Effects of drought on western pond turtle survival and movement patterns

Abstract

Drought has the ability to affect the persistence of small animal populations, especially those tied to aquatic habitats. We studied the response of western pond turtles Actinemys marmorata to California’s worst drought on record. From 2009 through 2015 we used telemetry to track movements and assess survival of 19 western pond turtles in a stock pond at the San Joaquin Experimental Range in the western foothills of the Sierra Nevada, in Madera County, California. In 2013 the pond dried in late summer and winter rains were insufficient for pond formation. The pond remained dry through the end of the study in March 2015. In years with below average precipitation the pond often dried completely in late summer; however, the lack of a pond forming in winter and spring had not been previously documented. We observed no mortalities of radiotagged western pond turtles in years with normal precipitation. All observed mortalities occurred in drought years and in years when the pond completely dried up in the summer or never formed. Results from known-fate survival models revealed that survival decreased with increasing drought. Model results also indicated that male survival was slightly higher than female survival (19.1% vs. 11.5%), although the 95% confidence intervals overlapped. We observed high variability in western pond turtle movement distances from the pond in the final 2 y of the study. Two individuals that survived to the end of the study showed unique movement patterns. One young male moved frequently, accumulating a large total distance, moved into new areas, and eventually found his way into a livestock water trough. The other, a young female, moved 2.6 km from the pond (a minimum total distance traveled of 3.3 km based on telemetry locations) and emigrated to a pond on a neighboring ranch. Turtles that died exhibited no distinctive behaviors. After the pond dried western pond turtles remained terrestrial for long periods, with one surviving individual remaining out of water for 617 consecutive days, which is an unprecedented finding for this species to our knowledge. Our findings suggest that increased frequency and severity of droughts can affect the resiliency of small, isolated western pond turtle populations, especially those in ephemeral aquatic environments. These small populations are essential to the long-term survival of the species because of the current fragmented distribution of the species.