Trajectories of Functional and Species Change During Plant Community Assembly in a California Serpentine Grassland

Elucidating the mechanisms that underlie species coexistence and community assembly is central for understanding basic ecological patterns of species' abundances, how global change may alter those patterns, and how to effectively manage ecosystems. Niche and neutral theory represent two opposite sides on a continuum of potential drivers of community composition and coexistence. Niche theory poses that species' differences facilitate coexistence by causing intraspecific competition to be stronger than interspecific competition. In contrast, neutral theory assumes that species are similar in fitness and their effects on one another, and that stochastic variation in births, deaths, immigration, and extinction allow coexistence. Rather than being mutually exclusive mechanisms, niche and neutral processes may operate at different levels of biological organization. We tracked the natural assembly of experimental serpentine grassland communities to test this idea. Specifically, we hypothesized: 1) complementarity, a niche-based deterministic process, would cause communities to converge toward a common functional composition; while 2) species composition within functional groups would be neutral and driven by stochastic forces such as dispersal. The communities were created in 1992 for a previous experiment and initially varied in both functional (and therefore species) richness and composition. Four functional groups, chosen based on complementary resource acquisition strategies, comprised the experimental communities: early season annuals, late season annuals, perennial bunch grasses, and nitrogen-fixing annuals. Starting in 2001, species from the surrounding grassland colonized the experimental communities. I analyzed data on species richness and relative abundance over the subsequent six years. Over the first four years of the study, both functional and species compositions converged across treatments. Subsequently, however, functional composition remained relatively constant while species composition diverged among communities. Niche complementarity likely drove overall convergence in functional composition and initial convergence in species composition. However, species composition was driven by additional forces, including trait-environment relationships, density-dependent plant feedbacks, and potential neutrality. Our results suggest that community composition may be driven simultaneously by both niche and neutral processes, and that the drivers of community composition likely shift over time.