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

Serengeti Lions eat a diversity of mammal species.

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

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, 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!

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.)

The roles of elephants...

Elephants, Tarangire NP, Aug 2011

As you've probably guessed, I've been away again, so thanks to Ethan for sharing his discoveries whilst I was away. I'm sure we'll come back to the migration again soon (especially as I'm hoping to be in the thick of it again this weekend!), it's such a fascinating subject. Meanwhile, part of my travels took us to Tarangire where the elephants can never fail to impress, so in a rare forray into the world of the 'big 5', here's a post about elephants... The Tarangire elephants are a population fast recovering from the poaching of the 1980s (though I'm sure some still goes on at times) - in 1960 there were only 440 animals in the park, by the last full census I can find numbers for in 1996 there were 2000. Many of these early arrivals migrated into the park from outside to escape the even heavier poaching in peripheral areas and have since become resident (or semi-resident) within the boundaries.  But since 1993 the closely monitored population in the north of the park has continued to increase at about 7% per year (pretty close to the maximum theoretically possible, given gestation rates, etc.), which is rolled out over the whole park to 2011 would give about 5500 animals. A not unreasonable estimate I'm told. As the park has an area of aroud 2850km2, that gives a density of nearly 2 animals per square kilometer. Compare that to the densities during periods of regular culling in Kruger NP of around 0.4 animals in the same area, and you can see the extremely high densities present in Tarangire.

Eles love to wallow - digging waterholes as they do and removing up to 1 m3 of soil a time.

In actual fact estimating densities of any animal is trickier than you might imagine - they're certainly not unifrom across the landscape, with local concentrations in certain areas, or in different seasons. So it's fairly hard to make direct comparisons of densities across different National Parks, but it's pretty clear that Tarangire is certainly among the top two or three elephant parks in Africa. So the question I'm innevitably asked, is what is the impact of these elephants on the landscape? Weighing in at around 3000kg and eating as much as 200kg of food per day, elephants can have a massive impact on the landscape - add to that the fact they're pretty good a toppling tasty looking trees (generally across the quieter tracks I like to frequent, it seems!) and there's a lot going on. In some corners of Africa it is certain that they've had massive impacts on vegetation - creating rather unsightly bare areas around permanent waterholes and rivers. However, whilst tourists might not like these places, increases in elephants are often associated with similar increases in buffalo and impala, and the biological impacts are not all negative. It's also difficult to discuss issues of elephant density from a well informed basis as we don't actually have any real idea about the starting conditions before massive hunting for ivory - most of Africa's elephants were hunted out alongside the slave trade in the 1800s, so even in the high density areas of today we really don't know how this compares to densities of even only 200 years ago, nor do we know what the environment looked like particularly well back then.

Tarangire Elephants deep in the swamp keep the water open. Aug 2011

In Tarangire, however, the elephant population increase has occurred at the same time as the density of trees in the park has increased (for reasons we might ponder in a later post), and whilst they certainly leave their mark on the baobabs, there's little evidence of major vegetation changes as a consequence of incredibly heavy elephant browsing. So, for now I'm going to skip discussions of potentially negative impacts of elephants and will illustrate just one of their particularly beneficial aspects that was plainly on view in Tarangire - their role in keeping waterholes open.

Open water created ideal habitat for water birds: Silale Swamp, Tarangire

From the picnic site I counted more than 380 elephants enjoying the Silale swamps - they were there for food and water, of course. But in the process, they keep the edges of this swamp free from vegetation. Elsewhere along the river their rolling and wallowing  was keeping pools of water open much more than would be possible without them (each animal can walk off from a mud bath with up to 1m3 of mud attached, a volume that takes me a serious effort to move!) - indeed, elephants are capable of digging in sand rivers to access the water (and at times salt) well over 1m below ground. So they create waterholes and maintain open, vegetation free areas in swamps (a role often also played by hippos, but not in Tarangire). They're also great fun to watch splashing in the water, and as well as explaining how fussy they are about the cleanliness of the water they like, it's also worth talking about how their activities benifit all the other animals around that need water too. They are certainly worth of the name 'eco-system engineer' as well as that of a 'keystone species'.

When you're tired of elephants you'd better stop guiding... Tarangire May 2011

Why Are There So Many Wildebeest Compared to Other Animals in The Serengeti?

Herds crossing into Kenya.

Having been on safari for the last couple months, I’m unworthy of being called a co-author of this blog considering the wonderful posts that Colin has been writing. In my travels I have been to the Serengeti ecosystem four times in the last few months, three times in Serengeti and once in Maasai Mara and of course we have followed the spectacular herds of wildebeest.

