Understanding the population structure within a species, and understanding the processes shaping those patterns are important for basic and applied ecology. Here, we used genetic data and theory about population genetics to investigate the spatial structure of the desert tortoise (Gopherus agassizii) in the Mojave Desert, and used genetic analyses to induce the potential factors that created population structure. The desert tortoise is listed as threatened under the U.S. Endangered Species Act of 1973 in the northern extent of its range, which occurs north and west of the Colorado River. This distinct population segment has experienced severe population declines mainly resulting increased human impacts in the southwestern United States. The life history traits (i.e., long life span) and cryptic behavior of the desert tortoise make extensive field studies on population dynamics difficult. Thus, it was necessary to use highly variable, neutral genetic markers and analyses based in population genetic theory to make inferences about the population ecology of this species. The goals of this research were to identify genetic population boundaries, assess levels of gene flow among subpopulations, and determine the biological and physical landscape features that influence movement of individuals through habitat in the Mojave Desert. Additionally, we provide several recommendations to revise conservation strategies for the Mojave desert tortoise.
Despite discovering low levels of genetic differentiation among tortoises across the geographic range, we were able to detect hierarchical structuring within the population. Three basal groups were identified that correspond to the mitochondrial DNA haplotypes identified by others two decades previously. Within these three basal groups, we detected seven subpopulations that loosely align with major geographic features. Geographic distance among subpopulations was a strong determinant of population structure, which suggests that localized dispersal is occurring across the geographic range. To investigate additional factors influencing movement of desert tortoises, we used a landscape genetics approach. We tested multiple hypotheses to determine which landscape features best correlate to patterns of gene flow. Landscape-genetic models supported the hypothesis that topographical features such as mountain ranges explain additional patterns in genetic substructure beyond a simple isolation-by-distance model.
The long generation time of desert tortoises contributes to a time lag in the genetic patterns identified by our analyses. Therefore, the inferred patterns of gene flow did not include any potential disruption from human activities such as habitat modification due to urbanization and the development of human infrastructure such as major highways. This unique situation allowed me to make conservation recommendations based on a genetic snapshot of historic population processes. Our main recommendations pertain to revising conservation units, maintaining landscape connectivity, and improving translocation of individuals. We provide suggestions for adjusting the boundaries of recovery units based upon genetic data as well as differences in ecology and behavior of desert tortoises that occur across environmental gradients in the Mojave Desert. We identify habitat corridors that have been historically important for connectivity among subpopulations, and we proffer potential management actions to maintain connectivity. Finally, we describe how genetic data can provide additional guidance for where individuals should be translocated.
|Advisor:||Tracy, C. Richard|
|Commitee:||Hoelzer, Guy, Peacock, Mary, Wilds, Leah, Zimmerman, Lynn|
|School:||University of Nevada, Reno|
|Department:||Ecology, Evolution and Conservation Biology|
|School Location:||United States -- Nevada|
|Source:||DAI-B 69/12, Dissertation Abstracts International|
|Keywords:||Gopherus agassizii, Mojave Desert tortoise|
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