Mechanisms for pollinator-mediated interactions between native and invasive plants

Pollinators represent a means by which different plant species may interact and influence one another’s reproductive fitness. The pollinator-mediated plant interactions that occur between native and exotic plants are of particular interest due to the increasing frequency of plant invasions worldwide. While effects of invasive plant species on native plant pollination have been documented, the mechanisms that drive such interactions are poorly understood. This dissertation focuses on the mechanisms of pollinator-mediated interactions between native and invasive plants through the study of (i) the effects of floral neighborhood on the pollination of a native plant (Chapter 1), (ii) the influence of invasive pollen deposited on native stigmas (Chapter 2), and (iii) the effects of an exotic plant on native reproductive fitness over the course of an invasion (Chapter 3). I explored how the heterospecific floral neighborhood affects pollinator visitation and composition of pollinator assemblages for native plant, Phacelia parryi. Through observations of pollination in natural patches of P. parryi, I found that floral neighborhood changed pollinator assemblage composition and that native bees were superior pollinators compared to nonnative honeybees (Chapter 1). Next, a series of hand pollination experiments was used to examine how pollen from the invasive Brassica nigra influences pre- and post-fertilization stages in Phacelia parryi. Mixed pollen applications resulted in deleterious effects on both seed set and pollen tube growth when compared to pure conspecific pollen deposition (Chapter 2). Finally, I tested the effects of invasive B. nigra abundance on the reproductive fitness of Phacelia parryi by simulating four stages in invasion and measuring pollinator visitation, pollen deposition and seed set. Native individuals near the invasion and within areas of low invasive density showed the highest reproductive fitness resulting from facilitation of pollinator visits, while natives within areas of high invasive density showed high levels of invasive pollen deposition. Isolation from the invasive reduced native fitness as a result of low pollinator visitation and conspecific pollen receipt (Chapter 3). Collectively, these results underscore the importance of determining the mechanisms for pollinator-mediated interactions between native and exotic plants and present valuable information for the mitigation of invasive plant species