Friday, 29 July 2011

On the shore

As I mentioned in the last post from the beach, there's both a huge density and diversity of invertebrates (mostly Annelid worms and molluscs) in the sand and mud. So much so, in fact, that the intertial mud and sand is a huge feeding resource for wading birds. I was once told that if you extract all the worms and molluscs that birds like to feed on from a sample of estuarine mud 1m square and 10cm deep, you'd have a food sample with a total energetic content of around about 1.5 Mars bars. Now, I've had a quick search online for the primary literature that back this statement up, and I wasn't too surprised to discover that althere the cubic metre equivalent is easily found on a google search (you get many answers from 13 mars bars per cubic metre, to 20, with most around about 14 or 15) none of them pointed me to the primary literature at all. And I can't do so either. But I did find papers like this and this that, once you do the sums, suggest it's not a bad approximation. And it's a nice thing to help people understand just how rich an environment this is - everyone knows what will happen to them if they eat to many Mars bars, and there's lots more than 1 square metre in most estuaries! What's more, the rate of productivity is prolific too, so even as it gets eaten, the resource is being continuously replenished. And, of course, the mud isn't the only place with massive densities of invertebrates either - if there's seaweed left to decompose on the beach it harbous massive densities of arthropods, another favoured food resource.
Spot the white-fronted sandplover on it's nest...

White-fronted sandplover nest - two eggs here.

Unsurprising, therefore, that many birds choose to forage in these rich habitats. Some of them, like the White-fronted Sandplover I found nesting at Ushongo (can you spot it on the nest?! This seems to be the first breeding record for the northern coast of Tanzania) are resident. But most of them are migrants fron the north and now, in mid July, most of them are busy finishing off their breeding season. Not all, though - a few first year birds will hang around in Africa all year, waiting to breed for another year, and already those adults who failed in their breeding attempt have come back. In the next few weeks more and more will pour south - and they don't just look for coastal estuaries either, the soft mud around many shallow lakes are similarly rich feeding areas so even if you're only passing the odd soda lake in the traditional safari areas you'll be able to see many of these birds as they start to return in ever bigger numbers. In fact, places like Lake Manyara will soon be home to truly spectacular gatherings of wading birds from the north - well over 1 million little stint are there in most years, and even more at times (up to 3 million have been estimated by Neil Baker of the Tanzanian bird atlas project).
Turnstones, curlew sandpipers and sanderling

Curlew sandpipers and turnstone

So, let's have a little look at some of these migrants that I saw last week - if only to illustrate some of the amazing migrations these birds are capable of. In the pictures above there's a mixed group feeding on invertebrates in rotting seaweed. Most of them - the ones with the orange legs and beaks - are (ruddy) turnstones. These birds, if they migrated this year, will have attemtped to breed right on the tundra beside the Arctic ocean - check here for a map. That's an extraordinary movement - and amazingly, these birds can easily live for 30 or so years, doing these phenomenenal migrations each year. (They also have a remarkably wide diet, with a memorable series of papers reporting ever increasingly bizarre food items from soap and other rubbish, to dead whales, until the editor finally stopped the correspondence when the title reached "Turnstones feeding on human corpse". Nice.). Anyway... You'll alsoalso see (at the right of the top picture, and several in the lower picture) lots of grey waders with rather longer beaks. These are Curlew Sandpipers, and have a similar breeding distribution in the Arctic (but only the Russian Arctic, not North America). These are clearly birds that didn't migrate this year, as the breeding adults at this time are a beautiful rusty red colour and we'll start seeing them soon. Long, decurved beak, long black legs and a white stripe above the eye help identify this species. And if you look carefully on the top picture you can spot one other grey bird running up the sand bank, a bit stockier looking, with a shorter beak - this is a sanderling, and when not breeding in the Arctic is a typical bird of sandy beaches around the world.
Greater sandplover and turnstones
Standing still among the rushing turnstones in this picture is a non-breeding Greater Sandplover, a species with a rather different migration route - this time breeding in the areas around the Caspian Sea in central Asia - not anywhere near as far as the other two, and obviously closely related to the other plovers we see around here, which a much shorter beak and big looking head. We also saw a few lesser sandplovers, which are extremely similar, but tend to have smaller beaks, small heads and shorter, blacker legs (you can see this is a bit greenish on this bird).

There were plenty of others around too, but I didn't get any good pictures I'm afraid. It's always worth checking through the wading bird flocks, though, as anything might turn up - many species show amazingly long migrations and occassionally extreme vagrants turn up among the flocks of commoner species. All very nice.

Thursday, 28 July 2011

Vachellia tortilis, or why there are no Acacias in Africa

I'd promised to write about marine things this week, but here's an interruption that can't be ignored as I read today that there really aren't any Acacia in Africa. (Bad news for the Acacia Rat and Acacia Tit, etc!) What's that, you say? Well, you may (or may not) have heard that for the last few years there's been some discussion among plant taxonomists about the correct taxonomy and names for the genus Acacia. And what seems to be the final final word on the matter was decided on Monday at a meeting of the International Botanical Congress over in Australia – the result? Africa no longer has any Acacia.
Vachellia tortilis will always make good sunsets, whatever you call them!
Now, this is going to cause some confusion (especially to poor ornithologists like myself), and it probably helps to understand a little about where these scientific names come from in the first place. Let's say you find an odd little plant, look around a lot and can't find anything like it. After quite a lot of asking the experts you conclude you've got a new species, so you need to give it a name. Now, ever since Carl Linnaeus back in 1735, scientific names have had two parts – one genus, one species. You can add extras designating sub-species if you like, but when you see a scientific name the important parts are the genus (always capitalised) and the species (never capitalised, even if named after a person). The aim of the name you chose is to tell you something about the plant and the first thing you have to do is identify the genus. Does your plant have any obvious relatives? If so, I'm afraid you're not free to chose the full name – you'll have to name your species with the same genus, though you can chose the species name (btw, it's generally considered bad form to name it after yourself, sorry. You could name it for a friend though – or get them to name it for you, if you feel particularly narcissistic). So in a scientific name, the genus should tell us about evolutionary relationships. And if our understanding of the evolutionary relationship changes then, bad news, the name will have to change too. Which is what has happened with the genus Acacia. In fact, we touched on the issue yesterday, when I wrote about sponges not being monophyletic, but we didn't take that discussion very far.

