The ecological niche is the conceptual foundation for the analysis of species distributions. Habitat variables and resources are therefore thought to determine the distribution of a species. However, metapopulation and source-sink models posit that population processes determine the local distribution of a species; metapopulation theory suggests that many suitable patches could be unoccupied and source-sink theory suggests that unsuitable patches could be occupied. We examined whether the distribution of 2 European anurans (tree frog [Hyla arborea] and natterjack toad [Bufo calamita]) was determined by habitat variables, local population size (as indexed by the number of calling males), or metapopulation characteristics are. We found that the distribution of both species was best predicted by the number of calling males in the previous years. This supports the idea that population processes are more important determinants of distribution than habitat characteristics. We suggest that future distribution models explicitly incorporate population dynamic processes. Our results suggest that there is a threshold number of calling males above which a population is almost certain to persist in the short-term. If more robust population data are not available, such a threshold should be useful for managing threatened anuran species because the number of calling males is easily determined.
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Vol. 69 • No. 3