Density-dependent mutualism in an aphid-ant interaction

Our objectives were to determine if the mutualistic effect of ant tending on aphid population growth varied with aphid density, and to consider possible mechanisms explaining such a context-dependent effect. We studied the interaction between Aphis varians and Formica cinerea on fireweed (Epilobium angustifolium) in the Rocky Mountains of Colorado. In 1988 we used two experiments to examine the interaction between ant tending to aphid density on the per capita growth of aphid populations at naturally occurring low aphid densities, and naturally occurring high aphid densities. In 1989, we conducted a similar experiment, but we manipulated aphid densities to include the full range from low to high. The results of these experiments were consistent: ant tending significantly improved the growth of small aphid populations, but the benefit from ant tending decreased or disappeared at higher aphid densities. We considered possible mechanisms for the density-dependent mutualistic effect by examining the relationships among aphid density, the number of ants tending, and predator abundance. We show that the relative number of ants tending declined as aphid density increased, and ant tending did not reduce predator abundance at any aphid densities. Therefore, the cause of the density-dependent mutualism is probably a decline in the direct effect of ants on aphids resulting from a decline in the relative number of ants tending aphids at higher aphid densities. We discuss the implications of density-dependent mutualism for the dynamics and evolution of mutualistic systems.