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Graduate Defense: Kristina Parker

October 29 @ 3:00 pm - 4:00 pm MDT

Thesis Information

Title: Assessing Cryptic Genetic Differentiation of Great Basin Rattlesnake and the Effects of the Cheatgrass-fire Cycle on Reptile Communities in the Sagebrush Steppe of Southwestern Idaho

Program: Master of Science in Biology

Advisor: Dr. David Pilliod, Co-chair, Biological Sciences and Dr. Jay Carlisle, Co-chair, Biological Sciences

Committee Members: Dr. Sven Buerki, Biological Sciences

Abstract

Reptiles inhabiting shrub-steppe ecosystems of the Intermountain West have adapted to harsh, unpredictable desert conditions, yet recent changes in disturbance regimes may put species at risk. In southwest Idaho, cheatgrass (Bromus tectorum) has altered the fire regime resulting in a vast conversion of shrub-steppe to mostly annual grasslands that burn too frequently to allow shrublands to recover. Southwest Idaho has the highest reptile diversity in the Pacific Northwest, yet we know little about reptile community dynamics in response to the cheatgrass-fire cycle. We hypothesized that wildfires and cheatgrass negatively affect reptile communities directly (i.e., mortality during fires) and indirectly through changes in the quality of reptile habitats at multiple spatial scales. We used trapping and visual encounter survey data to quantify the effect of previous wildfires, cheatgrass, and other habitat metrics on reptile richness, diversity, occupancy, and abundance at local (i.e., trapping array) and landscape levels. We found that vegetation cover, distance to a rock outcrop, and wildfire frequency were essential predictors c reptile abundance at both spatial scales. We found that many reptile species were not affected by cheatgrass cover but were affected by wildfire frequency. Lizard richness decreased with the number of times an area immediately around a trapping array burned. Our models indicated that occupancy for many reptile species declined in areas that burned, especially in areas with repeated burns at the local level. We found that only gophersnake abundance was significantly negatively affected by wildfire at the local level. Our research contributes to the growing body of evidence that the cheatgrass-fire cycle in the western U.S. negatively impacts many species, including reptiles. However, the effect on communities is nuanced, with winners and losers depending on a combination of habitat associations, life history, and environmental sensitivities.

Continual surveying efforts, via live-trap or visual encounter, are important for the survival of reptiles in southwest Idaho, especially for the species of concern. We further examined the occupancy of Great Basin rattlesnake (Crotalus oreganus lutosus) by assessing the genetic differentiation among and between highly occupied locations throughout the Morley Nelson Birds of Prey National Conservation Area (NCA). In addition to the cheatgrass-fire cycle, C.o. lutosus faces targeted persecution from vehicles and recreational shooters. These factors contribute to a decline in C.o. lutosus occupancy probability, which is potentially problematic— an understanding of the population genetics is powerful from a conservation standpoint. We predicted the presence of at least two subpopulations, due to the geography and urbanization of the area. The NCA encompasses a large area of land vital to wildlife, however, from a genetics standpoint it is a small landscape. Therefore, we used restriction site-associated DNA sequencing (RADSeq) to establish the genetic diversity and gene flow of C.o. lutosus subpopulations. To help identify genetic differentiation within the NCA, we included DNA samples from populations in southeast Idaho and central Nevada on the border of Utah. In addition, we used the prairie rattlesnake (Crotalus viridis) as a reference genome to assist the sequence alignment process. The inclusion of samples from distant populations within the C.o. lutosus range assisted our principal component analysis, which allowed us to identify two distinct clusters within the NCA. There are several possible reasons for two subpopulations to occur within the NCA; we examined the gene differentiation of C.o. lutosus to establish the current gene flow. Our results can assist land managers in maintaining the connectivity of subpopulations to prevent habitat fragmentation and enhance conservation efforts.