Can novel mutualisms with native species modify the community-wide consequences of ant invasions? A test using the Anoplolepis gracilipes invasion of the Samoan Archipelago

Background/Question/Methods

Although the ecological importance of positive species interactions has been historically underappreciated relative to that of antagonistic interactions, mutualisms appear to be increasingly likely to influence co-occurring species and communities. These interactions may be particularly important in the context of non-native species invasions, which can have substantial, negative community-wide impacts. For invasive ants, mutualistic associations with plants that bear extrafloral nectaries (EFN) and/or honeydew-excreting insects may play a key role in fueling both the population expansion and the subsequent negative impacts of highly invasive species. However, this ‘nectar subsidy’ hypothesis has not yet been examined experimentally. In this study, we tested this hypothesis, using the yellow crazy ant (Anoplolepis gracilipes) invasion of the Samoan archipelago as a study system. The factors in this plot-level factorial experiment were ant access to a common EFN-bearing shrub (Morinda citrifolia) and nectar levels of the pant. Treatment plots (4x4m) were replicated across sites that had A. gracilipes-dominated ant assemblages (>85% of all ground and plant-foraging ants were A. gracilipes) and those that were not dominated by A. gracilipes (<5% of all ground and plant foraging ants were A. gracilipes). Three and six months after treatment application, we surveyed arthropods on M. citrifolia plants, and sampled plot-wide arthropod communities using sweep nets.  

Results/Conclusions

We found that arthropod communities were significantly altered by mutualisms between ants and M. citrifolia plants. These effects of the mutualism on co-occurring arthropods were strongest when A. gracilipes dominated local ant assemblages. Additionally, arthropods from different feeding guilds displayed variable responses to experimental manipulations. Specifically, herbivores responded most strongly to manipulations of ant access to plants and detritivores responded most strongly to increasing nectar levels, with the strongest responses for both groups occurring at the plant level. Conversely, carnivore responses were only evident at the plot level, but were strong for manipulations of both ant access and nectar levels. These results demonstrate that positive interactions (including mutualisms) can strongly influence community-wide ecological dynamics and that there can be substantial community-level consequences when these positive associations are disrupted by highly invasive species.