A roadmap for incorporating positive species interactions into Spartina alterniflora restoration designs

There is a growing recognition of the influence that biotic factors have on restoration outcomes. In particular, the Stress Gradient Hypothesis predicts that harnessing positive species interactions (e.g., mutualism, facilitation) will enhance restoration success. Accordingly, there have been numerous calls for the incorporation of positive species interactions into restoration practice. To better guide restoration practitioners on the use of positive species interactions in wetland restoration, we performed a systematic review to document all facilitative interactions that benefit a dominant salt marsh species, Spartina alterniflora. In total, we identified thirty studies of facilitation, which revealed eleven unique species interactions that benefit S. alterniflora. These included interspecific interactions with molluscs, crabs, a plant and a parasite. The most supported interaction was with the ribbed mussel, Geukensia demissa, as it was shown to have a largely beneficial effect on S. alterniflora throughout much of its range. Notably, most of the identified species interactions were context-dependent (i.e., positive effects only existed under certain experimental or environmental conditions, and sometimes negative effects could occur). Altogether, this review provides a starting point for restoration practitioners that are interested in identifying species that may be suitable for co-restoration with S. alterniflora. Incorporating positive species interactions into restoration designs has the potential to improve restoration efficiency, but more work is needed to fully understand the mechanisms and context-dependency behind these interactions and explicitly guide their use in different locations.