The combined influence of the local environment and regional enrichment on bird species richness.

It is generally accepted that local species richness at a site reflects the combined influence of local and regional processes. However, most empirical studies evaluate the influence of either local environmental variables or regional enrichment but not both simultaneously. Here we demonstrate the importance of combining these processes to understand continental-scale richness patterns in breeding birds. We show that neither regional enrichment nor the local environment in isolation is sufficient to characterize observed patterns of species richness. Combining both sets of variables into a single model results in improved model fit and the removal of residual spatial autocorrelation. At short timescales, local processes are most important for determining local richness, but as the timescale of analysis increases, regional enrichment becomes increasingly important. These results emphasize the need for increased integration of multiple scales of processes into models of species richness.