Still, what's happened to Acacia? Well, the genus was first described back in 1754 (lots of these details from here, but it's behind a paywall I'm afraid) and until the recent changes referred to some 1300ish species, of which 960ish live in Australia, where they're known as wattles. It's long been noticed that within this large genus, there are some specific sub-groups (called subgenera) that are more closely related to one another - like those Australian wattles (two of which made it to Madagascar, btw) - but that's OK as long as the genus refers to all the relatives. The problem came when studies started showing that some of these sub-groups were more closely related to other members of the Mimosoidae subfamily, than to other members of the genus Acacia, which is what means the group is no longer monophyletic. Now, it's easiest to explain this with a little diagram, I think. So, here's what we used to think Acacia relationships were:
 You can see how all three groupings (subgenera) within the genus Acacia are grouped together, and all the Acacias are separate from the other members of the Subfamily Mimosoideae. That's good - it's a monophyletic group and everyone could have Acacias. (You know a few of the members of tribe Ingeae, btw - this is where Albizia are found.

Now, however, recent DNA evidence has shown a different pattern (it's not quite certain which pattern, so I've given an illustrative diagram from one paper):

Now this shows the problem - no matter how you twist and turn the branches, Acacia subgenus Phyllodineae comes out as more closely related to members of the Ingeae than other two groups of Acacia. As the genus therefore describes multiple branches of the tree, but not all the descendents, we know it's polyphyletic, and we're going to have to change some names. We could, of course, resolve the problem by naming all the plants within the tribe Ingeae as members of Acacia too - then the name would only refer to one group and would again be monophyletic. But imagine the chaos that would cause - suddely all the (obviously different) Albizia and their diverse relatives within the entire tribe, would have to become Acacia. No, it's much simpler to split the Acacia genus into several different genera, each of which are monophyletic. And because Australia has by far the most species in the old genus, they've got the Acacia name for their wattles, leaving the rest of the world to find some other names.

In fact, there are some names out there that look set to be standard. Many of our well known Acacia species look set to become Vachellia, whilst others will become Senegalia. Which become which, I've not sorted out yet. But, but I do know what was Acacia mellifera will be Senegalia, whilst A. tortilis will become Vachellia, and for many species it's fairly obvious which of these two are more similar, suggesting likely names - bushy, multi-stemmed versions look set to become Senegalia, big trees follow tortilis into Vachellia. The good news, of course, is that there's no reason to change the common names - we can still have Whistling-thorn Acacias if we want, even if their scientific name becomes Vachellia drepanolobium. Unless, of course, someone decides common names should change too, something I find much harder to understand (and I still see Crowned Plovers, even if they are Vanellus) - in which case there really will be trouble for the poor Acacia Rats...

Update: You can find a (nearly) complete list of the new species names here.

Wednesday, 27 July 2011

Marine life

I've been at the beach for the last week (hence silence on the blog!) having fun with family and friends. Despite the fact I grey up about as far away from the sea as is possible in the UK (which, I admit, isn't really that far...), I love beaches and sealife. It's a lot further to the sea from most of the safari circuit than where I used to live, but many visitors to East Africa will combine a safari with a beach trip, so you might find youself down there from time to time. And if you don't, I thoroughly recommend it for a fun trip some time. Tourists don't usually think of the beach as a place where wildlife happens, but actually marine and coastal habitats are completely fascinating and I thougth I'd make use of the pictures I took down there to give a few hints about what you might talk about if you do find yourself on the long drive to the beach. So today I'm going to start with what I think is most amazing about sea life - it's extraordinary diversity and plain weirdness.

This mollusc has an unusual foot - and you can see its eyes too!
Now, you have probably been told that after kingdom, the main division of all life is the phylum (in fact, if you were told this, it wasn't quite right - botanists refer to plant divisions, leaving phyla to zoologists - that's the  plural, btw, it's from the Greek phylon meaning race or stock). And the Animal kingdom is divided into around 40 phyla (12 divisions for plants). Being such a basic division of animal life, animals in different phyla can be expected to be remarkably different - insects belong to the phylum Arthropoda, and it's fairly clear they different to us humans, being representatives of the phylum Cordata. Similarly, snails belong to Mollusca and are pretty different to earth worms, belonging to Annelida. You get the idea - these divisions are pretty fundamental. Now, of the 40 phyla out there, I wonder how many you could name? (I don't think I'd do too well either, to be honest...) But what I do know is that whilst there are no extant Onychophora (velvet worms) living in the oceans, all the other phyla are found there but only 10 are found on land (if we exclude internal parasites of other species). So three-quarters of the most fundamental of divisions of the animal kingdom are entirely aquatic, most of them restricted to marine environments. Which, of course, means life is pretty different in the sea. Why this should be is obvious, of course - the original animals that formed the ancestors of modern phyla lived a very, very long time ago (during a period known as the Cambrian explosion, about 500-580 Million year ago) when there was only life in the sea (there's a nice time line of life here).
Tide-line discoveries from Ushongo beach, July 2011

Cuttlefish shell, also a mollusc
Why does this interest me? Well, when you go to the beach you have a chance of finding some of those rather more bizarre life-forms that never made it onto land. The above picture shows a few of the more obvious things we found on the beach. As everyone knows, there are lots of mollusc shells (phylum Mollusca) - but look at the major divisions within this phylum too - only gastropods, the snail type, have made it onto land, but the sea is full of bivalves and other classes we never see too. In fact, nearly a quarter of all classified marine species are molluscs and there's every reason to believe that some of them (octopus - the're also molluscs and belong to the same group as cuttlefish in the picture beside) have evolved remarkable intelligence, capable of solving problems and allowing them to practice deception.
Note crab carapace, top right, sponge top left and corals.


Hermit crabs have almost made the terrestrial transition but still return to sea to breed
Then you'll see a crab exoskeleton. Crabs are members of the Arthropoda, just like insects and spiders, and some of them have taken to the land - though they must return to water to lay eggs. The subphylum they belong to Crustacea does have truly terrestrial representatives - the woodlice are an example - but most are still aquatic creatures and there's another example here too in the goose barnacles near the back in the middle. Look further and you'll start seeing som true aquatic specials. There's a couple of sorts of coral there, belonging to phylum Cnidaria which also includes the sea anemones and jelly fish. These, in the form of hydra, make it to fresh water, but no cnidarian with their usually soft bodies have made it onto land. And there's also a cnidarian look-alike in the sponge belonging to Porifera. Actually, this phylum probably includes animals from more than one group and excludes some of the descendents of the original sponge (all the rest of the animal kingdom, in fact!), so we'd call if polyphyletic and expect taxonomists to break it into other groups when they understand the relationships better. Poriferans are an extraordinary group of animals that, when I was doing my first degree, everyone was excited to think might be a whole different kingdom (or even several kingdoms). Now this seems less likely, but it seems very likely that all the rest of animal life did evolve from a sponge, rather a long time ago.
Note sea-urchin top centre and goose barnacles bottom right

