Understanding the mechanisms that maintain species coexistence is a fundamental goal of community ecology. Coexistence requires stabilizing niche differences that cause species to suppress conspecifics more (or benefit them less) than heterospecifics, leading to negative frequency dependent growth. The role of niche differences in stabilizing coexistence is well established in modern coexistence theory, but few empirical studies have assessed their strength or the factors that influence them. Here, we tested the strength of stabilizing niche differences in a group of four locally co-occurring legume (Trifolium) species using a greenhouse experiment in which we grew each species at a range of frequencies in mixed communities. We examined the role of rhizobial mutualists in stabilizing coexistence by comparing frequency dependence when species were grown with rhizobial strains isolated from roots of either their conspecifics or their competitors.
The strength and direction of frequency dependent growth varied among Trifolium species, with one species (T. macraei) showing strong negative frequency dependence, one species (T. bifidum) showing strong positive frequency dependence, and two species (T. barbigerum and T. microdon) showing frequency independent growth. These results indicate a competitive hierarchy, in which growth was more strongly affected by the frequency of strong (T. macraei) vs. weak (T. bifidum) competitors than by frequency of conspecifics per se. Species had higher growth and stronger negative frequency dependence when grown with their competitors’ vs. their own rhizobial strains. As a result, three of four species (all except the weakest competitor, T. bifidum) performed best when they were both rare and associating with their competitors’ rhizobia—i.e. the conditions they would experience as invaders. Overall, our experiment demonstrates the simultaneous operation of stabilizing niche differences—driven by interactions with rhizobial mutualists—and competitive hierarchies among interacting plant species, with implications for patterns of coexistence and abundance in natural communities.