Recent shifts in taxonomic compositions of water frog populations (Anura: Pelophylax) inhabiting fish ponds in southern Poland

In Central Europe, water frog species coexist in assemblages consisting of Pelophylax lessonae, P. ridibundus and their hybridogenetic hybrid, P. esculentus. Population compositions are poorly understood, partly because of difficulties in distinguishing hybrids from parentals by means of morphology alone. Environmental change and human-mediated, cryptic introductions of non-native water frog species have modified local assemblages. In this contribution we examined the structure of nine water frog populations inhabiting mostly large fish ponds in the Upper Vistula river valley of southern Poland using morphology and PCR-based genotyping of single loci in the nuclear and mitochondrial genomes. We found four different water frog population compositions: two with only P. ridibundus, one composed of P. lessonae and P. esculentus, three with P. ridibundus and P. esculentus, and three containing all three water frog taxa. Historical data show that the area was previously inhabited by lessonae-esculentus populations. We propose that both ecological and genetic replacement by expanding P. ridibundus may have contributed to the decline of the former. Overall, 18% of P. ridibundus frogs contained introgressed P. lessonae mtDNA, however, the frequency of introgressants was most pronounced in populations with a high proportion of P. esculentus. Exotic water frogs were not detected in the study area. Our results bear significance for water frog population dynamics and conservation in Central Europe and highlight the importance of long-term monitoring for the detection of changes in population composition in these amphibians.