Population structure of freshwater turtles across North America: An Ecological Research as Education Network (EREN) collaborative project

Background/Question/Methods

Freshwater turtle populations can be profoundly impacted by urbanization. Adult females may experience high mortality with increased road density near nesting sites, leading to a male-biased population. Juvenile recruitment can be reduced by the high density of predators that often exist in human-dominated landscapes, resulting in an adult-biased population.  To determine if the population structure of freshwater turtles follows these patterns at widespread sites with varying degrees of urbanization, over 30 faculty and their students at 26 institutions extending from Massachusetts to Oklahoma conducted a mark-recapture study of turtles inhabiting lentic ecosystems. The research was conceived, organized, and operated through the Ecological Research as Education Network (EREN, www.erenweb.org).  EREN facilitates research and collaboration among students and faculty at primarily undergraduate institutions.  Between August and early October 2012, each participant placed a minimum of four hoop traps in each of their respective ponds for at least two consecutive days and marked, measured, and released any captured turtles. We sampled a total of 39 ponds.  For each pond with a minimum of ten individuals per species, we used chi-square goodness-of-fit tests to analyze whether the adult sex ratio differed from parity and the juvenile:adult ratio differed from 1:5 for each species.  

Results/Conclusions

We caught the painted turtle (Chrysemys picta), pond slider (Trachemys scripta), common snapping turtle (Chelydra serpentina), common musk turtle (Sternotherus odoratus), smooth softshell (Apalone mutica), and common map turtle (Graptemys geographica) across 34 ponds. Turtles were not caught in the other five ponds. Only the painted turtle was caught in sufficient numbers per pond to permit the statistical analysis of age-sex class ratio.   It was detected in 29 of the ponds, with 16 ponds yielding captures of at least ten individuals. The adult sex ratio was significantly biased towards adult males in eight ponds and did not differ from parity in the remaining eight ponds. A significant deviation from expectation for the juvenile:adult ratio was detected in six of the 16 ponds. Significantly more juveniles than expected were caught in three ponds and fewer juveniles in the other three ponds. Faculty and students will continue the turtle sampling in fall 2013 and will characterize urbanization by measuring landscape attributes around each sampled pond.  As the research continues, we will be able to assess more thoroughly how the population structure of freshwater turtles varies with urbanization across much of North America.