Regional Pools and Environmental Controls of Vertebrate Richness

The species richness of local communities depends on the richness of the regional pool and the filtering processes that preclude some regional species from occurring locally. These filters may include absolute attributes of the local environment and also how representative the local environment is of the surrounding region. The latter is consistent with a species-sorting perspective, in which regional species only occupy the local habitats to which they are adapted. Here we evaluate the relative effects of local environmental conditions, environmental representativeness, and environment-independent processes on the probability of local species occurrence, given their regional presence, of birds, mammals, and amphibians worldwide. In multipredictor models, environmental representativeness is a strong independent predictor of local species occurrence probability, with a relative contribution greater than that of absolute local environmental conditions. Furthermore, we find that local occurrence probability diminishes with increased regional richness independent of the local environment. This is consistent with reduced local occupancy in richer regions, which is a pattern that could stem from a largely neutral community assembly process. Our results support the importance of both environment-independent and species-sorting processes and suggest that regional richness and environmental representativeness should be jointly used for understanding richness gradients across scales.