LAUSR

ABSTRACT Large rivers can act as important ecological barriers for small mammals, but their exact role is debated. Here we examine the effect of the river Danube on populations of the European ground squirrel, an important representative of steppe communities, by studying the Quaternary history both of ground squirrels and of the Danube in the Pannonian Basin, in Hungary. The Danube experienced considerable channel changes during the Quaternary, with >100 km westward shift of its middle reach, interrupted by temporary eastward movements. Besides autogenic river processes, tectonically induced avulsions have played a role in re-routing channels. Three of the four Spermophilus species certainly extended their ranges on both sides of the Danube, referring to repeated river crossings during the Quaternary. Hydrological events, most importantly rapid river course changes (avulsions) but also meander cut-offs are viable mechanisms to enable ground squirrel populations to cross large rivers. On longer timescales, oscillating river course changes can maintain gene exchange across the river.