Resource mutualisms, such as the interaction between legumes and nitrogen-fixing rhizobia, not only directly influence the partners involved in the interaction, but they also have the potential to either inhibit or facilitate other species in the community by altering the abiotic and biotic environment. In particular, the legume-rhizobium mutualism can increase the competitive dominance of the legume host or affect patterns of coexistence, resulting in altered diversity, composition, and nutrient dynamics. Yet, it remains necessary to determine the contexts by which these interactions may be most likely to affect communities. Here, I build on my previous research to more fully understand how and when the legume-rhizobium mutualism may be likely to affect plant and arthropod communities by exploring how these interactions may be dependent on abiotic nutrient availability and biotic factors such as additional symbiotic interactions, competitor density, and intraspecific genetic variation in the host.
I found that substantial changes in diversity, composition, and abundances of species in both the plant and arthropod community arise because rhizobia have both facilitative and inhibitory effects. This likely occurs because rhizobia increase soil nitrogen availability and reduce light availability (by increasing legume growth) or because rhizobia reduce the niche overlap of the legume with other species by reducing the legume’s dependence on soil nitrogen. Therefore, rhizobia promote coexistence between the legume and some species, promote the exclusion of others, and alter competitive hierarchies, yet competitive effects are density-dependent. Rhizobia also scale up to affect higher trophic levels such as the arthropod community, including extrafloral nectar-tending mutualist ants and herbivores. These findings provide insight into predicting when mutualisms are most likely to strongly affect communities by showing how intraspecific variation in a dominant host plant, density, the presence of other mutualists, and abiotic environmental factors influence the magnitude of these effects.