| I conducted a series of studies to investigate possible mechanisms of persistence in rare plant species. Chapter one is a review of factors affecting rare species as well as a comparison of historically rare and formerly common species. I found mixed evidence for effects of genetic diversity, competition, pollination, and herbivory on rare species. Comparisons of historically rare and formerly common species showed interesting trends (e.g., less pollen-limitation in historically rare species and a weaker correlation between genetic diversity and population size in historically rare species) but the number of studies completed is too small to make any definitive determinations.; In chapter two I studied the effects of competition, timing of germination, site, and year to year variability on survival, growth, and reproduction in a rare serpentine endemic plant, Mimulus nudatus (Plantaginaceae). Timing of germination was found to have the largest effects on both plant weight and number of fruits produced, followed by spatial variability, and competition. There was a significant interaction between competition and site, indicating that the strength of competition depends on the particular site examined. Finally, at one site I found that plants with neighbors had higher survivorship than plants without neighbors suggesting some form of facilitation may be taking place. I found significant effects of all three factors on biomass and fruit production, as well as interactions between competition and site and competition and timing of germination.; In chapter three I studied pollination in Mimulus nudatus and found that patch area, floral density, population size, and date all had small but significant effects on pollination rate (coefficient of patch area = 0.0031, coefficient of density = 0.0018, population size = −0.000019, p = 0.045). Further, I found no effect of patch area on percentage of filled fruits in a patch and a small negative effective of patch area on mean fruit weight (coefficient of patch area = −0.007, p = 0.002). These results suggest that for this historically rare serpentine endemic plant, at a local scale, small patches are not population sinks but instead are viable populations that are not limited by pollinator visitation but by other factors, perhaps the size of the available habitat. |
