Comparing risk factors for population extinction

Extinction risk of natural populations of animals and plants is enhanced by many different processes, including habitat size reduction and toxic chemi cal exposure. We develop a method to evaluate different risk factors in ter ms of the decrease in the mean extinction time. We choose a population mode l with logistic growth, environmental and demographic stochasticities with three parameters (intrinsic growth rate r, carrying capacity K, and environ mental noise sigma(e)(2)). The reduction in the habitat size decreases carr ying capacity K only, whilst toxic chemical exposure decreases survivorship (or fertility) and in effect reduces both r and K. We derived a formula fo r the reduction in habitat size that decrease the mean extinction time by t he same magnitude as a given level of toxic chemical exposure. In a large p opulation (large K) or in a slowly growing population (small r), a small de crease in survivorship can cause the extinction risk increase corresponding to a significant reduction in the habitat size. This conclusion depends al so on the nonlinearity of dose-effect relationship. To illustrate the metho d, we analyse a freshwater fish, Japanese crucian carp (Carassius auratus s ubsp.) in Lake Biwa. (C) 2000 Academic Press.