State-wide population characteristics and long-term trends for eastern box turtles in North Carolina

Turtles are in decline worldwide, and the magnitude and recent acceleration of population declines requires immediate action to inform conservation and management plans. Long-term studies of population trends and characteristics covering multiple populations across a range of environmental contexts are needed to guide the most effective management decisions, yet such studies are uncommon. Eastern box turtle (Terrapene carolina carolina) populations have declined and require conservation action throughout much of their range. We assessed sources of variation and temporal trends in T. c. carolina population characteristics from 39 sites spanning four ecoregions in North Carolina using capture-mark-recapture. Surprisingly, there was no evidence of population decline at any site over a ten-year period (2008-2017). Population densities ranged from 0.2 to 6.0 turtles per hectare, decreasing with more urban development and increasing with more wetland habitats in the surrounding landscape. Populations varied in demographic structure but had similar sex ratios and more adults than immature turtles in each ecoregion even after adjusting for variation in encounter probability among groups and body sizes. Survivorship was similar among life stages and sexes and only weakly related to body size. After adjusting for emigration, annual survival probability ranged from 90.7% to 96.8% for the various demographic groups and body sizes. Growth rates decreased with increasing body size, with rates ranging from approximately 15-mm carapace length per year in the smallest individuals to near zero in the largest individuals. No aspect of population demographic structure or vital rates was related to surrounding landscape context. Population trends and characteristics were consistent among ecoregions, suggesting no need for region-specific conservation or management with respect to the population characteristics examined. However, given the high site-specific variability in nearly all estimated parameters, managers would benefit from targeting local threats such as urban land development and wetland destruction to ensure local population viability. The population characteristics reported here can serve as a useful baseline to compare population trends at these sites as monitoring continues. Our approach could be a useful model for other regions to follow in developing monitoring programs for this and other imperiled turtles.

Automated summary

The study found that eastern box turtle populations in North Carolina exhibited varying population densities in different environmental contexts, but no significant decline over the past decade. Population characteristics varied among different sites and body size groups, but overall trends were consistent. Urban development and wetland destruction were identified as local threats to the population viability.