Ecological mechanisms of extinction.

Abstract

Ecological theory offers predictions, sometimes conflicting, about the ecological characteristics of species that correlate with their risk of extinction. It is generally agreed that risk should be higher for species with small populations, small geographic ranges, and poor dispersal ability than for their ecological counterparts (1–3). How suites of life history characteristics affect risk of extinction is less clear. Species with high variance in the intrinsic rate of population increase (r), which is often associated with high fecundity, moderate to low survival rates, short generation times, and small body size, are predicted to be more susceptible to extinction because they are prone to large stochastic population fluctuations (4). Alternatively, species with a low r (because of low fecundity, high survival, and long generation times) are predicted to be at increased risk, because they would recover slowly from a severe reduction in population size and remain threatened longer by demographic and genetic stochasticity (5, 6). Such species are typically large. Thus, it is unclear whether the “fast lifestyle” associated with small body size and short generation times or the “slow lifestyle” represented by large organisms with long generation times makes species and lineages more or less likely to become extinct. Empirical studies of island fauna yielded contradictory conclusions about the effects of body size and lifestyle on the risk of extinction and produced conflicting explanations to account for the mechanisms underlying the patterns (5, 7–11). Resolution of such issues transcends academic debates, as governments and conservation organizations struggle to apply laws, like the United States Endangered Species Act, and decide how to rank threats and allocate funds among taxa that may differ in risk (12).