Plant-pollinator interactions in a highly invaded community: Network structure for native and non-native plants

Background/Question/Methods Recent analyses of networks of interactions among mutualistic partners have identified patterns consistent across different communities. Further evaluation of the possible determinants of network structure and examination of the temporal dynamics of these networks is needed to better understand mutualisms in the context of invaded communities. I characterized the plant-pollinator interactions of a highly invaded old field community in central New Jersey to describe temporal (intra-annual) changes in network structure and to test whether these networks are organized differently for non-native and native plants. I monitored pollinator visitations along fixed transects from May to October, the entire annual flowering period at this site. Comparisons of the subsets of interactions with native versus non-native plants with patterns expected in randomly generated networks tested whether non-native plants differ from native ones in the degree of pollinator specialization. Results/Conclusions The visitation network for one year of interactions in this community included eight hundred and eight observations of floral visitations. Insects visited 45 species of plants, 42% of which were non-native. Both native and non-native plants ranged from specialized to generalized in their insect visitations, but fewer insect species interacted with non-native plants in the community than with native plants. The community visitation network as a whole was highly nested when compared to randomly generated networks of the same size and connectance (p=0.0005). The nestedness of the sub-network containing only interactions with non-native plants did not differ from random (p=0.438). In contrast, the nestedness of the sub-network containing only native plants showed a marked but nonsignificant trend away from random (p=0.076). These results suggest that while the non-native species are well-integrated in the mutualistic interactions of this community, the nestedness structure of their interactions differs from that of the native plant species.