Abstract
Alien pollinators can disrupt mutualistic plant-animal interactions. Here we review studies on alien bee pollinators, demonstrating the ways in which invasive honeybees and bumblebees affect native plant seed production. Impacts of alien pollinators vary among pollination systems and among components of pollination success (visitation frequency, pollen transfer per visit, and pollen limitation). Reproduction of native bird-pollinated plants is little impacted by alien honeybees, because birds can pollinate even when honeybees visit target flowers. In bee-pollinated native plants, alien honeybee impacts on reproduction may be pervasive, but are not clearly defined in many cases; this lack of clarity arises because very few control sites without invasive bees are available, making comparative evaluation impossible. In bumblebee-pollinated plants, alien bumblebee effects are strong because native and invasive bee species are closely related congeners (of the genus Bombus) with similar pollinator mechanisms; alien bumblebees compete strongly with natives or replace them. In general, morphological, ecological, and behavioral similarities between native and alien pollinators are important for predicting their competitive interaction intensities and their effects on plant reproduction. The impacts of alien bees include (1) decreased pollen transfer per visit because of the loose morphological correspondence between flowers and alien bees, (2) increased competition for floral resources and exclusion of native pollinators because native and alien bees have similar ecological requirements, (3) changes in native pollinator visitation frequency and pollination efficiency caused by particular alien pollinator behavioral traits (such as stealing nectar or collecting previously-deposited pollen from stigmas). For a quantitative evaluation of invasion impacts, it is important to determine whether native plants suffer pollen limitation. To demonstrate pollen limitation, comparative studies with controls (that lack alien pollinators) are required. This approach is essential for discerning possible mechanisms responsible for observed patterns.
