| Summary: | Small populations are at increased risk of extinction due to their vulnerability to stochastic events. The population of fisher (<italic>Pekania pennanti</italic>, formerly <italic>Martes pennanti</italic>) in the southern Sierra Nevada Mountains of California is small and completely genetically isolated. My dissertation research investigates the timing and cause of this population's isolation, the degree of genetic subdivision within the population, the landscape features shaping gene flow, and the detection of population declines.
I detected a 90% decline in effective population size and dated the time of decline to over a thousand years ago. Analyzing historical and contemporary genetic samples, I also found a recent bottleneck signal in the northern portion of the southern Sierra Nevada, indicating the southernmost tip of these mountains may have acted as a refugium for fisher in the late 19th century. I conclude that this population became isolated pre-European settlement, and that portions of the southern Sierra Nevada subsequently experienced another more recent bottleneck post-European settlement.
I found that the southern Sierra Nevada fisher population is not highly genetically subdivided as previously thought. This population follows a pattern of isolation by distance with additional structuring that corresponds to geographic features and management boundaries. It can be characterized as having areas that are resistant to gene flow but without major barriers. I show that both sex-biased dispersal and spatial landscape heterogeneity can affect the determination of what landscape features structure gene flow, and that the landscape features influencing gene flow are different for each sex and within different geographic regions.
Using a spatially-based simulation approach, I investigated the power of the Sierra Nevada fisher monitoring program to detect population trend, and illustrate the relationship between occupancy and abundance in this population. I show that a simulated 43% decline in abundance over an 8-year period only resulted in a 23% decline in occupancy. I also found that increasing the effective sampling area, implementing biennial instead of annual sampling, and increasing the type I error rate all increase statistical power to detect trend. Overall this research provides a better understanding of the historical and contemporary connectivity of this population and our ability to monitor population trends over time that will contribute to the conservation of fisher populations in the future.
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