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The population biology of dispersal and gene flow in the desert shrub Acacia (Senegalia) greggii A. Gray in the Mojave National Preserve


Desert ecosystems are increasingly affected by the human-driven environmental pressures currently changing global landscapes and climates. Plant species are especially susceptible to these changes due to their inability to move except through propagules and a frequent reproductive dependence on dispersing animal species that could themselves be displaced. To identify the limitations climate and landscape already exert on the movement and reproduction of plant species in desert zones, we examined the desert shrub Acacia (Senegalia) greggii A. Gray in the Mojave National Preserve. In chapter one, we give an overview of the ecosystem and study species. In chapter two, we present determinants of reproductive success in A. greggii shrubs. Limited water availability in deserts leads to annual and spatial variation in floral production. The impact of this variation on the reproductive success of a plant depends on the response of dispersal agents and seed predators to fluctuating food resources. We conducted observational surveys and experimental manipulations, and found that greater floral abundance attracted a greater number of pollinators, increased fruit-set, decreased the seed predation rate, and thus increased the pre-dispersal reproductive success of the shrub. In chapter three we present the historic and contemporary dispersal in A. greggii. We compared the genetic patterns of adults and pollen in a 4 km2 area, and found widespread gene flow indicating pollen flow may be more extensive than seed dispersal. Despite this extensive movement, both adult and pollen genetic patterns were explained by separation between dry-washes, suggesting a potential dispersal corridor for pollen and seeds. In chapter four, we present our comparison of alternative landscape pathways derived from both climatic and topographic variables to explain regional genetic structure. We examined movement across 2,700 km2 to determine if the fine-scale patterns observed in chapter three translate into regional connectivity. We again found widespread dispersal was best explained by gene flow along dry-washes. Our work documents a species with a well-adapted reproductive strategy and historically widespread dispersal. In spite of these results, the movement pattern of A. greggii is shaped by landscape features suggesting a potential impact of landscape change and development on future movement.

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