|Savannas around the world are often open woodland like this Serengeti pic.|
When I talk of the savanna biome (or, indeed, the savannah biome, since I am English) I'm not referring to a habitat like grassland, or Acacia woodland; nor am I referring to a specific ecosystem like the Serengeti Ecosystem. Rather, I'm refering to the set of habitats that make up the savannah biome globally - the collection of grassland and woodland types that all have the same main processes operating on and in them, the savannah big four are, of course, nutrients, water availablity, herivory and fire. They might (and often do) contain remarkably different species, but the same processes are at work and, to remarkable degree, they show largely similar vegetation forms and structures. A biome may therefore be thought of as a set of habitats that share similar ecological processes wherever they occur across the world. By contrast, I tend to define an ecosystem as a single geographical area (like the Serengeti Ecosystem), within which nutrients are cycled with relatively little input or output from neighbouring ecosystems. Hope that's clear...
|Fire is crucial to maintain savanna, particularly in wetter areas. Tarangire July 11|
So, if we're going to understand what the savanna biome is, we need to look at the processes that are the dominant forces within it. If we can understand these, then we can predict where savannahs will be found throughout the world - and we can also understand what might happen to savannahs if we humans mess around with these processes through, for example, the effects of climate change. And this is exactly what Carla Staver and her co-authors have done in their paper (which I'm afraid is probably hiding behing the paywall here). They were particularly interested in discovering what determines the boundary between forest and favanna biomes (the boundary with desert is a much more obviously rainfall driven boundary), and decided that the most likely factors were rainfall and fire. Whilst that might well be true, there's plenty of evidence that herbivory is a major player too (at least here in Africa) and nutrients have been proposed as major players too (though there's also plenty of evidence to suggest this really isn't the main thing). However, they didn't look at these - herbivory they mention but exclude from their analysis with the very reasonable excuse that we just don't have a good idea how much herbivory there is around the world, but I think it's a bit of a shame they didn't have a go at nutrients too. Still, enough of what wasn't looked at - what did they find?
|Amani NR is forest - and WET!|
|Look how little really wet forest there is in Africa! From Staver et al|
Anyway, all in all a pretty convincing story showing once again that whilst climate is important, fire is also essential to many savannas (though we can still wonder about their necessity in those drier savannahs, below 1000mm) . And what's more it's another great example of how different stable states can exist in ecology largely, the authors argue, as a result of what's happened in the past: in the overlap zone where forests are currently found it's because these areas were once wetter, whilst in the current overlap zone where savannahs are found, it's because these areas were once drier.
Staver, A., Archibald, S., & Levin, S. (2011). The Global Extent and Determinants of Savanna and Forest as Alternative Biome States Science, 334 (6053), 230-232 DOI: 10.1126/science.1210465