When you’re driving through hundreds of thousands of wildebeest, or watching tens of thousands plunge into the Mara river because the grass is greener on the other side, its hard to wonder why there are so many of them. Why not zebra, topi, kongoni, impala, dikdik or one of the other antelopes?

So, I thought I would explore this topic and discovered this wonderful paper online, which you can download if you want to read a more scientific explanation. (Click here )

Part of Colin’s themes has been that there are things that shape or influence the environment, and that the environment then shapes the species in it. It’s a two-way interaction that steers what happens. E.g. When there is predation on plants they evolve defense mechanisms like thorns or chemicals.

So, what is it about the Serengeti that promotes these massive herds of wildebeest?

The simple answer:

Climate and soils.

The Serengeti ecosystem extends between two geologically significant features:

In the east, are the rift valley volcanoes that blew volcanic ash over the eastern part of the Serengeti, starting millions of years ago. These became the extremely fertile short grass plains between Maswa and Piyaya.

The short grass plains of Piyaya- the volcanoes in the distance.

In the west, Lake Victoria gives the north-western Serengeti a much higher rainfall (1200mm) than south-eastern Serengeti (500mm), especially when everywhere else is dry.  

Put these two factors together and you have high quality grazing every month of the year. In the wet months of the year (Feb, March, April), the soils in the short grass plains make the grass particularly excellent grazing with extra dose of calcium and phosphorous - perfect if you are a wildebeest trying to make milk for your calf. In the dry season- well, you migrate to where its raining and you find green grass which is much more nutritious than dry grass. (Wildebeest need 30% more energy, 5 times as much calcium, 3 times more phosphorous and 2 times as much sodium when they are lactating than pregnant and the short grass plains are perfect.)

A newborn wildebeest in Piyaya. It stands within 20 minutes
 to suckle. The milk is a high-cost to the mother but she
survives because of the minerals in the grass.

So, now we understand that the whole 25,000km2 Serengeti ecosystem always has nutritious grass (and drinking water) somewhere at all times of the year. The next question we have to investigate is- why wildebeest? Why not zebra, topi, kongoni, eland etc. etc?

The simple answer:

                   Wildebeest are special.

As you might know, wildebeest belong to a tribe of antelopes called the Alcelaphines. This means they are fairly closely related and if you want to know how close, well, they are about 4 million–year-old cousins. All of them are ruminants, which means they have a four-chambered stomach that they use to digest cellulose. Rumination is a very efficient way of extracting nutrients from plants but each species will have it’s own efficiency and Coke’s hartebeest are actually the most efficient of the three species. So why isn’t it Coke’s hartebeest?

Topi in the long less nutritious grass on the Lamai wedge

We can start by looking at the mouth structure of these animals and realizing that wildebeest actually have a mouth that is perfect for eating grass that is 3cm high, which is when the grass has the highest levels of protein.

The next thing they do is chose the parts of the grass that are also more nutritious- the leaves and fresh shoots. Coke’s hartebeest and topi eat more stems and leaf sheaths than wildebeest, zebra survive on almost only stems. But there’s a lot more grass stems than grass leaves so you would rather expect zebra populations to be in the millions but they aren’t- what is actually happening, is that zebras suffer very high losses of young, so predators keep zebra numbers down.

Now, you might ask, why aren’t wildebeest populations kept low by predators?

Answer: Synchronized reproduction and rumination.

80% of wildebeest calves are born in 3 weeks in February= 250,000 wildebeest calves= 500 per hour. It is an amazing sight. In scientific terms: extreme synchronous breeding outstrips predator’s ability to limit wildebeest recruitment.

Calves are most vulnerable when they are very young but they reach a certain age when they become equally vulnerable as the other wildebeest. There is a limit to how many calves predators can take per day, so by all having their babies at the same time, more calves have the chance to live past the age where they are vulnerable. Topi and hartebeest do not have as synchronized breeding as wildebeest.

Zebra on the extra nutritious short grass plains.

As we mentioned before, wildebeest are ruminants. They spend about 8hrs a day grazing so they have 16hrs a day to look for predators. Zebra on the other hand, spend 15hrs a day grazing so they only have 9hrs to look for predators. This is because they are hind-gut fermentators. This is obviously simplified.

Now, we’ve established the benefit of synchronized breeding but there are other advantages to being a wildebeest. Serengeti’s short grass plains are the best place for the females to get the nutrients they need to lactate, but they are also a great place to spot predators, which also helps to reduce the number of calves killed before they are out of the vulnerable stage.

