Effects of geitonogamy, habitat fragmentation, and population size on planet reproductive success: Ecological and evolutionary studies

The research presented here explores how geitonogamy (pollinator-mediated self fertilization), habitat fragmentation, and population size effects plant reproductive success. At the center of all three research questions is an attempt to understand better the core ecological interaction between plants and their pollinators. Chapter 2 investigates the importance of geitonogamy on the adaptive significance of the showy milkweed Asclepias speciosa . I conducted a field experiment involving hand-pollinations and inflorescence bagging to test how natural levels of geitonogamy impact fruit set. I found that (1) natural populations are significantly compatible-pollen limited, and (2) high rates of geitonogamy significantly increase fruit abortion and reduce fruit set in natural populations of A. speciosa . Both these findings are consistent with the plant's dilemma hypothesis of inflorescence design evolution, which proposes that evolution of larger inflorescences is driven by selection for greater pollinator attraction, but constrained by higher rates of geitonogamy. Chapters 3 and 4 investigate the pollination crisis, the recently recognized phenomenon of disrupted plant-pollinator interactions worldwide. Chapter 3 focused on the effects of habitat size on the reproductive success of Geranium viscosissimum. This study took place in the fragmented Palouse Prairie ecosystem of eastern Washington and compared plants occurring in different size habitat remnants. I found that seed set of Geranium viscosissimum declined with reduced habitat size, as did pollinator visitation, richness, and diversity. I conclude that smaller habitats are less able to support or attract robust populations of diverse and effective pollinators, thereby negatively influencing plant reproductive success of G. viscosissimum. Chapter 4 focused on the effects of population size on the reproductive success of A. speciosa, comparing plants from different population sizes. A unique feature of this study was the creation of smaller populations from larger populations to control for inherent qualities of larger populations that may favor plant reproductive success. I found that fruit set of A. speciosa declined with reduced population size, as did pollinator visitation rates, richness, and diversity. I conclude that smaller milkweed populations are less able to attract pollinators, including bumblebees, thereby directly reducing fruit set.