What else is there? Ah, a sea urchin, belonging to phylum Echinodermata. Here, at last, we find a purely marine phylum - starfish, sea urchins, sea cucumbers and crinoids are all here, and none of them has even moved up the rivers. They all have five-sided symmetry as adults (or there abouts), but as larvae they start life, as us, with bilateral symmetry. Very odd, but very important in some places, as grazing by these species can completely alter the ecology of a coral reef or sea-weed forest.
Casts from some annelid worm (I expect) in roots of a mangrove

Annelid tentacle trails radiate from the animals home in the centre
A few other things didn't make it into the group photo, but are interesting too - the vast numbers of 'worms' of one sort or another (mostly Annelida, but maybe one or two other phyla are around too, I've certainly found others before on these beaches). They're responsible for these casts and the strange patterns around this hole - at high tide the inhabitant stick lots of tentacles out of the hole to filter food back to the mouth at the top of the hole. And these worms and other invertebrates can occur at fantasic densities, providing masses and masses of food in the intertidal zone, for lots of nice birds and beasts. Those will have to wait for another time now, though, whilst we remain amazed by the extraordinarily diverse numbers of ways marine organisims seem to have evolved to survive. Remarkably, I'll leave you with the surprising thought that despite this huge diversity of live-forms, there are far more species of animals on land. In fact, far, far more. And most of them are beetles, phylum Arthropoda. Why should it be that such extraordinary diversity has evolved on the land and not in the sea, where evolution has been going on for an awful lot longer? Odd...

Tuesday, 19 July 2011

Connecting forests and savannahs

Pre-montane forest - note the high diversity of tree species, Lake Duluti, July 2011
In my last post I said I's intended to talk about the connections between forests and savannahs, but got distracted by pretty birds. So I thought I'd do it now instead, becuase it's still an interesting topic. Once upon a time, a surprisingly recently time ago, there were rather more forests in East Africa than there are today: even in the relatively well-wooded Eastern Arc mountains, it seems like nearly 80% of the prehistoric forests have been lost. And they were an important part of the landscape - both in their own right (harbouring massive biodiversity with hundreds of plant, vertebrate and invertebrate groups endemic to the Eastern Arc), but also through the role they play for many savannah animals.
Montane forest in Arusha NP is kept open by grazing and browsing - that's a C3 grss understory. June 2011

We've already seen that forest and savannah can both occupy areas where the rainfall is over 1000mm per year, and that fire seems to be the key factor determining which actually wins out. We've seen how riverine forest is a forest habitat within the savannah biome, and we probably also know that thickets are fairly similar too. But true forests are different, and we don't usually think of them as connected to the savannah at all. Yes, forests are different - they're not dominated by C4 grasses (though they can still have some pretty thick understory of C3 grass) as we've seen defines the savannah biome, but I think this distinction is unfortunate as they are connected, at times vitally so.
Forest C3 grass species - short, round leaves are typical. Duluti July 2011

Yes, it really is a grass!

For example, the loss of forest in the Mau escarpment that feeds the Mara river is believed to be responsible for the altered hydrology of the Mara river within the Serengeti-Mara ecosystem - less forest means less water is soaked up by the landscape, so more flows down the river in the wet season and less remains to flow during the dry season. More directly, East African forests provide homes for many of the species more typically associated with savannahs. You might not visit Arusha NP to see the elephants or buffalo, but they're there. And what's more, those elephants move - they go down the Ngara Nanyuki river and down onto the savannahs of West Kilimanjaro. Where, of course, they must meet elephants that have done the same from the forests of Kilimanjaro and those that have come out of Amboseli. The same seasonal movements of elephants into and out of forests is a regular pattern, whereever such movements remain possible. Typically, savannah elephants will move into the moister forest habitats during the dry season when food becomes scarcer and poorer quality on the savannahs. But elephants aren't the only animals making this movement - in other areas where the savannahs are moister than around West Kili, buffalo and several other species make the movements too, though it's usually a shorter-distance movement and less seasonal.
Buffalo (and other forest mammals...) Arusha NP, December 2009

We all know of the regular movements and migrations of many ungulates. This is usually driven in part by the need for dry-season forage (we'll have to post more about migrations later). But what's interesting even about the well-known Serengeti migration, if that's its variable both in route taken, and timing. And the key driver is food and water availability - if there's food and water around, the animals stay longer, if they see/smell rain somewhere else, they shift their routes to exploit it. This is a very sensible response to living in a highly variable environemnt, and it seems that many savannah animals are similarly flexible when they need to be, even if they don't show large-scale and well-known migrations. So the forest and thickets of the northern Mara can be heaving with wildlife during droughts, and provide an important buffer for animal populations when conditions get really harsh. Indeed, the loss of forest in Amboseli is believed to be one of the key changes that meant the drought there in 2009/10 had such a serious impact. It seems that many animals have their regular dry-season refugia (often wetlands like Silale in Tarangire) but if these dry up in a really bad drought, they'll move to the nearest forest and tough it out there. But if the forest patches have disappeared - particularly the lower-elevation forests - or are now separated by fences, then the animals can't do this ocassional movement and there's going to be trouble. So although true forests aren't usually considered part of the savannah biome, they certainly have a role to play in a well-functioning ecosystem. And what's more, they're a great change from the savannahs for visitors to East Africa, so don't avoid them just because you won't find lions there!

Saturday, 16 July 2011

Mixed species flocks

This morning we went for a nice walk in the forest around Lake Duluti. I was thinking I'd probably share some pictures of the forest and write about how the forest and savanna are connected by movements of all sorts of species. But then I encountered a classic mixed species feeding group of forest birds, and thought that might be a whole lot more interesting to talk about.

Red-capped Robin-chat, Lake Duluti July 2011
My 3-year-old son ran around a corner and flushed something from the path. Being (sometimes) well trained in these matters he stopped and informed us he'd seen a bird. I could hear greenbuls around and said it was probably one of those, but then a red-capped robin-chat hopped out. And whilst I was watching that, a scuffle in the bush behind me turned into one, then two and finally three Ruppell's (or were there white-browed too) robin chats were feeding.

In the bush behind a white-starred forest robin was hopping about (sorry, no pics), and then (finally) I found the greenbuls I'd been hearing - grey-olive greenbuls, and something of a speciality of Lake Duluti being easier to see there than just about anywhere else I know.
Grey-olive Greenbul, Lake Duluti 2011 - check the pale eye, pale legs and warm tail

Now, birding in tropical forests is often a frustrating experience. You hear a few calls, but see little. Until suddenly, as today, loads of birds all at once. Here in Tanzania you get this phenomenon both in the forests proper as today, but also in the dry forests and woodlands of the savannah (and particularly in miombo - if you've ever looked for bits in miombo you'll know quite how empty it can be for ages, before suddenly you find a little group). Why is this? Why do we get mixed species flocks in tropical forests? To most guides based here, there's nothing unusual about this, but it is pretty different to the experience most northern birders will be familiar with - in temperate forests there's usually birds all around, all the time - only in the harsh winter period do you get forests apparently empty of birds (and of course many have migrated for the season), and then a mixed group of several species. But perhaps this gives us some clue - the relative harshness of the environment.

