Recruitment dynamics, estimated vital rates, and population modelling of a Wood Turtle (Glyptemys insculpta) population in northeastern Minnesota

Abstract

Wood Turtles (Glyptemys insculpta) are medium-sized, semi-aquatic, freshwater turtles that are classified as endangered by the International Union for Conservation of Nature (IUCN) due to declines in populations across their distribution in the Great Lakes region and eastern North America. Threats to Wood Turtle populations vary across their range, with recruitment failure due to nest predation considered a major threat in northern Minnesota. In Chapter 1, we determine Wood Turtle nest predator species, nest predator visitation rates, and nest predation rates in northeastern Minnesota. We monitored 20 new nesting areas (NNAs) and 15 established nesting areas (ENAs) with camera traps for 1 to 4 summers (2015–2018). Turtle nests at some locations were individually caged, and an electric fence was installed at one ENA. We used camera traps to estimate predator visitation rates and to identify potential and observed nest predators. American badgers (Taxidea taxus) accounted for 55% and 4% of potential predator visitation events at ENAs and NNAs, respectively. All nests found were at ENAs, with 85% of depredation events by badgers. Mean hatching success was 62% and 6% at caged and uncaged nests, respectively. Most nest failures at both caged and uncaged nests were due to predation. Our study indicates that nest depredation rates are high in northeastern Minnesota, and that protecting nests from predation can be an effective management tool for increasing nest success. In Chapter 2, we use reconstruction and simulation models to estimate population vital rates and sensitivity of the population to changes in recruitment and survival. Since 1990, 1,117 unique Wood Turtles have been captured, aged, and marked in the northeastern Minnesota population. We used 2,774 recaptures of these marked turtles to estimate abundance changes through time using a population reconstruction approach. The population model tracks age classes from birth until death (up to 60 years of age). The reconstructed population was compared with an age-class structured population model to estimate survival rates and evaluate population sensitivity to changes in survival of different age classes. Nest hatching rates and survival to 1 year were estimated with sensitivity analysis. In order to match the reconstructed population, annual survival of adults >14 years needed to be >95% and average annual survival for juveniles (1–14 years) needed to be approximately 86%. Nest/hatchling survival was estimated to be between 10% and 14% to be consistent with the reconstructed population over the past 30 years. Population reconstruction and modelling support a stable population historically, but recent high mortality rates of adults are cause for concern. Continued monitoring, marking, and recapturing of turtles in this population is critical to estimate population trends and identify management actions that will help maintain this population in the future.