How pollinator-mediated mating varies with population size in plants

In most higher plants sexual interactions are mediated by animal pollinators that affect the number and differential reproductive success of mates. The number and sex of breeding individuals in populations are central factors in evolutionary theory, but the quantitative effect of plant population size on pollinator-mediated mating is understudied. We investigated variation in pollen removal (male function) and fruit set (female function) among flowering populations of different size of two bumblebee-and one butterfly-pollinated, rewardless, pollen-limited, hermaphroditic orchid species in Sweden. As the amount of pollen removed from plants by insects (either absolute or proportional) increased, so did the number of pollinations, whereas the proportions of plants with different pollinator-designated functional sex (male, female, hermaphrodite) depended primarily on the ratio between the amount of fruit set and pollen removed within populations. A larger population size was found to have several effects: (1) the total numbers of pollinia removed and fruits set increased; (2) the proportion of pollen removed from plants decreased; (3) the proportion of flowers pollinated decreased in the butterfly-but was not affected in the bumblebee-pollinated species; (4) the ratio between fruits set and pollinia removed increased linearly in the bumblebee-pollinated species but reached a maximum at c. 80 individuals in the butterfly-pollinated species; (5) the numbers of pollinator-designated pure male and hermaphrodite individuals increased; and (6) the variance in pollinium removal, but not fruit set, increased among individuals. These findings empirically verify the basic importance of population size for the mating structure of outcrossing plants, and indicate that selection for female sexual traits is reinforced when population size is smaller while selection for male sexual traits is reinforced when population size is larger.