Robin-chat - probably Ruppell's but see comments below! (both are present at Duluti), July 2011
Despite appearances, tropical forests can be a fairly difficult place to survive in, not because there's no food there, there's usually plenty of invertebrate life around, but for most birds just any old insect won't do - it's got to at least be the right size and, perhaps most importantly, has to be accessible and there really are lots of places to hide in a forest! So, despite the tourists view of tropical forests teeming with life, if you're at all fussy about what you eat (and, let's face it, most things are), they're tricky places to make a living. Add to that the fact that there are plenty of things out to eat you too, and life in the tropical forest is very hard indeed. Just as it is in the incredibly low nutrient miombo woodland, and most other savanna woodlands, at least seasonally.

Giant millepedes would be great food for some - but not everyone!
So, joining friends is a good way to deal with it – more eyes to look for predators, more eyes to look for food. Of course, you could just hang out with other individuals of the same species and many birds do this – but actually, when you do find food, you don't really want competitors to eat it all up before you can, and your biggest competitors are going to be other birds of the same species. They'll have exactly the same foraging strategy as you, and they'll be looking out for exactly the same food as you. So whilst I'd get the advantages of lots of eyes from hanging out with conspecifics, you might be better off hanging out with different species that are each looking for slightly different food from you. What's more, you might even find more of your food if you hang out with other species, as in the process of their foraging, they might disturb the food you're looking for in exactly the same way a lot of birds forage around grazing mammals. So many species prefer to join mixed-species flocks, to get the all the advantages of lots of eyes, but few of the disadvantages of intense competition. And if, as today, you come across a particularly rich rood resource like a bunch of termites on the ground, there'll be so much food for all, it's not a problem to be in a large group (which is why a lot of birds that habitually go around in large groups like finches feed on things that are, when you find them, extremely abundant, much more so than any one bird could eat on it's own).
Soldier Pansy, Junonia terea (I think) were common and probably good to eat - but only if you can catch them!

Some birds are really birds of mixed-species flocks , rarely occurring in single species flocks (in the savannah, how often have you seen red-billed buffalo weavers feeding in a single species flock? Much more likely they're with lovebirds, starlings, other buffalo-weavers and/or hornbills), whilst others seem to be more opportunistic in their joining of mixed flocks – again a savannah example would be of hornbills who seem quite happy either in small groups of their own, or together with several other species. In fact, where people have studied the phenomenon in some detail, it seems that it's often one or two species that start the mixed flock in the morning, and their calls attract the other species to them before they start the foraging patterns, yet some breeding birds will join flocks as they enter their territory, travel with the flock until it reaches the other side of their patch, and then head off on their own again. So the individuals in a mixed-species flock can be quite dynamic as the flock moves across the landscape. All very interesting I think...
Soldier ants - Siafu - are common in the forests but not followed by birds

Interestingly, the same sorts of thing happen in South American forests and many of them there are particularly associated with driver ants (the South American answer to our safari ants or Siafu). The birds forage along the moving front of the ant group, either eating the ants themselves, or the animals fleeing the upcoming invasion. I'm not quite sure why this doesn't seem to happen with siafu – there were several trails in the forest this morning, but as usual no birds attending. Any idea?

Thursday, 14 July 2011

Crowned eagles

African Crowned Eagle, Arusha NP Feb 2011


Montane race of East African Black & White Colobus, Arusha NP, Feb 2011
Driving around Arusha NP a couple of weeks ago we were busy searching for Black and White Colobus (among other things, of course) - my favourite monkey of the forests around here. And we did eventually find a troop, though they weren't playing as well as they often do. But as we were heading down the mountain I spotted a crowned eagle sitting in the top of a tree, and pointed it out to my colleagues (botanists), noting its probably the main predator of the colobus and other monkeys in this forest. It was a male, and sitting at the top of a tree, so not as big as they can appear and William was pretty amazed that they could catch monkeys at all, let alone carry them any distance (which, admittedly, they'll only do for smaller individuals up to about 6kg). Like most raptors, females are rather bigger than males (up to c. 5kg according tot he usual handbooks), but there's no doubt that these birds are the major monkey predator across most of their range (check here for some details of the impact on monkey populations). I knew all this, but was busy reading an article by one of the absolute experts on these birds today, and even I was pretty impressed by his desciptions of them killing Bushbuck! With an average mass of 36kg for a female, that means the bird can kill something 6 times it's own mass, and surely puts it in the top predator category!
Female bushbuck, Arusha NP Feb 2011

For animals of this size, apparently the birds will swoop in fast, hit, grab, squeeze and release. With a 10cm hind talon, they can easily puncture a lung and do serious internal damage then just sit and wait until the animal keels over. Once this happens, if it's not yet dead, they'll have a good stomp and squeeze some more until it bleeds to death. Not bad for a bird!

But now think about this - if a crowned eagle can kill prey 36kgs in weight, and shows a distinct liking for primates, doesn't that put rather a lot of children in the picture? And, indeed, the crowned eagle is the only extant (i.e. not extinct) bird that is known to kill humans (check here for one such event in Uganda reported in the scientific literature). And now think back a few million years to when our ancestors were rather more similar to monkeys than we are now - Crowned Eagles (or their ancestors, at least) were quite possibly our major predators, just as they are today on medium sized primates. And what's more, there's even fossil evidence of just once such predation event among our fairly recent relatives, Australopithecus africanus from South Africa. Certainly to our early (and somewhat smaller) ancestors these birds must have exerted a major selective pressure - it's even suggested that because predation by crowned eagles exerts a considerable selective pressure on primates to get larger and be able to fight off eagles (again, there's a record of a Sanje Mangabe killing a would-be eagle predator), they might be what forced our early ancestors to get bigger and have to leave the trees in the first place. Speculation, of course, but interesting to think that a bird may well have been a major part in our early evolution...

Tuesday, 12 July 2011

Dragonflies and ways to interpret wildlife sightings

I saw a beautiful dragonfly at the weekend. I have to say I can't identify it to species - I think it's probably Trithemis stictica, a fairly widespread and common species with a nice English name: the Jaunty Dropwing. If you know better than me (and I am only an ornithologist!), please tell me!

Jaunty Dropwing (I think!), Maji Moto, July 2011
It's pretty obvious why this genus of dragonflies are called dropwings from the picture...

Now, I know very little indeed about dragonflies, but as I had a nice picture to share, I thought it would be worth talking through a few of the things I do know, as an example of how wildlife sightings of any kind can lead to interesting conversations. And, of course, other intvertebrates (that's other than termites) are one of my ten things, of course, so any facts I can dredge up might be worth remembering.

