The effect of floral similarity between co-flowering native and exotic plants on pollinator visitation to a native forb (Achillea millefolium)
Exotic plant invasion may alter the strength and frequency of pollinator visitation to native flowering plants. Because co-flowering plant species often share pollinators, visitation to native flowers can be influenced by changes in the density and diversity of flowering exotic plants. It is expected that visitation rates to native flowers will be negatively affected when co-flowering exotic plants are phenotypically similar in color or symmetry to native flowers. This study experimentally tested whether the presence of exotic flowering plants and phenotypic similarity between exotic and native flowers influenced pollinator visitation to a co-flowering native, Achillea millefolium. The exotic plants were Leucanthemum vulgare, Trifolium pratense, and Vicia cracca. Leucanthemum vulgare and A. millefolium both have open white floral displays that exhibit radial symmetry (actinomorphic), while T. pratense and V. cracca have complex floral displays that exhibit biradial symmetry (zygomorphic). In the experiment, three treatments were used: i) A. millefolium in monoculture, A. millefolium in exotic plant assemblages ii) with and iii) without L. vulgare. The pollinators of A. millefolium and L. vulgare were primarily dipterans, including syrphid flies, and small halictid bees, primarily Halictus and Lasioglossum. The pollinators of T. pratense and V. cracca were primarily bumble bees, honey bees, butterflies and skippers. In the presence of exotic plants with a dissimilar floral phenotype, the visitation rate of syrphids and other dipterans to A. millefolium floral displays declined by 69% and 48%, respectively, while the visitation rates of halictids did not differ among treatments. Contrary to predictions, results from this experiment demonstrate that phenotypic dissimilarity between native and exotic flowers may result in reduced rates of pollinator visitation to native flowers.