Is the Reef's Destroyer a Natural Part of its Ecosystem?
Beneath the sun-dappled waters of the Great Barrier Reef, a slow-moving nightmare unfolds. A creature of striking, almost alien beauty, adorned with venomous spines, creeps across a vibrant coral garden. Where it passes, it leaves only a ghostly, white skeleton. This is the Crown-of-Thorns Seastar (COTS), or Acanthaster planci, and its population explosions have devastated vast swathes of coral reefs across the Indo-Pacific.
A single Crown-of-Thorns seastar can consume its body area in coral every day. During outbreaks, populations can reach densities of over 1,000 individuals per hectare.
For decades, the sight of these seastars has triggered large-scale, costly eradication programs. But what if we've been missing the bigger picture? What if this "villain" is actually a character in a much more complex ecological story? This was the central question when the world's top marine scientists gathered in Townsville in 1988, aiming to move beyond the panic and build a theoretical perspective on one of the ocean's greatest puzzles .
The initial view of the Crown-of-Thorns was simple: it was a pest, an outbreak species that needed to be controlled. However, by the late 1980s, a more nuanced understanding was emerging. The workshop sought to explore several groundbreaking theories that positioned Acanthaster not just as a destroyer, but as a potential force shaping the reef's very structure .
Could COTS be a "keystone species," a predator that maintains biodiversity by preying on the fastest-growing corals?
What triggers an outbreak? This theory points to conditions that lead to exceptional survival of COTS larvae.
Are we the real culprits? This theory suggests that overfishing of the seastar's natural predators has removed natural checks.
"The real challenge was untangling this web of cause and effect. To do that, scientists needed to move from observation to experimentation."
One of the most critical pieces of the puzzle was understanding the impact of an outbreak. Just how much coral could a population of COTS consume? A landmark study, a prime example of the work discussed at the workshop, set out to answer this with painstaking precision .
Before the seastars arrived, the team laid down transect lines across the reef and took detailed photographs and video of the coral at fixed points.
As the seastar front moved in, divers regularly counted the number of COTS within a defined area and observed their feeding behavior.
Individual seastars were monitored to see how much coral tissue they consumed per day by measuring the size of the feeding scars.
After the front had moved through the study area, the team repeated the baseline survey to measure the catastrophic loss of coral.
The results were stark. The data revealed that a single Crown-of-Thorns seastar can consume its body area in coral every day. When multiplied across a dense outbreak population, the effect is not just significant—it is ecosystem-altering .
| Coral Cover Before and After a COTS Outbreak | ||
|---|---|---|
| Survey Period | Live Coral Cover (%) | Observational Notes |
| Pre-Outbreak | 65% | Vibrant, diverse coral community |
| Post-Outbreak | 8% | Vast areas of white coral skeleton |
| Daily Feeding Rate of COTS | ||
|---|---|---|
| Seastar Size | Coral Consumed/Day | Area (approx.) |
| 25 cm | 1 x Body Area | 0.05 m² |
| 35 cm | 1 x Body Area | 0.10 m² |
The scientific importance was clear: COTS outbreaks are not a minor nuisance. They are a major disturbance event, on par with a severe cyclone or mass bleaching. The experiment provided the hard numbers needed to model their impact, showing that reefs could recover, but only if outbreaks were infrequent and other stressors (like poor water quality) were low .
Tackling the Crown-of-Thorns problem requires a diverse arsenal of tools and techniques. Here are some of the essential "reagent solutions" and equipment used by researchers in the field .
A chemical injection used to euthanize individual seastars. A small amount is injected into each starfish, causing rapid breakdown of its tissues without harming the surrounding reef.
A natural, reef-safe substance that, when injected, is lethal to COTS. It has been explored as a potential large-scale control method.
The "Single-Suction System" is a hydraulic device that allows divers to safely collect seastars from the reef for removal and study.
The fundamental tools for monitoring. A tape measure (transect) is laid across the reef, and framed quadrats are used to sample small areas.
Used to map the location and movement of outbreaks over large scales, often correlating them with water temperature and chlorophyll levels.
The 1988 workshop in Townsville was a pivotal moment. It forced a shift from seeing the Crown-of-Thorns seastar as a simple villain to understanding it as a component of a complex system. The central conclusion was that while Acanthaster is a native species that has likely always played a role on reefs, the frequency and intensity of modern outbreaks are likely not natural .
The evidence points to a perfect storm: human activities—specifically, nutrient runoff from land and the removal of key predators through overfishing—are likely tipping the balance, turning a potential keystone player into a catastrophic force.
The Crown-of-Thorns is not just a cause of reef degradation; it is a magnifying glass, highlighting our own impact on the ocean. The theoretical perspective born from that workshop reminds us that the solution lies not just in culling seastars, but in healing the reef's entire ecosystem—by cleaning our waters and protecting its natural balance .