Biased sex ratio and age distribution in a suburban population of Chrysemys picta (painted turtle)

Background/Question/Methods

The population structure of freshwater turtle species has been of interest as several researchers are finding biased age and sex ratios. These findings are often attributed to anthropogenic changes in the landscape. A high concentration of roads near turtle nesting sites may, for example, selectively increase mortality of adult females, leading to a male-biased population. Increased predation of nests caused by a high density of predators that flourish in human-dominated landscapes may reduce juvenile recruitment and cause an adult-biased population.   To determine if the freshwater turtle population in Elizabethtown, Pennsylvania is biased, we conducted a mark-recapture study of painted turtles (Chrysemys picta) in three ponds on the campus of Elizabethtown College in the fall of 2009, 2010, and 2011. We placed hoop traps in each pond for two consecutive days and marked, measured, and then released any captured turtles.  

Results/Conclusions

The total number of individual painted turtles caught was significantly biased towards males (22 adult females, 38 adult males; χ² = 4.27, p < 0.05) and to adults (60 adults, 5 juveniles; χ² = 56.54, p < 0.001). In each year, this biased population structure was similarly observed (2009: 13 adult females, 25 adult males, 4 juveniles; 2010: 13, 18, 1; 2011: 8, 19, 0). Survivorship and capture probability did not differ between the sexes. These results provide reasonable doubt for the long-term persistence of the painted turtle population at Elizabethtown College. With this single study site, we cannot perform the landscape-level analysis needed to assess the effect of land use on the turtle population. For this reason, we are using the Ecological Research as Education Network (EREN; www.erenweb.org) to coordinate a collaborative project in which faculty and students at 17 institutions across North America will simultaneously conduct turtle research. With these data, we will be able to determine how the population structure of freshwater turtles varies across an urbanization gradient. We will begin data collection for this collaborative research project in fall 2012.