Mutualistic interactions amplify saltmarsh restoration success

Mounting evidence shows that the functioning and stability of coastal ecosystems often depends critically on habitat‐forming foundation species such as seagrasses, mangroves and saltmarsh grasses that engage in facultative mutualistic interactions. However, although restoration science is now gradually expanding its long‐standing paradigm of minimizing competition to including intraspecific, or within species, facilitation in its designs, the potential of harnessing mutualistic interactions between species for restoration purposes remains uninvestigated. Here, we experimentally tested whether a previously documented mutualism between marsh‐forming Spartina alterniflora (cordgrass) and Geukensia demissa (mussels) can increase restoration success in degraded US saltmarshes. We found that co‐transplanted mussels locally increased nutrients and reduced sulphide stress, thereby increasing cordgrass growth and clonal expansion by 50%. We then removed above‐ground vegetation and mussels to simulate a disturbance event and discovered that cordgrass co‐transplanted with mussels experienced three times greater survival than control transplants.Synthesis and applications . Our findings indicate that mussels amplify cordgrass re‐colonization and resilience over spatial and temporal scales that exceed those of their actual mutualistic interaction. By experimentally demonstrating that mutualistic partners can enable foundation species to overcome stress barriers to establish and persist, we highlight that coastal restoration needs to evolve beyond the sole inclusion of intraspecific‐positive interactions. In particular, we suggest that integrating mutualisms in restoration designs may powerfully enhance long‐term restoration success and ecosystem resilience in the many coastal ecosystems where mutualisms involving foundation species are important ecosystem‐structuring interactions.