Experimental evaluation of the effect of co-flowering exotic species richness on pollinator diversity

Habitat degradation and exotic species invasion are suspected to be important factors driving global declines of insect pollinators. Recent research has demonstrated that exotic flowering plant invasion can alter the structure and dynamics of pollination communities. But few studies have evaluated the effect of exotic plant richness on pollinator diversity and foraging behavior. Here we used a two-factor experimental design to evaluate the effect of co-flowering exotic plant biodiversity on pollinator diversity and foraging behavior. Using a coastal old-field, we manipulated species identity and richness by planting four co-flowering exotic plants, Achillea millefolium L., Leucanthemum vulgare Lam., Trifolium pratense L., Vicia cracca L., in four monoculture treatments and in four combinations of three species assemblages. For each plot, absolute plant density remained constant and relative plant density varied. During observation periods, we measured insect visitor diversity and visitation rate. Bumblebees (Bombus spp.), honeybees (Apis mellifera), and lepidopterans, primarily skippers (Polites peckius, Hesperiidae) preferentially visited V. cracca and T. pratense. Dipterans and solitary bees, primarily Halictus spp. and Lasioglossum spp., preferentially foraged on L. vulgare and A. millefolium. A greater diversity of insect visitors visited assemblage treatments compared to monoculture treatments. Assemblage treatments had higher insect visitation rates compared to monoculture treatments. Insect visitor diversity was lower in all experimental treatments compared to the surrounding local habitat. These results indicate that exotic flowering plant species likely compete with native flowering plants for pollinators. Furthermore this evidence suggests that exotic flowering plants interact with pollinators differently depending on the surrounding floral neighborhood.