So, what's my strategy for interpreting wildlife sightings? I try and think of three basic questions to ask myself: (1) what is it? (2) What's it doing? And (3) What impact does it have / role does it play on the ecosystem and other plants/animals around? Not all of these will be useful to every sighting, but at least it gives me a framework to start thinking about - and I don't think I'll ever be left with just being able to say "oh, err, it's a dragonfly...". So, given my massive ignorace of dragonflies, how would this work if I'd been out with visitors (as it happened, I was with a friend, so it's a real situation).

Me: "Ah, look, isn't that a lovely dragonfly"
Friend: "Beautiful, what is it?"
Me: "No idea, I'll take a photo and we can look it up later..."
Click, click, click.
Me: "I wish I knew more about dragonflies, they do some pretty amazing things, you know?"
Friend: "Oh?"
Me: "yes,you know there's one around here called the global skimmer that migrates here from India."
Friend: "what?!? That's amazing!"

And so we went on - I'd identified it for him as much as I could at the time (a dragonfly, not too precise, but that's all I knew!) - and I've since discovered a fantastic website that will help you identify your African dragonflies here - if you register you can even send your unidentified picture and get help identifying it. Isn't it great what people do for free! So, now I know what it is (I think - I'll send it to them, and if I'm wrong I'll let you know here!), and I could also have carried on the identification part at the time - it was pretty obviously a male, as I already know that most blue dragonflies have females that are yellow. With other species the identification part can go much further - you can identify how old it is, what role it might be playing in a group (alpha male, etc.). For a dragonfly this wasn't going to go too far.

Then the next question was what's it doing? And it was sitting around (with its wings drooping), then buzzing about hunting. A typical sit and pounce type hunting behaviour. So it was feeding and resting - worth pointing out the strategy it uses for this too - there were other dragonflies I could have pointed out using a much more active search and destroy hunting technique. Given any animal, even one you've not seen before and don't know what it is, you can probably have a pretty good guess at what it's doing, as most animal activities fall into a fairly restricted number of classes: feeding/drinking, mating behaviour (including displays, dominance fights, calls, etc., if not actual copulation which probably doesn't take a guide to interpret...), territorial defence (whih can obviously also be related to mating), resting (in fact, I think that's all those boring lions ever do) and other social behaviour (play, groub bonding, etc.). If you think of these five categories as the main ones (and there are others of course), you can probably have a pretty good guess at what's going on. And by watching a bit more, you might learn something interesting!

In fact, in this case I didn't bother, just jumped straight to my most interesting story aout dragonflies - the global skimmer Pantala flavescens. It's rather scarce in Tanzania, but (as well as there probably being a resident population) it does indeed migrate here from India in November, and back again in May, 3500kms across the open oceans (some obviously rest on the Maldives). Amazingly, we think it often migrates at altitudes over 1000m above the sea, and even over 6000m when crossing mountains, literally skimming on the winds associated with the Inter-Tropical Convergence Zone (that's what gives us our rains). (You an read lots about the migration here). Interestingly, it's migration across the ocean occurs at the same time as a particularly good dragonfly predator is doing exactly the sae - the Amur Falcon. The dragonfly is quite a fat beast, yellow in colour and in the same family as my pretty above, so if you see a golden yellow looking dragonfly, it might be this amazing beast.

Finally, the question is what role does it play in the ecosystem. And you might think that a dragonfly doesn't do very much. But actually they do! Not only do they eat rather a lot of nasty bugs (and they're actually recommended as an efficient means of controlling mosquito numbers - a dragonfly can apparently eat it;s own body weight in bugs in just 30mins!), but they're also eaten by lots of things too - those Amur Falcons, and the two species of Hobby we get around here are major dragonfly predators. And the pygmy kingfisher in the riverine forest post was busy eating them too! But their biggest role in the environment is actually probably not as adults, but during the nymph stage. Now remember the dragonfly lifecycle? Eggs are laid in water, hatch into nymphs, nymphs (or naiads) live in pools for anything from a few months to up to five years, depending on species, and then after heaving themselves up some emergent pool-side vegetation a final moult sees the adult emerge direct from the nymph (no pupal stage for dragonflies). And during that long periods in the pond, dragonfly nymphs are major predators - they can move very quickly indeed (by squirting water out of their anal cavity, rather jet propelled. Good story for kids methinks...) and although they mainly eat invertebrates, they'll catch tadpoles and small fish quite frequently too. Not many invertebrates predate vertebrates, so that's pretty neat. They're actually top predators in many pools, and have massive impacts on the numbers and diversity of animals around pools - especially those nasty mosquitos! So, yes, just like top predators on land, they play a pretty important role in the wetland habitat too.

Hopefully something new and interesting will be in this - and if nothing else you might have a new way of interpreting your own wildlife sightings too. We'll try the technique on other vitual sightings in time, but I've had enough for tonight...

Sunday, 10 July 2011

Riverine forest- forest in the savanna biome

Majo Moto near Boma Ng'ombe - thick riverine forest is protected from browsers and fire

I spent the weekend with my family and friends camping at Maji Moto near Boma Ng'ombe. As the name suggests, this is a hot (well, warm at least) spring in the middle of some pretty dry Acacia – Commiphora scrub. There's a pretty impressive flow of somewhat warm water out of a cave forming a nice river that, as you might imagine, if a complete contrast to the surrounding dry, and it inspired me to think about riverine forest again. Already this year, it's pretty dry in lots of places (though I hear there's rain in western Serengeti), and everywhere is pretty dusty and dry. Except, that is, the riverine forests. Here the trees are still green, there's plenty of shade and the contrast is amazing, but this is just a hint of how different riverine forests will be from the surrounding vegetation in another couple of months as the dry season starts to bite.

Acacia commiphora scrub in the foreground with tall, green riverine forest in the back. Maji Moto, July 2011
So, where are riverine forests special and what role do they play in the savanna ecosystem? The first special thing that's immediately obvious is that they're green, when no-where else is. Ti understand what makes riverine forest special we're going to think again about nutrients, fire, water and grazing.browsing, the big four of the savanna ecology world. Lots of riverine trees are evergreen, or nearly so. And the reason they can keep green is that they typically have incredibly deep roots, penetrating right down into the water-table. With a few exceptions, the surrounding Acacia and Commiphoras tend to have shallow roots, spreading horizontally just below the surface of the soil, spread out and designed to catch as much of the water that falls on the surface as possible, very different from the tap roots of the evergreen riverine forests. Tap roots only work if the water table below ground isn't too deep, and alongside rivers that's fairly likely, so water-availability is the main factor altering the ecology of riverine forest.

