Intra-specific body size determines pollination effectiveness

Pollinators differ in morphological and behavioral traits. The effect of the resulting trait variation on pollination effectiveness at: the level of different species has received considerable attention, while the effect of intra-specitic trait variation at the population level is largely unexplored, We examined the impact of body size variation in the Red Mason bee Osmia rufa on the yield of oilseed rape in a caged field experiment. Crop yield was positively correlated to mean individual body size. Comparison with control plots showed that only large individuals increased yield to a level above pollinator-free variation. Small individuals, in contrast, even decreased yield compared to the variation within controls. Thus, the fitness of pollinator populations significantly affects pollination services, because adult body size is determined by the resource availability at the larval stage. Our results emphasize the need for complementing investigations on pollinator communities by analyses of the individual species involved. We conclude that only the availability of sufficient floral resources enables wild bee foragers to produce large, persistent offspring constituting effective pollinators for sustainable pollination services in the following season.