Do alien plant invasions really affect pollination success in native plant species?

A growing number of studies on naturally occurring plant species have shown that plant-plant interactions for pollination vary from competitive to facilitative. In reviewing the seven published studies on how alien species can affect the pollination success in natives, we found that all authors suggest competitive effects to dominate, either through reduced pollinator visitation rates or through increased heterospecific pollination of native flowers. Although certain pollinator interactions were competitive, the reproductive output in the native species was not always reduced. This implies that natives are not pollen limited, and/or that they compensate for the loss of pollinators by other animals. However, the few studies on pollination interactions between alien and native plant species show differing results. We therefore discuss other properties that can cause aliens to be strong interactors for pollination, and can modify how alien species affect the reproductive success in natives. We also emphasize the spatial scales addressed in the reviewed studies, as alien plant species may represent valuable food resources for many pollinators. Such plant species tend to be interpreted as competitive plants for pollination, whereas we stress their facilitative ability that may occur on a landscape scale by increasing pollinator densities. Additionally, while most studies tend to work within a shorter temporal scale, the impacts of the alien plant introductions may differ depending on the year or seasonal time leading to the differing results reported.