One of the critical challenges in conservation biology is to develop quantitative methods for evaluating the fate of populations that are threatened by human activities (Soulé 1987). Predicting population responses to various perturbations, such as habitat destruction, harvest, or supplementation via reintroduction, requires some practical analyses of population viability. These “population viability analyses” (PVAs) have come into increasing usage, and every indication is that their importance will rise in the future. A recent National Research Council panel convened to evaluate the Endangered Species Act vigorously recommended even greater reliance on viability models (NRC 1996), as have many other groups of biologists seeking to improve management of endangered and rare species (e.g., Carroll et al. 1996; Mangel et al. 1996; Ruggiero, Hay ward, and Squires 1995; Schemske et al. 1994).