The net fitness outcome of a facultative mutualism varies both in direction and magnitude depending on numerous contextual factors. While apparently common, the factors contributing to context dependence in facultative mutualisms are poorly studied in previous literature. We examined the temporal trends in community structure of an ant-plant protective mutualism to decompose the factors contributing to context dependence. This system includes a single nectar-producing host plant and multiple species of ants. Plant size and flowering data along with ant species identity and abundance data were recorded annually on the same group of plants for over a decade. Host plants typically have one species of ant attendant at a time; however, the plants occasionally change ant partner species between sampling dates. We investigated two potential drivers of ant species turnover, host plant reproductive state and temperature. First, we created a time series of ant abundance plotted against temperature and precipitation data to see how community structure responded to varying abiotic conditions. Second, we constructed an ant-transition matrix to derive the probability of acquiring or losing an ant species from one year to the next. We then amended the matrix to account for reproductive transitions (i.e. nonflowering to flowering and vice versa).
Results/Conclusions
Reproductive maturation and abiotic conditions (e.g. temperature) are both known to directly influence the content and availability of nectar on the host plant. The start of flowering typically coincides with an increase in the quantity of nectar produced and a change in the quality of the nectar. Conversely, stressful abiotic conditions can cause a slowdown in nectar production or even a total halt. We found that freezing conditions during the winter of 2011 could have caused a change in nectar availability which altered the structure of the ant community to favor vacancy temporarily. We found that plants that reach reproductive maturity experience reduced likelihood of becoming vacant or remaining vacant of ants and increased likelihood of retaining their current ant species. While the direct interactions of ant-plant mutualists and competition among different ant species have been extensively studied, greater attention should be paid to the indirect interactions between ants, plants, and the abiotic environment that surrounds them. The context dependent nature of these interactions is not unique to this ant-plant protective mutualism. With careful attention to the relative contributions of indirect drivers the mechanics of interspecific interactions might be better understood and more accurate ecosystem management decisions could be made.