Indirect facilitation mediated by pollinators in intermountain prairie

Pollinators can drive indirect facilitative and competitive indirect interactions among plant species. Most work on indirect facilitation via pollinators has focused on “magnet species” which enhance the pollination success of their neighbors because they are disproportionately attractive. However, focusing on magnet species may overestimate the generality of indirect facilitation and underestimate the occurrence of indirect competition among plant species via pollinators. We used experimental arrangements that included three flowering native intermountain prairie species (Achillea millefolium, Gaillardia aristata, and Linum lewisii), all of which are similarly attractive to pollinators, to explore how variation in species richness and density affected pollinator visitation rates, diversity, and behavior. All three plant species experienced significant increases in pollinator visitation and the species richness of visiting pollinator communities when grown with another species that was in flower at the same time. This “diversity” effect was stronger than the effects of the total density of individual plants in flower in a plot. We also found an increase in visitation time, per flower, for solitary pollinator species in plots with two species in flower compared to plots with one plant species in flower. Social pollinator species did not increase visitation time in two-species plots. Finally, seed set by Linum was significantly greater in two-species than in one-species plots. Our results indicate that indirect facilitative interactions mediated by pollinators may be common in intermountain prairie plant communities and that such indirect interactions do not have to mediated by benefactor species that are strikingly more attractive than their beneficiaries.