Over what distances can one plant species affect the pollination of another?
Plant species in a community can increase or decrease another’s pollination, if the abundance or foraging behavior of shared pollinators is altered when plants grow together. In particular, “magnet plants”, those with highly attractive flowers, can facilitate the pollination of other plants, because they draw pollinators into an area, which then spill over to visit coflowering neighbors. However, the benefit experienced by coflowering plants may vary with their distance from the magnet: if too close, the plant may be ignored by preoccupied visitors, while if too far, the plant may be outside the range of magnet-induced visitor abundance. Furthermore, the distances that pollination facilitation can extend are unknown. To explore this, we conducted a manipulative field experiment in a subalpine meadow with the putative magnet plant, Delphinium barbeyi, and two less common gentian species, which have different floral morphologies: Gentianopsis detonsa (“complex gentian”) and Gentiana parryi (“simple gentian”). We placed test arrays at 0, 3, 10, 30, and 100 m from a large patch of Delphinium, marked bumble bees foraging on Delphinium to observe their movements, and observed bumble bee visits and pollination success in the gentians.
The results differed for the two plant species. We saw very few visits to the complex gentian arrays, and bumble bee visits and seed set were unrelated to distance from the Delphinium patch. Individual bees rarely moved between the two species, so although they shared pollinator species, they did not share pollinator individuals. The complex gentian flowers may be too complex for bees to forage on simultaneously with Delphinium. In contrast, bumble bee visits and stigma pollen loads in the simple gentian were related to distance from Delphinium: they declined at 100 m, and we regularly saw individual Delphinium bees forage on the simple gentian up to 30 m away. In this system, Delphinium acted as a pollinator magnet for the simple gentian, an effect that extended beyond the distances that researchers typically examine. Furthermore, our study shows that coflowering plants must share pollinator individuals, and not just species, to affect the pollination success of each other.