|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|
|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!