Positive and negative interactions are each recognized to be important determinants of community structure, but there is little theory or empirical data on (1) the relative strengths of these effects or (2) whether their combined influence are non-additive and interact. I will discuss two studies that each document the combined influences of predation and mutualism on community structure. First, I will present the results of a three-year factorial field study where I measured the effects of insectivorous birds (predators) and aphid-tending ants (mutualists) on a pine canopy arthropod community and on pine itself. The effects of birds and ants on arthropods interacted strongly; birds disrupted the ant-aphid mutualism and in so doing shaped the pine arthropod community. The effects of birds and ants also cascaded to increase pine growth and alter the monoterpene composition of pine phloem. Second, I will present results from a quantitative genetic study of common milkweed. Here I document how ant-aphid interactions ranged from parasitism to mutualism depending on the genotype of milkweed on which they occured. Milkweed thus exerted indirect genetic control over ant abundance, and these recruited ants in turn defended milkweed against herbivory by monarch caterpillars. In summary, these studies each demonstrate how the community-wide consequences of mutualism and predation cannot be fully understood without considering the interactive, non-additive nature of inter-specific interactions.