Global warming is rapidly emerging as a universal threat to ecological integrity and function, highlighting the urgent need for a better understanding of the impact of heat exposure on the resilience of ecosystems and the people who depend on them1. Here we show that in the aftermath of the record-breaking marine heatwave on the Great Barrier Reef in 20162, corals began to die immediately on reefs where the accumulated heat exposure exceeded a critical threshold of degree heating weeks, which was 3–4 °C-weeks. After eight months, an exposure of 6 °C-weeks or more drove an unprecedented, regional-scale shift in the composition of coral assemblages, reflecting markedly divergent responses to heat stress by different taxa. Fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29% of the 3,863 reefs comprising the world’s largest coral reef system. Our study bridges the gap between the theory and practice of assessing the risk of ecosystem collapse, under the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems3, by rigorously defining both the initial and collapsed states, identifying the major driver of change, and establishing quantitative collapse thresholds. The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, both adding to and far exceeding the influence of recurrent cyclones and other local pulse events, presenting a fundamental challenge to the long-term future of these iconic ecosystems.
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We acknowledge support from the Australian Research Council’s Centre of Excellence Program and a Laureate Fellowship to T.P.H., from the Great Barrier Reef Marine Park Authority, and from the US National Oceanic and Atmospheric Administration. The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the authors and do not necessarily reflect the views of NOAA or the US Department of Commerce. We thank T. Simpson, who provided 225 aerial scores of bleaching from the Torres Strait; M. Jacobson for assistance with statistical programming; members of the Australian National Coral Bleaching Taskforce, marine park managers and rangers, and 30 student volunteers, who participated in extensive field studies on the Great Barrier Reef throughout 2016.
Extended data figures and tables
Aerial scores of bleaching on the x axis are: 0 (< 1% of colonies bleached), 1 (1–10%), 2 (10–30%), 3 (30–60%) and 4 (60–100%). Change in coral cover on the y axis was measured in situ between March and November 2016 on 98 reefs that were also scored from the air. Box plots are shown for each aerial category, showing median values (horizontal lines), boxes for values in the 25th–75th percentiles, vertical lines for values less than the 25th percentile and greater than the 75th, and data points for outliers. Medians were used when calibrating change in cover for each aerial category (see Fig. 1a).
Losses, measured on 110 reefs between March and November 2016, range from 0 (dark green) to 100% (1–5% (green), 5–25% (light green), 25–50% (yellow), 50–75% (orange) and 75–100% (red)). Map template is provided by Geoscience Australia (Commonwealth of Australia (Geoscience Australia) 2018).
The frequency distribution of coral cover on 110 reefs, measured in March 2016 (solid bars) and again in November 2016 (hashed bars). Reef locations are shown in Extended Data Fig. 2.
Box plots are shown for each taxon, showing median mortality (horizontal lines), boxes for the middle two quartiles, vertical lines for the first and fourth quartiles, and data points for outliers. a, The initial mortality of corals recorded on belt transects on 43 reefs with > 60% bleaching. b, Longer term loss of cover for taxonomic categories recorded between March and November 2016 on the 43 remeasured reefs with > 60% bleaching. Taxa in a and b are plotted in rank order along the x axis, from highest to lowest decreases in mean cover between March and November 2016.
Sensitivity is estimated from the loss of cover on 63 reefs for different groups of corals between March and November 2016, as a function of heat exposure (DHW). The horizontal axis is the slope of the relationship between the log-ratio of final and initial coral cover (response variable) and DHW (explanatory variable). Values plotted for each taxonomic grouping (ordered from most sensitive to least sensitive) are random effects estimates, with conditional standard errors.
The regression shows the relationship between the levels of bleaching by individual coral taxa on severely bleached reefs (where > 60% of all colonies were affected, n = 43 reefs), and their subsequent loss of cover eight months later. The non-significant correlation indicates that the winners–losers spectrum of bleaching among taxa is a poor predictor of which ones ultimately die.