Reduction in mutualistic ant aggressive behavior upon sugar supplementation

Mutualistic interactions between species are widespread and important for community structure and ecosystem function. In a changing environment, the proximate mechanisms that maintain mutualisms affect their stability and susceptibility to perturbation. In ant-plant mutualisms, ants defend their host plants against herbivores or competing plants in exchange for housing or food. While the phenomenon of species exchanging services for resources is well documented, how such arrangements are maintained is not. There are at least four hypothesized mechanisms through which plants use sugar to induce ant defense against herbivores. Three such hypotheses (“deficit”, “fuel for foraging”, “predictable rewards”) predict that the appearance of a new sugar source near the host plant would increase the rate of ant attacks against herbivores, but the fourth hypothesis (“attract and distract”) predicts the opposite. To examine how the mutualism between Triplaris americana and Pseudomyrmex dendroicus would be affected, we simulated the appearance of a novel sugar source at a random half of 34 T. americana trees with P. dendroicus colonies. Compared to control colonies, those with access to the sugar source were less likely to attack herbivorous insects (Nasutitermes sp. termites). Thus, our findings support the “attract and distract” hypothesis. We infer that this ant-plant mutualism could be destabilized by the appearance of an alternative sugar source, such as a nectar-producing plant or honeydew-excreting insect. More broadly, we conclude that the mechanisms responsible for maintaining mutualistic relationships are relevant for understanding how ecological communities are affected by environmental change.

Abstract in Spanish is available with online material.