| Summary: | Recognizing the abiotic and biotic parameters that favor one reproductive strategy over another can reveal much about population dynamics and aid in understanding diverging traits in different environments. This thesis work explores abiotic and biotic causes for a population of Zostera marina (eelgrass, a marine angiosperm) to partition re productively into subtidal perennial and intertidal annual zones. A tidal simulator experiment and field measurements of abiotic variables did not support a previous hypothesis that intertidal exposure favors an annual life history. However, experimental exclusion of Branta canadensis (Canada geese) identified an herbivore driven fall mortality event that occurs annually. Additional evidence documented plants within this population flowering and dying prior to the influx of Canada geese. These cryptic ???true annuals??? will over time contribute more progeny to future generations, while perennial plants are allocating energy to growth and storage, resources that go unutilized following geese grazing. Consumers have been hypothesized as a selective force for semelparity (programmed mortality following flowering); however, this may be the first empirical example of herbivore-mediated selection toward semelparity in any plant species.
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