The reproductive consequences of reduced population size in the biennial Sabatia angularis (Gentianaceae)
Spigler, Rachel Beth
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Efforts to understand the factors affecting population persistence are at the forefront of population biology. Accumulated evidence suggests that small plant populations suffer fitness disadvantages such as reduced reproduction and increased inbreeding that ultimately might influence population viability. In this dissertation, I sought to improve our understanding of these processes by investigating the reproductive consequences of reduced population size in the widespread, native biennial Sabatia angularis (Gentianaceae). First, I examined how fruit and seed set varied with population size, population density, and local neighborhood size, an individual-based index of conspecific density, across 20 natural S. angularis populations. I found that small populations have significantly reduced mean fruit set, perhaps due to reduced pollination in the smallest populations. Mean seed set per population did not vary with population size or density at the population level, and this result may be a net effect of significant, opposing facilitative and competitive interactions occurring on local spatial scales. These effects presumably weakened the effect of population size on subsequent reproductive success. Second, I examined how genetic diversity and the mating system, characterized by the outcrossing rate, biparental inbreeding rate, effective number of male parents, and inbreeding coefficient, vary with population size and density. I demonstrate that outcrossing rate is significantly positively correlated with population size. Small populations had selfing rates as high as 40% compared to approximately 7% in the largest. Genetic diversity was unrelated to population size and density, and evidence suggests that selection for outcrossed offspring maintains levels of heterozygosity in small populations. Lastly, I examined the presence and magnitude of inbreeding depression (ID) in S. angularis in two environments to understand the potential impact of increased selfing on population fitness. I demonstrate significant ID for germination and survival, which culminated in significant ID for a measure of cumulative fitness in both environments. Taken together, these studies suggest that small populations of S. angularis suffer significant reductions in fruit set and outcrossing rate, which might impact population fitness. These consequences are important as they have the potential to affect population viability and fundamental population processes such as colonization and local extinction.