Confronting contingency in restoration: Management and site history determine outcomes of assembling prairies

Background/Question/Methods

The outcomes of ecological restoration are notoriously unpredictable, but we have no general predictive understanding of this contingency. Management decisions can have strong effects on restoration outcomes, but in other cases they are overwhelmed by site characteristics (e.g., soil conditions), landscape context (e.g., abundance of similar habitat), or historical factors (e.g., priority effects). However, we generally cannot predict which of these four classes of drivers will affect restoration outcomes. Disparate aspects of restoration outcomes (e.g., species richness, beta diversity, and community composition) and their unique responses further complicate our understanding. Finally, these four classes of drivers might differentially affect subsets of the restored community; for example management might shape the abundance and distribution of species of the target community, while other species are more contingent on site, landscape, or historical factors. We used variation partitioning to compare the relative importance of management, site, landscape, and historical factors for determining the plant community outcomes of 27 prairie restorations in southwest Michigan.

Results/Conclusions

We found that management, especially seed mix composition, diversity, and density, and history, especially site age, were the most important drivers of prairie restoration species richness, beta diversity, and plant community composition. Site and landscape factors were rarely important for restoration outcomes we measured. Finally, we found that comparing the unique responses of sown and nonsown species typically increased our understanding of the dynamics contributing to community-wide restoration outcomes. By simultaneously measuring the influence of these four major drivers of restoration outcomes, this study represents an important step towards developing a more predictive framework for understanding contingency in restoration outcomes.