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Bermingham, Laura Hill

Biology

2009

PhD

Ecological and evolutionary factors affecting threatened and endangered species may compromise a population's ability to persist through time. Here, I determined how plant mating system, pollination biology, pollen source, habitat type, and white-tailed deer (Odocoileus virginianus) herbivory affected the persistence of a rare wetland endemic plant, Polemonium vanbruntiae (Eastern Jacob's ladder). In contrast to several of the more common species of Polemonium, I found no pollen limitation in populations of P. vanbruntiae. The lack of pollen limitation was best explained by the capacity for P. vanbruntiae to self-fertilize. However, pollinators play an important role as inter- and intra-plant pollen vectors in this system because female reproductive fitness was greatly reduced when pollinators were excluded. These results support the reproductive assurance hypothesis, whereby the ability to self assures fertilization for plants in small populations. A mixed-mating strategy, including the ability for clonal reproduction, may explain the ability for this rare species to persist in small, fragmented populations. However, mixed mating strategies may incur both costs and benefits.
The ability to self-fertilize may provide reproductive assurance that when pollinators and/or potential mates are scarce. Yet, selfing is potentially costly when the result is a reduction in offspring quantity and quality. I found that the relative performance of selfed offspring was lower than outcross offspring in terms of germination and offspring vigor. When pollen is received from a distant site, offspring exhibit heterosis with increased vigor in terms of more leaves, larger leaf area and height. Although I was unable to follow offspring survival to flowering, enhanced germination success and more vigorous growth suggest that gene flow among populations may increase plant vigor and enhance genetic variation within small, isolated populations of P. vanbruntiae. For long-lived organisms such as perennial plants, it is often difficult to determine which environmental factors will have the largest effects on long-term population dynamics. I incorporated the effects of habitat type and white-tailed deer herbivory into a population viability model for P. vanbruntiae to determine the effects of both habitat and herbivory on long-term ramet dynamics.
Polemonium vanbruntiae ramets in wet meadow habitats are expected to increase at a faster rate than ramets in the forest seep habitats, but P. vanbruntiae is expected to increase over time in both habitat conditions. White-tailed deer preferentially browsed adult ramets, primarily those occurring in forest seep sites. Deer browsing significantly decreased the predicted population growth rate under stochastic conditions, although P. vanbruntiae ramets are expected to increase at a slow rate in the future under herbivore pressure. Herbivory also increased the extinction risk to a detectable level. Deer browsing shifted the potential for younger life histories stages to change future population growth. Instead, survival and stasis of large vegetative ramets became the vital rates having the largest potential impact on future population growth. Thus, active long-term management of deer populations appears necessary for the guaranteed persistence of populations of rare, endemic herbs such as P. vanbruntiae.