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!

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...

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...