Models of Some Ant-Plant Mutualisms

Simple models of the mutualistic interaction between ants and the diaspores of some spring herbs (myrmecochory) predict that a stable, feasible equilibrium is unlikely. The two-species model is expanded to examine the effects of a predator or competitor because, in nature, myrmecochorous herbs often suffer heavy seed predation from small rodents or compete with nonmyrmecochores. Low to moderate levels of predation stabilize mutualism that is unstable in isolation. Numerical simulations indicate that the locally stable equilibrium point "attracts" reasonable initial population sizes of the three species. Competition between the myrmecochore and a nonmyrmecochore decreases the probability of a stable coexistence compared to the isolated mutualism. It is suggested that many mutualisms are stabilized, and not merely bounded by a third species. Predation is the most likely stabilizing interaction.