Grasslands
of the world:
In today's class we'll
discuss two types of grasslands, the savanna of Africa and the prairies of the
US. Although they have much in common in terms of main vegetation types and
herbivore similarities, they differ in extent of species diversity especially
that of predators.
Savannas are grasslands with scattered individual trees. Savannas of one sort or another cover almost half the surface of Africa, as well as large areas of Australia, South America and India.
Savannas can evolve under three conditions:
Climatic savannas are always found in warm or hot climates where the annual rainfall is from about 50 to 130 cm (20-50 inches) per year. Rainfall is concentrated in six or eight months of the year, followed by a long period of drought during which fires can occur.
Edaphic savannas are triggered by soil conditions and partially maintained by fires though this is not required. They occur on hills or ridges where shallow soils, or in valleys where clay soils become waterlogged in wet weather reinforce a periodicity of wet/dry conditions.
Derived savannas: In many tropical areas such as Latin America farmers practice slash and burn cultivation in which they cut down and burn forests for croplands. After years of cropping, land low in nutrients naturally, are abandoned. Grasses invade and if the area is sufficiently dry for a portion of the year, fires natural or manmade ( to sustain cattle ), may prevent reestablishment of forests.
In Africa, tribes people prevented from continuing their traditionally nomadic existence have settled to farm. In turn elephants which were free moving in the past are now restricted to protected parklands- in fact they have little desire to move out where they can be shot. They now have created new savannas by destroying trees of the woodlands in these limited spaces. Fires maintain these areas as savannas.
Today we'll concentrate on the Serengeti:
The Serengeti exists on a plain formed from volcanic activity during the Pleistocene era. Eroded mountain soils are poorest nearest the mountains ( coarse gravel) and richest furthest away where the finer more nutrient rich particles have been borne via erosion.
The second condition necessary is the dry/wet season phenomena. A shift in the direction of the winds on an annual cycle exists as explained below:
March through May is the wet season as winds arriving from the SE are moisture laden. Starting March, violent thunderstorms occur. When the rains come, the grasses grow rapidly with some of the larger grasses producing an inch or more in 24 hours. Heightened productivity results in herbivores calving at this time as the luxuriant growth can support the demands of milk production.In October, a series of violent thunderstorms, followed by a strong drying wind coming now from the NE, initiates the dry season. Once the soils dry out, fires either of natural derivation from lightning or more likely man incited to clear the view for hunting sweep the land.
In November or December a mini-wet season may prevail but typically it its followed once more by dry conditions till March
Continue to the next page on vegetation and soils and from there to the unique animals of the savannas...