The extinction risk of a population exploiting a resource

Fluctuations of population sizes affecting the risk of extinction may have different causes. The effects of environmental noise are well-known from different population models. Environmental noise results in fluctuating population sizes and an enlarged extinction risk. We present a stochastic simulation model investigating a different mechanism leading to such fluctuations. It is constructed for a consumer-resource population system. Explicit use of resources leads to fluctuations of the population size because the birth rates of the consumers depend on the amount of food available and thus vary with time. A different type of noise is created, denoted as internal noise. The effects of internal and environmental noise on the extinction risk are compared. We find that despite internal noise the mean lifetime of a population grows rapidly with capacity, whereas strong environmental noise causes a much slower growth of the mean lifetime. The implications of these findings for nature conservation are discussed. We argue that mere time-series of fluctuating populations provide an inadequate basis for population viability analysis, and that knowledge of the source of the fluctuations is required if the best conservation strategy is to be found.