What Are Species Pools and When Are They Important?

A regional species pool comprises all species available to colonize a focal site. The roots of the concept are imbedded in island biogeography theory, supply-side ecology, and early propagule addition experiments. The pool concept allows ecologists to examine large-scale effects—including geographic area, evolutionary age, and immigration and diversification—on the diversity, composition, and phylogenetic structure of local communities. Both theory and evidence show that pool influences are greatest when local communities are not strongly and predictably structured by species interactions (e.g., under frequent disturbance or if many species are rare). Practical and conceptual issues to consider when delineating species pools include choosing an appropriate spatial scale, whether to account for environmental filtering, whether to include the species within a fixed geographic area versus those whose geographic ranges overlap with a site, or whether to use databases or geographic data sources. Each issue is discussed in the context of 63 studies using the species pool approach. We conclude that the species pool concept has contributed greatly to our understanding of community dynamics by bridging the gap between large and small spatial scales. Future studies must compare pool characteristics with community structure across multiple regions for a more complete understanding of community assembly.