Historical contingency, niche conservatism and the tendency for some taxa to be more diverse towards the poles

Abstract

Aim: We test the ability of the biotic exchange across the Bering land bridge coupled to niche conservatism to explain current day mammalian diversity gradients. Location: The Holarctic. Taxon: Mammals. Methods: We compared the diversity within clades that participated in the exchange (colonizers), whose ancestors withstood the Beringian cold temperatures, with that within clades that did not participate (sedentaries). We contrasted biogeographical patterns, tested the ability of environmental models to predict species richness of colonizers and sedentaries across continents and, compared richness-climate relationships between colonizers and sedentaries controlling for phylogenetic effects. Results: We find that assemblages of colonizers are more diverse towards higher latitudes, opposing the traditional latitudinal diversity gradient which is followed by sedentaries. Despite the long passage of time since this major dispersal event, we find that the geographic distribution of colonizers is more strongly correlated with the distributions of other colonizers inhabiting a different continent than to the distribution of sedentary species. Main conclusions: Our results highlight the importance of historical migrations and dispersal in configuring present-day diversity gradients. We also suggest that colonizers may be particularly vulnerable to future climate change because of the predicted disproportionate decrease in climate space in the extra-tropical realm where they are currently most diverse.