Invasive plants affect native plant pollination through pollinator-mediated cross-boundary effects

One way that invasive plants may affect native plants, either positively through facilitation or negatively through competition, is by changing the foraging behavior and/or abundance of shared pollinators. Such effects have primarily been explored in the context of neighboring individuals within single habitats, however pollinators are highly mobile foragers that can readily cross ecological boundaries, raising the potential for invaders to affect native plants across ecological boundaries. Here, we examine the potential of invasive plant species to affect native plant species with which they share pollinators in a mosaic of 22 native-plant-dominated meadows on serpentine soil, embedded in a matrix of non-serpentine meadows. The non-serpentine meadows contain dense stands of two insect-pollinated invasive species: Vicia villosa, flowering in spring; Centaurea solstitialis, flowering in summer. The serpentine meadows contain two native species that co-flower and share pollinators with these invasive plants: Trifolium fucatum and Helianthus exilis, respectively. Across three years, we determined that the abundance ratio of each invasive species to its co-flowering native species, within radii of 250-1000 meters of focal patches of the native plant, was associated with decreased visitation rates, seed set, and contribution to plant-pollinator network structure for the native species. We conclude that invasive plants can have indirect cross-boundary effects on native plants in neighboring ecosystems, via competition for shared pollinators. These findings suggest that the spillover of pollinators across ecological boundaries may have consequences for plant reproduction and the structural stability of plant-pollinator interactions.