Abstract
Small scale local processes have traditionally provided the explanations for patterns of species richness. Competition avoidance as evidenced through mechanisms such as spatiotemporal heterogeneity or environmental stress have explained differences in species richness. Current evaluations of the impacts of large-scale processes address questions such as how many species can grow within a fixed community. Other issues under consideration include the selection of species to grow within a community, and the role of evolutionary processes and species dispersal in determining community differences. These issues are addressed with the species-pool theory. Different levels of species pools include the regional pool, the local pool, and the actual pool in the target community.