DOI 10.1007/s00442-007-0946-1PLANT-ANIMAL INTERACTIONS- ORIGINAL PAPER Contrasting eVects of invasive plants in plant–pollinator networks

Abstract The structural organization of mutualism net-works, typiWed by interspeciWc positive interactions, is important to maintain community diversity. However, there is little information available about the eVect of introduced species on the structure of such networks. We compared uninvaded and invaded ecological communities, to examine how two species of invasive plants with large and showy Xowers (Carpobrotus aYne acinaciformis and Opuntia stricta) aVect the structure of Mediterranean plant–pollina-tor networks. To attribute diVerences in pollination to the direct presence of the invasive species, areas were surveyed that contained similar native plant species cover, diversity and Xoral composition, with or without the invaders. Both invasive plant species received signiWcantly more pollina-tor visits than any native species and invaders interacted strongly with pollinators. Overall, the pollinator commu-nity richness was similar in invaded and uninvaded plots, and only a few generalist pollinators visited invasive spe-cies exclusively. Invasive plants acted as pollination super generalists. The two species studied were visited by 43% and 31 % of the total insect taxa in the community, respec-tively, suggesting they play a central role in the plant–polli-nator networks. Carpobrotus and Opuntia had contrasting eVects on pollinator visitation rates to native plants: Carpo-brotus facilitated the visit of pollinators to native species, whereas Opuntia competed for pollinators with native spe-cies, increasing the nestedness of the plant–pollinator net-work. These results indicate that the introduction of a new species to a community can have important consequences for the structure of the plant–pollinator network