Wednesday, 28 September 2011

Termite mounds

Bologonja termite mounds, September 2011
Mostly dead termite mounds visible from the air, Northern Serengeti Sep 2011
One of the most impressive things I saw whilst up in northern Serenget a few weeks ago was the incredible density of termite mounds around the Bologonja River. They were extraordinarily dense, with a mound every few metres. More generally the whole of northern Serengeti seemed very well endowed with termite mounds, both old and new. In fact, flying over I was first puzzled by the obvious bare patches visible across many of the mlains up there, and only once I'd been on the ground for a day or so did I finally convince myself that they really were the remains of old termite mounds. So this set me wondering just what determines the densities of mound-building termites, and when I got back I was able to look things up.
Dead termite mound from the ground. I'm sure that's what it was though...

The most interesting paper I found is this one, which is about the Kruger in South Africa, but I'm sure the same processes are at work in Serengeti. Now, as a bit of background it're worth recapping that (a) not all termites are mound building (in fact, most of them aren't), (b) there's a tremendous number of them out there, having a massive impact on nutrient cycling - three genera (Odontotermes, Macrotermes and Trinervitermes) of mound building termites in Serengeti, and total biomass very similar to that of ungulates or mega-herbivores, (c) they really are fascinating, and deserve more attention on this blog! So, massive densities of termite mounds are quite interesting to understand. Termite mounds are shaped by three main, interacting processes: behaviour of the builders (different species build different mounds); soil properties (the soil used is from the below ground resource); and climate (both how much they have to battle with heat, but also the water table and degree to which they might wash away in heavy rain). So to explain the very high densities in northern Serengeti we both need to be aware that there's obviously a lot of food in this very high rainfall savannah area, but also the soil properties might just be perfect.

Interestingly, in both the South African work and this work in Serengeti termite mounds were found to be at their highest density in general on the tops of hills. The South African study explains this very nicely in terms of soil and water - over very long time-frames, rain washed clay out of the soils on the hilltops, and deposits it lower down the slopes. This mean that when it rains, the soils on the hill tops are fairly free draining - which is important to termites as they have underground chambers that mustn't flood. But it also means that where the clay is washed down to, there's a layer around the hills where the clay content suddenly increases, and when it rains the water flows through the upper soil layers, then hits the clay layer and flows horizontally until it reaches the surface as a seep line - we've all seen them (and probably driven into them!) - the point on the hill slopes where there's a line of tiny springs. However, whilst the hill-top is very well drained, it's also rather poor in clay content, which the termites need if they're going to build a good mound - sand doesn't work very well. So although the top of the hills are the best areas, in some place the termite densities increase towards the seep line - but then below the seep line the soil is too wet, so they suddenly disappear. And I guess that these very high hillside densities in Bologonja must be just the ideal location where there's plenty of clay, but just above the seep line - certainly there were seeps very close to some of these mounds a couple of weeks ago.
Bologonja termite mounds, happily functioning.

There's one more interesting thing that struck me when I was reading up on all this though - in South Africa, the highest densities of termite mounds above the seep lines indicate the areas of (broad-leaved) savannah. Below the seep lines lie grasslands. The authors of that paper found a relationship between local rainfall and the relative position of the seep line (more rain had washed the clay further down the slopes and closer to the rivers, which makes sense), and suggested that termite mounds could therefore be used to predict where woodlands might be if rainfally patterns change. Now, the really high densities on the Bologonja slopes are definitely on grasslands - but there's a good chance these were wooded not so long ago. Are they a relict of that time, and can we take their presence to even indicate areas where there were woods not so long ago? Or not? And  if not, is something different happening in Tanzania to South Africa? Also, there seemed to me to be an huge number of dead termite mounds, now visible only as a bare patch in other parts of the Northern Serengeti. Has something happened? Is this, too, an indication of major change in the savannah? Or does it just take so long for a termite mound to be washed away after the colony dies that we'd expect this many dead mounds? Hmmm.... All interesting stuff I think, any ideas?

Sunday, 25 September 2011

Greater Painted Snipe

Female (centre) and two males


Just a quick blog tonight, with a few photos from our latest trip to Tarangire, where there seemed to have been a fairly major influx of Greater Painted Snipe along the river and down at Silale. I managed a few photos for a group that were by the German Bridge and thought they'd be a good prompt to remind people about some of the interesting things about these birds.