Look closer and you'll notice some other things – trees of the riverine forest are often not thorny, in complete contrast to the surrounding vegetation. Now why is this? Obviously thorns are to do with defence against herbivory, so something about these trees means they somehow escape herbivory in a way that the surrounding habitats can't. That's odd, given that rivers are generally excellent places to find animals. So the key here isn't that the herbivory is much lower than in the surrounding woodlands, but that the tree growth rates are much higher and the trees themselves are larger, all thatks to that ready availability of water. So once established, riverine trees can grow all year around, escaping from even the tallest giraffe in a fraction of the time water-limited plants in other habitats need. So although the animal densities can be high, the impact of herbivory is less, at least on mature trees. Now that is a problem for seedlings, of course – although they grow fast, they still need to escape heavy herbivory whilst they're short, and this the achieve by heavy reliance on nurse plants – plants of other species (bushes and shrubs) that are well defended against herbivores, but that allow the seedlings of the riverine species to sneak in among them and also escape herbivory. Once the forest is dense, of course, the mature riverine forest can be so dense (especially if there's a dense palm understory that nothing can get into) that herbivores are basically precluded anyway, making regeneration of the forest fairly straight forward. So herbivory also plays a part in the riverine system.

And finally, fire is a big issue in riverine areas. Typically, trees of the riverine forest (like other forest – and riverine really can be a forest habitat attracting many species typical of more montane habitats, particularly in our current cold season) are fairly sensitive to fire. That's not much of a problem for them in general though, as they're usually green and moist, dense and with little grass in the understory (especially if there are lots of buffalo around keeping it short around their preferred river habitats), so don't have lots of dry fuels waiting to burn. In fact, riverine forests can play an important role in fire suppression, allowing different fire regimes on one side of a forest strip to the other. This sensitivity, however, causes a problem if you burn an area too much so the nurse plants are burnt off (one of the reason we think many of Serengeti's riverine forests are currently on the way out). And once recruitment is lost in this system, it's really difficult to re-establish riverine habitats, as the natural fire suppression is lost, and the herbivores are also given much better access. A savanna with healthy riverine forest is therefore likely to be well managed savanna.
Otolemur garnettii, common in Riverine Forest. Maji Moto July 2011

Figs are a valuable year-round food resource in riverine forest

Fallen ones are eaten by everything!
 And riverine forest is, of course, a vital habitat for many species. I've already mentioned the fact that buffalo like to hang out in these areas, and they have their own set of animals too. They're great places in the shady spots to find leopards, or you might look for some of the more specialised riverine species too – lots of good birds, loads of bush-babies and maybe tree hyrax, etc. Obviously many game drives take in riverine forest, particularly in the dry-season when they're the only green areas around, especially as they're one of the only habitats at this time that has fruit on offer – figs are everywhere in the riverine forest, and there's always some ripe somewhere nearby. Figs are on all rivers, but up here Tamarind is a pretty specialised riverine tree too, and you'll often find Marulas on the rivers too, all excellent food sources for monkeys, baboons and birds, plus the fallen fruit are loved by bushbuck, elephants and just about anything really! At this time of year when food is hard to find in the open savanna, riverine forest really comes into it's own and lots of animals will make extra use of the habitat during the dry season. All in all, a very important part of the savanna ecosystem, both through proving food resources, but also from it's role as a fire break, etc. And you can have lots of fun in the river too!

Many special birds are easiest to find in riverine forest – a Pygmy Kingfisher, July 2011
In the cold season
higher elevation forest species like these Black Saw-wings follow rivers into the lowlands

Saturday, 9 July 2011

Plant signaling

Perhaps the subject that most surprised guides when we were chatting about things to talk about when there are no lions came up in my session on thorns. Thorns and other plant defences are quite fascinating and I'll certainly talk more about them in the future. But most people were more impressed to hear about how plants signal, than they were about thorns themselves, so I thought I'd give a bit more information about plan signaling in this post instead.


Now, imagine you're a plant that's getting browsed. Not much fun, huh? You'd want to do something about it if you could, wouldn't you? So, let's say you can detect browsing (how would you do that? Easy, as it happens, just look out for plant chemicals that should be contained within cells, in places they should be - if there's cell contents in places it shouldn't be, the chances are you'vebeen damaged - and we all know how damaged plants can smell), it would be nice to have a quick response and produce more nasty tasting chemicals straight away, and when you regrow, it would make sense to be extra thorny in this area. Even better if you could somehow warn other branches that you're been eaten and communicate with the other side of the tree, don't you think? And as it happens, plants can do this - rather than always producing lots of costly thorns and nasty chemical defenses, plants tend to just produce a minimal output, and then up the defenses if they actually come under attach. Very sensible really. And one of the key ways they have of signalling that they're under attack is through the use of a plant hormone. Now there are several plant hormones, but the simplest is called ethene (or ethylene, it's the same and I'll use the two interchangeably here) and is a colourless gas, consisting of two carbon atoms, and four hydrogen atoms:


H              H
    \          /
      C = C
    /           \
H              H


if you're into your chemical formulae. It's a very simple organic molecule, and plants use it for almost everything you can imagine: signalling that it's time to ripen fruit, so they all ripen at the same time (fruit importers in the west use this trick - they get people to pick unripe green fruit in the tropics, stick it on a boat to Europe (it last well if it's green), then gas it with ethene so everything ripens nicely before getting into the shop. Which explains why fruit tastes much better here than in UK... You can also use ripe bananas to speed up the ripening of other fruit, if you stick them together in a paper bag.); seed germination; fertilization, etc., etc. But for us, right now, we're interested in how they use it to signal stress, and in particular herbivory.
This avocado is ripening thanks to a ethylene signal from the ripe bananas

To be fair, it's not actually the main signal process for herbivory - it's probably too general in function for that task – but it does play a role and the concept is the same for the other signalling processes. Once chewed, a plant rapidly (within seconds) with start to produce ethylene. Being a gas, it can drift all around the plant, and receptors in other parts of the plant pick it up, decide what it means (“help!, I'm being eaten!”) and tell the plant to get on with appropriate defences.

All well and good. But the observant among you will already have picked up on one thing – there's nothing to stop the signal moving out of the plant being eaten and into the neighbouring plant. And if that plant can pick up the signal, then all of a sudden, plants in a neighbourhood can communicate with one another. And, in fact, this is what they do. Pretty impressive for a plant, I'd say! Since most plants use the more or less the same set of signals it's quite possible for the signal to come from an Acacia, but be picked up by a Balanites – inter-specifc plant communication. And you thought plants were just sitting there and taking it! I'll save the 'what can a plant do about it' question for another time, but for now just remember that plants can signal, even to other plants, that they're being eaten and perhaps there's something here to talk about next time you watch a bush being hammered by an elephant...
This poor Acacia xanthophloea is showing quite how many thorns it will grow when there are lots of nasty giraffe around. Arusha NP June 2011.

