What is the savanna biome?

Savannas around the world are often open woodland like this Serengeti pic.

Remember a few months ago I reported on some talks I'd enjoyed at the ATBC/SCA-Africa meeting here in Arusha? Well, the research behind one the the talks I enjoyed most was published late last year and I think it really helps explain what makes the savannah biome (which is probably why it was published in Science - great job Carla!).  However, before going into too much detail about this research, I think it's important to make sure we know exactly what I mean by a biome in the first place...

When I talk of the savanna biome (or, indeed, the savannah biome, since I am English) I'm not referring to a habitat like grassland, or Acacia woodland; nor am I referring to a specific ecosystem like the Serengeti Ecosystem. Rather, I'm refering to the set of habitats that make up the savannah biome globally - the collection of grassland and woodland types that all have the same main processes operating on and in them, the savannah big four are, of course, nutrients, water availablity, herivory and fire. They might (and often do) contain remarkably different species, but the same processes are at work and, to remarkable degree, they show largely similar vegetation forms and structures. A biome may therefore be thought of as a set of habitats that share similar ecological processes wherever they occur across the world. By contrast, I tend to define an ecosystem as a single geographical area (like the Serengeti Ecosystem), within which nutrients are cycled with relatively little input or output from neighbouring ecosystems. Hope that's clear...

Fire is crucial to maintain savanna, particularly in wetter areas. Tarangire July 11

So, if we're going to understand what the savanna biome is, we need to look at the processes that are the dominant forces within it. If we can understand these, then we can predict where savannahs will be found throughout the world - and we can also understand what might happen to savannahs if we humans mess around with these processes through, for example, the effects of climate change. And this is exactly what Carla Staver and her co-authors have done in their paper (which I'm afraid is probably hiding behing the paywall here). They were particularly interested in discovering what determines the boundary between forest and favanna biomes (the boundary with desert is a much more obviously rainfall driven boundary), and decided that the most likely factors were rainfall and fire. Whilst that might well be true, there's plenty of evidence that herbivory is a major player too (at least here in Africa) and nutrients have been proposed as major players too (though there's also plenty of evidence to suggest this really isn't the main thing). However, they didn't look at these - herbivory they mention but exclude from their analysis with the very reasonable excuse that we just don't have a good idea how much herbivory there is around the world, but I think it's a bit of a shame they didn't have a go at nutrients too. Still, enough of what wasn't looked at - what did they find?

TAWIRI conference discussions continued: Ruaha River

Wetlands, like Silale Swamp in Tarangire, are vital for feeding rivers

Returning to the TAWIRI conference back in December that I posted a bit about already, the other talk that set me thinking about economics of conservation was a fascinating talk by Eric Wolanski about ecohydrology. About what, I hear you ask?! Ecohydrology, the study of the interactions between water (hydrology) and ecosystems. Now it occurred to me that we've not done a post specifically about wetlands yet, which is a bit of a major ommission, given their importance in savannah ecosystems. We'll have to rectify that in time, but for now we're going to plunge straight into some important stuff.

Flows in the Mara River have been disrupted by deforestation in the Mau Forest

As we all know, the life-blood of a savannah ecosystem is its permanent water source(s). As we've talked about in our Serengeti Story, the Mara River is the only important permanent water source in the Serengeti Mara ecosystem, and the animals move a long way to get there. Tarangire has the Tarangire River. Ruaha has the Great Ruaha River, etc. What makes these rivers permanent and other rivers in the savannah only seasonal is that they're fed by sources that capture the rain in the wet season and slowly release it during the dry season, whilst sand rivers tend to just be rain fed. The Mara is fed by the Mau Forest in Kenya, Tarangire River by Silale Swamp, and the Great Ruaha by a series of wetlands, including those of the Usangu Flats. Meddle with these 'sponges' and you can get in all sorts of trouble with your permanent water sources.

