Context- and density-dependent effects of introduced oysters on biodiversity

Pacific oysters, Crassostrea gigas, have been introduced throughout much of the world, become invasive in many locations and can alter native assemblage structure, biodiversity and the distribution and abundance of other species. It is not known, however, to what extent their effects on biodiversity change as their cover increases, and how these effects may differ depending on the environmental context. Experimental plots with increasing cover of oysters were established within two estuaries in two different habitats commonly inhabited by C. gigas, (mussel-beds and mud-flats) and were sampled after 4 and 15 months. Within mud-flat habitats, macroscopic species living on or in the substratum increased in richness, Shannon–Wiener diversity and number of individuals with oyster cover. In mussel-bed habitats, however, these indices were unaffected by the cover of oysters except at one estuary after 15 months when species richness was significantly lower in plots with the greatest cover of oysters. Assemblage structure differed with oyster cover in mud-flats but not in mussel-beds, except at 100 % cover in one location and at one time. Within mud-flats at one location and time (of four total tests), assemblages became more homogenous with increasing cover of oysters leading to a significant decrease in β-diversity. These responses were primarily underpinned by the facilitation of several taxa including a grazing gastropod (Littorina littorea), an invasive barnacle (Austrominius modestus) and a primary producer (Fucus vesiculosus) with increasing cover of oysters. Although there were consistent positive effects of C. gigas on mud-flat biodiversity, effects were weak or negative at higher cover on mussel-beds. This highlights the need for the impacts of invasive species to be investigated at a range of invader abundances within different environmental contexts.