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 to 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.

Table S1 - Classification of Holarctic mammals into colonizer and sedentary speciesTable S1. Species list including a classification of each species as P-N colonizer, N-P colonizer, P sedentary, N sedentary, attending to their belonging to clades that participated or not in the biotic exchange through the Bering land-bridge during the Plio-Pleistocene. Note that P denotes Palearctic, and N is for Nearctic.Table S1. Species list and classification.xlsx