Understanding the Effects of Floral Density on Flower Visitation Rates and Species Composition of Flower Visitors

Pollinator responses to varying floral density have important implications for plant population dynamics, conservation, and the evolution of floral traits. Floral density can influence both flower visitation rates and the species composition of flower visitors, but neither of these effects is well understood. In my dissertation, I generate and test hypotheses explaining the relations of flower visitation rate and flower visitor species composition to floral density.In the first chapter, I present a foraging model that explores potential sources of variation in the effects of floral density on flower visitation rates. The model predicts that the relation of per-flower visitation rates to floral density will be nonlinear, with strong positive effects at low floral densities and weaker or negative effects at higher densities. Results from a field experiment using the annual composite Holocarpha virgata support this prediction. The model also shows that floral density in the surrounding environment and traits of both plants and their pollinators can influence the effects of floral density on visitation. In the second chapter, I describe an observational field study, again using H. virgata, that shows that flower visitors respond differently to floral density at site (12.6 ha) and patch (4 m2) scales and furthermore that floral density at the site scale influences flower visitors' responses to patch-scale floral density. This study also reveals an effect of floral density on the species composition of H. virgata's flower visitors. In the final chapter, I explore effects of floral density on the species composition of flower visitors, using a variation of the model presented in the first chapter. The model identifies several flower visitor traits, including flower search speed, flower handling time, and foraging currency, that can influence whether a species uses primarily dense or sparse patches when in competition with other species. Collectively, these studies fill a significant gap in our knowledge of pollinator responses to floral densities. By developing testable hypotheses based on biologically reasonable assumptions to explain many previously-observed phenomena, they lay the groundwork for understanding this important aspect of plant-pollinator mutualisms