Pollinator diversity affects plant reproduction and recruitment: the tradeoffs of generalization

One outstanding and unsolved challenge in ecology and conservation biology is to understand how pollinator diversity affects plant performance. Here, we provide evidence of the functional role of pollination diversity in a plant species, Erysimum mediohispanicum (Brassicaceae). Pollinator abundance, richness and diversity as well as plant reproduction and recruitment were determined in eight plant populations. We found that E. mediohispanicum was generalized both at the regional and local (population) scale, since its flowers were visited by more than 100 species of insects with very different morphology, size and behaviour. However, populations differed in the degree of generalization. Generalization correlated with pollinator abundance and plant population size, but not with habitat, ungulate damage intensity, altitude or spatial location. More importantly, the degree of generalization had significant consequences for plant reproduction and recruitment. Plants from populations with intermediate generalization produced more seeds than plants from populations with low or high degrees of generalization. These differences were not the result of differences in number of flowers produced per plant. In addition, seedling emergence in a common garden was highest in plants from populations with intermediate degree of generalization. This outcome suggests the existence of an optimal level of generalizations even for generalized plant species.