Mutualistic commodities, such as nectar or pollination services, constitute limiting resources for which mutualists compete. Thus, competition for commodities provided by shared mutualistic partners likely plays a fundamental role in mutualist coexistence. Recently, several theoretical studies have investigated whether mutualism is a mechanism for diversity maintenance by promoting the coexistence of competitors, such as co-flowering plants, via mutualist niche partitioning. Theory, however, has yielded contrasting predictions; thus, the question of how mutualism affects competitor coexistence remains largely unclear. Here, I develop a theoretical framework for investigating this question based on Tilman’s graphical-mechanistic approach to studying competitive coexistence within consumer-resource interactions.
The novel feature of the framework is that it explicitly incorporates competition for both commodities and resources external to mutualism, such as limiting nutrients. The key finding is that mutualism can promote or constrain the coexistence of competitors depending on the strength of intra- and inter-specific feedback loops within and between competitors. Coexistence requires that competitors partition commodities and external resources. In contrast to resource competition, however, in which species cannot affect resource supply rates, competitors for mutualists augment commodity availability via their mutualistic interactions with shared partners. As a result, a competitive asymmetry arises because increased commodity availability erodes the competitive advantage enjoyed by superior competitors for commodities. The model highlights the importance of indirect feedback loops within and between competitors via shared mutualistic partners and provides key insights into the role of mutualism as a mechanism of diversity maintenance.