Aims: In many taxa, the latitudinal span of species’ geographic ranges is positively correlated with median latitude (i.e., a Rapoport effect). This is frequently explained as adaptation to contemporary climate, however, variability in postglacial range expansion among species could also explain this pattern. Here, we analyze geographic data for North American salamanders to test the causes of Rapoport effects.

Location: Temperate North America

Taxon: Salamanders (order Caudata)

Methods: Using range maps, we tested for a Rapoport effect. We then manipulated species’ latitudinal ranges and species pools to test for an impact of postglacial range expansion in forming a Rapoport effect. In addition, we built ecological niche models for species found south of the Wisconsin Ice Sheet during the Last Glacial Maximum and transferred these models to postglacial areas. If dispersal is important in forming a Rapoport effect, then some species may be able to tolerate northern climates but have not expanded northward as a result of variation in geographic access to postglacial habitats.

Results: We found evidence of 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 postglacial 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: Our analyses suggest that variation in postglacial range expansion is likely important in shaping geographic range size variation among species in areas where climates have changed rapidly. Postglacial colonization and range expansion likely plays an important role in forming latitudinal biodiversity gradients in many taxa.  Historically, ecophysiology and biotic interactions have been emphasized as important contributors to diversity gradients, yet our study indicates that postglacial colonization also plays a key role in forming latitudinal biodiversity gradients.