Anthropogenic Effects on Freshwater Turtles Across Life Stages

Abstract

Turtles are one of the most imperiled vertebrate taxa and in light of rapid global urbanization, habitat destruction, and climate change it is crucial to quantify relationships between anthropogenic change and turtle responses. In my dissertation, I explore how various anthropogenic impacts influence freshwater turtle populations at multiple life history stages. In Chapter 1, I investigate the impact of urbanization on nest site choice among turtles in the family Emydidae. Nest site choice can have important implications for embryo and hatchling survival, and ultimately, population persistence. First, I determined whether turtles selected nest sites based on a range of microhabitat variables by comparing maternally selected natural nests to randomly chosen artificial nests. Additionally, I compared nest site choice across areas with different levels of human disturbance. Natural nests had less variance in canopy openness and average daily mean and minimum temperature than artificial nests, but microhabitat variables were similar across differing levels of disturbance. Overall, these results show that turtles are not adjusting their choices of nest microhabitat when faced with anthropogenic change, suggesting that preserving certain natural microhabitat features will be critical for populations in human-disturbed areas. In Chapter 2, I explore how early life environments, that can potentially be altered by climate change, influence hatchling pond slider (Trachemys scripta) performance. Specifically, I determined how incubation temperature and overwintering environment impact hatchling righting response. I incubated eggs at five constant temperatures (24°C, 26°C, 28°C, 30°C, 32°C). After hatching, turtles were distributed among housing treatments that simulated either overwintering in a terrestrial nest or an aquatic environment. In the spring following hatching, I conducted righting response trials, which are frequently used performance measures that may be associated with coordination and neuromuscular function. Results showed that incubation temperature, overwinter environment, and their interaction significantly influenced hatchling phenotypes and righting performance. Aquatically overwintering hatchlings that were incubated at cooler temperatures righted themselves faster than those incubated at warmer temperatures. This relationship did not exist in terrestrially overwintering hatchlings, which had faster overall righting times that were not affected by incubation temperature. Altogether, these findings highlight the sensitivity of turtles to environmental alterations, as early life conditions are crucial in determining hatchling survival. In Chapter 3, I investigate the relationship between urbanization and sex ratios in pond sliders (Trachemys scripta). Increased road density and other variables associated with urbanization can have profound impacts on species that must move across the landscape. Previous work has shown that male biases in many populations of freshwater turtles are related to increased road mortality of females as they travel over land to build a nest. However, analyses related to much of this work focus on sex ratios based on sampled numbers of individuals, introducing the potential for biases. I set turtle traps at five ponds along an urbanization gradient in Lee, Macon, and Chambers counties, Alabama from 2019 to 2025. I analyzed adult sex ratios in relation to a number of variables associated with urbanization. In addition, I built capture-mark-recapture models to estimate sex-specific population sizes at each site. I present results of analyses using both sampled proportions of males and females, as well as proportions derived from population size estimates. I found an unexpected negative relationship of urbanization with the proportion of males in the population. However, differences in human housing density and impervious surfaces at our most urban sites may be driving these results, highlighting how making distinctions between different landcover variables associated with urbanization is vital in understanding human impacts on populations. In addition, I show that there are drastic differences in the statistical significance of results between analyses using sampled versus estimated sex ratios. This finding highlights the importance of obtaining accurate estimates of male and female population sizes when making inferences related to urbanization effects on sex ratios. Collectively, my dissertation documents turtle responses to anthropogenic change at various life stages that are important to population persistence. In addition to illustrating the unique responses that may occur at each stage, my dissertation sheds light on potential management strategies that could improve outcomes for urban turtle populations.