Size and structure of two populations of spotted turtle (Clemmys guttata) at its western range limit

Determining demographic properties for threatened and endangered species is paramount for crafting effective management strategies for at-risk populations. Collecting sufficient data to quantify population characteristics, however, is challenging for long-lived species such as chelonians. One such species in Illinois is the state-listed as Endangered Spotted Turtle (Clemmys guttata). While demographic data exist for populations from other extremes of the range of the species, no similar investigation has been published for Illinois, in which only two isolated populations remain extant. We used a long-term mark-recapture data set to analyze changes in sex and stage structure, abundance, and population growth between 1988 and 2016. Both populations exhibited a strong adult bias (76.5–90.6%) and an even adult sex ratio throughout the duration of the study. At one site the estimated population abundance increased, although there was a decreasing trend in the growth rate over time. Population size and growth rate remained relatively stable at the other site. Sex and stage distributions in the Illinois C. guttata populations were consistent with those of other populations, but the two populations are not experiencing the steep declines documented throughout the remainder of the range of the species despite threats from habitat limitations, subsidized mesopredator abundance, poaching, and traffic. We recommend increasing available habitat as the most effective strategy to reduce risks to C. guttata persistence in Illinois.