Finally, calves are born precocial with a very strong imprinting instinct. The mother and calf learn to recognize each other immediately by smell and the calf stands as soon as it can and then stays as close to its mother as possible. The calf then also tends to run on the hidden side of the female so that predators have a harder time seeing them. The effect= reducing predation.

Wildebeest calve's coats change color to look like their
mothers at 2 months. Predation drops drastically.

There are other minor influences and for more details download the paper, but to try to sum it up in a sentence: The Serengeti’s unique climate and soils provide the perfect conditions to allow wildebeest to live in such large migratory herds because of wildebeest’s unique biology.

Burning Mwiba

Back from my trip now, I thought I'd make a few posts based on the things we've seen on safari. As I've been writing up my thoughts about one place already today, I thought I'd use that as the basis for this first post.

Buffalo are a major grazer in Mwiba, note the relatively short grass.

Mwiba Game Ranch is a new private game reserve within the Serengeti ecosystem. I've been before when I put together a bird list for the area. This time it was a trip to see the place in the dry season with a view to including it within my big Serengeti Fire experiment. For those who know Serengeti , Mwiba is squeezed into the corner between NCA and Maswa GR, right down in the south of the ecosystem. (NB, we usually define the Serengeti/Mara ecosystem as the area that encompasses the wildebeest movements - Mwiba includes some of the calving grounds, particularly important during drier years.) This puts it right in the driest region of the ecosystem, with around 400mm of rain per year and as you'd expect at that end of the gradient it's largely Acacia-Commiphora woodland, though there's a surprising number of nice Albizia in there too. It's also interesting because it's got a number of interesting mammals not found or not easy to see in the rest of Serengeti - we saw both Greater Kudu and Roan Antelope again this trip. Anyway, I was there to talk fire, but knowing that we're in a low rainfall part of the ecosystem is important, because water availablity is one of the big four drivers in the savannah (fire, grazing/browsing and nutrients being the other three, of which we'll visit two more shortly). Low rainfall means low productivity - the grass even on the highest nutrient soil never grows tall and thick like in other parts of the Serengeti, but what does grow tends to be nutrient rich annuals, so pretty good grazing, even if it isn't plentiful.

Zebra are the other big grazer - the grass here has already been grazed a bit

Rich grass means plenty of game, with the main dry-season grazers being large populations of buffalo and zebra thanks to the numerous perrenial springs around the ranch. Already, only half way through the dry season the grass in the areas around the waterholes and by the denser thickets is heavily grazed - by October it seems unlikely there'll be much left at all as the grazing impact spreads further from the water points. A lot of the area is pretty dense bush though, with some good thickets in places along the (seasonal) rivers.

Nearby areas with many cattle are already completely denuded, what will they do until the rains come?

 In recent years this area (as with most of Serengeti) has been subject to an early burning management regime - fires being set as soon as the vegetation starts to dry in June. Fires are important in the savannah for a number of reasons we've gone into elsewhere, but the two most important issues to bear in mind here are bush control - there were lots of seedlings in the grassy areas of Mwiba that have been prevented from forming to thick bush by regular burning - and grazing management, maintaining and encouraging new growth of nutritious grass. The early burning policy that has become the norm in Serengeti and most other Tanzanian protected areas ensures fires are controlled and generally are rather cooler than fires set at the end of the dry season when the fuel is drier.  But here in the drier areas with lower water availability it also means the fires burn regularly - you can be fairly sure there's enough fuel to burn early in the dry season, whilst later on all those animals will have eaten so much there might not be anything left to burn. That's good if you want regular firest to control bush encroachment, but isn't so good if you happen to be a buffalo wanting to eat during the dry season and all the nice grass gets burnt at the start. In fact, the ranch manager is of the impression that this year, when for the first time in a long time no fires have been set during the early season, there are lots more animals on the ranch than last year thanks to the availability of unburnt grass.

There's a sand river that forms a firebreak between the grass and the bush - frequent early burns have removed thicket vegetation from the upwind side of the river.

Mwiba's springs attract a lot of wildlife (and reflect sunset)

So, what to do? Maybe late burns will be possible at a lower frequency - give the land a few years to build up sufficient fuel reserves left over at the end of the dry season to allow a fire to take with regular enough frequency to control bush encroachment. Or maybe early burns are the only option to allow fires in these low rainfall areas to carry - though they probably don't need setting each year, balancing fire options against forage loss. The only real way of finding out will be through a big experiment, of course, which is exactly what I intend to do! Watch this space for the answers...

