Pollinator-mediated interactions between native plants and the invasive alien Himalayan balsam

Invasive alien species threaten global biodiversity and can impose severe economic costs. Some invasive alien plants can be strong competitors for pollinators due to a high abundance of attractive flowers, which can disrupt native plant-pollinator interactions and reduce native plant reproductive success. This thesis explores pollinator-mediated competition between native plants and the invasive alien Impatiens glandulifera, Himalayan balsam. Previous studies have found conflicting effects of I. glandulifera on native plant-pollinator interactions. This study aimed, using a combination of direct field observations and controlled experiments, to explore the reasons behind these differences by examining the relationship between impact and the abundance of I. glandulifera at multiple spatial scales, and the responses of a wide range of cooccurring species. Chapters two and three tested the hypothesis that the composition of plant and pollinator communities and bumblebee-flower visitation vary in response to I. glandulifera abundance and spatial scale of invasion. Chapter four tested the hypothesis that I. glandulifera pollen will reduce the reproductive success of the native Lamium album. Chapter five tested the hypothesis that pollinator-mediated competition will alter the genetic quality of pollen received by co-flowering L. album. I found a relationship between the plant and pollinator community composition and the abundance of I. glandulifera, which was generally stronger at a broad scale. Responses to invasion differed according to pollinator taxa and plant traits, which could be useful for identifying and protecting potentially vulnerable native species. Impacts differed according to the mechanism used to examine its effects: bumblebee-flower visitation patterns changed, and L. album experienced reduced seed set and disruption to its mating system; however alien pollen did not prevent L. album from setting seed. In conclusion, the direction and magnitude of pollinator-mediated effects varied with I. glandulifera abundance, spatial scale, and the way in which impact was assessed. A wide range of approaches are necessary to understand the impact of invasive alien plants.