Sources of spatio-temporal variation in habitat provisioning by the Sydney rock oyster, Saccostrea glomerata

Habitat-forming species provide structure that facilitates colonisation and survival of associated species. Along the east coast of Australia, there is growing interest in restoring reefs once formed by the Sydney rock oyster, Saccostrea glomerata, to enhance biodiversity, fisheries productivity and water quality. To provide information on when and where restoration efforts might be most successful, I investigated sources of spatial and temporal variation in the colonisation of oysters and their facilitation of invertebrates. In many areas oysters are substrate limited and live or dead shell is added to facilitate reef growth. I assessed how the timing of substrate deployment, and its status as live or dead shell, in loose or consolidated arrangements influences oyster recruitment and the development of associated communities. I found that the timing of substrate deployment influenced oyster recruitment and community assembly by determining whether oysters were able to colonise prior to competitors such as barnacles and algae. Dense and diverse communities of invertebrates, however, colonised irrespective of whether substrate was live oysters or dead shell, or was consolidated or loose. Hence, the timing of substrate deployment will be critical to the success of restoration projects. Although many restoration projects have focused on the rehabilitation of a single habitat-forming species, in most instances biodiversity is underpinned by interactions between multiple co-occurring habitat-forming species. I assessed how the presence of a second habitat-forming species influences habitat provisioning by S. glomerata. I found that in mangrove forests, S. glomerata and the co-occurring, habitat-forming alga Hormosira banksii had distinct, and additive effects on invertebrate recruitment and on mitigating predator-prey interactions. Hence, there may be benefits of restoring multiple habitat-forming species at a site. Finally, I investigated variation in facilitation of invertebrates by oysters across a wave exposure gradient. Using a survey and a manipulative field experiment, I partitioned effects of wave energy on oyster communities into direct effects and indirect effects arising from responses of oyster morphology and density to wave energy. Across a wave exposure gradient, invertebrate abundance and richness, and oyster size and density were negatively correlated with wave energy. Indirect effects of wave energy on invertebrate communities, arising from effects on oyster density and morphology, were more important than direct effects. Overall, my results support the important role of habitat-forming species morphology and environmental context in shaping positive interactions. To maximise the success of restoration, programs should target environments in which the growth forms of habitat-forming species have the greatest positive effect on biodiversity.