Model selection analysis of conditionality and multiple mechanisms of benefit in an ant-treehopper mutualism.
Morales, Manuel*,1, 1 Williams College, Williamstown, MA
ABSTRACT- Recent studies of mutualism have emphasized context-dependent variation (i.e., conditionality) in the net benefit to participants. At the same time, an increasing number of studies is finding that the net benefit of mutualism can be a complex function of community-level interactions or can include multiple mechanisms of benefit. In the interaction between ants and treehoppers in the genus Publilia, ants collect the sugary excretions of treehoppers as a food resource and treehoppers benefit both directly (e.g. by feeding facilitation) and indirectly (e.g. by predator protection). Previous studies have ascribed conditionality in the net benefit to treehoppers to temporal or spatial variation in background predator levels, the species of ant-tending partner, or density-dependence. Here, I use a model selection analysis based on data collected over the period 2002-2004 to integrate these previous results with biologically-based models that incorporate proposed direct and indirect mechanisms of benefit. In 2002 only, predators were excluded from all treehopper aggregations. Results suggest that model selection analyses can effectively decompose multiple mechanisms of benefit. In particular, a direct-benefits only model is selected as best for 2002 (predator excluded data) whereas a direct+indirect benefits model is selected as best for 2003 and 2004. Overall, predation on untended treehopper is best described by a type I functional form and predator protection by ants is best described by a step function. In contrast, direct benefit to treehoppers is directly related to the ratio of ants and treehoppers. Preliminary results suggest that the temporal variation in treehopper survivorship within a season is mediated by variation in predation on untended treehoppers rather than by variation in direct benefits of ant tending. Model selection analyses represent an important approach for understanding variation in complex species interactions, especially when combined with experimental manipulations.
Key words: mutualism, Publilia concava, model selection, Membracidae
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