A Conceptual Model of Arid Rangeland Degradation

A major concern of managers of arid and semiarid rangelands is the progressive loss of secondary productivity and diversity over the past century (Breman and de Wit 1983, Dean and Macdonald in press, Downing 1978, Friedel et al. 1990, Le Houerou 1989, Schlesinger et al. 1990, Talbot 1961, West 1993). Losses in productivity and diversity have been attributed to the overuse of rangelands by a narrow suite of domesticated herbivores. Arid rangelands and those with a relatively short evolutionary exposure to mammalian herbivory appear to be more sensitive to domestic livestock than mesic rangelands and vegetation types that evolved with mammalian herbivores (Mack and Thompson 1982). Severely degraded rangelands may not return to their original state, even when rested for decades (O'Connor 1991,Westoby et al. 1989). Conceptual frameworks for rangeland change have suggested that the probability of reversing grazing-induced change may be inversely related to the amount of disturbance involved in the transition. For example, the cup-and-ball analogy (George et al. 1992) envisages that the same amount of energy is required to alter species composition of vegetation as is re-

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