Can oysters provide a refuge to coastal biodiversity in a changing world?

Climate change is forcing species to adapt to the rapidly changing environment, migrate, or face extinction. Ecosystem engineers can ameliorate environmental stress experienced by associated organisms, and may provide climate refugia for biodiversity. However, for their conservation to be an effective strategy for climate change adaptation, we need to know where, when and how ecosystem engineers have the greatest influence on biodiversity. This thesis focuses on ecosystem engineering by intertidal Saccostrea oysters, examining (1) how the mechanisms of facilitation by oysters change across environmental gradients, (2) whether specific Saccostrea populations are more resilient to temperature extremes, (3) whether greater resilience is driven by sub-cellular stress responses to high temperatures, and (4) how intraspecific variation in key structural traits of oysters influence their capacity to ameliorate temperature extremes. Manipulative experiments replicated across 900km of coastline revealed that although provision of structure by oysters was a key mechanism by which they facilitated biodiversity, at warmer sites amelioration of heat and desiccation stress was an increasingly important mechanism of facilitation, whereas in cooler climates, amelioration of predation was more important. Oysters selectively bred for fast growth and disease resistance were more susceptible to rising temperatures than unselected oysters, and consequently, were less effective microhabitat refugia to invertebrates under scenarios of warming. However, selectively bred oysters demonstrated greater upregulation of genes involved in maintaining cellular homeostasis under warmer climate scenarios, suggesting that breeding programs targeting climate resilience may successfully increase the resilience of Saccostrea. The structural traits of oyster habitat influenced their capacity to ameliorate climate, with vertically but not horizontally orientated oysters alleviating physical stress experienced by associated species. Overall, Saccostrea appear to have the capacity to endure the predicted temperature increases for the coming decades, and where they form dense, vertically orientated habitat, their conservation can provide a climate-adaption strategy for coastal biodiversity.