Abiotic factors and the conditional outcome of an ant-plant mutualism

Mutualisms are inserted in a network of direct and indirect biotic interactions built within the framework imposed by the abiotic setting. We carried out an experiment to test how the availability of light and nutrients modulates the interaction strength between Inga vera (Mimosoideae), a Neotropical extrafloral-nectary-bearing plant, and its associated ants. From July 2001 to July 2003, 48 plants were grown in an old-field site following a three-factor randomized blocked design where treatments were: ant (ants present and absent), nutrient (without and with nutrient addition), and light (sun and shade). During the experiment, in the absence of ants, plants growing in sunlight were shorter, developed fewer leaves and leaflets, were more damaged by herbivores, had half the total dry biomass, and died more frequently than plants visited by ants. In contrast, ants had no effect on the growth, survival, and total biomass of shaded plants. Therefore small-scale heterogeneity in the light environment turned mutualism (+,+) into commensalism (+,0). This study demonstrates experimentally that the outcome of mutualistic interactions can be conditional upon the abiotic setting.