The role of termites in the savanna biome

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

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

Nitrogen in the savanna biome

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

Middens are amazingly rich patches!

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

Ecology of broad-leaved woodlands in the savannah

Terminalia Woodland, Sasakwa Hill, Grumeti Reserves, July 2009

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

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

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

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

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

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

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

The Serengeti Story 2: the great migration

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

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

Average Serengeti Rainfall, adapted from here

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

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

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

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

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

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

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

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

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

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

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

TAWIRI Conference discussions

I've spent most of this week at the Tanzania Wildlife Research Institute (TAWIRI) conference here in Arusha. This is an event that happens every two years and involves a very high proportion of researchers active across Tanzania, so it's always a good place to hear about interesting things going on in these areas. I thought I'd give a few of my highlights today. The two talks that most exicted me were from two different aspects of ecology - one by Dr. Grant Hopcraft on the Serengeti and how climate change might impact wildlife there, the other also related to Serengeti, but this time by Dr. Dennis Rentsch from Frankfurt Zoological Society on the economics of the bushmeat industry. I know both of these folk fairly well, so was able to press them for lots of extra information about both talks, and what I'm going to descibe here represents both their presentations and some of the other stuff we talked about - I hope they don't mind me putting this information out before it's all polished and published!

Wildebeest and zebra migrating through Grumeti Reserves, Feb 2010

Grant knows rather a lot about Serengeti and, in particular, the herbivores of the system. His work has focussed on how nutrition impacts herbivores and his talk fitted well into the overall theme of the conference on climate change, by asking how climate change will affect the nutrient content of the grasses and how this might impact the animals that feed on them. You might think it's crazy to suggest that climate change impacts grass quality (i.e. nutrient content), but actually it can have some pretty profound impacts indeed. Grass growing in high rainfall areas gets very tall very quickly, but also tends to be poor in nutrients - it might be that the grass can only collect the same amount of nutrient from it's roots, but in wet years it grows faster, so there's less nutrient per leaf than in dry years when the plants can't grow as much and pack all the nurients into a smaller volume. So more rain means lower quality grass, but more of it, less rain would mean less, but higher quality grass. In fact, lots of people showed plots of rainfall in Serengeti and demonstrated that the area is getting wetter (though I also suspect there might be shifts in the dry season length which could be even more significant, but no-one really talked about that), so we should be seeing more, lower quality grass. What is the consequence of this? Well, according to Grant, perhaps it means different things for different species, since all the herbivores prefer slightly different combinations of nutrient quality and grass quantity. In particular, hind-gut fermenters like zebra are happy with lots of relatively low quality food, whilst wildebeest are typically selective ruminants and need higher quality grass. Now, Wildebeest in Serengeti are food limited, not predation limited or anything else, so a decline in food quality might be bad for them - but they are, of course, interested in quantity too, particularly during the dry season when any rain is going to provide grazing which is clearly better than no rain at all. So a wetter Serengeti, if it impacts the dry season too, is probably going to mean more food at this crucial dry-season food shortage period, and we can expect that even in a wetter dry season the rain will still be scarce, so the grass will be relatively nutritious. So on the one hand poorer-quality forage during the wet season might be bad news, but more grass in the dry season is certainly going to be good news - which effect wins out isn't yet clear. My money will be on the dry season effects, but we'll wait to see! On the other hand, it seems pretty unambiguously clear that a wetter Serengeti will be good news for zebra, provided again that the dry season remains at least a bit wet too. So more zebra will always be good - though how that will affect everything else is also tricky to forsee. Does more zebra mean better facilitation for the wildebeest? Or might there be more competition? Who knows, as usual, more research needed (and if you want to fund Grant on his next project, do let him know - he's searching for money right now!).

The migration reaches Seronera, Nov 2010. Don't get eaten!