Thursday, 7 July 2011

Fire!

I've been thinking a lot about fire recently. Burning is, of course, Tanzania's national pass-time in this season, and is one of very few active management activities occuring in protected areas, and we found a nice fire to admite in Tarangire on our safari a couple of weeks ago.
Lots of smoke, and some big flames as a fire goes through bushes, Tarangire June 2011


But again, I've been pretty surprised by how little guides know about fire in savannas. I'm currently trying to put togher a big experiment that wll see us burning lots of areas around the Serengeti National Park (it burns every year anyway, all I want to do is tinker with the management in a controlled way to understand the impacts). As part of thinking about this I organised a meeting at the ATBC-SCB confere of lots of people interested in both the Serengeti ecosystem, and of those interested in fire throughout the savanna world. I also tracked down a number of people in South Africa when I was there in May to get their thoughts. As part of all this thinking, I've put together a document that pulls together lots of ideas, so I thought I'd share some of the introduction to that here:

Fire is generally considered a vital component of savannah ecology, with trees, grasses and animals all adapted to a fire prone ecosystem: in fact, globally, 85% of fires occur in savannah habitats. In most of Tanzania's protected areas, rangers deliberately set fires each year for a number of different purposes, including the encouragement of new grass growth for grazers and the control of bush spread. Recent evidence from a global study suggests that at least for those savannahs occurring in areas with over 1000mm of rainfall, the forest/savannah edge is often maintained primarily by fire, and that in it's absence many savannahs will revert to forest. There is some debate, however, about how much animals may take over this role when at particularly high densities and the benefits of fire in certain areas have been questioned. For example, in some areas of Tarangire National Park with high numbers of migrant animals during the dry season, fire has been totally suppressed for over 30 years with relatively little obvious difference between fire-suppressed and more frequently burnt areas: it is argued that in these areas fire simply burns potential animal forage. Similarly, in Kenya, the Kenya Wildlife Service has recently instigated an overall policy of fire suppression except where the fire is caused by natural events (essentially, lightening storms) in an attempt to encourage one view of naturalness. There are, of course, alternatives to these two extremes, where fire is sometimes suppressed and sometimes encouraged. Given that fire is one of very few activities under active management in these areas it seems sensible to understand more fully the consequences of these different management decisions.
Note the fire burns the grass, bigger bushes are untouched.
Traditionally, pastoralist communities in northern Tanzania have set fires towards the end of the dry season, in anticipation of the rains: they make a calculated gamble between burning possibly useful forage if the rains are delayed, and waiting too long and being unable to burn (and encourage the new growth that will come with the rains) if the rain comes before they have time to burn. Obviously, by this time of the year grazing animals will already have reduced fuel loads to relatively low levels in many areas, so late dry season fires will only occur in less grazed areas, with other areas remaining unburnt in any one year. Such management was also the norm in many protected areas (including Serengeti) until around 1970, when concern over the regeneration of trees prompted a switch in fire management towards controlled burns in the early dry season (mainly in June). Such fires are generally cooler and patchier, with possibly less impact on woody plants. They also prevent the late season burns that are likely to be hotter and less easy to control with consequent reduced concerns about tourism infrastructure. Recently, managers of the Grumeti Reserves in western Serengeti have attempted to suppress early season fires until later in the dry season, allowing the migrant wildebeest and zebra to graze unburnt areas and increasing the forage available to these animals to the extent that their residence times within the reserves have tripled. Fires are also set during the short dry season, in February, and grass growth rates are such that some areas of the national park burn twice per year. Observations suggest that areas burnt in February may be preferred by migrant ungulates during June, but beyond these patterns and the immediate, short-term responses of grazing animals, we understand relatively little about how fire impacts the ecology of Serengeti or other East African protected areas in the bimodal rainfall area. Whilst we know a little about the impacts on large mammals and early vegetation responses, we know almost nothing about the impacts on other taxa, or the soil fauna and nutrient flows of the savannah ecosystem.

In practice, over much of Serengeti, fires occur once or twice per year. If there are species within the ecosystem that are rather more sensitive to fire than others – for example, ground nesting birds, or various plant species – such frequent fires may have negative impacts on the population. On the other hand, burning in different seasons may affect different species in different ways. Whilst it is generally considered 'a good thing' to burn Serengeti whenever it can burn, maybe reducing this frequency doesn't actually have the negative impacts expected of it and could, instead, benefit other taxa not usually considered in protected area management plans? Quite how many unburnt seasons does it take before grass quality starts to decline? Would a general switch back to late-season burning be beneficial?

Lots of birds were making use of the fire to catch fleeing insects, including this lilac-breasted roller - both animals and plants in the savannah are adapted to fires.
There's lots more to fire in savanna too that's of interest to guides and their clients (like what do the animals actually do?!), so we'll revisit the topic. But maybe the ideas here will help some understand why I'm so interested in fire. It's not just that I want a bonfire big enough to see on Google Earth. Honest!

Tuesday, 5 July 2011

Rinderpest erradication

I'm often suprised when talking to guides about how few know of the great African rinderpest epidemic of the late 19th Century, as it has had a huge impact on the wildlife we see around us today. I'm prompted to write of it now for two reasons - the first being news I missed whilst busy training in Tarangire at the end of May and have only now noticed: on 25th May this year, the Food and Agriculture Organisation (FAO) announced the global erradication of rinderpest. This is surely something to be celebrated, as the first global erradication of any animal disease. And as we'll see in a moment, what a good disease to erradicate! The second reason is somewhat less pleasant, as I hear there's a measles outbreak here in Arusha at the moment. Why should this prompt me to write abour rinderpest? Well, evidence suggests that measles is actually a mutated form of the rinderpest virus that affects people. In fact, it seems most likely it evolved during the 11th or 12th century during one of the periodic rinderpest epidemics in Europe at that time, showing once again how nasty diseases have a habit of jumping across species boundaries when they get very common.

If you're wondering why I'm interested in cattle diseases at all, you've not heard about the African rinderpest epidemic, and of the impact of the disease and its control on wildlife. So let's start back in 1887, when the Italians were busy trying to colonise Ethiopia (afterall, the rest of Europe had colonies, why shouldn't they?). It seems as though some time early in the year they imported cattle from Asia that had the disease - whether as a deliberate act of biological warfare or accidently within their own supplies we're not certain (there's apparently no evidence to support the often heard claim of biological warfare), but whatever the reason it didn't take long for the disease to spread. Within they year up to 90% of Ethiopian cattle died, and at least 30% of the human population too (some estimates put it up to 60%). By 1897, after a short pause at the Zambesi and a couple of southern cattle fences, the disease had reached Cape Town, destroying 60-90% of Africa's cattle along the way. (You can read more about the human cost of this here if you want.)

