Genetic monitoring reveals loss of genetic variation and increased isolation in an endangered pond-breeding amphibian

Abstract Empirical knowledge on how genetic diversity changes through time in populations in the wild is extremely limited. This study aims to contribute toward a better understanding of the dynamics of genetic diversity over time in an amphibian species Red-Listed as endangered in Switzerland, the natterjack toad, Epidalea calamita , which lives along the Swiss Reuss valley, an area Dominated by human activity. We collected samples in 1998 and 2020 from the same area and, when possible, from the same sites (i.e., populations). Observed and expected heterozygosity, and allelic richness (H o , H e , and A r , respectively) were significantly higher in 1998 than in 2020, while the fixation index (F st ) was significantly higher in 2020 than in 1998. The inbreeding coefficient and the effective population size (F is and N e , respectively) did not differ between 1998 and 2020. Bottleneck events were detected in two populations in 1998 and in five populations in 2020. Spatial genetic differentiation among the four populations which could be sampled in both 1998 and 2020 was greater in 2020 than in 1998. In conclusion, genetic monitoring revealed that populations of an endangered amphibian lost genetic diversity and became more isolated over a period of 22 years, corresponding roughly to seven generations. Future research should investigate the causes and consequences of this loss of genetic diversity for individual fitness and population viability and to develop conservation strategies to mitigate further losses of genetic diversity.