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Connecting the Plots: Anthropogenic Disturbance and Mojave Desert Tortoise (Gopherus agassizii) Genetic Connectivity
AuthorDutcher, Kirsten Erika
AdvisorHeaton, Jill S.
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Habitat disturbance impedes connectivity for native populations by altering natural movement patterns, significantly increasing the risk of population decline. The Mojave Desert historically exhibited high ecological connectivity, but human presence has increased recently, as has habitat disturbance. Human land use primarily occurs in Mojave desert tortoise (Gopherus agassizii) habitat posing risks to the federally threatened species, which has declined as a result. As threats intensify, so does the need to protect tortoise habitat and connectivity. Functional corridors require appropriate habitat amounts and population densities, as individuals may need time to achieve connectivity and find mates. Developments in tortoise habitat have not been well studied, and understanding the relationship between barriers, corridors, population density, and gene flow is an important step towards species recovery. Genetic tools provide a framework to examine processes like movement and incorporating landscape enhances our understanding of genetic patterns. For tortoises a historically connected landscape coupled with limited dispersal produced a pattern of isolation-by-distance. This dissertation highlighted more recent genetic connectivity by: (1) assessing population genetic structure and relatedness across a recently disturbed landscape, (2) evaluating the impact of barriers and corridors using simulations of genetic processes, and (3) investigating the relationship between landscape metrics and genetic connectivity using simulations of disturbance scenarios. I genotyped 299 tortoises at 20 microsatellite loci from the Ivanpah Valley region along the California/Nevada border. A fine-scale sampling scheme was applied to evaluate recent gene flow and historical genetic structure. Because the genetic effects of disturbance are often observable after a substantial time lag, I used individually based spatially explicit forward-in-time genetic simulations to test hypotheses related to barriers, population density, and habitat disturbance. I used landscape resistance surfaces representing 17 locations (525 – 625 km2) in southern Nevada and evaluated connectivity success using genetic differentiation. Three genetic clusters that generally corresponded to valleys and one mountain pass were detected with second order relationships up to 60 km apart, suggesting a greater range of interactions than previously suspected. The correlation between pairwise genetic distances and cost distances revealed reduced genetic connectivity across a railway and a highway bisecting the study area. In simulations of linear barriers, genetic connectivity improved with corridors, but was also influenced by population density. Low density landscapes experienced reductions in population size and genetic diversity with or without barriers as the result of individuals moving without finding mates and genetic drift. Simulations found that anthropogenic disturbance increased demographic and genetic effects. Disturbed landscapes with high levels of genetic connectivity tended towards low levels of landscape fragmentation with high amounts of suitable habitat. Urban growth is predicted to exacerbate declines in tortoise populations and genetic connectivity unless intact habitat and populations are adequately protected and connected. This research provides a basis for management actions to protect desert tortoise habitat between existing conservation blocks and restore connectivity along current barriers.