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题目内容
题目材料：
Where grazing reduces species diversity, a rangeland can be considered overgrazed. Conversely, where species diversity is increased by grazing, that grazing can be considered well managed. Although popular consensus characterizes the intensive grazing of rangelands surrounding the Mediterranean Basin as overgrazing, such intensive grazing is not necessarily destructive.
Crawley argues that the primary way in which herbivores affect plant species diversity is not by eating plants to extinction but by selective feeding, which modifies competition among plant species. For example, some species can survive because competition from more aggressive species that have been grazed is reduced. This mechanism is one of several that underlie the intermediate-disturbance hypothesis, which holds that biodiversity is higher with moderate disturbance of the ecosystem than with low or heavy disturbance. The impact of grazing in Mediterranean ecosystems can be considered in terms of this hypothesis. However, the grazing intensity that is necessary to cause intermediate disturbance in the grazing-resistant Mediterranean shrublands is generally quite high, and low- to moderate-intensity grazing is often ineffective in preventing the encroachment and dominance of woody vegetation. In a study comparing two Greek islands, one "heavily overgrazed" by the Cretan wild goat and the other undergrazed, Papageorgiou found that there were many more species on the "overgrazed" island.
Crawley argues that the primary way in which herbivores affect plant species diversity is not by eating plants to extinction but by selective feeding, which modifies competition among plant species. For example, some species can survive because competition from more aggressive species that have been grazed is reduced. This mechanism is one of several that underlie the intermediate-disturbance hypothesis, which holds that biodiversity is higher with moderate disturbance of the ecosystem than with low or heavy disturbance. The impact of grazing in Mediterranean ecosystems can be considered in terms of this hypothesis. However, the grazing intensity that is necessary to cause intermediate disturbance in the grazing-resistant Mediterranean shrublands is generally quite high, and low- to moderate-intensity grazing is often ineffective in preventing the encroachment and dominance of woody vegetation. In a study comparing two Greek islands, one "heavily overgrazed" by the Cretan wild goat and the other undergrazed, Papageorgiou found that there were many more species on the "overgrazed" island.
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