Why is Africa so full of thorns?

Assorted browsers and browse lines, Selous GR, June 2010

I'm actually away this week, but left this to post itself whilst I'm off and keep people interested! One of the places I'm headed to is Mwiba Ranch, south of Serengeti and, as you might know, mwiba is swahili for 'thorn'. No doubt I'll have some specifics to talk about when I'm back, but one of my 10 things to talk about topics is thorns. Why? Well, many visitors from the north live in places where tere aren't lots of thorny trees, so going to a walking safari and discovering that just about every bush and tree is covered in massive needles is a bit of a shock, even if those of us lucky enough to live here barely notice them (until they get infected, of couuse...).

Giraffe (and shorter!) browse line, Arusha NP, June 2010

So, why should Africa, or at least African savannahs in particular, be so thorny? The answer, of course, is fairly simple - why do tourists visit the savannah? To see the animals, and what do the animals eat? Well, rather a lot of them like to eat bushes, which is no fun at all if you happen to be a bush. In fact, in many places the grazing pressure is so heavy, very distinct browse-lines form and the plants take on structures as if someone was pruning them into interesting sculptures. So, defending yourself against Africa's abundan browsing animal population is a very good idea, and I've always thought thorns must be pretty nasty things to eat. But, you say, if the thorns are a defense against browsing, how come I still see impala and giraffe and all the others happily choping on thorn trees? Obviously the defence doesn't work? That's a good point, and allows me to introduce one of the many ways in which nothing in biology makes sense except in the light of evolution. Imagine living in an Africa before there were any thorns - all the bushes are undefended, and equally appealing to a browser. Now imagine there's a little mutation in one of the offspring of those bushes, that means it grows small spines - all the other bushes are still nice and undefended. If you were an impala, which bush would you eat? I suspect you'd go for any of them, except the one with the small spines. Which, of course, means that the spiny bush is going to do very well and will produce lots of babies, also with spines. In time, all the bushes will have little spines and the poor impala, if he wants any lunch, just has to tuck into that thorny bush. But, of course, one bush might have a mutation making it's spines a little harder, a little longer, a little nastier - and you can see immediately what's going to happen now - bigger spines evolve.

Greater Kudu, carefully nibbling around thorns, Kruger NP, May 2011.

Of course there might also be heritable variation in the impala browsing technique or mouths - maybe for thicker skin, or a narrower nose that can squeeze between the thorns. Giraffe, of course, have evolved a huge long sticky tongue so they practically lick the leave out from between the thorns, rather than have to go to close. But both species certainly are aware of the thorns, even though they have no choice but to eat the prickly trees, of course - imagine what would happen to the poor thorn tree that, though some mutation, had no thorns. Ooops, poor thing! So clearly the thorns do have an impact - what's interesting to me to look for in places with very heavy browsing is evidence that the thorns do work, even though the plants get eaten.

Heavily browsed yellow-barked Acacia, Arusha NP, June 2010

To see this, you need to look at the shapes of trees and bushes. Here's a nice Vachellia xanthophloea (see, I'm trying to get you used to the new names!) that's been very hevily pruned - a favourite with the girafe. So how's it ever going to make the leap from heavily pruned bush to fully fledged tree? The secret is to grow wide, before growing tall. While the bush is still short and relatively narrow it doesn't stand a chance - any giraffe will bend down, and chew off the top bits. But if it can get wide enough the giraffe don't like the thorns on their skin, and they'll just nibble to top bits within easy reach - leaving a tuft in the centre to break away. And once that is done, those short, wide branches at the base of the tree are no longer important and soon die back, in favour of the taller tree. Not clear? She here's a little diagram showing how to escape giraffe broswing pressure...

Bushes escaping browsing. Honest!

Nearly there! This bush behing the giraffe has started to escpe from the centre, Lake Manyara NP, April 2010

Not the most artistic ever, I admit - but I'm an ecologist, not an artist... (It's a giraffe bending down on the left, not a funny, long-necked kangaroo). So on the left you have a tiny seedling been chewed by a giraffe. This makes the bush grow flat, like in the photo above. Then, when the plant is wide enough, the throns around the edge stop the animals from being able to reach the centre and in the third part of the diagram a new shoot is escaping browsing, and finally in the last picture a mature tree has grown, with just a little whisp of the original short, fat bush that had to escape browsing remaining as evidence.

