Crown-of-thorns starfish thrive in degraded coral reefs

Research into one of the most persistent coral predators on the Great Barrier Reef has unveiled a concerning paradox: crown-of-thorns starfish (COTS) seem to flourish in the degraded coral reef conditions they help create. 

This new insight into reef ecosystems comes from the work of Professor Maria Byrne from the University of Sydney and marine science researcher Dr. Kennedy Wolfe. 

Resilient species have the upper hand

The study highlights the complex relationship between coral health and the population dynamics of COTS, offering vital information for conservation efforts.

“This research reinforces our understanding that resilient species have the upper hand when adjusting to altered habitats,” Byrne said. “Unfortunately, in the case of the Great Barrier Reef, one of these resilient species – the crown-of-thorns starfish – is one of its most persistent coral predators.”

Threats to coral reefs

Coral reefs, often dubbed the “rainforests of the sea,” are essential ecosystems that sustain a wide variety of marine life. However, they are under serious threat from a range of stressors, including climate change, pollution, and overfishing. 

One of the greatest challenges to coral reefs is outbreaks of crown-of-thorns starfish, which can devastate coral populations and accelerate reef degradation.

Degraded reef hypothesis 

Published in the journal Marine Environmental Research, the study proposes a “degraded reef hypothesis” to better explain how COTS populations persist despite substantial efforts to control them. 

This hypothesis suggests that the very degradation of coral reefs, largely caused by factors like heatwaves and pollution, creates conditions that benefit the juvenile stages of COTS.

As live coral becomes degraded, the rubble habitats that emerge provide a nursery for juvenile crown-of-thorns starfish. This allows them to accumulate and remain hidden, waiting for the coral to begin recovering before emerging as coral-eating adults. 

A vicious cycle of reef degradation 

The delayed surges in COTS populations exacerbate coral loss and create a vicious cycle of reef degradation.

“Our previous research has shown that juvenile COTS can wait for just the right moment to emerge in large numbers; sometimes up to six years,” Byrne explained. This ability to bide their time adds to the challenge of managing COTS outbreaks effectively.

The study emphasizes that the same conditions contributing to coral degradation, such as cyclones, heatwaves, and pollution, are favorable for crown-of-thorns starfish. 

Degraded coral and starfish outbreaks

The resilience of COTS presents a significant challenge for conservation efforts aimed at restoring coral health, as any recovery in coral populations could also fuel new COTS outbreaks.

“Our findings reveal a critical feedback loop in reef ecosystems,” said Wolfe, who began his research career as a PhD student at the University of Sydney. “COTS populations may thrive as coral reefs degrade, leading to further coral loss. This cycle not only threatens corals but also the range of species that depend on healthy reefs for survival.”

According to Byrne, this feedback loop explains repeated outbreaks on the Great Barrier Reef, despite exhaustive management through manual culling.

Effective management strategies are needed

The study underlines the importance of a multifaceted approach to coral reef conservation that not only targets the root causes of degradation but also considers broader ecological interactions. 

Effective management strategies must address the resilience of COTS and their ability to exploit weakened coral environments. “These coral predators are a symptom of ongoing environmental impact,” Byrne said.

By gaining a deeper understanding of how coral health and COTS populations interact, conservationists can devise more effective strategies to mitigate the starfish’s impact. 

This could involve focusing on habitat restoration projects aimed at strengthening coral resilience, as well as measures to control nutrient runoff and reduce sedimentation that can contribute to reef decline.

Coral degradation and COTS outbreaks 

The study adds to the extensive research conducted by Byrne and her team, which has greatly improved the understanding of how COTS continue to thrive as a pest species on the Great Barrier Reef. 

Through continued research and targeted intervention, there is hope that the cycle of coral degradation and COTS outbreaks can be broken, offering a brighter future for the world’s coral reefs.

“In a changing climate, COTS population resilience responds well to, and likely even benefits from, degrading reef condition and environmental stochasticity. From issues stemming from water quality, altered food webs, and reef degradation, the response of COTS is rapid and adaptive,” noted the study authors. 

“Management aiming to conserve coral reefs must consider COTS not as a single issue, but as a symptom of ongoing environmental impacts that requires a broad response with knock-on benefits to reef ecosystems and their inhabitants.”

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