Predicting Broad-scale Patterns in Species Distributions

Abstract

Species richness of virtually all high-level taxonomic groups is strongly statistically related to climatic variables such as temperature and precipitation, and consistently so across space and time. These observations are consistent with a causal link between the number of species that occur in a given region and its climate. Although dozens of hypotheses have been proposed, the main mechanisms underlying this pattern remain largely unresolved. And, few ecological studies have attempted to identify regularities in the individual species distributions that make up the richness–climate relationship. Despite the complexities of species’ biologies, I found that, to a first approximation, species’ probability of occupancy at continental scales were generally well statistically explained by a Gaussian function of temperature and precipitation. This simple model appeared general among species, taxa and regions. However, although individual species’ ranges are strongly statistically related to climate, spatial variations in richness cannot be explained by systematic variations in species’ climatic niches. And, individual species track changes in climatic variables through time much more weakly than species richness tracks these changes, suggesting that richness is at least partly constrained by mechanisms independent of species identities. Moreover, at macro-scales, species richness was also not strongly predictable from the temperature at which clades have originated, from historical variability in climatic variables nor from local short-term extirpation rates. In sum, I rejected several prominent hypotheses aiming to explain richness–climate relationship and found several lines of evidence inconsistent with the common idea that climatic constraints on individual species, by themselves, can explain richness–climate relationship. I propose a mechanism to explain, as a first approximation, the continental biogeography of species distributions that relies on neutral processes of dispersal and local extinctions within species’ broad deterministic thermal tolerances.