Obviously, they're nomadic, moving around wetlands across Africa. In fact, the species has a huge range across Asia and (though some people split the species) into Australia, and in very few places are they ever resident, wandering around according to processes as yet undertermined. Which means, of course, they're always worth keeping an eye out for!

And, of course, they rather famously have reversed sexual dimorphism - the female is the brighter of the two, with the male being rather more cryptically coloured, a trait the share with their rather closely related Jacana cousins. (The similarities to common or African snipe are due to convergant evolution.) Thus in the top picture the nice brigtly coloured female in the centre is flanked by two males. And as you'd expect in such cases, parental care - both incubation and care of the chicks, is left to the males, whilst after completing laying of the clutch (usually around 4 eggs) the female goes on to find another male to mate and lay with. It's interesting to puzzle about why this response to polyandry may have evolved  - the only birds that have it all belong to Charadriformes, and it's most developed in a few tropical species. Worth puzzling over, as scientists haven't solved it yet either! Let me know if you have any ideas...

Thursday, 22 September 2011

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!

Tuesday, 20 September 2011

East African climate

One of my 10 things to talk about that I haven't touched on very much so far is the weather, which is a bit of a surprise as I'm British, and apparently talking about the weather is the top thing that identifies us Brits. Even more so, as studying the impacts of climate change on the savannah is what pays my salary at the moment. So, it's about time that was rectified a bit and our recent trips to northern Serengeti and Tarangire have given me the ideal inspiration to do so.
Southern Serengeti is dry whilst the north is already green: Mwiba August 2011

Why do I think that the weather (or more generally, the climate) is something worth talking about (apart from because I'm British, of course)? Well, firstly it makes a huge difference to the ecology of the system - we've already talked a lot about how water availability make a huge difference, and most (though not all) of that is due to differences in rainfall. So understanding the seasons will help make sense of what's happening throughout East Africa. But also because the climate in East Africa is completely different to the climate where most of your clients come from - visitors from the north are mostly used to four seasons of different length and severity depending on quite where they live. There's winter, from about Dec - Feb, when it's cold wet (or snowy), the days are short (seriously, before I moved out here we lived in Aberdeen and in the December the sun would rise at 8.45 and set at 3.30pm) and trees loose their leaves and wildlife finds it tough. Then there's spring, March - May, when the days start to lengthen, the temperature warms up, rainfall declines somewhat, the trees grow leaves and there's a flush of invertebrate life. Summer arrives June - August, with long days (Aberdeen sunrise at 4am, set after 10pm) and madly busy breeding season for wildlife, with lower rainfall (and very little in more southern areas). Then the days start shortening, the temperature falls and September - November is Autumn (or Fall if you're not in UK), the leaves start to fall, rains pick up and the migrants all head south.
Wildebeest enjoy the green in northern Serengeti, September 2011

Wildebeest endure the dry, Tarangire September 2011
Here, of course, things are very different - in nothern Tanzania and through Kenya, there are two rainy seasons and two dry seasons with very little variation in day length (none on the equator, of course). Southern Tanzania has a single rainy season, running from November to May (or there abouts). In general, our rain comes from a process called the Inter-Tropical Convergance Zone (ITCZ) which is global band of rain that sits more or less under the sun as the eath tilts on it's axis. Essentially, wherever the sea is directly below the sun, evaporation is highest and winds bring the clouds inland to fall as rain - but as the year passes the earth tilts and so different bits of sea are closest to the sun at different times - close to the equatior the sun passes directly overhead twive a year, bringing two rainy seasons, further away (but still within the tropics) the sun is directly overhead only once a year, bring a single rainy season. Hence when the north pole is tipped towards the sun the sahel region get's it's rainy season (June - August), and when the south pole is tipped towards the sun in December - Feb), Southern Africa get's it's rainy seaon. In between, there are two rainy seasons as we have here. (lots more details here on Wikipedia if you really want to understand it). But Lake Victoria confuses the issue greately by generating its own local climate, which means that even in August and September when the rest of the region is dry, northern and western Serengeti are green and wet. What's more, these parts of Tanzania are rather far from the coast where most of the rain comes from and they show the opposite patterns to the rest of the place - in years of good rains elsewhere the coastal winds are strong, bringing rain inland. But the same wind blows the Lake Victoria rainfall further west into Uganda, and this side of Lake Victoria is drier than normal. But when the winds fail and the rains don't come to the rest of the region, Serengeti enjoys all that Lake Victoria rainfall too.

