Influence of stochastic and deterministic community assembly on beta-diversity in a long-term grassland experiment

Background/Question/Methods

Species turnover in space (beta-diversity) is an important pattern of biodiversity and is closely linked to processes of community assembly. It is well known that species sorting along environmental gradients contributes to beta-diversity. However, there has long been interest in gauging the extent to which historical contingencies may influence beta-diversity patterns by producing alternative trajectories of community assembly, even among sites that share common environmental conditions. Here we report results from a long-term grassland experiment to test alternative models of community assembly. Experimental plots were sown in a common environmental background to a wide variety of initial community states with respect to plant species diversity, composition, and initial abundances, and the extent to which these plots converged or diverged in composition over the course of community development was evaluated. Community convergence and decreasing beta-diversity is expected if highly deterministic, niche-based species interactions such as competition overcome initial conditions and drive community development. Alternatively, divergence (or a lack of convergence) is expected if priority effects are common or if species are competitively equivalent, leading to community drift; these processes will maintain or even increase beta-diversity over time.

Results/Conclusions

Considering all plots in the experiment, communities are exhibiting a significant pattern of community convergence, largely attributed to a select few, highly dominant species that have increased in abundance over time. Interestingly, similarity analyses conducted on presence-absence matrices showed that plots have significantly diverged over time in community membership. Taken together, these results indicate that increased abundance and spread of a few competitively dominant species across the experimental metacommunity are deterministically increasing the similarity of communities. However, when all species are weighted equally, communities are instead growing dissimilar over time, suggesting that community membership is diverging and beta-diversity is increasing. These results illuminate an interesting relationship between deterministic and stochastic assembly processes – deterministic processes lead to dominance by a few species, but the impacts of these species on the subordinate community appears to be stochastic in nature.