Identifying and tracking resilience to ocean warming in marine ecological communities using the Community Temperature Index

The community temperature index (CTI) summarises the average thermal niche of the species and individuals present in a local community. It has been shown to increase with long-term warming in birds and butterflies and in... [ view full abstract ]

The community temperature index (CTI) summarises the average thermal niche of the species and individuals present in a local community. It has been shown to increase with long-term warming in birds and butterflies and in global fisheries catches, and offers a theoretically-specific indicator for warming-driven biodiversity change. There has been no empirical evaluation of whether it responds linearly with temperature/warming, however, nor any evaluation of broad spatial trends in CTI of whole ecological communities in the marine environment. Using the Reef Life Survey dataset, we undertook a global analysis relating the CTI to local sea surface temperatures (SST), and identified considerable non-linearities – steps where the CTI jumps disproportionately with small changes in SST. These steps have interesting underlying ecological, biogeographic and physiological causes, but also substantial implications for predictions of warming impacts on marine biodiversity and associated management options.

Our results confirm the utility and importance of CTI as a biodiversity indicator relevant at local and global scales, when interpreted in light of the non-linearity (and reduced scope for change in tropical locations). It allows the magnitude of warming-related change to be reported to the public and policy-makers in an easily interpretable manner. The CTI should be a critical inclusion in monitoring, evaluation and reporting (MER) for marine protected areas, and evaluation of other marine management (e.g. fisheries), providing a unique means to assess indirect ecological effects which improve resilience to warming-related biodiversity change. Tracking CTI not only provides managers with an additional layer of detail on which to base evaluation of management effectiveness, but with standardised monitoring over broad spatial and temporal scales, commonalities can be drawn which can guide future management aimed at more broadly addressing multiple threats to marine biodiversity.