The Ecological Context of Pollination: Variation in an Apparent Mutualism

What makes a good pollinator? As pollinators perform the function of pollination, exporting and delivering grains between conspecific plant species, the floral visitor's foraging interests and the messy process of pollination interfere; many pollen grains are removed but not delivered. "Conditionally parasitic" floral visitors "waste" so much pollen (from a plant perspective) that they sometimes reduce total pollen delivery, the total number of pollen grains transferred to conspecific plant stigmas. Most pollinators visit plants with a diverse assemblage of other floral visitors; as pollinators remove, transport, and deposit pollen, they interact indirectly with one another through the supply of available pollen. These interactions can result in conditional parasitism because they affect the number of pollen grains that are successfully transported to stigmas. In this thesis, I explore how pollinator wastefulness and variation in pollinator assemblages may affect pollen export and delivery. I document a particular ecological situation in which a pollen-specialist ("oligolectic") bee and a nectar-collecting fly visit concurrently. In this system, pollen removal by the oligolectic bee results in substantial and rapid pollen depletion, which suggests that pollen wastefulness may be costly. The assemblage of pollinators varies widely throughout the geographic range of the plant; citizen scientists and I documented a consistent geographic pattern. These geographic differences correspond with variation in floral traits that are linked to pollen transfer by bees and flies. Finally, I use a simulation model to explore what ecological contexts will cause a floral visitor to increase or decrease overall pollen delivery. As a whole, these data provide substantial evidence that conditional parasitism may occur, and help elucidate aspects of plants, pollinators, and mechanisms of pollen transfer that may make it more likely. Moreover, I document plant traits that may be a result of selection mediated by conditionally parasitic floral visitors.Ph.D