week I've blogged about one of the big five. Shock! Must find smaller fare for next time...)

The landscape of fear

Serengeti Landscape of Fear - where would you feed? On the green by the river where predators hide? In the bare bits on the plain with no grass left but a good view? Or risk the woodlands somewhere in between?

Lions are often in thickets (N. Serengeti)

But sometimes on kopjes... S. Serengeti

where you might also find a cheetah! N. Serengeti

It might sound like the sort of novel you's find abandoned at a camp by passing visitors, but understanding what ecologists mean by the 'landscape of fear' - how predators have impacts on the ecology of a savannah that go well beyond their direct predation events - is such an important concept that I'm going to break my usual rule of not talking about the big five! Actually, understanding the concept is simple - it's about looking at a landscape and working out where you'd be (most) scared to be walking. Long grass? Yup, scary. Thick riverine vegetation? Not for me! Nich bushy kopjie? I'll give it a miss, thanks. You get the idea - any place you might think about looking for predators whilst on a game drive, is going to be a scary place for herbivores too. And it's not simply a function of the numbers of predators that are present, but how efficiently they might be able to hunt within that habitat - I'd be much happier walking a short grass plain with a high density of lions than I would walking through some tangled thickets with a much lower density of lions. (Obviously I'm also a bit warier of buffalo and elephant than most herbivores have to be, but if you've done a few walking safaris you'll have the idea anyway.)

So what? These patterns are so obvious, we don't really think about them, or think they have an important part to play in very much - but we'd be wrong. In places where top predators have been removed, we rapidly see changes in the behaviour of herbivores and, soon after, we'll see changes in vegetation. Perhaps nowhere more famously than in Yellowstone National Park in the US (described here) - when wolves were eliminated elk and bison were released from their major predator and the populations changed - they didn't change in numbers very much, becausee like Serengeti's wildebeest and zebras (and, of coruse, elephants and the rest of the mega-herbivore group) they're limited by bottom-up processes of food availability, not top-down processes like predation. But they changed in behaviour, spending much less time looking around for predators and not moving around very far from their favoured willow patches. Which mean that after 50 years of no wolf predation, those patches of willows were in a bad way - it looked possible this form of riverine vegetation would vanish forever. Until 1994, when wolves were reintroduced. Within a matter of months the female elk and bison were spending significanty more time looking around, avoided open areas and only stayed in one place for a little period before moving on. And, in time, the riverine areas started to regenerate. We'd witnessed a 'trophic cascade' - removal of a top predator had had a massive impact on vegetation and landscape, the impacts 'cascading' down from top predator through the herbivore to the basal layer.

Leopards like riverine too, C. Serengeti

And lions often hunt by rivers and small ridges, Tarangire

These Fringe-eared Oryx have spotted something from their vantage in the Tarangie plains

The same processes are in operation on our east african savannahs all the time. Lions are far, far more efficient predators in areas where they can conceal themselves (in bushes, around kopjies, along even small river lines and shelves) than in the open - and as we know they plan their hunts accordingly. Similarly, leopards are best looked for around kopjies and riverine forests, where they're both able to avoid lions and can hunt sucessfully. Even cheetahs are often around kopjies to get a good view. So these are scary places for animals and, as we've seen so often before, if you start altering grazing pressure (one of the big 4 processes in the savannah), you'll see a change in the vegetation - riverine forests get a headstart if there's a big predator population living in them, scaring all the wildlife out! There's even the possibility that this processes becomes a positive feedback - as bush increases, so does predation success, making bushes even scarier, leaving fewer herbivores to keep the bushes back, meaning the bushes spread further, etc. And on the other hand, grazers like to graze areas where they have a good view - short grass - but if there are enough of them, their very grazing ensures the grass stays short, reinforcing the benefits.

Of course, things are complex for a herbivore - you can't simply decide never to forage in a wooded area because you might get eaten, because maybe half-way through the dry season you'll have eaten all the grass on the plains, and all that's left is in those scary woods. So you can either starve to a certain death in the plains, or head into the woods and risk predation, but at least stand a chance of avoiding starvation. Animals must constantly be assessing and weighing up the costs and benefits of foraging in high reward (grass under legumes like Vachellia is often of higher nutrient content than elsewhere) but risky areas, versus the safer but less beneficial areas on the plains. Not only will seasons make these decisions change, but so too will the details vary during the day - it's far more important to be in the plains at night than it is during the day, resulting in a evening movement of animals out of woods and onto plains - woodland edges are a great place to be at sunset!

Tarangire Wildebeest treck from the woodlands to the plains every evening

Spotting predators on Serengeti's short-grass plains is easy - no fear here!

Anyway, once you've got the idea of trophic cascades and the landscape of fear, you'll start seeing how it works all the time, and I hope I've given you enough here to start thinking about at least. (You'll get a much more in-depth and very readable discussion of tropic cascades and the landscape of fear in this nice article here.)

Kopjes

Moru Kopjes, Serengeti, Jan 2011 - stacked boulders form by erosion around cracks

One of the prettiest things about Mwiba is the large number of kopjes (pronounced 'kopees') found down here. Massive and ancient, the rocks that form kopjes are great added value in many safari destinations. They also form an important part of the landscape and should never be seen simply as a photogenic backdrop, or a great place to enjoy the sunset! 