Spot the snare: many animals are poached in Serengeti. Moru Jan 2011

Meanwhile Dennis has been working on bushmeat trade on the western side of Serengeti for many years now. His approach to studying what is, after all, an illegal activity has been to deal not with the hard end in the park of finding and apprehending poachers and trying to get them to tell him how many animals they hunt (they're very unlikely to give an honest answer in such circumstances!). Instead he's focussed mainly on trying to work out how much bushmeat is being consumed in the villages around the Serengeti by asking them about the various protein sources they eat during the week. Although there might still be some resistance to tell the absolute truth in this context, it's likely his numbers are underestimates of the full impact of the harvest (especially as it doesn't include any of the meat that gets exported from the region commercially). Underestimates they might be, but the numbers are still staggering. In the villages surveyed, the average number of meals of wildebeest eaten per family per week was 2.4. Obviously that goes up during the period when the wildebeest are migrating through the particular village, and down when they're far away, but 2.4 meals per week is the average for the villages immediately around Serengeti NP. And knowing the number of households in each village, plus the number of villages Dennis estimates that somewhere between 90,000 and 100,000 wildebeest are harvested (illegally) from Serengeti each year. To put that into context, it's equivelant to a harvest greater than the entire wildebeest population of South Africa each year!

At between 500 and 1500TSh / kg (depending on seasonal availablity), and assuming a conservative 100kg of meat per animal that gives a a total market value of $2.5 - $8.5 Million per year. Compare that to TANAPA income from Serengeti gate fees 10 years ago (the latest I can find online) at about $5.23 Million, and we're talking the same size economy. (Bear in mind that these TANAPA fees are used throughout the national park system to subsidise less well visited parks, so Serengeti NP actually has an opperating budget of only around $2Million per year.) That's a pretty remarkable figure on it's own, but Dennis went on to talk about how consumption is related to price of other meat in the area - if the price of beef goes up, more wildebeest is eaten. Which suggests that it might be possible to reduce the amount of wildebeest eaten, if you bring the price of beef down. Now unfortunately I wasn't quick enough to get all the figures off Dennis's slide to do the calculation here, but I think I'm right in saying that if you want, say to halve the wildebeest harvest, his figures suggest you need to bring the price of beef down by about 3 times as much - so 50% of 50% of 50%, which is an 87.5% reduction in price. That's probably going to be tricky to achieve, unless you fill Serengeti with cattle, which is hardly going to help! So you're rather stuck there. Instead, the only effective solution is to make the wildebeest more expensive - and Dennis suggested you can do that either by giving poachers alternative employment and dry up the supply of meat, or by even more strictly enforcing the regulations within the park. But bear in mind that this is a sustainable harvest - there's no impact of this level of poaching on the wildebeest population overall. The problem is the bycatch - people want to trap common wildebeest, but instead their snares catch resident game sometimes and have had a missive impact. So instead of strictly enforcing current regulations, perhaps TANAPA should be looking at ways to encourage sustainable use and minimse the negative off-take. Perhaps making a few million $$ in the process. What do you think? Should we go this way? Or how should we feed these people?

Nutrients in the savannah biome

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

Termite mound, Mwiba Ranch, August 2011

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

Impala Midden, Manyara Ranch, Nov 2010

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

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

Elephant diggings near Gibbs Farm, Dec 2010

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

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

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

Dung Beetles

Continuing the theme of small but rather important pieces of the savannah jigsaw puzzle, I thought I'd write a little about dung beetles. As usual, I'll try and follow my three questions for interpreting wildlife sightings - what is it? What's it doing? And what's it's role in the ecology of the environment?

Scaning for the route?!

So, let me first confess that I've never even identified a single dung beetle to species level. Unless you're a real specialist, I think you can forget it. Our dung beetles are are insects of the order Coleoptera and, as a rather prominent biologist (J.B.S. Haldane) once (may have) said when asked what we can learn about the Creator through studying His works: "He must have an inordinate fondness for beetles". In fact, the latest estimates  - published here in August - of terrestrial biodiversity are around 8.7 million species, of which about 7.8 million are animals (we've only described about 1.7, though). It's estimated that about 80% of all species are insects, and of these about 40% are Coleopteran beetles, which would suggest there are about 2.5 million species of beetles out there. In one tiny corner of the Serengeti plains alone, over 100 species were recorded in a relatively small study. As only a tiny proportion of these are already described I conclude (a) if you want to discover a species new to science, look at beetles, and (b) there are far too many beetles to spend time trying to identify them specifically. Still, most of our dung beetles belong to the Scarabaeidae family, and most people will have heard of Scarabs, especially if they know anything about the ancient Egyptians, who considered them sacred (holy), since it's clear that the world must be kept in motion by a giant dung beetle rolling it about.

