Date of Award
Doctor of Philosophy in Biological and Environmental Sciences
Nancy E. Karraker
Beginning several centuries ago, European settlement brought great change to the landscape of New England. Vast deforestation, subsequent reforestation, and rapid development that continues to this day have had a profound impact on wildlife populations. Elements of this legacy of landscape change have been studied, but the ways in which past and current landscape patterns affect freshwater turtles remains poorly understood. The primary objective of this research was to better understand the influence of the landscape of Rhode Island on populations of freshwater turtles. It is in many ways a work in landscape ecology, but with components of spatial ecology, habitat selection, and population genetics.
Chapter 1 is a stand-alone, opportunistic study of the effects of a small forest clear-cut on a population of spotted turtles (Clemmys guttata). We conducted a radiotelemetry study for one year before, and one year after a 3-ha forest clear-cut in close proximity to wetlands known to contain a resident population of the species. The annual home range size of turtles was 18.5% larger post-cut, possibly due to changes in the distribution of resources and suitable habitat after the harvest. However, turtles exhibited fidelity to hibernacula and communal hibernation despite nearby disturbance, and patterns of activity and habitat use were similar in both years and were generally consistent with those of other spotted turtle populations. Our results suggest that timber harvesting of this spatial scale and management approach may not have any short-term effects on the spatial ecology or habitat use of populations of spotted turtles. It is a strong caveat though, that further research is needed to understand longer-term effects.
Chapters 2, 3 and 4 consist of data collected during a state-wide sampling effort of freshwater turtles in small, non-riparian wetlands across a gradient of forest cover. By systematically and intensively sampling 88 randomly selected wetlands across this gradient, we intended to capture the variability in landscape composition and configuration found in Rhode Island and determine how this variability is related to species distribution, abundance, demography, and population genetic structure. In Chapter 2 we report abundances based on capture per unit effort, and use occupancy analysis to determine which among a suite of environmental and within-wetland covariates are driving species occurrence. Eastern painted turtles (Chrysemys p. picta) and snapping turtles (Chelydra serpentina) were widespread (occurring in 83% and 63% of wetlands, respectively), relatively abundant, and exhibited wide niche breadth. Spotted turtles were far less common, occurring in 8% of wetlands, and exhibited a strong association with forested, shallow, natural (i.e., not manmade or heavily modified) wetlands. Non-native red-eared sliders (Trachemys scripta) occurred in 10% of wetlands and exhibited a strong, positive association with road density, likely as a function of human population density. In Chapter 3 we further examine eastern painted turtle abundance, demography, and the relationship between sex ratio and road density. There was no difference in abundance or any demographic trait between natural and manmade wetlands. A negative relationship between abundance and forest cover surrounding wetlands emerged as the best model, but explained very little variation. Contrary to expectations, there was a significant, but weak relationship between increasing road density and the proportion of females in a population. Collectively, these results suggest that eastern painted turtles are exhibiting little to no detectable variation in population demography across the range of landscapes found in Rhode Island and are resilient in the face of human-induced landscape change. Finally, in Chapter 4 we used microsatellite markers to compare the population genetic structure between the common and widespread eastern painted turtle, and the rare and more specialized spotted turtle. Due to their relative rarity and smaller populations, we predicted that spotted turtles were more likely to have experienced the detrimental effects of habitat loss and fragmentation associated with landscape change, and that these effects would manifest in the form of more inbreeding, reduced genetic diversity, and greater population genetic structure. As expected, eastern painted turtles exhibited little population genetic structure, showing no evidence of inbreeding or strong differentiation among sampling sites. For spotted turtles however, results were consistent with certain predictions and inconsistent with others. We found tentative evidence of recent population declines in spotted turtles, as well as a greater degree of inbreeding in the species when compared to eastern painted turtles. Genetic diversity and differentiation among sites were comparable between species, however. As our results do not suggest any major signals of genetic degradation in C. guttata, the southern region of Rhode Island may serve as a regional conservation reserve network where the maintenance of population viability and connectivity is prioritized.
Globally, turtles are among the most threatened of vertebrate taxa. Information on how populations respond to human-induced landscape change has important implications for conservation. The work herein will provide wildlife biologists with a better understanding of the current state of populations of freshwater turtles in the state and region, and allow for more informed management decisions.
Buchanan, Scott W., "The Influence of Altered Habitat: Landscape Ecology of Freshwater Turtles in Rhode Island" (2017). Open Access Dissertations. Paper 665.
Available for download on Sunday, December 01, 2019