Short-term plasticity and variation in acacia ant-rewards under different conditions of ant occupancy and herbivory

In ant-plant defense mutualisms, plants known as myrmecophytes provide food and shelter to ant partners in exchange for defense against herbivores and pathogens. To ensure interaction pay-off, myrmecophytes must regulate their investment in ant-rewards depending on local conditions and herbivore pressure. We investigated how myrmecophyte investment in multiple ant-rewards relates to herbivory, ant defense, and ant occupancy over time. Specifically, we examined the plasticity of ant-rewards produced by swollen-thorn acacias (Vachellia collinsii) under different ant occupancy and herbivory conditions. We compared food rewards (number of extrafloral nectaries and pinnules as a proxy for food bodies) and housing rewards (domatia dimensions) of V. collinsii for three conditions: (1) occupied (defended by the obligate mutualist Pseudomyrmex spinicola) versus unoccupied trees, (2) occupied trees subject to an experimental herbivory manipulation versus control trees, and (3) trees occupied by different ant species varying in their level of defense (P. spinicola, P. simulans, Crematogaster crinosa). We found that food rewards were more likely to vary in time depending on ant occupancy and resident species. Conversely, housing rewards varied with the condition (occupancy or species of partner) and less through time. A one-time herbivory manipulation did not cause any changes to the ant-rewards produced. Our results reveal short-term plasticity in V. collinsii ant-rewards and demonstrate that myrmecophytes with constitutive rewards can adjust their investment in ant-rewards depending on the presence and identity of ant partners.