Dispersal rates and distances in adult water frogs, Rana lessonae, R. ridibunda and their hybridogenetic associate R. esculenta

Abstract

I investigated dispersal of adult water frogs between adjacent ponds, located within 2 km2 of each other, containing Rana lessonae, R. ridibunda, and their hybridogenetic associate R. esculenta. Recent models indicate a strong influence of species specific dispersal on the dynamics of such mixed populations. However, empirical data on dispersal are still rare and populations are often defined through individuals that reproduce at the same site. In my study area near Zurich, Switzerland, 12.2% of the animals changed ponds in 1995 and 1996. Dispersal occurred throughout the year and was not restricted to specific periods. The dispersal rate decreased with increasing pond-to-pond distance and degree of isolation. In addition, I found differences in dispersal rate and distance related to size, sex, and genotype of the animals. Females moved relatively more often when smaller or younger. Males, in contrast, dispersed more often when larger or older. In terms of genotypes, dispersal rates decreased and distances increased from R. lessonae through R. esculenta to R. ridibunda. Although based only on movements, these results suggest that there is gene flow among different ponds, because 96% of the migrating animals remained at the new pond for at least one breeding season and 92% did not move back at all. Hence, with the exception of one isolated pond, all water frogs from the investigated ponds seem to belong to one breeding population rather than to different populations.