Coral reefs form under conditions that are, on a planetary scale, rather narrow. They require warm, clear, shallow water, stable salinity and a rocky or sandy base on which larval corals can settle. These conditions occur in a band roughly thirty degrees north and south of the equator, and although they are a small fraction of the ocean's surface, they are home to something like a quarter of all known marine species. Understanding how reefs manage this density of life, and how they respond to disturbance, has become central to marine biology over the last forty years.

A coral is not a single organism but a colony of small polyps, each of which secretes a stony cup around itself. The polyps are cnidarians, related to sea anemones and jellyfish, and they draw most of their food from tiny algae called zooxanthellae that live inside their tissues. The algae receive shelter and a reliable supply of carbon dioxide; the coral receives sugars produced by the algae through photosynthesis. This exchange is so important that most reef-building corals cannot survive in the long term without their algal partners. Marine biologist Dr. Tariq Ahmed has described the reef as 'a rock built by a partnership'.

The partnership is also its weakness. When water temperatures rise even a degree or two above the long-term summer maximum, the corals expel their zooxanthellae, a response known as bleaching. Without the algae, the corals turn white and lose their main food supply. If conditions return to normal within a few weeks, the algae may recolonise and the coral may survive. If elevated temperatures persist, the corals starve. Severe bleaching events on the Great Barrier Reef in 2016 and 2017 killed large areas of shallow coral, and further events in 2020 and 2022 affected reefs that had partially recovered.

Recovery, where it occurs, depends on the supply of new larvae. Adult corals spawn on a small number of synchronised nights each year, releasing gametes that drift with the currents until they settle. A reef that has been damaged can therefore be reseeded from upstream reefs, provided those reefs are healthy and the currents connect them. Dr. Ahmed's group at James Cook University has mapped these connections for the Great Barrier Reef and identified a smaller set of so-called source reefs whose protection is disproportionately important. Concentrating conservation effort on these reefs, rather than spreading it evenly, may make a larger contribution to overall recovery.

Not all threats are thermal. Sedimentation from coastal development clouds the water, reducing the light available for photosynthesis. Nutrient run-off from agriculture encourages fast-growing algae that can smother coral. Outbreaks of the crown-of-thorns starfish, a native species that periodically becomes destructive, can strip live coral from large areas in months. And ocean acidification, a slow chemical consequence of increased atmospheric carbon dioxide, makes it harder for corals and other calcifying organisms to build their skeletons. Each of these factors interacts with the others, and none is easily reduced in isolation.

Several restoration techniques are now in trial. In Indonesia and the Maldives, coral fragments are grown on underwater nurseries and transplanted onto degraded sites, where they sometimes establish quickly. In Florida, heat-tolerant strains of coral, selected from naturally warm lagoons, have been propagated for use on reefs likely to experience higher temperatures. Critics point out that such projects are expensive and have not yet been shown to work on a scale that makes a measurable difference to the species they aim to save. Dr. Ahmed accepts this but argues that the techniques are still necessary as insurance, precisely because the primary causes of reef decline lie beyond the control of any single reef authority.

Reefs, in short, are under pressure on several fronts at once. Their capacity to recover from past disturbances has been their most reassuring feature, but that capacity depends on conditions that are changing rapidly. The science of reefs is now as much a science of rates of change as it is of community ecology: how fast corals grow, how quickly algae bloom, how long a bleached colony can hold on before it starves. What the next decade shows will depend less on new biological insight than on whether the large-scale conditions that support reefs can be stabilised in time.