The coexistence of competing species depends on dynamic interactions between competitor and resource populations, including mutualistic effects of competitors on resource survival and fecundity. We consider how the dynamics of mutualism modify competitive exclusion and coexistence in a well-known model of resource competition, and show that mutualism can powerfully stabilize competitive coexistence in the absence or presence of resource heterogeneity. We use a transition matrix approach to describe the population dynamics of two species engaging in lottery-style competition for a shared resource, where each of the competitors can also facilitate the population growth of the resource. For example, two plant-defending ant species may compete for nesting space within ant-adapted plants.

Results/Conclusions

We show that the competitors’ mutualistic influence on resource dynamics can facilitate competitive coexistence through negative feedbacks.  By altering resource population dynamics, the competitors can modify a tradeoff between resource acquisition and resource retention in a way that can stabilize the neutral coexistence of the classic lottery model.  Mutualism is thus a novel mechanism allowing stable competitive coexistence. In addition, rather than being threatened by the presence of a relatively exploitative competitor, the dynamics of a mutualism can facilitate the persistence of exploiters. We show that the biological details of a mutualism can affect the strength of the coexistence mechanism: for example, in a mutualism between ants and myrmecophytic plants, aiding plant fecundity has different dynamical results for competing ant species than does aiding plant survival. Finally, we consider a heterogeneous resource (such as two types of nesting site), and show that resource heterogeneity also can stabilize competition through niche partitioning.  When resource heterogeneity is dynamic, arising through stage or size structure in the resource population, niche partitioning can still promote coexistence and also provides additional dynamical scope for the operation of mutualism.