Can damaged coral reefs recover?
Coral reefs cover less than one per cent of the ocean floor, yet they are thought to support around a quarter of all known marine species, as well as the fishing and tourism on which many coastal communities depend. Over the past few decades, rising sea temperatures have caused repeated episodes of bleaching, in which stressed corals expel the tiny algae that live inside their tissues and provide most of their food. For a long time, scientists assumed that a reef which had lost most of its living coral in this way would take decades to return, if it returned at all.
Recent research has challenged this view. A team led by Dr Amara Okonjo spent twelve years monitoring a group of reefs in the Indian Ocean that had all been severely bleached in a single summer. Others, however, showed almost no improvement over the same period, even though they had suffered similar damage and lay only a few kilometres away.
The team set out to explain this difference. Where the dead coral skeletons remained standing, they offered young corals countless sheltered surfaces on which to settle and grow. Where storms had reduced the skeletons to loose rubble, the larvae that drifted in had nowhere stable to attach, and most were swept away or buried. Fish also played a part. Without this grazing, seaweed quickly spread across the bare surfaces and prevented new coral from taking hold.
These findings have practical consequences for the way reefs are managed. Some coastal communities have already introduced limits on catching grazing species, and conservation groups in several countries are experimenting with fixing artificial frames to the sea floor where the natural structure has collapsed. Early results from these frames are encouraging, although they are expensive to build and so far cover only small areas, so they are unlikely ever to replace a healthy natural reef.
Dr Okonjo is careful not to overstate the case. Local measures, she argues, can buy time, but they cannot protect reefs from heatwaves that are becoming more frequent and more intense. Recovery after one severe bleaching event is possible; recovery after one every few years is far less likely.