Functioning in Grasslands

Grasslands are the potential natural ecosystem type on approximately 25% (33x lo6 km2) of the land surface of the earth (Shantz 1954). Current estimates of the global extent of grasslands range from 16% (Whittaker and Likens 1973, 1975) to 30% (Ajtay et al. 1979). The difference between the estimates of the potential extent of grasslands and the current extent provides an indication of the degree to which humans have, and are, modifying this ecosystem type. In the temperate regions much of the area of natural grasslands has been converted to cropland. In the subtropical and tropical regions the area occupied by savannas is increasing as a result of conversion of forest to pasture for domestic livestock. Humans have had an enormous influence on the structure and function of grasslands worldwide. The scope of this chapter employs a broad definition of grasslands encompassing those regions covered by natural or seminatural herbaceous vegetation, predominantly grasses, with or without woody plants (Singh et al. 1983). The largest areas of grasslands are found in central and southern Asia (Lavrenko and Karamysheva 1993; Singh and Gupta 1993; Ting-Cheng 1993), southern South America (Soriano 1992), Africa (Herlocker et al. 1993; Le Houkrou 1993a; Tainton and Walker 1993) and central North America (Coupland 1992) (Figure 6.1). Smaller areas occur in Europe (Lavrenko and Karamysheva 1993; Le Houkrou 1993a) and Oceania (Gillison 1993; Mark 1993; Moore 1993). To a large extent the potential distribution of grassland ecosystems is determined by climatic variables, principally temperature and precipitation (Whittaker 1975). In general, grasslands occur between forests and deserts. They are located in areas in which water availability falls below the requirement for forest at some time during the year but is sufficient to support grasses as the dominant plant type. Many grasslands have an important