Abnormal Demographic Structures May Preclude Population Viability in Isolated Populations of Freshwater Turtles

Freshwater turtles are one of the most threatened vertebrate groups in the world, mainly due to the degradation of their habitats. Some species, including the European pond turtle (Emys orbicularis), have been the subject of long-term monitoring programmes carried out within large, well-preserved wetlands and based on robust capture–mark–recapture (CMR) protocols. Yet demographic studies of small isolated populations are very scarce. In this study, a highly isolated population of E. orbicularis was intensively monitored in the Durance River Valley in southeastern France over five consecutive years (2013–2017). In total, 153 adult individuals were used to estimate demographic parameters using two different CMR Robust Design model formulations. These models estimated a relatively constant adult population size, annual survival and seniority probability over time, similar for both sexes, but a highly unbalanced sex ratio in favour of females (0.24 male per female on average). The results showed that this isolated population exhibits atypical and little-known demographic parameters in this species. The highly unbalanced sex ratio in favour of females could be the result of factors impacting sexual determinism during embryonic development. This imbalance could ultimately reduce the reproductive output and increase the risk of extinction of the population. These results highlight the need for both a better understanding of the mechanisms underlying the origin of such an imbalance, and in identifying the critical thresholds that must not be crossed to guarantee the viability of sex-biased populations.