Facilitative interactions among plants via shared pollinators

Outcrossing in plants is influenced by the availability of pollinators and compatible mates, both of which may be modified by the population and community context in which plant-pollinator interactions occur. Although indirect interactions among plants through shared pollinators are often expected to be competitive, pollinator sharing may be beneficial when plant species jointly attract or maintain populations of pollinators. In this study, I tested the hypothesis that pollinator-sharing congeners facilitate reproduction in a focal taxon, Clarkia xantiana ssp. xantiana, and that positive interactions are most pronounced in small and sparse populations. Population surveys revealed that C. x. xantiana frequently coexists with pollinator-sharing congeners except at the periphery of its range. Populations varied extensively in size and density, with small populations more likely associated with pollinator-sharing congeners; conversely, populations occurring alone were more likely large. Flowering schedules in Clarkia communities ranged from segregated to aggregated. Although there was not strong evidence of character displacement, modes in flowering time were often staggered among Clarkia species resulting in a protracted flowering season within plant communities. Studies of bee pollinator availability in 17 populations and pollen limitation to reproduction in 39 replicate populations revealed that populations occurring with multiple congeners had high pollinator availability and low pollen limitation of reproduction compared to populations occurring alone. Population size was inversely related to pollen limitation but did not affect pollinator availability, suggesting that Allee effects were caused by mate limitation. Intraspecific interactions were also positive at a fine spatial scale where pollen deposition increased with the density of closely neighboring conspecifics across 11 populations. Overall, inter- and intraspecific interactions through shared pollinators were generally facilitative, suggesting that population viability and the coexistence of ecologically similar Clarkia species may be promoted by positive reproductive interactions.