Monitoring the thirty-year decline of a freshwater turtle in protected habitat

Background/Question/Methods: Long-term studies on wildlife populations are necessary to track population abundance and shifts in demography over time, yet such studies are difficult to plan, fund, and conduct and are therefore rarely undertaken. Such studies are especially important for long-lived species that are able to persist for a long period of time with little reproductive output. Spotted turtles are threatened in Canada, listed as endangered on the IUCN red list and declining throughout their range. However, there have been no published long-term studies within the US tracking their population declines. Using a combination of trapping and hand collection, we conducted two population studies spanning a thirty-year time frame on the imperiled spotted turtle (Clemmys guttata) on protected land in the center of their range. The goals of the study were to describe the changes that occurred to the population during this time frame, assess the efficacy of the protected area at maintaining a stable population, and produce a population viability analysis (PVA) of the population to predict long-term viability. Using program MARK we produced estimates of abundance and annual adult survivorship. Using these data and other life-history parameters we built a PVA model in VORTEX to model this populations long-term viability.

Results/Conclusions:

Between 1992 and 2017, population size decreased by 49% (a loss of 117 individuals) and the proportion of older individuals increased significantly. Results from the PVA indicate that both wetland complexes have a high (63-72%) chance of reaching quasi-extinction. Using the baseline PVA and the abundance from 1992, the population was predicted to decrease by 60% by 2017. The population size estimate from the 2014-17 study showed that a 49% decrease had indeed occurred, indicating high accuracy of the PVA model. It is clear from the decline in estimated abundance and using a PVA that the population at this site is likely to be extirpated unless some sort of management strategy is implemented. The decline at a site that has seen little habitat loss between sampling periods questions the efficacy of unmanaged protected areas to slow declines of imperiled species. This study highlights three important issues in conservation: (1) long-lived species may persist for long periods of time despite negative population growth rates, (2) an ecosystem approach to conservation may not be enough to ensure long-term population stability, and (3) conservation efforts need to incorporate occupancy assessments and more intense population studies to effectively monitor changes of long-lived species.