Why do crown-of-thorns starfish sometimes swarm a reef?

Crown-of-thorns starfish swarm a reef when far more of them survive to adulthood than usual, but scientists still argue about what causes that, so the honest answer is that nobody knows for sure. The starfish itself is not a newcomer or a monster. It is a large starfish covered in venomous spines that eats hard coral polyps, and it is part of most coral reefs. At low numbers, it eats coral more slowly than the coral grows. Some ecologists even suggest that it helps keep a reef diverse, by stopping fast-growing corals from overpowering slower ones.

The trouble starts when the numbers explode. It feeds by pushing its stomach out over the coral and digesting the living tissue, leaving a white skeleton that is quickly covered in algae. One starfish can consume up to 6 square metres of living reef a year. When there are too many starfish for the coral to feed them, coral cover goes into decline. In 2000 an outbreak contributed to a loss of 66 percent of live coral cover on reefs sampled in a study on the Great Barrier Reef. Large outbreaks have been confirmed at 21 reef locations between the 1960s and 1980s.

Why do they happen? One idea is that more larvae survive. A female releases millions of eggs, so a small change in survival makes a huge difference: a survival rate of one in a thousand, instead of a tiny fraction, would turn one spawning into thousands of adults. Charles Birkeland noticed outbreaks about three years after heavy rain that followed a drought, and suggested that runoff feeds plankton blooms for the larvae, whose survival does depend strongly on how much plankton they find. Yet some later studies found that runoff has little or no effect on larval survival. Another idea blames the loss of predators, yet only about 11 species are reported to eat healthy adults, occasionally, and none seems to prefer them. The common view is that outbreaks come in natural cycles, made worse by poor water quality and overfishing of the starfish's predators.

The argument got so heated that it is sometimes described as the "starfish wars". Meanwhile people fight outbreaks directly: in early 2018 one team of volunteer divers culled 47,000 of them on Swains reef.