Aphids alter the community-wide impact of fire ants

Positive species interactions have the potential to strongly influence the structure and dynamics of ecological communities, yet surprisingly few studies have documented their general importance. We tested the hypothesis that the mutualistic association between fire ants and aphids enhances the impact of fire ants on the herbivorous and predaceous arthropod community of cotton. We found that the presence of aphids attracted foraging fire ants onto cotton plants. This shift from ground to foliar foraging resulted in more frequent interactions between fire ants and arthropods associated with cotton. The survival of herbivores (caterpillars) and predators (ladybird beetles and lacewings) was lower in the presence of fire ants and aphids compared with fire ants alone in greenhouse experiments. Similarly, fire ants and aphids deterred plant bugs from spending time on cotton foliage. Using large-scale field manipulations of fire ants along with naturally occurring aphid populations, we found that the ant-aphid mutualism accounted for much of the variation underlying ant impact on cotton herbivores and predators. For example, in 2001, similar to 27% of herbivore taxa and 54% of predator taxa were adversely affected by the mutualism. The overall community response, however, was highly conditional with strong temporal variation in the effects of the ant-aphid association. Our results indicate that the relationship between fire ants and aphids serves as a key interaction that alters the structure of cotton arthropod communities. This conclusion has far-reaching significance for understanding food web dynamics in natural systems and the efficacy of biological control in ant-infested areas.