Stable coexistence in communities requires that each species can increase when it is rare in a patch of heterospecifics. These stabilizing dynamics are possible when conspecifics limit each other’s fitness more than they limit the fitness of heterospecifics, leading to negative frequency-dependent growth. We assessed stabilizing forces in the coexistence of diverse assemblages of the legume Trifolium, for which there is a 15-year record of co-occurrence of 8 species in 4-m2 plots at our field site in Bodega Bay, CA. We tested stabilizing forces in the field by transplanting each Trifolium species into the field niches of each species, with neighbors present or removed. We quantified density- and frequency-dependence of species’ performance (survival and growth) and the strength of stabilization for each species pair in each neighbor treatment. We related stabilizing forces to species associations in natural communities, using co-occurrence data from 562 0.09-m2 plots surveyed over two years.
Overall, Trifolium survival was greater in sites with greater natural Trifolium density, reflecting more suitable habitats. The strength and direction of frequency dependence varied among species and neighbor treatments. In the absence of neighbors, survival of most species decreased with increasing conspecific frequency, suggesting stabilizing soil-mediated feedbacks. In contrast, most species had positive frequency-dependent survival when neighbors were present, indicating destabilizing feedbacks mediated by interactions with neighbors. Similarly, when examining stabilizing forces (the degree to which species limited conspecifics vs. heterospecifics) across species pairs, we found stabilization for most species pairs in the absence of neighbors, whereas most pairwise interactions were destabilizing in the presence of neighbors. The strength of stabilization without neighbors was positively related to species’ spatial associations in field plots, suggesting that soil-mediated stabilizing forces promote species coexistence in natural Trifolium assemblages.