Competing consumers draw down the abundance of their shared resource, but a growing body of research demonstrates that consumers can also affect the survival, growth, and reproduction of the resource species. These effects on resource vital rates can be negative, but in the interesting case of consumers that are mutualistic with their resource species, they are positive. How do the dynamics of competition between consumers depend on the consumers' effects on resource demography? Previous work has considered competition between obligate mutualists for access to their hosts; here, we consider more facultative mutualisms, as well other short-lived interactions such as parasitism and herbivory, for example. Building on the classic MacArthur-Levins model of resource consumption and consumer competition, we assume a single resource species with age, stage, or size structure. Consumers can use the different resource stages to different extents, as they do different resources in the classic model, but the stages are dynamically linked through the resource vital rates, which in turn depend on consumer identity.
Results/Conclusions
Using mathematical analysis of special cases and simulation, we show that consumers' effects on resource vital rates cause two important departures from classic niche partitioning results. First, competitors can coexist even if they use the resource stages identically, provided they shift the resource distribution in different ways. This means that, for instance, two mutualistic consumers can coexist despite identical use of the resource species if they differ appropriately in their mutualistic effects. Second, competitors specializing on different resource stages do not necessarily coexist: competitive exclusion can occur via the consumer-mediated relative abundances of resource stages. These results, and the ease with which our framework enables comparison with classic expectations, highlight the centrality to competitive dynamics of consumers' effects on resource vital rates.