Complex structure of pollinator-plant interaction-webs: Random, nested, with gradients or modules?

Abstract

We analysed the interaction web of a plant-bee pollinator community (Hymenoptera: Apidae, honeybees excluded) for two years. Based on the ordination of the incidence matrix, both webs showed coherence and clumping but no species turnover. While this may indicate a moderate set of nested subsets and sub-communities, further analysis of nestedness did not reveal uniform results. A null-model analysis of different nestedness metrics showed no evidence despite the asymmetric structure of bipartite graphs. However, further analysis revealed significant modularization within the community with connected hub species within modules and module-interlinking connector species. The web is characterized by 4-6 dominant connector plant species, representing four main flower types. The pattern depends on the year. DCA demonstrates that the connector plant species support resources for bees of different body sizes and behaviour. The pattern is characterized by modularity and the existence of specific connector plant species. © INRA/DIB-AGIB/ EDP Sciences, 2009.