Birds and other wildlife also love wetlands like Silale! Sep 2011

Eric told a fairly simple story, but a fascinating one none the less (especially when you start doing the sums that I've been looking at). If you have vegetation covering a waterbody, he said, the water loss through evaporation and transpiration (plant breathing) is about 50% of the evaporation you have from open water. Somore water flows from vegetated wetlands into rivers than from ones taht have lost their vegetation through excessive grazing. Which is exactly what had happened during the 1990s and 2000s on the Usangu flats, above Ruaha. Much of the water that flows from Usangu at the end of the wet season is the water that subsequently fills Mtera Dam, so keeping the water flowing - as well as providing a vital resource for the wildlife of Ruaha National Park - is pretty important for electricity generation in Tanzania! Uncontrolled (but illegal) immigration had allowed hundreds of people with an estimated 300,000 cattle to occupy the Usangu Game Reserve (as it was), and the cattle ate all the vegetation over the wetland. As a consequence the evaporation rates increased and less water flowed from Usangu into the Great Ruaha. In 2006 the government decided to evict these people and incorporate the Usangu Game Reserve into Ruaha National Park (in so doing creating the largest National Park in Africa), hoping to restore water flows, though among serious concerns about human rights. And Eric was able to watch the consequences in the flow rates through the Usangu flats and into the Great Ruaha river. Amazingly, this operation alone has resulted in the Ruaha river flowing for an extra month. Now that result is a great success for conservation, but it's not the end of the story by any means and I did some quick back of an envelope calculations of my own that are pretty staggering, but before we go there let me post a few caveats - firstly, I'm not a hydrologist and I'm just reading a few things quickly, I'm not guaranteeing these figures in any way. Also, as I've not always found the exact figures I've tended to err on the side of caution - for example, I've found statements that show dry season flow is unimportant to Mtera water levels (as you'd imagine), but that the dam mostly fills when the Usangu wetlands are flowing full rate at the end of the wet season, but I've not got the relative figures for this so I've assumed that the flow operates continuously - which should underestimate the importance of Usangu. Still, here are some interesting numbers...

Early in 2011 the IMF downgraded it's forecasts for growth in the Tanzanian economy from 7.5% to 6%, due to the costs imposed by TANESCO power cuts (up to 16hrs per day in much of the country!). Tanzanian GDP in 2010 was abour $23 Billion, so the cost to Tanzania of the powercuts, according to the IMF, is $345M. [Interestingly, that GDP is a tiny bit more than George Soros and family have tucked away, is similarly a tiny bit more than the value of the treasure recently discovered in the vaults of an Indian temple and is not even half the annual turnover of GlaxoSmithKlein!] Most of these power cuts were caused by low water levels in dams preventing power generation - Mtera dam (fed by Usangu through the Great Ruaha) feeds two power stations, Mtera and Kidatu, producing between then 284MW of power, which is about 40% of Tanzania's total capacity of 769MW (Thanks TANESCO for these figures). I've not found how many months the stations were going for, but I did discover that for the last few years the Ruaha has flowed for only 9 months, so let's assume it's similar. Now if we assume that 40% of the cost of the blackouts is caused by Mtera not producing (actually, I'm sure it's much higher since it's mainly the Hydro part of the production that's failing, but that's the proportion of overall capacity sourced by Mtera and will give a conservative estimate), those thee months of non-flow each cost the Tanzanian economy $46M. So the government's action to remove cattle, by providing an extra month's flow from Usangu, might well ahve saved about $46M per year. Not a bad investment, I think, even if they had paid the going rate of $150 per cattle the $45M required would have been paid off in one year and as it was many of these cattle moved elsewhere where they caused less damage to sensitive wetland habitats.

Now the final question you'll be asking, I guess, is what about the remaining 3 months of no-flow? How can we get that back? Well Eric and colleagues estimated that if the rice farms that are the other major water user returned only 25% of their water, there's be no problem at all. And I've just done a quick check and found that the estimated cost of completely closing the rice farming industry in this area would cost the national economy 'only' $15.9M per year. If that's what it takes to keep Mtera flowing, it doesn't seem a particularly hard decision for me, and think of the wildlife benefits too!

Anyway, I hope those figures are of some interest - it just goes to show that conservation really can be a 'win, win' option, even when hard decisions need to be made. Let's just hope it doesn't take too long before someone sees sense here - good luck to those people and organisations trying to build awareness of these issues!

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!

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!

Burning Mwiba

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

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

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

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

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

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

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

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

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

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

Waterholes and dams

A nice waterhole never did any harm, did it? Sasakwa Dam, Grumeti GR July 2009

I often hear questions about the impacts of artificial waterholes and I've spent some of today writing about this issue in a paper we're working on about management practices in the savannah enviornment, so I thought that whilst it was fresh in my mind I'd share those thoughts here. The topic usually comes up when we talk about South Africa, and the far more intensively managed parks that exist down there, but it's also often an issue that crops up when talking to managers of lodges and camps who are, understandibly, keen to have a nice waterhole infront of their property.

