Date of Graduation

5-2016

Document Type

Thesis

Degree Name

Master of Science in Biology (MS)

Degree Level

Graduate

Department

Biological Sciences

Advisor

John D. Willson

Committee Member

Dan Magoulick

Second Committee Member

Steve Beaupre

Keywords

Biological sciences; Locomotion; Seminatrix pygaea; Snake; Water loss

Abstract

Aquatic ecosystems and their associated wildlife are threatened by alteration in the frequency and intensity of precipitation events predicted to affect many regions as a consequence of global climate change. Semi-aquatic snakes serve important roles as predators and prey within aquatic ecosystems, yet little is known about the effects of drought on these species due to their secretive behavior. Long-term studies at an isolated wetland in South Carolina found that drought caused populations of banded watersnakes (Nerodia fasciata) and Florida green watersnakes (Nerodia floridana) to crash. Alternatively, black swamp snakes (Seminatrix pygaea) fared well, exhibiting a resistance strategy by aestivating within the wetland. This thesis presents two studies that build on our understanding of these observed patterns. The first chapter evaluates the generality of these interspecific differences in drought susceptibility by conducting a landscape scale analysis of semi-aquatic snake responses to drought using an occupancy modeling approach. We compared changes in occupancy and detectability from 20 isolated wetlands in South Carolina before drought to after drought. Our results confirm that interspecific patterns of drought sensitivity observed previously at one wetland are paralleled at the landscape scale. Specifically, site occupancy decreased dramatically for N. fasciata (0.95 to 0.69) and N. floridana (0.32 to 0.05) but was relatively unchanged for S. pygaea (0.41 to 0.45), Farancia abacura (0.49 to 0.49) and Regina rigida (0.10 to 0.28). Our results confirm that supra-seasonal drought can have negative effects on some, but not all, semi-aquatic snake species. The second chapter investigates interspecific variation in two traits that may mediate species’ responses to drought: evaporative mass loss (EML; the rate of water and product lost through respiratory, cutaneous and ocular pathways) and locomotor endurance (total distance, total time, and speed traveled before exhaustion in aquatic habitats and in hydrated and dehydrated state in terrestrial habitats). We find that for EML, species group into three categories that do not follow taxonomic relationships, with N. floridana exhibiting unexpectedly high and Regina rigida (glossy crayfish snake) exhibiting unexpectedly low rates of EML. Species also vary in their ability and willingness to move over land, suggesting that some species may be more willing or able to make terrestrial overland movements in response to drought than others. Our results identify important mechanisms that may drive variable responses of semi-aquatic snakes to environmental variation. In particular, the apparent sensitivity of N. floridana to drought may be driven in part by its high rate of EML and reluctance to move terrestrially. Taken together, this thesis builds our understanding of patterns of interspecific variation in sensitivity to drought and traits that may underpin this variation. This information is a critical component of understanding, predicting, and mitigating populations threated by climate change and other anthropogenic alterations to the environment.

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