I D EAS AND PERSPECT IVES Stoichiometric relations in an ant-treehopper mutualism

Adam D. Kay,* Sara E. Scott, John D. Schade and Sarah E. Hobbie Department of Ecology, Evolution and Behavior, University of Minnesota, 100 Ecology Building, 1987 Upper Buford Circle, St Paul, MN 55108, USA Department of Biology, College of St Benedict, St Joseph, MN, USA School of Life Sciences, Arizona State University, Tempe, AZ, USA Department of Integrative Biology, University of California, Berkeley, CA, USA *Correspondence: E-mail: [email protected] Abstract Carbon : nitrogen : phosphorus (C : N : P) stoichiometry can underlie physiological and life history characteristics that shape ecological interactions. Despite its potential importance, there is much to learn about the causes and consequences of stoichiometric variation in terrestrial consumers. Here we show that treehoppers (Publilia modesta) tended by ants (Formica obscuripes) contained lower N concentrations than treehoppers on plants from which ants were excluded. Ant presence also affected nutrient concentrations in host plants: on plants with ants, leaves contained uniformly low concentrations of N; on plants without ants, N concentrations were low only in the few leaves fed upon by treehoppers at the time of collection. We suggest treehopper feeding reduces leaf nutrient levels and ants positively affect treehopper abundance, producing a top–down effect on plant quality. Determining the causes of these stoichiometric changes should help elucidate factors guiding the dynamics of conditional mutualisms between ants and homopterans.