Along the Great Barrier Reef, a creature long cast as villain may be something more ambiguous — a disruptor whose presence, in measured doses, quietly sustains the very ecosystem it is blamed for destroying. New research from the University of Sydney finds that crown-of-thorns starfish, when held at low densities by healthy predator populations, selectively prune dominant corals and open space for slower-growing species to take hold. The finding does not absolve the starfish of the devastation wrought during population explosions, but it reframes the question: not whether the starfish belongs,
Study finds controlled crown-of-thorns starfish populations boost reef diversity
Low-density starfish create space for diversity by feeding on fast-growing corals
Mark
So the study is saying crown-of-thorns starfish are actually good for reefs?
Mimi
Not exactly. It's saying that when their populations stay low and controlled, they can help maintain diversity. The key is the density and the health of the surrounding ecosystem.
Luke
How low is low? The paper doesn't give us a specific threshold, does it?
Mimi
No, it doesn't. That's one of the gaps. They studied areas where fish populations naturally keep starfish in check, but they don't quantify what that equilibrium looks like.
Mark
And the 75 percent figure—that's how often the starfish left coral tissue behind?
Mimi
Yes, in the feeding events they surveyed at One Tree Island. The starfish would eat and move on rather than strip the whole colony.
Luke
But One Tree Island is a protected area with healthy predator populations. How does that translate to reefs that have already lost their tritons and reef fish?
Mimi
That's exactly what Foo is saying—the research can inform culling practices, but it doesn't solve the underlying problem of depleted predator populations.
Mark
So the real work is restoring the fish and tritons that naturally control the starfish?
Mimi
That's the argument. You can cull an outbreak, but without rebuilding those predator populations, you're just treating the symptom.
Luke
The intermediate disturbance hypothesis is interesting, but it assumes the ecosystem is otherwise stable. How stable is the Great Barrier Reef right now?
Mimi
That's beyond the scope of this study, but it's a fair question. The research is narrowly focused on what happens at low densities in protected areas.
Mark
So this doesn't change how we should respond to an active outbreak?
Mimi
No. The findings suggest that once an outbreak is suppressed, the goal shouldn't be to eliminate every starfish. But preventing the outbreak in the first place still requires healthy predator populations.
The Pulse
- Four major outbreaks since the 1960s have left the Great Barrier Reef scarred, with coral communities struggling to recover across years — the crown-of-thorns starfish has earned its reputation as one of the reef's most destructive forces.
- The collapse of natural predators like tritons and reef fish has removed the ecological brakes that once kept starfish numbers in check, making periodic explosions nearly inevitable every 15 years.
- Researchers at One Tree Island found that in nearly 75 percent of observed feeding events, living coral tissue survived — the starfish grazed and moved on rather than consuming entire colonies.
- By targeting fast-growing Acropora corals, low-density starfish inadvertently cleared space for slower, less competitive species like Montipora and Porites, increasing overall reef biodiversity.
- The findings are shifting the management conversation from eradication toward calibrated suppression — culling to sustainable levels while restoring the predator populations that do the long-term regulatory work.
Along the Great Barrier Reef, a creature long cast as villain may be something more ambiguous — a disruptor whose presence, in measured doses, quietly sustains the very ecosystem it is blamed for destroying. New research from the University of Sydney finds that crown-of-thorns starfish, when held at low densities by healthy predator populations, selectively prune dominant corals and open space for slower-growing species to take hold. The finding does not absolve the starfish of the devastation wrought during population explosions, but it reframes the question: not whether the starfish belongs, but how many is too many.
The crown-of-thorns starfish has long been treated as a straightforward threat to coral reef health. Its venomous spines and coral-consuming appetite have made it a symbol of reef destruction, and four major population explosions along the Great Barrier Reef since the 1960s have done little to soften that image. But research from the University of Sydney is complicating the picture in ways that matter for how reefs are managed going forward.
The vulnerability of the Great Barrier Reef to these outbreaks is itself a symptom of ecological imbalance. Tritons and reef fish that once naturally suppressed starfish numbers have declined, leaving the reef exposed to population surges that now occur roughly every 15 years. Each one leaves coral communities struggling to recover.
