Benefits of ant attendance for aphid colonies of varying density

Abstract

Benefits between species engaged in mutualistic interactions are often dependent on ecological context. For instance, in mutualisms in which aphids exchange food for ant protection from predators, the degree of benefit that aphids receive from ants depends on the presence of other arthropod predators. In this study I examined the influence of aphid density on the benefits that aphids (Artemisaphus artemisicola) receive from ants (Formica obscuripes). Treatments of ant presence and starting aphid density (N=5, 10, or 15) were factorially crossed on individual sagebrush stems in a field experiment. A significant increase in aphid per capita growth rate was found in the intermediate density treatment with ants present. Mirroring this relationship, the abundance of predators was highest when ants were excluded and aphid populations started at an intermediate density. No ant exclusion treatments managed to establish successful aphid colonies. These results reveal a strong benefit of ant attendance for aphids that is dependent on aphid density, likely reflecting changes in the balance of costs and benefits of the mutualism across aphid densities.

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