Towards Developing a Mechanistic Understanding of Coral Reef Resilience to Thermal Stress Across Multiple Scales

Coral reefs are a globally threatened ecosystem due to a range of anthropogenic impacts. Increasing sea surface temperatures associated with global warming are a particular threat, as corals grow close to their upper thermal limit. When this limit is exceeded for a sufficient length of time during thermal stress events, corals lose their algal symbionts, resulting in coral bleaching and possible mortality. Coral reefs have experienced the most severe and extended global bleaching event to date extending from 2014 to 2017. The most recent global climate models predict that similar global bleaching events are likely to become an annual occurrence by the middle of the present century. Current uderstanding of coral reef recovery following disturbance events is based around decadal to sub-decadal impacts, making the adaptive capacity of corals as bleaching events approach an annual frequency unknown. However, there is considerable spatial heterogeneity in bleaching impacts across a range of scales, from global reef provinces to local reef areas and between coral species. Understanding of the mechanisms responsible for this observed coral resilience to thermal stress is increasing within a variety of disciplines, with particular recent advances at the sub-cellular level, facilitated partly by technological developments. This understanding suggests that some resilience factors have the potential to operate within the predicted annual frequency of thermal stress events, whilst others act over longer time-scales. The ability of coral reef management actions to successfully support coral resilience is a significant challenge and requires increased empirical evidence to support and refine actions. However, any protective action first necessitates a focus on identifying
reef locations that have the potential to exhibit resilience to thermal stress events, either via resisting them or recovering quickly following impact. Here, we present a spatially explicit overview of the potential resilience factors and mechanisms that can be considered in such an approach.