Mwiba kopjes give a good view! Jan 2011

In fact, geology is one of my 10 things to talk about, and there's no more obvious prompt to talk about geology than when sitting on a kopje. But if you're going to do that, it's important you know something about them before plunging in. If you want a really good overview of the soils and geology of the Serengeti ecosystem, you'd do no better than looking here, this is more of a general overview and whilst mostly focussed on the Serengeti ecosystem, the processes involved are similar across Africa and it shouldn't be too hard to find out the location specific details once you understand the process if you want other areas. 

So, what is a kopje, geologically? Essentially, it's a pile of ancient rocks that protrude through the more recent soils and surface rocks – that's what gives them their other name of inselbergs: 'mountain islands'. In Serengeti they're either gneiss [pronounced 'nice'] (a metamorphic rock that looks rather like granite but doesn't have the little flat crystals of mica or similar – some gneiss has originally been formed from granite) or diorite/granite. And they're all OLD! The kopjes of the north west are the oldest and those of the western corridor the youngest, but all are Precambrian, which means over 500 million years old. Compare that to the volcanic ash deposits they poke through on the Serengeti plain, which are only 3million years old, and you see how different they are. All originally formed under the surface of the earth from volcanic activity that didn't make it to the surface, cooling below ground, and then over the ensuring millions of years the surrounding softer rocks have eroded, leaving the harder metamorphic or igneous rocks to become exposed as they are today.

Figs on a Mwiba kopje, Aug 2011

Horned Rockdweller, Bradinopyga cornuta, perhaps a surprising rock specialist!

Gloriosa superba the Glory lilly are common on kopjes: Naabi, Jan 2011

Klipspringer, a kopje specialist. Kruger May 2011

So, that's what they are and it's a good start on geology (though there's much more to talk about there too). But what about the role they play in the savannah ecosystem? In thinking about this it's first of all good to remember that they're a pretty unique habitat, with a specialised group of animals and plants. Kopjes are the place to find klipspringer and hyrax, they're also the only place to find one or two more esoteric bird species like rock-loving cisticola. Rock figs, as the name suggests, are common on kopjes but not many other places. As well as these really rather specialised species, there are a number of species here that are more often associated with riverine vegetation: there's a range of figs, you'll often see tamarind on kopjes and plenty of the animals often found by rivers are there too – both kopjes and rivers are great places to find leopard, for example. Why's this? Well, we need to think about the savannah 'big four' again, and how kopjes affect them.

(1) Water availability: you might think kopjes are dry barren places, but not at all – whilst the rocks are definitely dry, there are often hollows and cracks within them that keep the water for a long time. Figs are particularly good at sending roots over rocks to find the damp pockets and grow really well in such wet places. Animals and birds often know about the puddles too and will use them throughout the dry season, allowing water-dependent species to utilise areas of savannah that would otherwise be too dry for them.

(2) Grazing/browsing: ever seen an elephant on top of a kopje? A giraffe? No, me neither. Whilst there is a specialist browsing community of bush hyrax and klipspringer, for example, the huge impacts of mega-herbivores are minimal on kopjes, allowing plants that are poorly defended to thrive in a way that they can't manage on the flatter plains.

Bush hyrax and Mwanza Agama share this kopje! Moru, Serengeti, Jan 2011

(3) Fire (my favourite of course!): Yes, kopjes are fire breaks – large areas of bare rocks obviously can't burn and plants growing in the gaps in the rocks are safe from fires. As we know fire can be the main process determining whether there's forest or savannah in the wetter areas, it should be no surprise that the fire refuge offered by kopjes should have a forest type vegetation.

(4) Nutrients: just as the bare rocks allow water to concentrate in hollows and cracks, so nutrients from hyrax dung, baboon dung, leaf litter and all the rest concentrate in the cracks too, providing a relatively nutrient high (though thin) soil for those plants that can get to it. What's more, several animal species (like leopards hunting the plains, but bringing their kills back to the trees on the kopje, or eagles nesting on the crags but foraging over a wide area) use the kopjes at certain times of day or night and the plains at other times, but bring their food (and/or dung) back with them to the kopjes, further concentrating nutrients on the kopjes and resulting in a net flow of nutrient out of the surrounding plans, onto the kopje on a truly grand scale.

So, with all of the big four savannah processes being substantially different on a kopje from the surrounding landscape, it should be obvious that there's going to be a big difference in the ecology of the kopje, just as we see. And as the differences are mostly fairly similar to those in a healthy riverine area, it's not surprising that some of the elements are shared between the two otherwise rather different habitats.

Simba on Simba Kopjes, Serenegti. Jan 2011

As one last thought to leave you with, I've already mentioned how kopjes can be seen as a large-scale nutrient pump, pulling nutrients out of the plains and focussing them on the rocks, but there's one other big way in which they alter the ecology of the surrounding plans: predators. Just as leopards like kopjes, so too do lions. They're great places to warm up in the morning or evening sun, and provide an ideal viewpoint to survey the plains for food. Moreover, the rocks and thicker vegetaion make it much easier to sneak out and ambush your prey. So kopjes have a real impact on the 'landscape of fear' if you're a tasty antelope. Out on the short grass plains you've got a great view and it's tough for lions and the like – the plains prides of Serengeti have a home range over 200km2, compared to only 15km2 in the woodlands around Seronera (no wonder they're easy to find there!). So if you're a wildebeest, zebra or kongoni, you'd be wise to avoid the immediate area of kopjes – just as they do a lot of the time. And removing that grazing pressure from the plains around the kopjes, of course, is going to result in a changed ecology of the grasslands there too. So the impact of kopjes spreads out far wider into the landscape than just the rocks themselves. All very interesting – and easy to see whilst sitting about waiting for the sun to set!

Mwiba sunset from a kopje, Aug 2011