On a world-wide basis the response of grasslands to the major human use, domestic livestock grazing, has been variable (Milchunas and Lauenroth 1993). In some areas where the native vegetation is well adapted as a result of evolution, changes in biodiversity have been very small (Milchunas et al. 1988). In other areas changes have been very large. In some cases, and especially in tropical and subtropical grasslands, the large changes have involved a shift from a grass-dominated vegetation to one dominated by woody plants (Walker et al. 1981; Van Vegten 1983; Archcr 1989). In other cases the large changes have involved invasions of exotic plants that have profoundly altered the ccosystems. Conversion of grasslands to croplands or seeded pastures has also had a major influence on biodiversity and ecosystem function. In many cases these converted grasslands have become net sources of carbon and nutrients accelerating global change. These major transformations of grasslands and their effects on biodiversity modify the water, carbon and nutrient cycles to an extent that significantly contributes to jeopardizing the earth's life-support system.
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