Grazers love artificial waterholes - the migration reaches Seronera, Nov 2010

Certainly, waterholes and perrenial rivers are a fantastic place to find wildlife, especially during the dry season. You only have to sit by a water hole for a few minutes and the animals (and sometimes clouds of birds!) start trickling in for a drink. And, of course, the predators know this too, so theyll just sit and wait until lunch laks up to them! So what's the problem? Everything loves a waterhole, it's great for tourists to see animals, let's dig them everywhere! And that's what has happened in some places in southern Africa - both in National Parks like Kruger, and even more in private grame reserves people have dug waterholes and pumped water. In some of the private reserves waterholes were dug every 2km, so even the most sedentary of animals could always have access to water. And, as expected, this made a huge difference to the distribution and populations of animals - particularly Zebra and other grazers, animals that are very strongly dependent on water and don't like to walk far to find it. So, everyone's happy, right?

Lions wait for thirst animals by a waterhole, Selous GR, June 2010

Roan Antelope, Mwiba Ranch, Jan 2011. Declines have been related to water provision

Wrong. Zebra are also pretty much the favourite food of lions, so if you increase the zebra population and let them go to areas they couldn't use before, you also increase the lion population and let them spread further afield in their hunt for zebra. That might sound good too - but it's not if you happen to be an animal that deals with predation risk by hiding away in areas predators don't go - like Roan and other large, slow antelope. In fact, mainly in recognition of the damage that has been caused by making water available throughout the landscape there has been a recent policy within Kruger and some other areas of closing (some of) the artificial waterholes to allow such sensitive species spaces where they can survive.

Elephant near dam, Kruger NP, June 2011 - and look at the invasives on the dam!

But that's not all artificial waterholes do, they also attract elephants. Now, there's a long running discussion between East and South Africans about elephant impacts (dare we say damage?) in protected areas that I'm not going to go into here. But impacts they certainly do have, especially in areas near waterholes as they're another species that doesn't like to travel far from water if they don't have to. So spreading waterholes across the landscape means elephants can easily get everywhere, especially during the crucial dry season when they are feeding on bushes (grasses during the wet season, remember) and have bigger impacts on the habitat. So providing waterholes for elephants everywhere means the vegetation never gets a chance to recover from browsing impact, and can easily start to cause problems throughout the landscape. So, another thing to be aware of.

Zebra drinking, Manyara Ranch, Sep 2010. Note how heavily grazed the land is around the dam.

Now, the last thing you might want to argue with me about, of course, is what happens in a dought? If you don't provide water for the animals, surely they die? And that's true, of course - but it turns out that if you do maintain artificial waterholes everywhere, the animals still die - not of thirst, but of starvation. During a drought there far less food around, especially in areas where everywhere is usually accessible to most animals because there are waterholes everwhere. If there are fewer waterholes and things get tough, animals (even those lazy zebra!) will make the decision to walk further to feed in areas they don't normally go and there'll be enough food to go around. So in a serious droung in the 1980s in southern Africa, one private game reserve with lots of waterholes where studies were made lost nearly 90% of grazing animals, whilst the next door Kruger, with fewer waterholes, only lost 35% of animals. So, no, I don't think waterholes are essentially a good idea, even during drought.

The unifying process at work here, with the lions, the elephants and the drought stress, is a loss of heterogeneity - variability in the landscape. With waterholes everywhere, lion predation risk was the same everywhere, elephant impact is the same everywhere, grazers are the same everywhere. Without artificial waterholes, there are refugia from lions, elephants and heavy grazing that can be used by other species and at other times when needs are greatest. Heterogeneity, as we'll see time and time again, is a key issue in maintaining functioning ecosystems.

Birds like waterholes too! This is a Black-winged Stilt near Ndutu, NCA. Jan 2011

 So, do I always think artificial waterholes are bad? In an ideal world, I'd say yes. But we don't live in an ideal world, so to be realistic I have to say usually. Some protected areas - such as Hwange in Zimbabwe - would once have only been wet-season dispersal areas, with no permanent waterholes. But here the seasonal movements have been blocked by people, so waterholes are needed to enable any of the ecosystem to survive. That said, I don't think they should be spread all over the park - rather, groups of pools in one or two areas would maintain the heterogeneity that is needed. And if climate change and human water abstraction continues to increase aroudn parks, we'll start loosing perrenial rivers, in which I'd I'd advocate maintaining some limited pools of permenant water along these routes artificially if necessary. Finally, of course, if there's already a nearby source of perrenial water, another waterhole nearby will probably have little additional effect (good news for lodge owners who are near rivers, but didn't negotiate the view!). But interferring with savannahs is not easy, and should never be attempted without considering all the impacts you might have. Let's keep waterholes natural!

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.