The extinction of animal species has undergone acceleration in the past century (Ehrlich & Ehrlich 1981 ). The main causes of animal extinctions are overkill, habitat impoverishment, and the impact of introduced species (Diamond 1989). The stablishment and protection of nature reserves is a necessary first step to limit the extinctions, but not much more than that. The reserves are often too small to sustain so-called minimum viable populations of especially the larger species (Belovsky 1987; Newmark 1987). These large species, mammalian herbivores in particular, may have a keystone function in communities by creating niches for other species (Laws 1970). The extinction of keystone species will cause the extinction of other species which are dependent on them, leading to chains of extinction (Diamond 1989).