Crown-of-thorns starfish may be helping reefs when their numbers stay low
New research from the University of Sydney finds that living coral tissue survives nearly 75% of crown-of-thorns starfish feeding events, and that controlled populations may boost coral diversity rather than destroy it.
The crown-of-thorns starfish has spent decades as the Great Barrier Reef’s designated villain — a spiny, coral-eating predator whose population explosions are routinely blamed for stripping whole reef sections bare. New research from the University of Sydney suggests that reputation is at best incomplete. The study, published in Ecology and Evolution, found that living coral tissue survived nearly 75% of crown-of-thorns starfish feeding events, which is a striking number for an animal usually described in terms of the coral it consumes.
That survival rate is the hinge the whole argument turns on. If a feeding event typically removes only a fraction of the tissue it touches, then a starfish at ordinary density is not a bulldozer but a grazer — one that takes a bite and moves on, leaving the colony alive to keep growing. The picture that emerges is less of a killer and more of a disturbance regime, and disturbance regimes are something coral reefs have evolved alongside.
At low densities, the researchers argue, that disturbance does useful work. Crown-of-thorns starfish can have a positive impact on coral prevalence and diversity when their populations stay controlled, according to Dr. Shawna Foo’s team. The mechanism is a familiar one in ecology: the starfish preferentially feeds on fast-growing, dominant coral species, and in doing so opens up space on the reef for slower-growing corals that would otherwise be outcompeted for light and substrate.
That reframes the animal as a kind of maintenance crew rather than a pest. A reef monopolised by a handful of quick-spreading branching corals is less diverse than one where something periodically prunes the dominant competitor back, and the crown-of-thorns starfish appears to be one of the things doing that pruning. The same feeding behaviour that makes the species catastrophic at outbreak densities is, at background densities, part of what keeps a reef varied.
The research, reported by ScienceDaily, draws the obvious distinction between a starfish and a swarm of them. The damage associated with crown-of-thorns outbreaks is real, and nothing in the new work disputes it — the argument is about what the species does when its numbers are not exploding, which is most of the time. The 75% survival figure applies to individual feeding events, not to a reef under sustained attack by thousands of starfish.
The two accounts of the study agree on the substance: the near-75% tissue survival rate, the role of low-density populations in maintaining coral biodiversity, and the mechanism of feeding on dominant fast-growers to free space for slower species. Neither suggests the starfish should be left unchecked, and both frame the finding as a reason to think about population control rather than eradication.
That is a meaningful shift in how reef managers might think about a species that has been the target of culling programmes for decades. If the goal is a diverse reef rather than a starfish-free one, then the question stops being how to eliminate crown-of-thorns and becomes how to keep their numbers inside the range where they do more good than harm — a harder question, and a considerably more interesting one.