European native oysters and associated species richness in the presence of non-native species in a southern North Sea estuary complex

There are growing calls to restore populations of European native oysters (Ostrea edulis), on the premise that restored populations will support a range of ecosystem services with an emphasis placed on restored oyster habitats promoting biological diversity, however benefits associated with naturally occurring O. edulis remain unclear. We undertook biannual surveys in the Blackwater, Crouch, Roach and Colne Estuaries Marine Conservation Zone (BCRC.MCZ), a highly sedimented estuary complex in the southern North Sea, to investigate links between natural densities of O. edulis (0-4.2 m-2), the prevalence of other dominant habitat features such as non-native slipper limpet (Crepidula fornicata), dead shell abundance and epibenthic macroinvertebrate species richness. Increased epibenthic species richness was associated with O. edulis, even at densities below the OSPAR Commission recognised definition of an oyster bed (5 oysters m-2). Our analysis predicts increased associated species richness with density of native oysters (e.g. +1.6 additional species at 1 oyster m-2 or + 2.8 species at 5 oysters m-2), but only in areas with lower density of C. fornicata. Where C. fornicata are at higher density, the potential benefits of oyster restoration for associated species were curtailed. This may explain the observed asymptotic relationship between oyster density and diversity at 1 oyster m-2. In these and other high Crepidula density areas we recommend extending native oyster habitat even at low density. This may be of particular interest to areas with the protozoan oyster parasite Bonamia ostreae, which spreads more easily in high density areas. These lower density thresholds should also be considered for future management decisions – closing harvests so they do not reduce density further and impair biodiversity services of the habitats. In conclusion, while C. fornicata may be a useful oyster settlement substrate, we find that it limits the potential increases in associated species gains of oyster restoration.