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