New published research: Climate change and Spotted Turtle sex ratios

An adult Spotted Turtle from southeastern Georgia – Houston Chandler

The Eastern Spotted Turtle Working Group is composed of state and federal biologists and university and NGO partners pursuing turtle conservation from Maine to Florida. This group has been working for several years to quantify the status and distribution of Spotted Turtles across the eastern portion or their distribution. The combined survey efforts and data analysis of this group ultimately led to the publication of the Status Assessment and Conservation Plan for the Spotted Turtle in the Eastern United States. Part of this document included an assessment of the potential effects of certain threats to existing Spotted Turtle populations. These analyses, led by Patrick Roberts, have now begun to make their way into the published literature.

The first of these papers examines the effects of land-use change and climate change on Spotted Turtle sex ratios in populations across the eastern United States (Roberts et al. 2023). There is significant concern that many turtle species may be especially susceptible to changes in temperature because they undergo temperature dependent sex determination — the sex of the embryo is determined by the incubation temperature of the eggs. In Spotted Turtles, warmer incubation temperatures tend to produce more female hatchlings. Land-use change can also impact sex ratios both through sex-specific mortality rates (i.e., females can be more susceptible to road mortality when making nesting movements; Steen and Gibbs 2003) and by altering the available nesting habitats. Ultimately, there is concern that a combination of these effects could skew sex ratios in turtle populations to the point where the viability of the population is challenged.

A Spotted Turtle hatchling is a rare encounter in the southeast – Houston Chandler

To examine the effects of climate change and land use on sex ratios in Spotted Turtle populations, we used a large dataset collected by the Spotted Turtle Working Group. From 2018–2020, surveys for Spotted Turtles were conducted in 531 survey areas that spanned 12 states from Florida to Maine. Of these 531 survey areas, 58 were included in the current project because at least 10 Spotted Turtles were captured during the survey event. The proportion of Spotted Turtles captured that were males in these 58 plots ranged from 0.15–0.90. There was similar variation in both temperature and land use characteristics across the sampled sites.

We found that there were complex relationships between temperature and observed sex ratios in Spotted Turtle populations. In locations with warmer temperatures (the southern end of the distribution), there was an increased proportion of female turtles as temperatures increased. However, this pattern was absent in populations experiencing intermediate temperatures and actually reversed in locations with lower temperatures. Populations tended to be more male biased towards the northern end of the distribution in response to warming. While these results are only correlative, they suggest that climate change may be impacting the sex ratios in Spotted Turtle populations across a large portion of their distribution. The differences in the relationship between sex ratio and temperature across the range may be driven, at least in part, by differences in nesting habitats and the ability of certain populations to respond to warmer temperatures by nesting in different locations. For example, Spotted Turtles in the southeast almost always nest in high canopy areas (Chandler et al. 2022) and may not have the ability to select cooler nesting locations, leading to the observed increase in female-biased sex ratios.

Relationships between Spotted Turtle sex ratios and temperature across a temperature gradient ranging from cool to hot. – Patrick Roberts

The effects of land use on Spotted Turtle sex ratios were just as interesting. The amount of cultivated crops surrounding a wetland was found to impact the turtle sex ratio, but this relationship was different depending on the clustering of wetland habitats. When wetlands were dispersed, an increasing proportion of crops led to a female bias in Spotted Turtle sex ratios. However, at intermediate and high levels of wetland aggregation, the relationship between crops and sex ratios was relatively weak. At broader spatial scales, sex ratios tended to become more male biased as the proportion of crops increased. Interestingly, we did not find a strong effect of road density on Spotted Turtle sex ratios in our land use analysis. This lack of affect may be related to bias in our sample site selection or inability to account for temporal factors in land use change. Overall, our results demonstrate that anthropogenic land uses can impact sex ratios in Spotted Turtle populations but that these relationships likely depend on the configuration of the wetland habitat.

Relationships between Spotted Turtle sex ratios and the proportion of cropland surrounding wetland habitat across a gradient of wetland aggregation ranging from low to high. – Patrick Roberts

Skewed sex ratios are often cited as a potential impact of climate change on organisms experiencing temperature dependent sex determination. However, there is limited empirical evidence documenting this effect. Our results suggest that changing temperatures may already be influencing sex ratios in Spotted Turtle populations and that the effects are not uniform across the distribution. This could have important implications for ongoing conservation projects as skewed sex ratios can lead to lower viability long-term. Furthermore, our work highlights that land use can also be a strong driver of sex ratios in freshwater turtle populations. Anthropogenic land use may ultimately be a larger threat to Spotted Turtle populations than climate change as it can affect not only sex ratios but also mortality rates and microclimates.

Spotted Turtle populations will continue to face a variety of threats in the coming years. Large-scale datasets and analyses like the work described here will allow us to better understand and combat these challenges. The full publication is available here.

Literature Cited

Chandler, H. C., B. S. Stegenga, and J. D. Mays. 2022. Compensating for small body size: The reproductive ecology of southern Spotted Turtle (Clemmys guttata) populations. Ichthyology & Herpetology 110:268–277.

Roberts, H. P., L. L. Willey, M. T. Jones, T. S. B. Akre, D. I. King, J. Kleopfer, D. J. Brown, S. W. Buchanan, H. C. Chandler, P. G. deMaynadier, M. Winters, L. Erb, K. D. Gipe, G. Johnson, K. Lauer, E. B. Liebgold, J. D. Mays, J. R. Meck, J. Megyesy, J. L. Mota, N. H. Nazdrowicz, K. J. Oxenrider, M. Parren, T. S. Ransom, L. Rohrbaugh, S. Smith, D. Yorks, and B. Zarate. 2023. Is the future female for turtles? Climate change and wetland configuration predict sex ratios of a freshwater species. Global Change Biology 29:2643–2654.

Steen, D. A., and J. P. Gibbs. 2003. Effects of roads on the structure of freshwater turtle populations. Conservation Biology 18:1143–1148.

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