Partner supply mediates elevational gradients in the outcome of ant‐hemipteran mutualistic associations

Aim Along elevational or latitudinal gradients, the dynamics of mutualistic interactions may be driven by shifts in partner abundance, themselves driven by changes in environmental factors. We test the hypothesis that partner abundance mediates the frequency of associations and investment incurred by partners while associating. Location Western and Eastern slopes of the Andes of Ecuador. Taxon Ants (Hymenoptera, Formicidae) and treehoppers (Hemiptera, Membracidae). Methods Using a combination of visual surveys and sucrose baits, we scored the abundance of ants, treehoppers, and their associations at 500‐meter intervals along two 4000 m elevational gradients in the equatorial Andes (three locations per elevation interval, 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 analysed our data using GLMM with transects as units of analyses and locations as random effects. Results 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. Main conclusion 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 engage in mutualism at higher elevations 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. Resumen Propósito A lo largo de los gradientes altitudinales o latitudinales, las interacciones mutualistas pueden ser mediadas por cambios en la abundancia de socios, los cuales a su vez están mediados por cambios en factores ambientales. Estudiamos la asociación mutualista entre hormigas y membrácidos a lo largo de dos gradientes de elevación de 40