Phylogeny, ecology, and the origins of climate-richness relationships

Many studies show that species richness is correlated with climate, especially among local sites within a region. However, few studies have addressed how these climate-diversity relationships actually arise. Only a few processes can directly change species richness (i.e., speciation, extinction, dispersal), and these processes may be best studied by incorporating a phylogenetic perspective. Here, we used a phylogenetic approach to address the causes of climate-diversity relationships in plethodontid salamanders by combining data on richness, climate, and phylogeny for 250 species. Our results suggest that species richness patterns in plethodontids are explained primarily by how long each region and climatic zone has been occupied, rather than by the effects of either area, species density (i.e., ecological limits), or climate on the rates of speciation or extinction. Across regions, diversity is related to time rather than climate. Within regions, significant climate-diversity relationships are also related to time, with higher richness in climatic regimes that have been occupied longer. Although some might think that phylogeny is unimportant at local scales and when climate and diversity are strongly correlated, we show that niche conservatism and phylogenetic history (time) combine to create species pools of different sizes in different habitats (climatic regimes), leading to variation in local species richness across these habitats within a region.