Stabilization of a salamander moving hybrid zone

When related species meet upon postglacial range expansion, hybrid zones are frequently formed. Theory predicts that such zones may move over the landscape until equilibrium conditions are reached. One hybrid zone observed to be moving in historical times (1950-1979) is that of the pond-breeding salamanders Triturus cristatus and Triturus marmoratus in western France. We identified the ecological correlates of the species hybrid zone as elevation, forestation, and hedgerows favoring the more terrestrial T. marmoratus and pond density favoring the more aquatic T. cristatus. The past movement of the zone of ca. 30 km over three decades has probably been driven by the drastic postwar reduction of the "bocage" hedgerow landscape, favoring T. cristatus over T. marmoratus. No further hybrid zone movement was observed from 1979 to the present. To explain the changing dynamics of the hybrid zone, we propose that it stalled, either because an equilibrium was found at an altitude of ca. 140 m a.s.l. or due to pond loss and decreased population densities. While we cannot rule out the former explanation, we found support for the latter. Under agricultural intensification, ponds in the study area are lost at an unprecedented rate of 5.5% per year, so that remaining Triturus populations are increasingly isolated, hampering dispersal and further hybrid zone movement.