Functional complementarity and specialisation: The role of biodiversity in plant-pollinator interactions

Ecological niche breadth (specialisation) and niche differentiation (complementarity) play a key role for species coexistence and hence biodiversity. Some niche dimensions of a species represent ecosystem functions or services such as pollination (functional niche). When species differ in their contribution to some collective function (functional complementarity), this implies that functions from several species are required for a high overall functional performance level. Applied to plant pollinator interactions, functional complementary suggests that a higher diversity of pollinators contributes to an increased pollination success of the plants or, in turn, that a higher diversity of flowers may better sustain the consumers' requirements. Complementarity can affect functioning at different scales: the collective functioning of the target community, a single species, an individual or even a part of the individual, e.g. a single flower. Recent network analyses revealed that plant pollinator interactions display a relatively high extent of complementary specialisation at the community scale. We propose several mechanisms that generate complementarity. From the consumers' viewpoint, differences in flowering phenology and/or nutritional variation in floral resources (nectar, pollen) may explain a complementary role of different flower species. From the plant's viewpoint, temporal or environmental variation in the pollinator species' activities may contribute to complementary effects on pollination of plant communities. In addition, different species may also pollinate either more exposed or more sheltered flowers from the same plant individual, or vary in their functions within single flowers. So far, empirical evidence for complementary effects in general, and particularly mechanistic explanations of such effects are scant and will require comparative investigations at multiple scales in the future. Such studies will help us to understand if and how biodiversity maintains the quality and quantity of plant pollinator functional relationships.