Dr. Shawna Foo and her team at One Tree Island Research Station focused on what happens when starfish populations remain small — in areas where fish communities still provide natural control. Surveying coral feeding scars and surviving tissue, they found that in nearly 75 percent of observed feeding events, living coral persisted after the starfish moved on. The starfish, it turned out, ate in measured portions and departed rather than consuming entire colonies.
The ecological consequence was unexpected. Fast-growing Acropora corals, which tend to dominate reef space, showed little recovery after being fed upon. Slower-growing species like Montipora and Porites moved into the cleared areas and held on. Low-density starfish were, in effect, preventing competitive monopolies and making room for broader coral diversity — a pattern consistent with the intermediate disturbance hypothesis, which holds that moderate, periodic disruption can sustain biodiversity.
Foo was careful to distinguish this dynamic from the devastation of full-scale outbreaks. The research does not rehabilitate the starfish as harmless — it draws a line between a population functioning within ecological limits and one that has exceeded them. The practical implication is a shift in management philosophy: rather than pursuing total elimination during culling operations, reef managers might aim to suppress populations to levels where the starfish can play its natural role, while investing in the recovery of the predator species that keep it there.
The crown-of-thorns starfish carries a fearsome reputation in marine biology. Its venomous spines and appetite for coral have made it synonymous with reef destruction, particularly along the Great Barrier Reef, where four major population explosions since the 1960s have left swaths of bleached skeleton in their wake. But new research from the University of Sydney suggests the story is more complicated than the starfish's menacing appearance implies. When populations remain small and controlled, these creatures may actually help reefs thrive.
The Great Barrier Reef's vulnerability to crown-of-thorns outbreaks stems partly from ecological imbalance. The starfish have natural predators—tritons and various reef fish species—that once kept their numbers in check. As those predator populations have declined, the reef has become increasingly susceptible to the periodic explosions that occur roughly every 15 years. Each outbreak leaves devastation in its path, with coral populations struggling to recover for years afterward.
Dr. Shawna Foo and her team at the University of Sydney's One Tree Island Research Station set out to understand what happens when crown-of-thorns populations remain at low levels, in areas where healthy fish populations naturally suppress their growth. The researchers surveyed coral feeding scars and the tissue that survived predation events, publishing their findings in Ecology and Evolution. What they discovered challenged conventional wisdom: in nearly 75 percent of the feeding events they examined, living coral tissue remained after the starfish moved on.
The pattern was revealing. Rather than consuming an entire coral colony in a single feeding session, the starfish ate a measured amount and departed, leaving behind fragments capable of survival. This behavior created an unexpected ecological benefit. The fast-growing Acropora corals that dominated reef space showed little regeneration after being fed upon by the starfish. Slower-growing species like Montipora and Porites, however, took advantage of the cleared space and persisted. By selectively feeding on the aggressive competitors, low-density starfish populations inadvertently created room for coral diversity to flourish.
Foo emphasized that this finding does not minimize the catastrophic impact of full-scale outbreaks. The distinction matters: a starfish population performing its natural ecological role differs fundamentally from an outbreak that triggers sustained reef decline. The research instead suggests that the intermediate disturbance hypothesis—the ecological principle that occasional, moderate disruption can maintain biodiversity—applies to crown-of-thorns dynamics on healthy reefs.
The implications extend to future management strategies. Rather than pursuing complete elimination of starfish during culling operations, reef managers might instead aim for suppression to sustainable levels, paired with efforts to restore the predator populations that naturally regulate starfish numbers. Foo noted that strong marine management protecting tritons and reef fish is essential to keeping starfish in check once an outbreak has been controlled. The goal shifts from eradication to balance—maintaining the conditions under which a starfish population can perform its ecological function without triggering the cascade of destruction that defines an outbreak.
Notable Quotes
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. Shawna Foo, University of Sydney
It's important to distinguish between COTS populations performing their natural role as coral predators and outbreaks causing sustained reef decline— Dr. Shawna Foo
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