When competition doesn’t matter: Grassland diversity and community composition

Numerous ecological processes, including competition, are thought to control the diversity of plant communities. In grasslands, root competition is intense, but few studies have directly measured whether that competition has any consequences for species diversity or community composition. In this study I used Structural Equation Modeling (SEM) to examine the networks of interaction through which competition, soil resources, and light availability control plant community biomass, species richness, evenness, and composition. Soil resource levels had direct consequences for root and shoot biomass but only weak effects on competition intensity. Competition intensity had few effects on species richness, evenness, or community composition. In rough fescue grasslands root competition can reduce plant performance by an order of magnitude or more, yet despite these severe effects on individual plants competition appears to have almost no consequences for plant community structure. The lack of relationship between competition and species richness and evenness suggests that belowground plant interactions may not play an important role in structuring plant communities.