Biotic resistance and facilitation of a non-native oyster on rocky shores

The Pacific oyster Crassostrea gigas varies dramatically in density throughout its introduced range in the northeast Pacific; this could be driven by environmental constraints or by species interactions that change across habitats. I studied the effects of native species on this invader across a range of environmental contexts where it is common (mid-intertidal zone, low wave exposure) vs. where it is rare (low intertidal zone, high wave exposure) in Barkley Sound, Vancouver Island, Canada. I carried out factorial manipulations of 2 guilds of native species (neighbors, i.e. filter feeders such as mussels and barnacles, and predators such as crabs and whelks) and recorded survival and linear shell growth of transplanted, newly settled oysters (juveniles similar to 1 cm in size). C. gigas responded dramatically to tidal elevation: growth improved but survival declined at lower elevations, where feeding time and predation were greater. Wave exposure reduced shell growth over 2 mo from 11.4 mm at protected sites to 7.2 mm at exposed sites, but variation in survival was not statistically significant. Both guilds of native species exerted biotic resistance: predators reduced oyster survival and neighbors reduced growth. Surprisingly, neighbors improved oyster survival at some wave-exposed sites, thereby facilitating one demographic variable related to invasion while restricting another. Finally, predation had equivalent effects on survival at wave-exposed and protected sites, although different predators were probably involved. Post-recruitment phenomena were unable to account for variation in oyster density across sites, and propagule pressure is a likely driver instead. Abiotic and biotic factors jointly contributed to the risk of proliferation of a non-native species, and in this case their influence was often (on growth) but not always (on survival) additive.