The role of termites in the savanna biome

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

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

Nutrients in the savannah biome

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

Termite mound, Mwiba Ranch, August 2011

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

Impala Midden, Manyara Ranch, Nov 2010

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

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

Elephant diggings near Gibbs Farm, Dec 2010

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

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

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

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?