How do Kopjes form?

It's a question I regularly get asked by guides and also one that seems to bring a lot of google-searching visitors to the site, but I've not actually posted much of an answer yet although we have covered it briefly here, so here goes...

Cross-section through a kopje in the process of formation
from smooth, uninterrupted landscape at the top to
typical kopje at bottom, following millions upon
millions of years of erosion.

We start by remembering that Africa is old - most of the surface rocks are pretty ancient (and consequently washed clean of most nutrients - an issue we've talked about repeatedly). During these millenia, mountains have been formed and then worn down to small hills, whilst the valleys, plains lakes and seas have been buried in the sands and gravels of this erosion process. Over time and with immense pressure these sands and muds too have sometimes been 'recycled' into sandstones and mudstones in someplaces. It's not just been static though: later volcanic events sometimes push magma (un-errupted lava) through the layers of rock towards the surface where it cooled and formed an intrusion of new rock within a mass of older layers. (As shown in the diagram!)

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!

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!

Kopjes

Moru Kopjes, Serengeti, Jan 2011 - stacked boulders form by erosion around cracks

One of the prettiest things about Mwiba is the large number of kopjes (pronounced 'kopees') found down here. Massive and ancient, the rocks that form kopjes are great added value in many safari destinations. They also form an important part of the landscape and should never be seen simply as a photogenic backdrop, or a great place to enjoy the sunset! 

Mwiba kopjes give a good view! Jan 2011

In fact, geology is one of my 10 things to talk about, and there's no more obvious prompt to talk about geology than when sitting on a kopje. But if you're going to do that, it's important you know something about them before plunging in. If you want a really good overview of the soils and geology of the Serengeti ecosystem, you'd do no better than looking here, this is more of a general overview and whilst mostly focussed on the Serengeti ecosystem, the processes involved are similar across Africa and it shouldn't be too hard to find out the location specific details once you understand the process if you want other areas. 

So, what is a kopje, geologically? Essentially, it's a pile of ancient rocks that protrude through the more recent soils and surface rocks – that's what gives them their other name of inselbergs: 'mountain islands'. In Serengeti they're either gneiss [pronounced 'nice'] (a metamorphic rock that looks rather like granite but doesn't have the little flat crystals of mica or similar – some gneiss has originally been formed from granite) or diorite/granite. And they're all OLD! The kopjes of the north west are the oldest and those of the western corridor the youngest, but all are Precambrian, which means over 500 million years old. Compare that to the volcanic ash deposits they poke through on the Serengeti plain, which are only 3million years old, and you see how different they are. All originally formed under the surface of the earth from volcanic activity that didn't make it to the surface, cooling below ground, and then over the ensuring millions of years the surrounding softer rocks have eroded, leaving the harder metamorphic or igneous rocks to become exposed as they are today.

Figs on a Mwiba kopje, Aug 2011

Horned Rockdweller, Bradinopyga cornuta, perhaps a surprising rock specialist!

Gloriosa superba the Glory lilly are common on kopjes: Naabi, Jan 2011

Klipspringer, a kopje specialist. Kruger May 2011

So, that's what they are and it's a good start on geology (though there's much more to talk about there too). But what about the role they play in the savannah ecosystem? In thinking about this it's first of all good to remember that they're a pretty unique habitat, with a specialised group of animals and plants. Kopjes are the place to find klipspringer and hyrax, they're also the only place to find one or two more esoteric bird species like rock-loving cisticola. Rock figs, as the name suggests, are common on kopjes but not many other places. As well as these really rather specialised species, there are a number of species here that are more often associated with riverine vegetation: there's a range of figs, you'll often see tamarind on kopjes and plenty of the animals often found by rivers are there too – both kopjes and rivers are great places to find leopard, for example. Why's this? Well, we need to think about the savannah 'big four' again, and how kopjes affect them.

(1) Water availability: you might think kopjes are dry barren places, but not at all – whilst the rocks are definitely dry, there are often hollows and cracks within them that keep the water for a long time. Figs are particularly good at sending roots over rocks to find the damp pockets and grow really well in such wet places. Animals and birds often know about the puddles too and will use them throughout the dry season, allowing water-dependent species to utilise areas of savannah that would otherwise be too dry for them.

(2) Grazing/browsing: ever seen an elephant on top of a kopje? A giraffe? No, me neither. Whilst there is a specialist browsing community of bush hyrax and klipspringer, for example, the huge impacts of mega-herbivores are minimal on kopjes, allowing plants that are poorly defended to thrive in a way that they can't manage on the flatter plains.

Bush hyrax and Mwanza Agama share this kopje! Moru, Serengeti, Jan 2011

(3) Fire (my favourite of course!): Yes, kopjes are fire breaks – large areas of bare rocks obviously can't burn and plants growing in the gaps in the rocks are safe from fires. As we know fire can be the main process determining whether there's forest or savannah in the wetter areas, it should be no surprise that the fire refuge offered by kopjes should have a forest type vegetation.

(4) Nutrients: just as the bare rocks allow water to concentrate in hollows and cracks, so nutrients from hyrax dung, baboon dung, leaf litter and all the rest concentrate in the cracks too, providing a relatively nutrient high (though thin) soil for those plants that can get to it. What's more, several animal species (like leopards hunting the plains, but bringing their kills back to the trees on the kopje, or eagles nesting on the crags but foraging over a wide area) use the kopjes at certain times of day or night and the plains at other times, but bring their food (and/or dung) back with them to the kopjes, further concentrating nutrients on the kopjes and resulting in a net flow of nutrient out of the surrounding plans, onto the kopje on a truly grand scale.

So, with all of the big four savannah processes being substantially different on a kopje from the surrounding landscape, it should be obvious that there's going to be a big difference in the ecology of the kopje, just as we see. And as the differences are mostly fairly similar to those in a healthy riverine area, it's not surprising that some of the elements are shared between the two otherwise rather different habitats.

Simba on Simba Kopjes, Serenegti. Jan 2011

As one last thought to leave you with, I've already mentioned how kopjes can be seen as a large-scale nutrient pump, pulling nutrients out of the plains and focussing them on the rocks, but there's one other big way in which they alter the ecology of the surrounding plans: predators. Just as leopards like kopjes, so too do lions. They're great places to warm up in the morning or evening sun, and provide an ideal viewpoint to survey the plains for food. Moreover, the rocks and thicker vegetaion make it much easier to sneak out and ambush your prey. So kopjes have a real impact on the 'landscape of fear' if you're a tasty antelope. Out on the short grass plains you've got a great view and it's tough for lions and the like – the plains prides of Serengeti have a home range over 200km2, compared to only 15km2 in the woodlands around Seronera (no wonder they're easy to find there!). So if you're a wildebeest, zebra or kongoni, you'd be wise to avoid the immediate area of kopjes – just as they do a lot of the time. And removing that grazing pressure from the plains around the kopjes, of course, is going to result in a changed ecology of the grasslands there too. So the impact of kopjes spreads out far wider into the landscape than just the rocks themselves. All very interesting – and easy to see whilst sitting about waiting for the sun to set!

Mwiba sunset from a kopje, Aug 2011