Thursday, 2 June 2011


They're the thing you probably think of first when you start thinking of savannah habitats, so probably a good place for me to start too. As we've already said, savannah is defined by grasses and all the grazing animals (by definition) depend on grass to a greater or lesser degree, so there must be something interesting to be said about grasses. Surely?! Not one of our guides in training had ever been brave enough to tell their clients about grass though, so certainly room for improvement.
Only ecologists could wander around Serengeti looking for grasses, surely?!

So, what is there to say? Well, it's probably worth first wondering why grass is so popular with so many animals, and how it survives being eaten all the time? The answer to the first part is probably as simple as there being a lot of grass out there, so clearly a niche waiting to be filled. The second is, perhaps, more interesting, even though we all know it anyway: the growing tip of grass is right at soil level, so you can eath the tips without damaging the growing point - not like most other plants at all. This is clearly a great evolutionary advantage for grass - the first grass that grew from the bottom must have had a much easier life than the other plants around. But it also raises the interesting prospect that, once evolved, the plant may actually harness grazers to keep the environment suitable for itself. The main competition grasses face on the savannah is from trees for water and (probably to a lesser degree) light. If grazers munching on grass encourage those same grazers to occassionally nibble (and thereby seriously damage) the seedling acacia nearby without suffering any serious cost themselves, they've effectively harnessed the animals to keeping the environment suitable for grasses. Neat trick for a bit of grass, huh?! Further more, we think there's a chance grasses may have also evolved flamibility for the same reason - by encouraging fire in the ecosystem (and grasses always carry the fires from place to place in the savannah), they keep seedling trees out of the picture in grasslands. As they suffer little from fire, this is again a neat trick (if it's true - there's some debate still, but the topic is an interesting one all the same!).

And then, despite first apperances, there's grasses and there's grasses. Which is to say, not all grasses are the same. In fact, there are somewhere between 9 and 10,000 grass species, depending on who's counting. And that includes (in only eight grass species) 70% of all crops - maize, wheat, sugar, millet, rice, etc. are all grasses. So there's somthing interesting to begin with!

Given the incredible diversity of grass species out there, it's no surprise that for grazing animals there are some pretty serious differences too, particularly in the dry season. One of the most important ecological divisions of grasses is into the two groups or "Sweet grass" or "Sour grass" - the familiar sweetveld and sourveld for our South African colleagues. This fundamental distinction is based on what the grasses do to the nutrients they've captured during the dry season. During the dry season all grasses stop growing and most above ground parts die, life remaining only in and near the roots. Indeed, for annual grasses, the entire plant dies during the dry season. But plants preparing for the dry season have two options - either they withdraw all the nutrient they can into the roots, leaving a very poor cellulose only dry matter in the dry season, or they don't and the nutrients remain available above the surface during the dry season. Obviously, grazing animals will prefer the latter. In general grass species do one of these or the other, but a few adapt their strategies slightly depending on how nutrient rich the soil is, being less careful about the nutrient that gets left to grazing animals in areas where the soils are richer.

This fundamental distinction between strategies for preparing for the dry season has huge impacts in grazing animals. Indeed, there's some evidence that the (now largely extinct) large mammal migrations of South Africa were primarily driven by animals leaving the sourveld in the dry (winter) season and moving to areas with better dry season grazing options. The same impacts can be seen in many areas of East Africa too.

This is far from the only difference between grasses, however, as even during the wet (growing) season there are considerable differences in the nutritional content of grasses growing in different soils. In particular, grasses growing on the recent volcanic soils associated with the Rift Valley (famously the short-grass plains of southern Serengeti and Ngorongoro, less widely know being the Simanjiro plains north east of Tarangire NP, etc.) are particularly rich in nutients. Indeed, it is largely the search among pregnant and lactating female wildebeest and zebra for grasses rich in Calcium (needed for growing healthy bones in particular)  and Phosphorus (needed for growth in general) that drive the famous migrations, with calving occurring precicely when the animals can use the nutrient rich grasslands in these areas.

Grasses can also be beautiful!
The other main variable that determines how palatable a particular grass might be is, of course, how well defended it is. Grasses use silica, a natural glass, to defend themselves - it's what makes grasses sharp enough to cut you as you walk past some. But not all species are as well defended as others, and (perhaps even more importantly) at different stages of growth the same plant may be better or worse defended. Silica takes time to produce and deposit once growing has begun, so new growth - as well as being rather nutrient rich - is typically rather poorly defended. Which is great for grazers! So you often find that once an area has attracted a number of grazers and they've kept it free of old, well defended course grasses, a grazing lawn develops where, just as in our garden lawns, as soon as the grass gets a bit high (and starts to defend itself), something comes along and chops the top off. In East Africa these grazing lawns ocur comonly on particularly rich grasslands (indeed, the whole of the Serengeti's short-grass plains may be a grazing lawn), but also on a smaller scale around local nutrient sources, like termite mounds. It's incredibly easy to find them at the end of the wet season, where most grass is tall, except at the grazing lawns. And a nice feature to point out, once you start seeing them. Interestingly, in areas with Hippos, grazing lawns are really common by rivers, they're very good at forming them. But in the few places where White Rhino are still relatively abundant they also play a very important roll in opening and maintaining grazing lawns that are then also utilised by many other species - this is a process that is now (sadly) lacking from many parts of Africa, and we can only wonder about the overall impact.
Grazing Lawn in Kruger, May 2011 - note characteristic very short grass among larger patches of tall, less palatable grasses. White Rhinos at work?!

Anyway, that's enough for now. I hope I've persuaded you that there are some interesting things to say, even about grass! And I'm sure there'l be more to come too...

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