Predicting the outcome of direct and indirect effects of interacting species in diverse, highly connected food webs can be extremely challenging. Community ecologists developed the keystone species concept to help simplify this task. Keystone species are species that have disproportionately large effects on the abundance of many interacting species. In some communities, beneficial interactions among species (mutualisms) may dramatically alter the community-wide effect of one of the interacting species. Recent research has demonstrated that mutualisms involving ants and honeydew-producing hemipterans such as aphids and scales can function as ‘keystone interactions’ that alter the abundance and distribution of many interacting species via increased ant predation of arthropods inhabiting plants that host these mutualisms. A recent review of the ecological consequences of these mutualisms suggests that in most cases hosting ant-hemipteran mutualisms is beneficial to plants. This occurs because aphids and scales typically impose relatively small fitness costs to their host plants when compared to the foliage and fruit-feeding herbivores that are removed via enhanced ant predation. Recent research also suggests that plants may be active participants in ant-hemipteran mutualisms that signal aggressive ants to tend non-damaging aphids. Thus, ant-hemipteran mutualisms may function as extended phenotypes of some plants that ultimately play an important role in plant defense against herbivory.