|Diversity, Globalization, and the Ways of Nature (IDRC, 1995, 234 p.)|
Savannas are extensive tropical grassland areas that cover more than 10 million square kilometres of the Earth. The largest expanses are found in Africa (nearly 4 million square kilometres), South America, and, to a lesser degree, Asia and Australia. The dominant vegetation in savannas is usually grasses, intercalated with shrubs and trees. In some cases, shrubs and trees dominate, and savannas evolve into other ecosystems, such as:
· cerrado - a type of forest with a lower canopy and a bushier, grassier undergrowth (see Chapter 4);
· caatinga - a low-density, xerophytic ecosystem of bushes and shrubs with some trees characteristic of northeastern Brazil;
· chaco - a more xerophytic forest of low trees and bushes in extensive flatlands of Paraguay and northern Argentina; or
· sudan - herbaceous ecosystems with intercalated trees extending from the valley of the Nile to the Atlantic Ocean along the low tropical latitudes north of the equator.
The main characteristic of present-day savannas is their periodic (usually annual) water deficit. Both vegetation and fauna have adapted to it. Plants, for example, have developed deciduous aerial systems (leaves) and an annual cycle of growth.
Not all savannas are primary ecosystems (which evolve with little or no human intervention). Many, perhaps most, are secondary ecosystems (which result from human action). Frequently, shrubby, bushy, or forest areas have been and are being burned to make way for agriculture, raising cattle, or hunting. It is difficult to know to what extent todays savannas are the result of a transformation of other preexisting ecosystems rather than naturally occurring. In Africa, the savannization process started very early. Setting forest fires is a common hunting strategy in many societies and Africa was the home of hunters for several hundred thousand years (if not millions).
It is likely that fragile, nongrassy ecosystems gave rise to savanna or even steppe environments perhaps as early as the mid-Pleistocene. Obviously, climate also changed during this geological epoch. During humid periods, savannization of deserts and encroachment of forests into savannas took place. However, the effects of geological changes during the last few hundred thousand years have been somewhat obscured by the impacts of continued human action.
After domestication of herbivorous animals and the spread of agriculture (starting about 10 thousand years ago, but intensifying during the last 3 thousand years), the processes of environmental savannization progressed further. Savannization of the Sudanese region is probably related to domestication of sorghum and millet and to the adaptation of previously domesticated animals to local environments - mainly cattle, goats, and sheep, but later also dromedaries in the Sahelian periphery. In the late 19th century and first half of the 20th century, after widespread European colonization, new commercial crops were introduced (such as peanuts), population growth accelerated, and new forested areas were cleared, transforming most of intertropical Africa into savannas. The remaining forests are retreating at a rapid rate.
In South America, the process of savannization is recent. When the Europeans arrived in the 16th century, there were few, if any, typical savannas. Most South American intertropical ecosystems are in subhumid climates with seasonal rains. Apparently during the humid subperiod of the mid-Quatemary, these areas were covered by trees or bushy vegetation (cerrado and chaco). In more arid areas, a less-dense, shrubby or bushy steppe vegetation developed (caatinga). Grasslands were restricted to temperate areas, particularly in the southern cone - the pampas and to the northern Ilanos.