The tolerance of island plant–pollinator networks to alien plants

Summary1. Invasive alien plant species pose a severe threat to native plant communities world-wide,especially on islands. While many studies focus on the direct impact of alien plants on nativesystems,indirecteffectsofplantinvadersonco-floweringnatives,forexamplethroughcompetitionforpollinationservices,arelesswellstudiedandtheresultsarevariable.2. We used six temporally and taxonomically highly resolved plant–pollinator networks from theisland of Mahe´, Seychelles, to investigate the indirect impact of invasive alien plant species onremnantnativeplantcommunitiesmediated by sharedpollinators. Weemployedfullyquantitativenetwork parameters and information on plant reproductive success, and pollinator diversity andbehaviour,todetectchangesinplant–pollinatornetworksalonganinvasiongradient.3. The number of visits to and fruit set of native plants did not change with invasion intensity.Weighted plant linkage and interaction evenness, however, was lower at invaded sites than at lessinvaded sites. These patterns were primarily driven by shifts in interactions of the most commonpollinator, the introduced honey bee Apis mellifera, while weak interactions and strong nativeinteractionsremainedunchanged.4. Synthesis. The implications of these findings are twofold: first, quantitative network parametersare important tools for detecting underlying biological patterns. Secondly, alien plants andpollinatorsmayplayagreaterroleinshapingnetworkstructureathighthanlowlevelsofinvasion.We could not show, however, whether the presence of invasive plants result in a simplification ofplant–pollinatornetworksthatisdetrimentaltonativeplantsandpollinatorsalike.Key-words: Indian Ocean, indirect interactions, inselbergs, interaction connectance, invasionecology, invasive alien species, plant communities, pollination webs, Seychelles IslandsIntroduction