The return of apex predators like sharks can restore the critical ecological balance to coral reef ecosystems, creating healthier and more abundant environments, a new collaborative study between researchers in the UK and Australia has found.

Strong correlations have been drawn between a rise in the population of sharks on Ashmore Reef, off the north-west coast of Western Australia and an increase in other large and mid-sized predatory reef fish alongside the decline in smaller species.

The study – a joint effort between the University of Glasgow, University of Western Australia’s Ocean Institute, and the Australian Institute of Marine Science – has been published in the scientific journal, Animal Ecology.

It details a project through which researchers analysed video footage captured using baited remote underwater video systems before the enforcement of the area’s ‘no-take’ status in 2004, when there were few sharks within the area. This was then compared to footage captured in 2016, after shark populations had recovered.

Among the findings, scientists discovered that small mesopredatory reef fish exhibited anti-predator behaviours – including less foraging for food – when they were under threat by larger predators. 

An overabundance of any species can disrupt the ecosystem balance in a coral reef. In particular, a large and unchecked population of small mesopredatory fish can have impacts down the food chain, with essentially free reign to over-consumer young fish or invertebrates leading to the decimation of their numbers.

Under the watchful eye of an apex predator, like sharks, however, this balance is restored. It’s precisely what researchers observed in the case of Ashmore Reef, presenting their findings as ‘how reefs should function’ when they are in relatively pristine conditions. 

It all offers a deeper insight into the role that big predators such as sharks play in maintaining these ecosystems – essential information when it comes to improving the resilience and survival of reefs.

Professor Shaun Killen, Professor of Ecophysiology, Ecology, and Environmental Change at the University of Glasgow, said: “This study really shows the importance of conserving top predators like sharks. Their return doesn’t just restore numbers, it restores critical ecological interactions that are vital for healthy marine ecosystems.

“Removing large predators and allowing mesopredators to proliferate can destabilize ecological balance, leading to unpredictable changes in species abundance and interactions that can negatively affect overall reef health.”

These findings have been echoed in a similar study spanning two-decades looking into the gradual disappearance of Great white sharks from False bay in South Africa.

Historically abundant in the region, Great white sharks have experienced a dramatic decline and subsequent disappearance over the last 20 years, with the finger of blame being directed at a combination of factors, including decades of unsustainable capture in nets and some more recent instances of predation by orcas.

In this study – similar to the findings in Ashmore Reef – the loss of the apex predator gave greater space to mesopredators which in turn led to the stark over-consumption of smaller fish and prey species.

It all illustrates the ripple effect that losing a top ocean predator can have on an ecosystem already walking a fine line of equilibrium.

Dr Mark Meekan from the University of Western Australia’s Ocean Institute, said: “Reef shark populations on Ashmore Reef have increased significantly since effective enforcement of the no-take Marine Protected Area status of the reef started in 2008.

“This has enabled us to examine how these large predators have the potential to structure reef communities through the behavioural effects they have on their prey. In this situation, fearful prey reacts to an increase in predation risk by exhibiting traits to reduce exposure, such as being more wary.

“These responses can alter the prey’s behaviour so they spend much more time avoiding predators, which limits the amount of time they have available for important activities, including reproduction and foraging – when they forage they may be forced into poor quality habitats.”

The work now contributes to a growing body of evidence to support the idea that predators have important indirect effects on other species in food chains by triggering in them behaviours in prey that attempt to lower their risk of predation.

The study – Recovery of Reef Shark Populations Invokes Anti-Predator Behaviours in Mesopredatory Reef Fish in a Coral Reef – is published in the scientific journal, Animal Ecology.

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