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 di fferent population models. Environmental noise results in fluctuating popul ation sizes and an enlarged extinction risk. We present a stochastic simula tion 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 birt h rates of the consumers depend on the amount of food available and thus va ry with time. A different type of noise is created, denoted as internal noi se. The effects of internal and environmental noise on the extinction risk are compared. We find that despite internal noise the mean lifetime of a po pulation grows rapidly with capacity, whereas strong environmental noise ca uses a much slower growth of the mean lifetime. The implications of these f indings for nature conservation are discussed. We argue that mere time-seri es of fluctuating populations provide an inadequate basis for population vi ability analysis, and that knowledge of the source of the fluctuations is r equired if the best conservation strategy is to be found. (C) 1999 Elsevier Science S.A. All rights reserved.