Do mutualisms between the Argentine ant and cotton aphids structure arthropod food webs in cotton?

Background/Question/Methods

In the last decade mutualisms have been affirmed as important ecological drivers with impacts resonating beyond pairwise interactions to entire ecosystems. As a result, mutualisms with a long history of research are being evaluated for their role in structuring food webs. Ants and honeydew-producing Hemiptera participate in food-for-protection mutualisms where hemipterans provide sugar-rich honeydew rewards in exchange for defense from natural enemies. In this study we examined whether mutualisms between aphids and an invasive ant affect other components of the food webs in which they belong. We manipulated cotton aphid (Aphis gossypii) abundance on cotton (Gossypium hirsutum) in a field dominated by the Argentine ant (Linepithema humile) in La Jolla, CA.  Cotton plants were either inoculated with aphids, or aphids were removed. Arthropod surveys were conducted weekly from June to August 2010 and plant biomass was measured at the end of the study.

Results/Conclusions

As expected, aphids drew ants up onto plants and there was a significant positive relationship between aphid and ant abundance (r=0.81, p<0.0001). As aphid abundance increased, the ratio of aphids to ants increased (r=0.79, p<0.0001), with fewer ants tending per capita than at lower aphid densities. Although the abundance of predators and parasitoids increased with ant abundance, the ratio of predators to aphids (r²=0.46, F_1,104=90.89, p<0.0001) and parasitized to healthy aphids (r²=0.29, F_1,104=44.72, p<0.0001) decreased with increasing ant abundance. This result suggests that high ant densities reduced per capita risk.  Although overall arthropod species richness and plant biomass increased with ant abundance, these trends appear driven by the fact that larger plants supported more arthropods. Previous studies on red imported fire ants demonstrated that ants decreased leaf-chewing herbivore abundance on plants and in doing so indirectly benefitted plants. However, the Argentine ant does not seem to provide similar protective services to plants in cotton. If plants rely on ants to provide indirect benefits, then plants may lose those benefits when the Argentine ant is the resident ant species.