Ant and treehopper abundance in relation to their mutualistic associations across elevations in the equatorial Andes
Along elevational or latitudinal gradients, patterns of mutualistic interactions may be driven by shifts in partner abundance, in turn the result of changes in environmental factors. We studied the mutualistic association between ants and treehoppers along two 4000-meter elevational gradients on the equatorial Andes to test the hypothesis that partner abundance mediates the frequency of associations and investment incurred by partners while associating. Using a combination of visual surveys and sucrose baits, we scored the abundance of ants, treehoppers, and their associations at 500m intervals along two 4000m elevational gradients in the tropical Andes (three locations per elevation, two 70 m transects per location). We used the size of the treehopper aggregations and of the ant worker force involved in the associations as proxies for the investment partners incur in the interaction. We analyzed our data using GLMM with transects as units of analyses and locations as random effects. We show that the number of ant-treehopper aggregations declined with elevation, in parallel with a decline in the abundance of both partners, but with ants declining at a faster rate. Despite this, the numbers of ants and treehoppers within individual aggregations increased with elevation, but this time with the number of treehoppers in the aggregations increasing at a faster rate. We suggest that this elevational gradient in the size of ant-treehopper aggregations arises from consumer resource-dynamics, where the partner in short supply elicits greater investment from the other. Within the range of elevations where the interaction is possible, the observation that a smaller proportion of treehopper species at higher elevations engage in mutualism further suggests decreasing benefits of the associations with elevation. Our study suggests that the geography of abundance and supply, and its consequences on cost and demand of services, may dictate the outcome of mutualistic interactions along gradients, with possible implications for the maintenance of biological diversity.