Multi‐species restoration accelerates recovery of extinguished oyster reefs

Abstract

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.

Oyster larvae swim along gradients of sound

“…By increasing larval settlement, restorers also have the capacity to reduce the risk of poor settlement during low recruitment periods; spatial and temporal variability of recruitment being notoriously variable in marine systems. Moreover, restoration may be accelerated if sound technology is used in tandem with times of natural peak recruitment and the provision of competitor‐free substratum (Lipcius et al, 2021; McAfee et al, 2021). By putting sound back into currents, these gradients of larval transport (sensu Garth, 1966) may become more navigable, such that they represent gradients of sound to aid restoration.…”

Repairing recruitment processes with sound technology to accelerate habitat restoration

“…For example, synchronizing soundscape enrichment with the timing of recruitment or interventions to provide competitor‐free substratum (McAfee and Connell 2020, McAfee et al. 2021) may increase the success of restoration initiatives.…”

“…First, we recorded soundscapes associated with three distinct marine habitats of southern Australia (Gulf St. Vincent, South Australia): a temperate rocky reef ecosystem, a sedimentary barren where oyster reefs historically existed but were lost to dredge fishing (Alleway and Connell 2015), and a large restoration site (20 ha) on which lost oyster reefs are being restored (McAfee et al. 2021; Appendix : Section S1). Based on sound pressure level and snapping shrimp snap counts, we found that acoustic characteristics increased from degraded to healthier habitats; the sedimentary habitat was the quietest, the rocky restoration site louder and natural rocky reef the loudest and contained the most snapping shrimp snaps (Appendix : Figs.…”

“…Ongoing environmental and anthropogenic changes to marine ecosystems will likely continue to diminish the acoustic function of marine soundscapes (Rossi et al 2016a(Rossi et al , 2017. And in many cases, the historical extraction of marine habitatforming species (e.g., oyster reefs) would have transitioned soniferous marine habitats into sedimentary barrens centuries ago (McAfee and Connell 2021). With wholesale habitat loss, the function of marine soundscapes is also lost, potentially limiting the capacity of these habitats to naturally recover.…”

“…To assess whether habitat restoration can transform degraded soundscapes ("muted-scapes") back to functional soundscapes, we first compared the soundscapes of natural, partially restored and degraded marine habitats, and then assessed how these differences alter recruitment processes. First, we recorded soundscapes associated with three distinct marine habitats of southern Australia (Gulf St. Vincent, South Australia): a temperate rocky reef ecosystem, a sedimentary barren where oyster reefs historically existed but were lost to dredge fishing (Alleway and Connell 2015), and a large restoration site (20 ha) on which lost oyster reefs are being restored (McAfee et al 2021;Appendix S2: Section S1).…”