When Sharks Return, Reefs Respond

In the turquoise waters off the north-west coast of Western Australia, Ashmore Reef has become a living laboratory for understanding what happens when big predators come back. Once depleted, shark populations around this remote reef have been rebounding since 2008, when the area was granted fully enforced “no-take” Marine Protected Area status. And with the return of these top predators, the newly published study gives us a glimpse into how predators do more than eat their prey — they change the very way ecosystems work.

A study published in the Journal of Animal Ecology led by Dr. Mark Meekan and Dr. Emily Lester from The University of Western Australia’s Oceans Institute, alongside Dr. Zoe Storm from the University of Glasgow and researchers at the Australian Institute of Marine Science, compared fish communities before and after shark numbers recovered. “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,” said Dr Mark Meekan. “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.” Using baited remote underwater video systems, the team analyzed footage from 2004 (when shark numbers were low) and from 2016 (when sharks had bounced back).

The goal was to track how rising predator numbers influenced the reef’s residents. What they found was a clear shift.

Alongside the increase in sharks came a rise in other large and mid-sized reef predators. And not only had the number of large and mid-sized predators gone up, but many smaller predatory fish had declined. It wasn’t just a case of sharks eating those smaller species. The fish that remained were acting differently too. Fish that previously roamed more freely were now sticking closer to shelter. Some became more cautious, spending less time feeding or mating and more time simply trying not to get eaten — or even seen! — by the returning sharks.

According to Lester, this shift in behavior is an example of how predators can affect ecosystems even without making a kill: “Fearful prey react to an increase in predation risk by exhibiting traits to reduce exposure. They may hide more, become warier and forage in lower quality areas. This limits how much energy they can get from food and can reduce reproduction too.” In other words, the mere threat of a predator can change how prey live, a phenomenon known as non-consumptive effects.

Storm added that these indirect effects are just as important as actual predation when it comes to shaping ecosystems. “Predators structure ecosystems not just by what they eat, but by influencing how their prey behave,” she explained. “This can affect population sizes and community makeup in big ways.” That means sharks, as top predators, play a vital role in keeping reef ecosystems balanced — even when they’re not actively hunting.

This dynamic, where predators influence prey behavior and that behavior in turn shapes the entire ecosystem, helps explain why top predators like sharks are so important. Understanding how these effects work is key for conservation and reef management. Coral reefs are among the most complex and fragile ecosystems on Earth, and they face numerous threats, ranging from overfishing to climate change. This study, says Meekan, shows what a relatively healthy reef looks like when top predators are back in the mix and highlights how important those predators are for maintaining the balance of the ecosystem. “Our work contributes to the growing evidence that predators can have important indirect effects on other species in food chains,” Storm concludes, “by triggering behaviours in prey that attempt to lower their risk of predation.”

Ashmore Reef offers a real-world example of what happens when sharks are allowed to return and reclaim their role at the top of the food chain. Their presence is about more than predation — it’s about presence itself. The silent power of fear, it turns out, is just as important as the bite. And it’s a reminder to all that protecting predators doesn’t just save one species… it helps safeguard the entire web of life beneath the waves.