Total rainfall (from here), but seasonality might be more important
There's more variation too, of course - mountains tend to catch rain and be wetter places, whilst the areas on the inland side of the mountains tend to be drier (we say they're in the 'rain shadow'). But in general for East African ecology, I think it's the length of the dry season, rather than the total amount of rainfall, that's most important. Lots of water all at once isn't particularly helpful, but a small amount of dry season rainfall can make a huge difference to the ecology of an area - think of the coastal forests growing in areas not that different to many savannah areas, but getting enough dry season rainfall to keep things green. Which, in turn, keeps the fires out, meaning forest wins over savannah and showing yet again how the big four processes interact with one another to shape the ecology of East Africa.

Now, not only is it good to be able to chat about these seasonal differences with clients, (especially at this time of year when they might well go from somewhere dry like Tarangire straight to somewhere in wet and green in northern Serenegti) but it's impact is massive. Why do you think the Wildebeest are up in Northern Serengeti and the Mara at this time of year? That dry season rainfall is critical for providing good grazing for such large numbers of animals. In fact, the Serengeti ecosystem is a great place to explore the impacts of climate on the savanna, because it has such different seasons across relatively small areas - dry season rainfall in the north is actually equivalent to total annual rainfall in the south, where the crater highlands catch most of the rain coming from the coast, but I think I'll leave a more detailed analysis of Serengeti's climates for another post, as this is long enough already.

Now might be a good time to talk weather! Grumeti GR, July 2009

Last thing though - it's worth thinking about how you might actually impart this sort of knowledge to clients in an interesting and relevant way. It might, of course, come up easily enough if their ask you directly (and of course, if their British, they're bound to...). But otherwise it might be something to chat about when you're watching a big thunderstorm brewing on the horizon, or when your are about to take clients from a dry place to a wet one (putting them on the plane to go from Tarangire to northern Serengeti at the moment would be a good time!) or vice versa. Or even, of course, as you watch the migration unfolding in Serengeti, just to explain some of the processes involved in why they're even bothering to make these dangerous trips!

Wednesday, 14 September 2011

Temminck's Courser

Temminck's Courser pair, Tarangire NP, Aug 2011. Male behind
One of the highlights of my recent trips was coming across this pair of Temminck's Coursers with two small chicks in Tarangire. They were nesting in an area burnt earlier this year, as it typical for this species - it's certainly one of the bird that are strongly associated with recently burnt areas. In fact, as you can see from the photo there's very little ground cover left in the area they were nesting, and there's unlikely to be anything until the rains come several weeks from now. The same species was also common last weekend on the nicely regrowing, but heavily grazed grass of northern Serengeti - but breeding doesn't seem to have started up there as the birds were all in small flocks. Breeding seasons for many birds are pretty confusing in the tropics, and it doesn't help that different parts of Tanzania have different climates either: much of northern and western Serengeti is now well into their rainy season, whilst southern Serengeti and most of the rest of the country are still parched and will remain so for several more weeks yet. The few breeding records there are in the Tanzania Bird Atlas database  from northern Serengeti are all August/September, so maybe they're just about to start up there which suggests that, as in southern Africa, the species has breeding season that are in some areas - like Tarangire - associated with the dry season, but in other areas are associated with the wet season. Curiously, there it seems to be the other way around, with wet-season breeding in low rainfall areas, and dry-season breeding in higher rainfall areas. Clearly there's a lot to learn about the ecology of these common birds - and if you come across nests with eggs or chicks please do let Liz and Neil know via the Tanzania Bird Atlas so we can start to put the full picture together: the more contributors the better (at the time of putting the map together for their website there were NO breeding records at all for Tarangire!).
Male Temminck's Courser feeding chick

As a little background information on the birds, it's good to remember that coursers are actually wading birds - order Charadriiformes, just like plovers, lapwings and sandpipers, but they below to a specialised family Glareolidae within that order, including the pratincoles and other coursers. Most coursers are nocturnal, and Temminck's is also active at night, but easily found during the day (unlike some other species). That's quite handy, as they mainly feed on termites, and most of them are mainly active at night too. (Just one more of the very many termite predators around the savannah!)
Temminck's Courser, Male

