Trait-mediated indirect interactions in invasions: unique behavioral responses of an invasive ant to plant nectar

Exotic species often form beneficial, facultative associations with indigenous species. However, we still have a limited understanding of the influences that these positive associations may have on the dynamics and impacts of species invasions. Highly invasive species may respond differently than less invasive species to resources that are exchanged in mutualisms, leading to trait-mediated indirect interactions between native species via invaders that may reshape native communities. In this study, we tested the hypothesis that the highly invasive ant species, Anoplolepis gracilipes, exhibits stronger trait changes in response to increasing levels of nectar than co-occurring, less invasive ant species. Across two islands in the Samoan Archipelago, we located multiple sites dominated by A. gracilipes and multiple sites dominated by other, less invasive species. At each site, we manipulated nectar levels on a common extrafloral nectary-bearing shrub and assessed short-term changes in ant worker recruitment and aggression. We found that the recruitment response of the highly invasive ant species A. gracilipes was not unique: other dominant ant species also increased recruitment in response to increasing nectar levels. However, A. gracilipes did show unique changes in aggressive behaviors: as nectar levels increased, the proportion of prey discovered, attacked and removed by A. gracilipes workers and the speed at which they performed these aggressive behaviors all increased strongly. Other ant species showed no such responses. In addition, fewer subordinate ants persisted on plants at sites invaded by A. gracilipes. Finally, plot-level, simultaneous manipulations of ant access to the plants and nectar availability demonstrated that Morinda citrifolia-ant mutualisms influenced the beta-diversity of local arthropod communities differently when A. gracilipes dominated local ant assemblages. These results suggest that mutualisms between invasive ants and native plants can modify interactions between invaders and co-occurring arthropods, possibly leading to more negative consequences for native communities. They also underscore the importance of incorporating both positive species interactions and indirect pathways into our studies of both community ecology and invasion biology.