Mutualism between consumers and their shared resource can promote competitive coexistence.

Competitive coexistence depends on dynamic interactions between competitor and resource populations, including mutualism between the resource and each competitor. We add mutualism to a well-known model of resource competition and show that it can powerfully stabilize competitive coexistence in the absence or presence of resource heterogeneity. We use a transition matrix approach to describe lottery competition, while allowing each of two competitors to affect the population dynamics of their shared resource. For example, two plant-defending ant species may compete for nesting space within ant-adapted (myrmecophytic) plants. We show that mutualism between consumers and a resource species can stabilize competitive coexistence of the consumers by allowing each competitor to influence resource dynamics in a way that benefits the other. The effect of this novel coexistence mechanism depends on a mutualism's biological details: for example, altering myrmecophyte fecundity affects competing ant species differently than does altering plant survival. Finally, we consider a heterogeneous resource (e.g., two types of nest site) and show how niche partitioning can stabilize coexistence in the absence of resource dynamics. When resource heterogeneity is dynamic (e.g., small and large plants of the same species), niche partitioning also provides new routes for additional stabilization via mutualism.