Neutral effect of an invasive plant species with specialized flower structure on native pollinator communities

Invasive plants modify native plant communities with serious consequences on plant-pollinator interactions. Invasion by common milkweed (Asclepias syriacaL.) threatens natural and agricultural habitats in Europe, with unknown effects on pollinators. Its special flower structure, habitat requirements and phenology offer novel insights into pollination ecology aspects of plant invasion. We compared flowering plant and pollinator communities between invaded and control sites, and the flower visitors between native plants and common milkweed. Wild bees and hoverflies did not differ in abundance, diversity and community composition between the invaded and control sites. However, honey bees and bumble bees preferred milkweed above native plants during milkweed flowering. In contrast to many studies, our results suggest neutral effect of plant invasion on the sampled aspects of diurnal wild pollinator community, while providing resources for a few pollinator taxa. This neutral effect might be explained by the long-term, wide scale distribution of milkweed and/or its typically relatively low coverage compared to many other invasive plants, enabling the persistence of some native flowering species. However, its special flower structure offers nectar only for a few common pollinators, including honey bee, and it decreases abundance of native flowers in spring with unknown consequences on wild bees' reproduction success. Despite the lack of direct negative effects on wild pollinators, restoration of invaded habitats to promote native floral communities is suggested to enable diverse, longer lasting foraging resources for wild pollinators and honey bees. Promoting actively wildflower habitats might be vital for beekeepers in the case of milkweed eradication.