Post-Pleistocene dispersal explains the Rapoport effect in North American salamanders

Aims: In many taxa, the latitudinal span of species' geographic ranges are positively correlated with median latitude (the Rapoport effect). This correlation is frequently explained as adaptation to contemporary climate; however, variability in post-glacial range expansion among species could also explain this observation. We analyse geographic data for North American salamanders to test the potential causes of Rapoport effects. Location: Temperate North America. Taxon: Salamanders (order Caudata). Methods: We tested for a Rapoport effect by estimating correlations between the latitudinal midpoint and latitudinal range among species. Next, we manipulated species' latitudinal ranges by removing post-glacial habitat and assessing the impact of species demonstrating post-glacial range expansion in forming a Rapoport effect. We built ecological niche models for species found south of the Wisconsin Ice Sheet during the Last Glacial Maximum and transferred these models to post-glacial areas. If dispersal is important in forming a Rapoport effect, then some species may tolerate northern climates but have not expanded northward as a result of variation in geographic access to post-glacial habitats. We created binary ecological niche models by thresholding using the equal sensitivity and specificity value. Results: We recovered a Rapoport effect that was robust to the null models we tested. Analyses that manipulated ranges and species pools supported a role for variation in post-glacial range expansion among species, especially for eastern North America. Results from transferring ecological niche models indicated that species have suitable habitat north of their range limit. Main conclusions: Variation in post-glacial range expansion is important in shaping geographic range size clines among species in areas where climates changed rapidly, though we also found support for the climatic variability hypothesis. Post-glacial colonization and range expansion likely plays an important role in forming latitudinal biodiversity gradients in northern taxa. While ecophysiology and biotic interactions have been emphasized as important contributors to diversity gradients, our study indicates that post-glacial colonization also plays a key role in forming latitudinal gradients.