Multi-species restoration accelerates recovery of extinguished oyster reefs.
A multi-species approach to habitat restoration may boost the key processes (e.g. recruitment) that enable foundation species to overcome barriers to recovery. Natural systems tend to be formed by co-occurring foundation species whose synergy drives ecological productivity and resilience beyond that of single foundation species. Yet, restoration remains primarily a single-species focus enterprise where positive interactions are seldom incorporated into planning. A multi-species approach that prioritizes species combinations that create emergent properties for their persistence may accelerate habitat recovery and the success of restoration programmes. On the largest oyster reef restoration project in the Southern Hemisphere, we experimentally established canopy-forming kelp to test whether they could accelerate the natural recruitment of oysters to substrata monopolized by turf-forming algae. To understand whether facilitation of oysters was a function of the kelp themselves (biological facilitation) or the physical environment they create (physical facilitation), we compared recruitment to the understorey of living kelp and synthetic kelp mimics. Despite observing high density oyster recruitment to the turf-free underside of reef boulders (8,300 oysters/m2), turf algae appeared to inhibit oyster recruitment to the exposed surfaces of the reef, limiting their capacity to grow and form complex, three-dimensional habitat. Transplanted kelp, whether living or synthetic kelp mimics, effectively reduced the biomass of turf and enhanced oyster recruitment, creating turf-free substrata on the upper reef surfaces with up to 26 times the oyster recruitment than turf-covered substrata. Synthesis and applications. Our results provide proof-of-concept that incorporating the transplant of canopy-forming kelp to reefs constructed to restore oysters is not only achievable, but may be imperative to successfully restore oyster reefs in turf-dominated systems. Kelp transplants suppressed the turf algae that otherwise excluded oysters from the reef surface, effectively shifting the competitive advantage toward oyster recovery by maintaining bare substrata for oyster recruitment. By demonstrating that a multi-species approach to restoration accelerates the recovery of a restored habitat, we emphasize the value of incorporating the multi-species concept into restoration planning.