Ecology and evolution of distribution patterns in vernal pool annual plant populations

The nature of species' distribution patterns has long been a focus of ecology, and has recently received increasing attention from evolutionary biologists. Through field experiments and observations, I explored the roles of both ecological and evolutionary processes in determining the local distributions of annual plants endemic to California's vernal pools.; In southeastern Sacramento County, I observed that the population boundaries of five dominant vernal pool plant species were relatively consistent across years that differed markedly in the amount and distribution of precipitation. A transplant experiment revealed that each species is broadly adapted to the hydrological conditions in which it is naturally most abundant, but that competition and recruitment limitation may reinforce population boundaries by restricting the ability of species to track year-to-year fluctuations in the inundation regime.; After establishing the ecological context associated with population boundaries, I examined the evolutionary processes underlying the distribution of one vernal pool endemic, Lasthenia fremontii. In this species, differences in the inundation regime across the environmental gradient have dramatic effects on phenology that may influence the potential for cross-gradient gene flow by pollen. In a shallow vernal pool and particularly dry year, flowering was nearly synchronous between center and edge plants, creating an opportunity for extensive pollen movement across the inundation gradient. There was no diversifying selection across the inundation gradient in this year, although the opportunity for selection was significantly reduced in the edge habitat. Very different patterns of gene flow and genetic differentiation were found in a pool with a broader inundation gradient and a year with higher levels of precipitation. Under those conditions, more dramatic phenological isolation and highly selective pollinator behavior restricted cross-gradient pollen transfer to generate differences between lineages from the center and edge of the L. fremontii population. These differences did not reflect adaptive differentiation, but instead suggest that genetic isolation of plants at the population edge may lead to inbreeding depression even over these small spatial scales. Together, these results emphasize that the ecological and evolutionary factors enforcing local range limits may vary dramatically among sites and years due to spatial and temporal variation in the environment.