Cattle aren't the only animals to suffer from rinderpest though - most ruminants were susceptible, some even more than cattle with up to 90% mortality of wildebeest and buffalo. Descriptions from travellers in Serengeti during 1898 suggest there were huge mortalities among the wildebeest, with the plains covered with carcasses. And for the next 60 years the numbers of animals in the Serengeti - Mara ecosystem were vastly reduced, held constantly in check by disease: in 1963 there were an estimated 250,000 wildebeest in the ecosystem (compared with 1.4million today) and immediately post disease outbreak possibly as few as a few tens of thousands of  individuals. The key change allowing the populations to recover was the erradication of rinderpest in the cattle around the park, following a large-scale vaccination programme. In fact, across most of British colonised East Africa at the time a plan was formed to erradicate wildlife from the area as a way of controlling rinderpest, but the colonial administration decided that whilst they could manage to shoot everything in most places Tsavo (in Kenya) and Serengeti/Mara were just too big, so around these areas they'd institute a 'cordon sanitaire' where cattle would forever have to be immunised, isolating the disease within the parks. This cordon was completed in the 1950s and by 1963 rinderpest died out of the wildlife population - the disease was so effective at killing wildlife that it couldn't actually be sustained within the population and had only be maintained by continual re-infection from cattle in the surrounding areas.
There are a lot of Wildebeest in Serengeti today - the view from Naabi hill, Jan 2011

Which led to a six-fold increase in wildebeest populations from 1963 to 1977 (and other increases in buffalo, etc), with a huge increase in the amount of grass being consumed by animals. (Read more about it here.) Of course, eating all the grass had a huge impact on how much fuel there was to carry fire in the ecosystem, and the frequency of fire started to decline in the 1970s. And as we know, fire has massive impacts in the savannah and it's decline is, perhaps, responsible for the return of acacia woodlands across much of Serengeti from the 1980s. (And so many visitors think of Serengeti as an ecosystem in the same state as it was thousands of years ago! Unchanging Africa is a myth...) But that's also a complicated story we'll deal with another time - for now, let's be glad there's no more rinderpest anywhere, let alone in Serengeti.

Sunday, 3 July 2011

Global patterns in forest and savannah species

So I memtioned in my last post how I'd enjoyed taking some of the conference attendees on a little safari on the weekend following the conference. We had a night in Tarangire and then a day in Arusha National Park. Now I'm really an ornithologist, who plays at being a savannah ecologist. I'm not a botanist at all. So driving around with people who really are is always educational, and the number one insight that I got from the weekend was the extraordinary degree to which Bill (who works in Brazillian savannahs) and William (working in African savannahs) could identify a plant - say a Xanthoxylem and William would turn to Bill and ask if they had the same genus in Brazil. To which, almost always, the answer was yes. Even more remarkable (to me) was the fact that on occassions they even had the very same species.
Botanists getting serious, Tarangire June 2011. I think it's a grass.
 Interestingly, whenever there was a genus match between continents, if we were in a savannah, the same genus was always a savannah plant in Brazil, whilst if we were in the forest on Meru, the South American members of that genera are also forest plants. To me as an ornithologist thinking quickly, I can come up with no more than two or three bird genera that are shared between the continents (there are a few Turdus thrushes in both places, Tyto barn owls, etc.), and that's it.

I was brought up as an ecologist understanding that biogeographical (bio - from biology, geographic, from geography of course - biogeography being the study of distributions of species) similarities between continents could usually be explained by a process known as vicariance. This idea essentially explains the distribution of related species by assuming that a common ancestor of the current species lived on a continent that then moved around through the process of continental drift. A typical example might be the distribution of Ratites  - the group of large flightless birds that includes the ostrich. The closest relatives to the ostrich include the emu in Australia, the rhea of South America and the kiwis of New Zealand. Their distribution in these southern continents is explained by their shared ancestor living on the ancient continent of Gondwana, a single continent that eventually broke up (around 200 million years ago) to form the southern continents (plus India and the Arabian peninsular). Each fragment carried a population of this ancient ratites and today we see a distribution of birds across the southern hemisphere.
Ostrich are ratites, a typical Gondwanan group with a distribution explained by vicariance

This explanation of shared ancestry, each population of which floated off on it's own continent it the one that immediately springs to the minds of ecologists of my generation where confronted with similar species across southern continents. But the break-up of Gondwana took place around 200 million years ago - and that's a very long time for evolution to have been acting. Although most ratites are fairly similar and the shared ancestry immediately obvious (though check the kiwis!), they're actually very different and certainly not in the same genus. Now, it's important to remember that, unlike species, genera are not very well defined groups - rather they are a taxonomists attempt to identify common ancestors and group similar species - but whether we group 50 similar species into five genera in one family, or one large and diverse genus within a family is rather more arbitrary than the similar decisions about species (though even there it's actually surprisingly tricky!).  So I already knew that the best predictor of how long ago the common ancestor of any particular genus lived is nothing to do with the variety within the species, but everything to do with the number of taxonomists that work on the group - the more taxonomists, the more genera, the more recent the common ancestor. So my first question was whether the common ancestor of these plant species really lived more than 200 million years ago and are just kept in the same genus because there's such a shortage of taxonomists. And I learnt that whilst my head has been full of other things, I've missed one of the biggest revolutions in biogeography of the last decade.
Meru's forests were full of genera also found in South America (and Australia!) Erica have interesting distributions, but not in the New World.
Now we can use DNA to provide fairly accurate dates on when individual species shared common ancestors, we've been able to see that, contrary to the vicariance ideas I've been brought up with, that imply aces over 200 million years, most of the shared genera across the southern continents seem to be far more recent that Gondwanan in origin, which implies that they must, time and time again, have managed to disperse from continent to continent. Wow! What's more, it seems that more often than not, Africa has been the source of the movement, rather than the recipient. Amazingly (to me at least) even some plant species that are dioecous - i.e. have male and female plants - have amnaged to generate almost global distributions through regular long-distance colonisation events. (Unfortunately plant names have a habit of slipping my mind and I can't remember the one that impressed me most - and my pencil was broken so I couldn't take notes. Rubish, huh?!) That is pretty extraordinary I think! So next time you wonder how a seed disperses from a tree, and how it could ever move more than a few metres, remember that most of these genera have managed to get from one continent to another, probably several times! Surprisingly, though, despite these multiple movements across continents, whenever a plant does make the jump it has never (or nearly never) colonised a different biome - savannah plants have to find themselves a slot in a savannah, forest plants in a forest. Which probably tells us all sorts of interesting things about how plant communities are put together, but that will have to wait for another post...