So, that's what they are, identified as far as I feel the need. But what are they doing? Well, when we usually notice them they're rolling balls of dung along the track. Why? Because they eat it. Yumm. Most of the ruminants feeding on grass only extact about 50% of the nutrients from their forage, and obviously much less for hind-gut fermenters like elephants, so there's still significant resource left in the dung of these animals. And dung beetles love it for everything - they eat it themselves (some species eat it on site, some under the dung and some roll it off to snack on elsewhere), they roll it off and eat it as part of their courtship procedure (if you find two on the same ball, they might off on honeymoon with a nice snack to keep up their energy...), and they take it away to provision their young. For breeding, often the males dig large holes where they'll store several dung balls, a female laying a single egg on the top of each one (and in some cases coating the balls with a layer of clay that hardens around the dung ball. These broods are, in turn, a favourite food of honey badgers and some mongooses.

Dung beetle nest predated by honeybadger or mongoose, Lake Manyara, July 2010

The really exciting thing about dung beetles (I promise!) is, however, the impact they have on the ecology. First, let's appreciate the task they perform in tidying up dung. Your average zebra produces about 4.1 kg of dung per day, and a Grant's Gazelle about 0.75kg (never let it be said I'm not full of useful facts!), so let's assume about 200,000 zebra and 1.4M wildebeest for Serengeti, and guess that wildebeest, being bigger than Grant's Gazelles do about 2kg per day, and we're looking at a massive 1.3 Million tons of dung per year in Serengeti - nearly 3620 tons per day! It's just as well there's an army of dung beetles out there, just waiting for their meals (as the Australians learnt, when they started cattle ranches and the Australian dung beetles, used to a fine quality product from kangaroos, turned their noses up at the offerings from cows, with a massive fly problem the result - and they had to import African dung beetles to clean the mess up!). And, of course, we all know that dung is a pretty good fertiliser - whilst most of what they bury they also eat, the sheer numbers ensure that huge amounts of nutrient cycling are carried out by these beasts. What's more, they a bt picky about where they dig - it must be moist enough for them to dig, so that's why they're out there rolling balls long distances, looking for somewhere suitable. And as not everywhere is suitable, they tend to concentrate the dung in certain areas, creating a nutrient hotspot. Which, of course, attracts more wildebeest, to produce yet more dung, which is immediately returned locally - a major source of heterogeneity in the Serengeti plains. So, absolutely critical for nutrient cycling in the savannah - in fact, the huge volumes of dung involved alows you to realise that, thanks to dung beetles removal and burying, almost all the soil you walk on in Serengeti must, at one stage not that long ago, have been a dung ball. Lovely thought...

Rift valley geology and soils

Many visitors to East Africa are looking forward to seeing the rift valley, but often aren't quite sure what they're seeing when they get there, especially here in Tanzania, where it's a remarkably complicated feature and doesn't show the typical east and west escarpments of a rift valley - only some obvious western edges. This means, of course, that here in Tanzania it's impossible to point out exactly when you enter the edge of the rift valley, which is a bit confusing and disappointing to some travelling from Arusha to, say, Manyara for the first time - you're definitely in the rift valley at Manyara, and the western escarpment is obvious - but when did you actually arrive there?! Now, I'm not a geologist and am not going to go into huge detail about the rift's formation here, just the general idea should do. But I am an ecologist, and the presence of the rift valley has huge consequences for the ecology of East Africa too, so I might go into more detail about that!

