The mechanisms by which oysters facilitate invertebrates vary across environmental gradients.

The effective use of ecosystem engineers to conserve biodiversity requires an understanding of the types of resources an engineer modifies, and how these modifications vary with biotic and abiotic context. In the intertidal zone, oysters engineer ecological communities by reducing temperature and desiccation stress, enhancing the availability of hard substrate for attachment, and by ameliorating biological interactions such as competition and predation. Using a field experiment manipulating shading, predator access and availability of shell substrate at four sites distributed over 900 km of east Australian coastline, we investigated how the relative importance of these mechanisms of facilitation vary spatially. At all sites, and irrespective of environmental conditions, the provision of hard substrate by oysters enhanced the abundance and richness of invertebrates, in particular epibionts (barnacles and oyster spat) and grazing gastropods. Mobile arthropods utilised the habitat provided by disarticulated dead oysters more than live oyster habitat, whereas the abundance of polychaetes and bivalves were much greater in live oysters, suggesting the oyster filter-feeding activity is important for these groups. In warmer estuaries, shading by oysters had a larger effect on biodiversity, whereas in cooler estuaries, the provision of a predation refuge by oysters played a more important role. Such knowledge of how ecosystem engineering effects vary across environmental gradients can help inform management strategies targeting ecosystem resilience via the amelioration of specific environmental stressors, or conservation of specific community assemblages.