Made it! Just a few whisps of short left, West Kili, April 2010

So, that all sounds very nice, but why, you ask, do some plants get away without thorns, even in the savannah? Both Terminalia and Combretrum are fairly thorn-free, but a typical savannah plants. So how do they do it? Well, the difference here is the nutrients. Remember that Acacias (or whatever we're going to start caling the group now) are legumes and are absolutely full of nutrients, whereas these other two species live on the nutrient poor ridges and represent pretty low-quality browse. So, if you're a tasty plant growing in nutrient rich areas, you're going to be browsed much more than those specialising in nutrient poor environments, and here you want to invest more in defence.

And that, for now, is that. Hope you're having as much fun as I will be having in Mwiba!

Savannah Ecology

Most East African safaris spend a lot of time in the savannah biome. Forests and coastal areas are also popular, but the savannah is where the safari focussed and a basic understanding of the ecology of this biome will make a visit much more interesting. You can read more about savannahs and the savanna biome here on Wikipedia, of course, and there's a large team making sure that post is up to date. But I like to break into the subject rather differently so will do my own thing here, with future posts picking up the threads we identfy here.

Let's start by defining the savannah biome. Note first that I'm trying to be careful to talk of a biome here, not simply a habitat - the savannah biome is made up of many different habitats from grasslands and woodlands, to kopjes and swamps. Each of these habitats (and others) play an important role in the savannah biome and we'll visit them individually in future posts. In fact, the biome is defined as a grass dominated system - the grasslands are obviously part of the savannah ecosystem, but the woodlands and other habitats also have an understory dominated by grasses. The two photos above show typical grassland savannah from Kruger NP (South Africa) in the top (plus White Rhino) and an Acacia woodland (plue Oryx) with thick grassy understory in Tarangire NP (Tanzania). Other savannahs might looks less familiar to East African safari types - check the nice shot of a Guinea savanna in West Africa here, and the interesting savannah woodlands of Australia here. All savannahs, as all grass dominated ecosystems.

Right, definition out of the way it's time to introduce the Big Four of the savannah (sorry, moved on from the original three, but still can't make five!) - the four processes that shape the savannah biome globally. With an understanding of each of these, you can start to understand savannah ecology and begin to guess at what drives the patterns you see in this biome.

Firstly, there's climate and particularly water availability. Temperature and rainfall/precipitation combine to define the earth's major biomes - to get savannah, you need to be warm and fairly dry. Too wet and you'll end up with a forest of one type or another, to dry and you'll head rapidly towards desert. In fact, globally the savannah biome tends to dominate in tropical areas with rainfall above about 400mm, and below something between 1400 and 1650mm. Within this range, depending on how the other big processes combine, you'll probably get savannah habitats of one form or another - though how they look depends exactly where you are on the rainfall gradient. And, of course, understanding seasonal rainfall patterns are vital to understanding the seasonal movements of wildlife.

Three processes in one! Wildebeest near Naabi in Serengeti are gathering in the rainy season when surface water is drinkable to graze the nutrient rich grasslands of the short-grass plains. The impact of so many grazers is extreme!

Secondly, there's the impact of animals - grazing and browsing in particular. Savannahs are often full of animals - it's why people come to visit after all! And the impact of all those animals is not to be underestimated - take them away and the savannah can change dramatically from grasslands to woodlands and even (depending on the other processes) forest. Animal impacts can be seen all over the savannah and again, we'll visit these issues in subsequent posts.

Thirdly, there's fire. Savannahs burn and always have done so - today, many fires are deliberately set as part of the management, but people have probably been burning savannahs as long as there have been people around and before that lightning would have set fires naturally - probably about every 3-6 years we think. This is an ecosystem that has evolved with a constant presence of fire, the trees regrow, the grass regrows and (most) of the animals are perfectly capable of escaping fires by running or hiding in holes, etc. But fire frequency and intensity can certainly shape the savannah and it's a vitally important process to understand.

Finally, there are nutrients. Many savannahs are found on ancient and highly nutrient poor soils where every little patch of nutrients will be highly valued by something. Other areas are on recent volcanic and nutrient-rich soils, providing ideal grazing opportunities and different niches for vegetation types. Where nutrients are found (and how they get moved about) dramatically shapes the ecology of the savannah biome from the small scale of termite mounds to the larger scale of soil types, determining seasonal patterns of movement for animals and many of the habitat differences found from place to place.

And that's it! Future posts will develop all these issues further, but it's a great start in savannah ecology to have in mind the processes that shape the biome before we look too far at each one.