Environmental forcing and the competitive dynamics of a guild of cactus-tending ant mutualists

In generalized, multispecies mutualisms, competition among members of one guild can influence the net benefits that each species in the other guild receives. Hence seasonal factors that affect the dynamics of competition can also affect net benefits, especially if the benefit or cost of mutualism also varies seasonally. In the Sonoran Desert, two common species of generalist ants compete for access to extrafloral nectaries on the fishhook barrel cactus Ferocactus wislizeni, but their relative abundances vary seasonally; one ant dominates more cacti in winter/spring, and the other in summer/autumn. Recently, a third ant species, which also varies in abundance seasonally, has appeared at our study sites and is competing With resident ants for access to cactus nectaries. This empirical system motivated us to examine a metapopulation model of competition for patches in an open system with periodic forcing. We use the model to: (1) illustrate three ways in which competing species may differ in their sensitivities to environmental conditions that are consistent with seasonally displaced patterns of abundance; (2) ask under what conditions the invasion of a third competitor into a two-species system could alter the sensitivity of the system to environmental forcing at low vs. high frequency; and (3) show how differences among competitors in the pattern of seasonal forcing alone can dramatically alter the outcome of a competitor invasion. We discuss implications of these results for the ways in which seasonal variation among competing mutualists, and the benefits and costs of mutualism, may affect the functioning of generalized, facultative mutualisms.