Grasslands are the potential natural ecosystem type on approximately 25% (33 x 106 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 at. 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 at-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 at. 1993; Le Houerou 1993a; Tainton and Walker 1993) and central North America (Coupland 1992) (Figure 6.1). Smaller areas occur in Europe (Lavrenko and Karamysheva 1993; Le Houerou 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
Functional Roles of Biodiversity: A Global Perspective -?
Edited by H.A. Mooney, J.H Cushman. Ii. Medina, O.E. Sala and E.-D. Schulze © 1996 SCOPE Published in 1996 by John Wiley & Sons Lid <L* Uni f i V
Tropicalisubtropical grasslands and savannas r'J ^
Tropicalisubtropical grasslands and savannas j.
Figure 6.1 Map of the global distribution of temperate grasslands (adapted from Bailey 1989)
woody plant component. In temperate and subtropical regions, shrubs often provide the woody component of grasslands.
Three factors of grassland environments distinguish them from other ecosystem types (Anderson 1982; Milchunas et at. 1988), these arc drought, fire, and grazing by large ungulate herbivores. The influence of these three factors on grasses and grasslands has resulted in some of the most characteristic features of grasslands. All three factors provide selection pressures for high turnover of above-ground plant organs, location of perennating organs near the soil surface, and a large fraction of biomass and activity below-ground. The influence of any one of these factors on the structure and function of grasslands depends upon the details of the particular environment. Drought is a more frequent influence on dry grasslands than it is on those in humid regions. Conversely, fire is a much more frequent force in shaping grasslands in humid regions than it is in dry regions. Grasslands in humid regions have higher fire frequency because they have higher production and accumulate more fuel than those in dry regions. The effect of grazing is also related to the dry-to-humid gradient but in a different way from fire. Grazing can be an important evolutionary force across the moisture gradient, but its specific influence depends upon the moisture status of the site (Milchunas et al. 1988). In dry areas, the effects of drought provide a selection pressure that is complementary to that of grazing - that is they select for a similar set of characteristics. If fire was an important force in dry areas, it effects would also be complementary. In humid regions, drought is a less frequent control on ecosystem structure and function than in dry areas. One result of this is that the structure of plant communities in humid regions depends as much, or perhaps more, on a species ability to compete for above-ground resources (light) than its ability to compete for below-ground resources (water and nitrogen). Therefore, selection pressures exerted by the need to competc for canopy resources and those of grazing by large herbivores are antagonistic. For example, competition for light selects for tall plant types and grazing selects for short ones. Fire is an antagonistic selection pressure to competition but at a different temporal scale than grazing.
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