here, and the interesting savannah woodlands of Australia here. All savannahs, as all grass dominated ecosystems.
Right, definition out of the way it's time to introduce the Big Four of the savannah (sorry, moved on from the original three, but still can't make five!) - the four processes that shape the savannah biome globally. With an understanding of each of these, you can start to understand savannah ecology and begin to guess at what drives the patterns you see in this biome.
Firstly, there's climate and particularly water availability. Temperature and rainfall/precipitation combine to define the earth's major biomes - to get savannah, you need to be warm and fairly dry. Too wet and you'll end up with a forest of one type or another, to dry and you'll head rapidly towards desert. In fact, globally the savannah biome tends to dominate in tropical areas with rainfall above about 400mm, and below something between 1400 and 1650mm. Within this range, depending on how the other big processes combine, you'll probably get savannah habitats of one form or another - though how they look depends exactly where you are on the rainfall gradient. And, of course, understanding seasonal rainfall patterns are vital to understanding the seasonal movements of wildlife.
|Three processes in one! Wildebeest near Naabi in Serengeti are gathering in the rainy season when surface water is drinkable to graze the nutrient rich grasslands of the short-grass plains. The impact of so many grazers is extreme!|
Thirdly, there's fire. Savannahs burn and always have done so - today, many fires are deliberately set as part of the management, but people have probably been burning savannahs as long as there have been people around and before that lightning would have set fires naturally - probably about every 3-6 years we think. This is an ecosystem that has evolved with a constant presence of fire, the trees regrow, the grass regrows and (most) of the animals are perfectly capable of escaping fires by running or hiding in holes, etc. But fire frequency and intensity can certainly shape the savannah and it's a vitally important process to understand.
Finally, there are nutrients. Many savannahs are found on ancient and highly nutrient poor soils where every little patch of nutrients will be highly valued by something. Other areas are on recent volcanic and nutrient-rich soils, providing ideal grazing opportunities and different niches for vegetation types. Where nutrients are found (and how they get moved about) dramatically shapes the ecology of the savannah biome from the small scale of termite mounds to the larger scale of soil types, determining seasonal patterns of movement for animals and many of the habitat differences found from place to place.
And that's it! Future posts will develop all these issues further, but it's a great start in savannah ecology to have in mind the processes that shape the biome before we look too far at each one.