Keystone predators are defined by their high trophic status and ability to alter the distribution and abundance of multiple species in complex food webs. The impact of keystone predators, however, is not constant. Variation in community composition, food web structure, and the abiotic environment help determine the strength of interactions between keystone predators and other members of the community. In this study, we investigated the role of a fire ant-aphid mutualism on the community-wide effects of fire ants in cotton agroecosystems. We hypothesized that the fire ant-aphid mutualism acts as a keystone interaction that stimulates fire ants to function as keystone predators in this system. In a series of greenhouse and field experiments, we found that fire ants are primarily ground-foragers, but that honeydew produced by cotton aphids induces ants to forage in the canopy of cotton plants. In addition, fire ants behaved more aggressively in the presence of honeydew-producing aphids and were more likely to attack other arthropods when they were present. As a result, fire ants reduced the density of beneficial arthropods (predators and parasites) as well as herbivores in the canopy of cotton plants when cotton aphids were present. The interaction between fire ants and aphids, therefore, serves as a keystone interaction that ultimately alters the structure of arthropod communities. This result has far-reaching significance for understanding the community structure of natural systems and the efficacy of biological control in fire ant-infested areas.
Species 1: Hymenoptera Formicidae Solenopsis invicta (red imported fire ant)
Species 2: Homoptera Aphididae Aphis gossypii (cotton aphid, melon aphid)
Keywords: keystone predation, ant-aphid mutualism
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