Reef's Deadliest Coral Killer May Actually Keep It Alive

The result: coral tissue survived roughly three out of every four observed feeding events.

"At One Tree Reef, COTS appear to consume a relatively consistent amount of coral during each meal and move on, rather than demolishing the entire coral colony," Dr. Foo said. "This meant live coral tissue remnants were frequently left behind."

How can a coral killer boost biodiversity?

Here's the mechanism, and it's simpler than you'd expect. Some corals, notably the fast-growing Acroporidae family, spread aggressively and crowd out slower-growing species. By grazing on the dominant ones, low-density starfish open space for the underdogs.

The data backs this up. Surviving fragments of fast-growing Acroporidae corals did not regenerate over the areas the starfish had damaged. Regrowth occurred mainly among slower-growing Montipora and Porites corals.

"By feeding on fast-growing corals such as Acropora, low-density COTS populations create space for slower-growing coral species to persist, helping to increase coral diversity," Dr. Foo said.

Co-author Professor Maria Byrne framed the finding within a familiar ecological idea. "The results are consistent with the intermediate disturbance hypothesis, an important framework in ecology and conservation," she said. In plain English: moderate, occasional disturbance can stop any single species from monopolizing an ecosystem.

Should we stop culling starfish?

Not so fast. The researchers themselves stress that outbreaks remain a serious threat, capable of wiping out extensive reef areas. The entire argument hinges on whether starfish stay at naturally manageable levels or explode into destructive swarms.

"It's important to distinguish between COTS populations performing their natural role as coral predators and outbreaks causing sustained reef decline," the team cautions.

There's also a real limitation here. The research was conducted mainly in protected areas with healthy fish populations and little risk of major outbreaks. We don't yet know whether the same pattern holds on degraded, predator-poor stretches of reef, which is where most outbreak damage actually happens.

Still, the practical implication for conservation managers is clear enough. Once an outbreak has been suppressed, wiping out every last starfish may be the wrong move.

"Once an outbreak is suppressed, the goal should not necessarily be to eliminate COTS entirely," Dr. Foo said. "Culling needs to be complemented by strong marine management to protect COTS predators and keep COTS populations in check."

So what does this change for the reef? Potentially, the target of culling programs, and a renewed push to protect the fish and tritons that do the population control for free. What it doesn't change is the threat of outbreaks, which remain as destructive as ever.

The study appears in Ecology and Evolution, volume 16, issue 9 (DOI: 10.1002/ece3.74285), authored by Shawna Andrea Foo, Matthew Clements, Adi Zweifler, Georgie Eggleton, Hayden Millican and Maria Byrne.