On the risks of extinction

Well-known theoretical predictions are that the risk of extinction should decrease with maximum population size (K) and should increase with the temporal coefficient of variation in population size (CV). In an unvarying environment, where extinction is caused solely by demographic accidents, the risk of extinction should decrease steeply with K; the greater the contribution of environmental variability to the risk of extinction, the less steep should be the dependence on K. Large-bodies species tend to have long lifetimes but low rates of increase, which have opposite effects on the risk of extinction per year. In comparisons of a large- and small-bodied species at the same average population size (N), the large-bodied species should be at less risk at low N but at greater risk at high N. These predictions were tested using a data base of short-term survivals of 355 populations belonging to 100 species of British land birds on 16 islands. Mean N and risk of extinction are known for these populations, and CVs for 39 species can be calculated. Risk of extinction does decrease sharply with N. After correcting for much of the effect of N, the theoretical prediction is confirmed that the relative susceptibility to extinction of large- and small-bodied species reverses with increasing population size. Above 7 pairs, larger-bodied species are at greater risk than smaller-bodied species; the reverse is true <7 pairs. Migratory species are at greater risk of extinction that resident species. After accounting for the effects of N, body size, and migratory status, the risk of extinction does increase with the CV. -from Authors

Duke Scholars

Author Stuart L. Pimm Environmental Sciences and Policy