Demographic Consequences of Inbreeding in Remnant Populations

Although traditional population fragmentation theory and management has been strongly oriented toward concerns arising from inbreeding depression, recent papers suggest that small populations will be eliminated by demographic and/or environmental events before inbreeding becomes a problem. We explore the interaction between these factors by developing a stochastic, discrete time Leslie model that incorporates inbreeding depression. We model small population dynamics with three realistic demographic schedules: low growth rate "ungulates," medium growth rate "felids," and high growth rate "rodents," examining the impact of survival and fertility depression commensurate with inbreeding effects reported in the literature. Focusing on the first few generations after habitat fragmentation and isolation, we find that (a) high growth rate populations are affected only by strong inbreeding depression, but low growth rate populations are extremely vulnerable to even minor inbreeding depression; (b) vulnerability to extinction is affected more by survival depression than by fecundity depression; and (c) reductions in the sex ratio exacerbate inbreeding accumulation and hence extinction rate. Counter to the current fashion, which downplays the importance of inbreeding in stochastic environments, we conclude that, while inbreeding depression is not necessarily the primary cause of extinction, it can be critical.