Wood Turtles (Glyptemys insculpta) in Northeastern Minnesota: An Analysis of GPS Telemetry and a Population Assessment

Abstract

Wood Turtle (Glyptemys insculpta) populations have experienced declines across their North American extent of occurrence and are listed as a state threatened species in Minnesota. To improve our understanding of the current conservation status of the species in northeastern Minnesota, we performed a snapshot comparison study using data from population surveys in 1990 and 2015. Our snapshot comparison indicated relative abundance, adult sex ratio, and juvenile-adult ratio did not differ between years. Thus, we found no evidence of Wood Turtle population change in a 40 km river system in northeastern Minnesota over the last 25 years. We also evaluated the positional data quality of snapshot GPS devices at a study area, for use in freshwater turtle research. GPS stationary tests in four different cover types were used to evaluate location accuracy, fix success rate, and directional bias of GPS devices in Wood Turtle habitat. These tests demonstrated that positional data quality is reduced in closed canopy conditions. Utilizing the results for these tests, we developed a GPS screening procedure for turtle locations. We collected 122,657 locations and 399,606 temperatures readings from the carapace of 26 Wood Turtles from May to September 2015 and 2016. We removed locations with high horizontal dilution of precision values, locations farther from surrounding points than a predefined distance, biologically impossible movements, and then used moving averages to estimate turtle locations. The screening procedure removed 10% of GPS locations, and reduced mean location error from 26 m (SD = 33) to 11 m (SD = 12). We also developed a methodology to compare ambient temperature profiles from water, sunny, and shaded locations to the temperature of a turtle’s carapace, to define a turtle’s location as land or water. Our results suggest that snapshot GPS technology and temperature loggers provide temporally unbiased and abundant GPS data useful in describing spatial ecology and habitat use of semi-aquatic turtles.