Date of Award

12-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Jeffery L. Larkin, Ph.D.

Second Advisor

Joseph Duchamp, Ph.D.

Third Advisor

Josiah Townsend, Ph.D.

Abstract

The shorthead garter snake Thamnophis brachystoma has one of the most restricted ranges of any snake species in the United States, with approximately 90% residing within northwestern Pennsylvania. Although recent surveys indicate that the shorthead garter snake is relatively abundant in certain populations, there is at least some evidence of overall population decline. Furthermore, the specific requirements that may contribute to the restricted range of this species are currently unknown. A better understanding of shorthead garter snake habitat requirements and areas of occupancy is a necessary first step toward the development of an appropriate species-specific conservation strategy. In 2010, I conducted shorthead garter snake occupancy surveys and associated habitat sampling at 40 sites in northwestern Pennsylvania. I used logistic regression to model the relationship between shorthead garter snake occupancy and habitat covariates. I detected shorthead garter snakes at 18 of 40 sites surveyed. Based on the regression analysis, canopy cover (p=0.0242) and distance to water (p=0.0372) were found to be the best predicting factors for shorthead garter snake occupancy (AUC=0.809). Canopy cover was lower at occupied sites (avg=1.4%) than unoccupied sites (avg=10.9%), and distance to water was shorter from occupied sites (avg=397m) than unoccupied sites (avg=1598m). My results suggest that extensive canopy cover at sites with otherwise appropriate conditions may influence shorthead garter snake presence, and that the decline of this species may be linked to patterns of landscape-scale regrowth of forests that have occurred throughout much of the species restricted range.

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