Rift valley scarpment above Lake Manyara

Firstly, what is the rift valley? Well, some very readable details are available here, if you want the full thing. In summary, it's a great series of cracks in the earth's crust that can be traced right from eastern Turkey, through the Middle East and down trhough Ethiopia, Kenya, Tanzania, Uganda, Rwanda, Burundi, etc., as far south as Mozambique. Here in East Africa there are two parts to it - the western, or Albertine Rift, than runs through Uganda, Rwanda and Burundi back to Tanzania, and the eastern rift, running through western Kenya and the middle of Tanzania. These cracks are around 20-30 million years old (oldest in Ethiopia) and are believed to form because there's a large plume or two of magma (molten rock) beneath the earth's crust that pushes up on the crust, creating large bulges (the Ethipian Highlands, and the Kenyan/Tanzanian higlands are both pushed up from below) and, in places on top of the bulge, cracking the crust and leaving a rift valley. Some times, of course, the magma has burst through as volcanos, with erruptions still fairly regular in various locations. The older volcanos associated with this (such as Monduli, and the Crater Highlands) date from 20-30 million years ago, though the major faults (big escarpments) are only about 2 million years old. It's a divergant fault, splitting the African contient in two and gradully moving even now - eventually it seems likely that this fault will split Africa in two - but I don't think we'll be seeing that for the next few years at least...

Oldonyo Lengai is spectacular from the air!

So, that's (very briefly) what it is. The important things from an ecological persepective are it's incredibly recent geological age - compared to most of Africa, these mountains and plains are new - even the oldest are only 20-30 million years old, and volcanic activity has probably been pretty much constant since then. Now, this age is important, because (as a first approximation), material recently thrown out of the earth is full of unusual chemicals that, over several million years, will be washed away. Consequently, soils derived from new rocks are usually nutrient rich, whilst older soils derived from older rocks have been washed clean (leached) and are generally rather nutrient poor. And as we know, nutrient availability is one of the big four processes driving savannah ecology. I couldn't find any very fine-scale pictures of this, but I've found a global map of nutrients available to plants in soils here that I've included below.

The important thing to note is that in general, soils in Africa are incredibly nutrient poor (yellow and orange on the map), but that there's a clear green bit associated with the rift valley. That's the consequence of volcanic activity in this part of the world. This large scale doesn't show too much, but focussing in on the underling rocks will give us some idea about nutrients too - so here's a map I've edited from here that shows the geology of East Africa.

In this map you can see the fault lines creating the escarpments nicely, but you can also see how the only areas with relatively recent rocks are those associated with the rift, from northern Tanzania up through Kenya, except for the Tana river area, where the recent rocks have other origins. Note especially that Serengeti/Mara only has recent bedrocks in the southern, short grass plains, and Tarangire only just gets into that new complex right in the northern tip. In both places these nutrient rich soils explain in part the migrations we see, with calving always happening on nutrient rich grasslands. It's also obvious why wildlife densities in the rift valley are so much higher than elsewhere in Tanzania - and indeed Africa. The scarcity of nutrient rich soils, and hence food availability, probably limits animal populations in many of these other areas, right down to South Africa. No doubt we'll come to this in more detail in the future, but for now, what's probably enough - as well as being a spectacular geological feature, the nutrient rich grasslands associated with the volcanic activity help explain both the number of animals up here, and their seasonal migrations. Geology matters!

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.

Acacia woodland

Next on the list of major savanna habitats must be the Acacia woodlands. The sun setting behing a lat-topped acacia provides one of the iconic images of an African savanna, and many of the most interesting game drives involve meandering through Acacia woodland. In fact, there are a very large number of Acacia species in Africa - something over 150 - and they're al a little different (let's ignore the current taxonomic discussions about Australia taking the Acacia genus for it's species leaving the African's with none...) . Ecologically, Acacia species play a vital role in the savanna ecosystem, but before we think too much about that, let's start by thinking about why Acacia woodlands are found where they are.

