Trait-mediated interactions drive local diversity

Identifying overarching processes that maintain biodiversity in natural communities remains a challenge in ecology. Although functional traits help explain regional species distributions, they often fall short at the local community level. We investigate whether traits can offer mechanism-based insights into local diversity maintenance due to associations with the sign and strength of plant interactions. We examine the effect of 12 plant functional traits on the sign and strength of pairwise species interactions across two Mediterranean annual plant communities. Results show that traits mediate a spectrum from facilitative to competitive interactions and are influenced by neighbor density and identity. At low densities, species with conservative resource-use traits are consistently stronger facilitators than acquisitive species. The traits of the focal species, however, explain facilitation at low densities better than the traits of neighbors or the pairwise differences in traits. When neighbor density increases, facilitation switches to competition, a pattern we hypothesize reflects a density-mediated reduction in how much neighbors' traits drive interactions. Moreover, species frequently receiving heterospecific facilitation also display traits associated with self-competition and low intrinsic population growth rate. This study of community-level trait sorting provides insights into the persistence of co-occurring populations, showing trait-based generalizable insights into the local context dependency of species interactions.