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CHAPTER 19 DIVERSITY IN AND AMONG GRASSLANDS
Vice President for Research, University of New Mexico, Albuquerque, New Mexico
Grasslands cover broad areas of both temperate and tropical regions, but they occur primarily in climatic zones with a pronounced dry season (Axelrod, 1985). They are characteristically found in regions where there is insufficient soil water to support an arboreal canopy yet adequate moisture to permit the existence of a grass-dominated canopy rather than desert vegetation. Technically, grasslands can be described as types of vegetation that are subjected to periodic drought, that have a canopy dominated by grass and grasslike species, and that grow where there are fewer than 10 to 15 trees per hectare. The number of grass species in these areas, however, is frequently lower than the number of forbs, e.g., composites such as daisies and sunflowers (Curtis, 1959). Grasslands are found in such diverse locations as the steppes of the Soviet Union, the Serengeti of Africa, the dry grasslands of Australia, the pampas of Argentina, and the Central Plains of the United States. Given this wide range of variations, it is not surprising that the grasslands of the world contain a large amount of native biodiversity.
GRAZING AND AGRICULTURE CONVERSIONS
All grasslands support an array of native herbivores. In terms of energy consumption, the impact of herbivores is usually quite low but differs among the various grassland types. However, the greatest impacts on most grassland ecosystems are caused by domestic herbivores that have been introduced by human societies. Grasslands can withstand moderate grazing, especially when weather conditions are favorable, but overgrazing frequently causes important changes in the composition of the plant and animal population. A common response is that the