Predator-mediated ant response as an indication of mutualism with aphids on milkweed plants.

Abstract

How species interactions affect the individuals involved is an important aspect of the study of ecological systems. One well-studied interaction is that of mutualism, where two species interact in a manner that benefits both. Using the mutualistic ant-aphid relationship on milkweed, Asclepias syriaca, we tested the hypothesis that the number of ant behaviors that are classfied as protective increases when aphid predators are introduced to the plant. We manipulated six milkweed plants on the Douglas Lake shoreline of northern lower Michigan by introducing predators and observing and classifying ant response. The predators used were ladybugs and lacewing larva. We found that ants responded protectively to both predators with similar frequencies. We also found this protective behavior to be much greater in the presence of predators. These results suggest that mutualistic species interactions can be studied and quantified in the context of behavior.