Context dependence is a hallmark of species interactions: a given interaction will have different effects depending on when and where it occurs. To incorporate species interactions into our understanding of large-scale ecological patterns and responses to environmental change, we must identify the major causes and effects of context dependence. In this talk, I will consider the environmental drivers of context dependence in interactions among plants, ants, and hemipteran insects. I will then consider how the context-dependent outcomes feed back to the environment to structure local communities. Through the study of a geographically widespread interaction among a myrmecophytic tree and its symbiotic ants and scale insects, I will describe how evolutionary history, historical biogeography, and ecology shape how strongly trees benefit from their symbiotic insects. I will then describe the mechanisms that structure these variable outcomes, including the chemical stoichiometry of ant-plant-hemipteran interactions and the roles of microbial partners. Finally, I will describe the effects of these variable outcomes on local and regional communities of leaf-eating herbivores and other plant-dwelling ants.