Competition and facilitation among plants for pollination: can pollinator abundance shift the plant–plant interactions?

We hypothesize interactions among plants for pollination may depend on pollinator abundance, which always varies among years and habitats and has different effects on plant reproductive success. Honeybee-pollinated plants, Lotus corniculatus , and its commonly coflowering neighbor, Potentilla reptans var. sericophylla , were used in a two-year project. We designed six types of plant combinations with different conspecific and interspecific flower densities in 2011 and repeated this in the same site in 2012. Meanwhile, we artificially increased pollinator abundance by hiring beehives only in 2011. Pollinator abundance as well as flower density significantly affected pollination of L . corniculatus plants from both the conspecific and interspecific plots. Total number of bees visiting a plot was enhanced by an increase in both the conspecific and interspecific flower densities regardless of high or low pollinator abundance. However, changes in visitation rates and fruit sets in the focal plants when flower densities were increased depended on pollinator abundance. Under high pollinator abundance, an increase in both the conspecific and interspecific flower densities significantly enhanced pollinator visits to L . corniculatus . However, under low pollinator abundance, the pollinator visitation rate remained unchanged as conspecific flower density increased, but decreased when there was an increase in interspecific flower density. Coflowering plants enhanced fruit sets of L . corniculatus only when the pollinator abundance was high. The findings suggest that the interactions among plants for pollination are influenced not only by a plant density threshold, but also by a pollinator abundance threshold.