Pollinator floral provisioning by a plant invader: quantifying beneficial effects of detrimental species

Aim: Although the negative effects of invasive species are globally recognized, little is known about the potential positive impacts they can have on other species in the ecosystems they invade. However, the persistence of invaders in a wide range of ecological communities may mean that they provide resources and refuge for threatened native species, or supplement ecosystem services. Location: Agroecosystems in the USA. Methods: We use a 2-year field experiment to explore the potential positive and negative impacts of an invasive thistle (Carduus acanthoides) on the composition of the resident community of foraging insects. The presence or absence of the thistle was the only difference in experimental and control plots comprising a background community of ten flowering annual species. Results: We demonstrate that the invasive thistle is both highly visited and strongly preferred by bees relative to other flowering species. Bee abundance was 302% higher and bee species richness 35% higher in habitat patches where the thistle was present compared to where it was absent. In addition, the abundance of Bombus species, a native group recently found to be in decline, was 479% higher when the thistle was present. Main conclusions: Our results suggest that, despite causing significant problems, the invasion of this non-native species may also provide crucial benefits via floral resources for pollinators. Benefits, such as the floral resources that invaders provide to pollinators, should also be taken into account in conservation and invader management plans. Eradication or complete removal of invasive species which provision insects with floral resources could have unintended negative impacts on the associated pollinator community.