Persistence in foundational seagrasses: Genetics, demography, and community implications

In this dissertation, I use genetic techniques, field observations, experiments, and mathematical modes to understand the evolutionary origins and ecological consequences of extreme female-biased sex ratios in the seagrass genus Phyllospadix (surfgrass). I develop genetic markers for sex and apply these markers in concert with field experiments to identify the origin of male rarity. I distinguish among competing evolutionary hypotheses for the origin and maintenance of skewed sex ratios and show that seedlings are born with equal ratios and female bias most likely develops gradually across surfgrass' lifespan.;Using variation in sex ratio among seven populations I assess the population consequences of male rarity. I show that pollen dispersal is limited; females adjacent to males receive pollen and produce seeds while plants just meters away receive little pollen and produce few seeds. Such pollen limitation translates to population differences in reproductive success, with fertilization and seedling recruitment rates scaling positively with the abundance of males. Male abundance regulates the success of sexual reproduction, but the overall effect of male rarity for population persistence is unclear.;To understand the effect of the loss of sexual reproduction in the context of surfgrass' life-cycle, I developed a size-structured, two-sex demographic model. This model allows me to ask how sex differences in life-history generate skewed sex ratios, how sex ratios affect surfgrass' prospects for long-term persistence, and to illuminate the evolutionary forces that maintain sex differences in life history. I show that sex differences in survival have little effect on population growth rates and that very small sex differences can produce extremely biased sex ratios. In addition, I show how the presence of multiple reproductive modes in surfgrass---sexual as well as asexual---interact in complicated ways and may play a significant role in the creation and maintenance of skewed sex ratios.;Finally, I perform a surfgrass removal experiment to connect the presence of surfgrass to its effect on an entire community of organisms. I show that in tidepools where surfgrass is removed, pool temperatures increase dramatically relative to control pools and that tidepool species composition also change significantly.