Shifting rivers and stationary ground: biogeographic history of slimy salamanders on an isolated bluff in the Mississippi River floodplain

Geographic barriers are integral to the processes of dispersal and vicariance, but some barriers, such as rivers, may vary spatiotemporally, potentially affecting biodiversity patterns in significant ways. Fountain Bluff is an isolated upland in the Mississippi River floodplain along the Illinois-Missouri border, USA. The Mississippi River presently runs on the west side of Fountain Bluff, but the course ran on the east side prior to the Illinois glacial stage (similar to 150000years ago). The northern slimy salamander (Plethodon glutinosus) occurs to the east in Illinois, whereas the morphologically similar western slimy salamander (Plethodonalbagula) occurs to the west in Missouri. We used mitochondrial and nuclear DNA sequences to determine the species identity of slimy salamanders on Fountain Bluff and test whether they originated via vicariance (P.albagula) or dispersal (P.glutinosus). We found that slimy salamanders on Fountain Bluff are P.glutinosus, which supports the hypothesis that they originated via dispersal from the river bluffs similar to 6km to the east. Divergence time estimates corroborated that dispersal likely occurred after the westward diversion of the Mississippi River. The shifting courses of large rivers have had important impacts on biodiversity through the creation and removal of geographic barriers as well as by facilitating passive dispersal across riverine barriers.