As a final little snippet, the name comes from Coenraad Jacob Temminck, a Dutch zoologist (and aristocrat) who has a pretty long list of birds named for him, including the Temminck's Stint that will be familiar to many birders from Europe and Asia, and which winters in very small numbers in East Africa. (He's also got a pangolin named for him, which is rather cool!) He lived in an interesting time for zoology - when he took up his final job as first director of the National Natural History Museum in Holland in 1820 zoology was essentially the preserve of a few rich people like himself, who occupied all the important positions. But within a few years other, ordinary people without long family histories were starting to get involved in zoology, and started challenging the authority of the established aristocrats. Temminck couldn't handle the competion and wanted things to remain as they were - poor people should be happy being poor and not have anything to say about science. And eventually, in about 1940 he stopped being involved in ornithology at all as he couldn't handle his views on taxonomy being challenged by people he didn't consider to be his peers - and consequently lost all the authority he once had. All in all, this 'democratization' of science can only be seen as a good thing - all the more reason not to leave science to the scientists and contribute your own observations and thoughts whereever you can!

Temminck's Courser Chick - 1 or 2 days old. Note 'egg tooth' on tip of bill to cut through the egg shell.

Sunday, 11 September 2011

Honeyguide brood parasitism

Male Greater Honeyguide, Tarangire NP, August 2011
Here are some particularly gruesome images of what happens inside a nest parasitised by Greater Honeyguides. In it you'll see how the chick, hatched about 3 days earlier than the hosts (in this case bee-eaters nesting in a termite mound) has already grown rather large and has a massive and nastily hooked beak with which it quickly and efficiently slaughters the hatching bee-eaters, one after the other. Very mean.

There accompanying text says more than enough for me to just point you there to look at – massively high parasitism rates, but most of those parasitised nests are immediately abandoned; female honeyguide tried to stab the host eggs when she lays hers, but the chick is perfectly capable of killing off the competition (even in pitch black, remember!), etc., etc. Nasty business, brood parasitism... Obviously fairly successful though, given the number of groups that practice it here in Africa – cuckoos, honeyguides, whydahs, etc. It must have evolved several times. Anyway, you can get all the details over here, and the original research is published here.

Saturday, 10 September 2011

Evaluating Safari Ecology blog...

This is a brief  cheating blog, as I'm away (again!) and I've just scheduled it for whilst I'm off. We started the blog back at the end of May, and I promised we'd try and keep things going if we had enough interested visitors. So, I've just been having a look over the stats for the first little while to see if we can justify carrying on, and I think we probably can. So, who are you all, and where do you come from?!

Firstly, about 1/4 of you come direct to the site, which is a good sign I guess - you're following fairly regularly and have a bookmark or similar. Another 2/3rds come from referring sites, some fairly regularly, others just once, never to return. And the rest come from search engines where the most popular search is for information about Vachellia tortillis. Some of the search engine arrivals even stop to read the page, which must be good news too!

And, probably most importantly, where are you all? First on the list (by a very narrow margin) is Tanzania - hurrah! (I don't think its all me either, as I've met a couple of guides in the bush on my recent trips who've seen the site and read it regularly, plus we have visitors from all over the country who regularly read lots of pages...) With around another 1/4 of the visitors, it's the US next and whilst that wasn't our intended audiance it's lovely to see you - though most US visitors seem to have found us by mistake and move quickly on! Then there are some regulars out in the UK and Sweden before we return to Africa, with nearly 10% of our visitors from South Africa - whilst our East African focus probably isn't exactly what you're looking for, at least many of the processes are (or should be) similar. We've also had visitors from a further six African countries, so I think we're meeting some local needs at least. We've also had steadily growing visitor numbers and followers, which is enough to keep me tapping for another few months at least.

Having made that decision, you'll see I've added a few extra things to the right-hand side. There's a list of sites I've used to identify birds, butterflies and dragonflies in the last few weeks that might well prove helpful to anyone looking up beasts in this part of the world - do send me more sites if you find anything else handy that others keen on nature in East Africa should know about. And I've added an opportunities section that so far has only one link to a bird training programme (not run by us, but I'm sure it will be good!) - as other opportunities for training come up in East Africa we'll try and publicise them here, so again, let me know if you hear of anything. Ethan will be doing things in November too, and I know of at least one other course on the go as well, so once I've got details they'll go up there.

Let us know if you want more from the blog in other areas too!

Thursday, 8 September 2011

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

Thursday, 1 September 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.