Researchers found a rise in the shark population on Ashmore Reef, off the north-west coast of Western Australia, coincided with an increase in other large and mid-sized predatory reef fish and a decline in smaller species.
Dr Mark Meekan and Dr Emily Lester, from The University of Western Australia's Oceans Institute, collaborated with Dr Zoe Storm, from the University of Glasgow, and researchers at the Australian Institute of Marine Science (AIMS) on the study published in the Journal of Animal Ecology.
"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," Dr Meekan said.
"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."
Researchers analysed videos from baited remote underwater video systems collected before enforcement of the no-take status in 2004, when there were few sharks, and in 2016, after populations had recovered.
The findings indicated that small, mid-ranking predatory reef fish exhibited anti-predator behaviours when they were under threat of consumptive and non-consumptive impacts; being eaten or scared by the presence of bigger predators.
"In this situation, fearful prey reacts to an increase in predation risk by exhibiting traits to reduce exposure, such as hiding in the reef structure or being more wary," Dr Lester said.
"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 do forage, they may be forced into poor quality habitats."
The results suggested the decline in small mid-ranking predatory reef fish, following the recovery of reef sharks could be due to both the consumptive and non-consumptive impacts of the larger predators.
"Predators can structure ecosystems and change community dynamics not just by consuming target species, they also change the behaviour of their prey in ways that can affect the fitness and ultimately the demography and abundance of prey species," Dr Storm said.
"Our work contributes to the growing evidence that predators can have important indirect effects on other species in food chains by triggering behaviours in prey that attempt to lower their risk of predation."
The results show how reefs should function in a relatively pristine state and gives insights into the role big predators such as sharks play in maintaining these ecosystems, which is essential information to improve the resilience and survival of reefs.
Some of the field data collection was part of the Global FinPrint Project funded by Paul G. Allen with additional support from AIMS.