They make great backdrops, even for Flamongos! Lake Magadi, Serengeti NP Jan 2011

First, of course, we need to identify where the Acacia woodlands are and it's obvious if you're looking for it: Acacia woodlands are typically found on the lower slopes of hills and on the flatter land at the bottom of valleys. On the ridges there are generally broad-leafed woodlands, such as Terminalia and Combretrum woodlands, or in areas with a single rainy season you might find Miombo woodlands on the ridges. Boardering rivers, of course, you often find true riverine forest, another habitat again (although one that often features Acacia species). But between the riverine forest and the broad-leaved woodlands is the area where grasslands and Acacia woodlands are commonest. And the reasons for this position are probably to be found on those four main drivers of savanna ecology: fire, nutrients, water availability and grazing pressure. On the ridges, nutrients are very scarce thanks to millenia of washing by heavy tropial rains. Lower down there are more nutrients, but in consequence the grazing/browsing pressure is going to be higher - to survive in these areas you need to be very heavily defended - like the big throns on many Acacia species. The soil moisture content is also important: like other members of the Fabaceae (peas and beans being obvious examples, of course), Acacia species have a symbiotic relationship with nitrogen fixing bacteria (called Rhizobia) that live in nodules in their roots, and these bacteria are somewhat fussy about where they live. (In fact, they can life in soil away from the plants, but they are unable to fix nitrogen in isolation.)

This relationship with Rhizobia is responsible for what is probably the most important role Acacia woodlands have in the savanna: they're incredibly important nutrient pumps. Thanks to their nitrogen fixing bacteria, Acacia species have a pretty much unlimited supply of organic nitrogen, vital to producing proteins and growth. In the generally rather nutrient poor soils of Africa, this has a massive impact. All the browsers love a snack on the nutrient rich Acacia leaves, despite their thorns and high tannin content. But most importantly, at the end of the wet season Acacia trees drop their leaves like most other savanna plants - but because they have such a plentiful supply of nitrogen, they don't bother withdrawing all the nutrient before they do so. This is immediately obvious if you drive the savanna at this time of year: Acacia trees remain greenish right until the leaves fall, other broad-leaf species withdraw as much nutrient as possible, resulting in yellow or orange leaves, before they fall. And the consequence of this is that Acacia leaf litter is much richer than the little of other species and fertilises the soil under the trees. So effective is it, that in some places Acacia litter is used as a major fertiliser for poor soils.

Again, the impact of the richer soil is immediately obvious if you go and look under an Acacia - there's a whole lot more diversity in the herb layer than under a neighbouring non-Acacia species.

Lots of herb diversity in the understory of an Acacia woodland thanks to the fertilisation effects. Near Mbalageti River, Serengeti, Jan 2011.

Not much under a Balanites but grass (oh, and a few animals) - Grumeti GR, Sep 2010

This diversity, combined with the fertilisation effect making everything rather more nutritious than elsewhere in the savanna explains the reason you spend so much of your time on game drives in Acacia woodlands: everything loves them! Of course, the problem with this is that it attracts lions and other predators who hunt much more efficiently from the cover of woodlands, so if you're a browsing animal you've got to choose: do you got for the nutrient rich woodlands and under-story of the Acacia belt but face the higher risk of being eaten yourself, or do you avoid the richer habitats and forage in safer places where you can keep an eye on predators much more easily? If you're sensible, of course, you'll probably balance the two options up and make decisions based on exactly how much you need those nutrients at any one time – early in the dry season there's plenty of forage in the open grasslands and you've had plenty of nutrients recently during the wet season anyway, so you might spend more time in the grasslands. Later in the dry season those open areas may have been grazed to nothing and you're more in need of nutrients as you might well be preparing for pregnancy, so you might decide to take the risk and forage in the woodlands: at different seasons, different strategies make most sense and within the savanna ecosystem such movements are very sensible. Of course, you might also decide to just live on the woodland margin – nipping into the woods when you're fairly sure there are no predators around, but in easy reach of the open areas if you're more worried. This, of course, means that those ecotones – the transition from one habitat (Acacia woodland) to another (open grasslands) – are going to be fantastic places to explore on your game drive.

And that, for now, is probably enough about Acacia woodlands. There's lots more to say about both Acacias and the woodlands, but as an introduction to the habitat it's not a bad place to start.

Acacias do make for nice sunsets! Here some herons roost on a bit A. tortilis near Lake Ndutu, Serengeti, Jan 2010

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.