The decline of shark populations has profound consequences on marine ecosystems, as highlighted by researchers investigating the disappearance of great white sharks in South Africa and the depletion of reef sharks in northern Australia. The removal of apex predators disrupts the marine food web, triggering cascading effects that alter species dynamics and ecological balance.
Dr. Neil Hammerschlag of the Shark Research Foundation underscores the significance of top predators in maintaining marine ecosystems. He and his colleagues documented the disappearance of great white sharks from False Bay, near Cape Town, South Africa, while they were based at the University of Miami, Florida, along with researchers from the University of Lancaster, U.K. Their study, published in Frontiers in Marine Science, reveals the consequences of this loss on the local food web.
Historically, great white sharks preyed on Cape fur seals and sevengill sharks in False Bay. However, beginning around 2016, these predators vanished, likely due to a combination of beach nets and predation by a pod of orcas that had developed a preference for shark livers. The absence of great whites allowed their usual prey to proliferate, which in turn placed excessive pressure on smaller marine species such as anchovies, horse mackerel, smooth hound sharks, and pyjama cat sharks. These disruptions extended throughout the food web, demonstrating the critical role of apex predators in ecosystem stability.
The study employed underwater video surveys conducted over a decade apart to compare the marine environment before and after the sharks’ disappearance. Co-author Yakira Herskowitz emphasized that these surveys provided valuable insights into food web changes resulting from the loss of top predators. Hammerschlag further stressed that without these regulatory species, measurable ecological shifts occur, potentially impacting ocean health in the long term.
Dr. Mark Meekan of the University of Western Australia concurs with this assessment, highlighting how predator loss can instigate trophic cascades that disrupt food web dynamics. Meekan and his colleagues propose that shark depletion may be a driving factor behind crown-of-thorns starfish (CoTS) outbreaks on the Great Barrier Reef. Their research, published in Nature Communications Biology, examines dietary analyses using DNA, predation studies, and experimental evidence to support this hypothesis.
Coral reefs, among the most threatened tropical ecosystems, face significant challenges due to CoTS infestations. The Australian Institute of Marine Science (AIMS) reports that these outbreaks have led to extensive coral loss on the Great Barrier Reef and other regions. Meekan attributes the decline of certain reef shark populations in northern Australia to overfishing, particularly by Indonesian fishers operating under a Memorandum of Understanding (MoU) between Australia and Indonesia.
Scott Reef, located 270 km off northwestern Australia, falls within the MoU’s traditional fishing zone. While the agreement permits non-motorized fishing for sea cucumbers, trochus, reef fish, and sharks, it prohibits harvesting of protected species such as turtles and dugongs. Nevertheless, shark fishing remains unregulated, driven by demand for their high-value fins. The practice is predominantly carried out using basic methods by impoverished Indonesian fishers who dry shark fins onboard for export.
Sharks’ low reproductive rates and natural curiosity make them particularly vulnerable to overfishing. According to Meekan, the removal of these predators enables mid-level species, such as snappers and emperors, to shift their feeding patterns. Instead of consuming invertebrates like marine snails and starfish, these fish begin preying on small fish and squid, altering ecosystem balance. This shift provides a predator-free environment for invertebrates, including the crown-of-thorns starfish, allowing their populations to surge unchecked.
Meekan suggests that this “predator release” effect plays a crucial role in CoTS outbreaks. Without emperors to consume juvenile CoTS, a critical mortality bottleneck is removed, leading to explosive population growth. While there is ongoing debate over whether these outbreaks result from increased nutrient availability due to algal blooms or predator loss, Meekan points out that CoTS infestations occur even in isolated, nutrient-poor environments such as the Red Sea, Seychelles, and Maldives. In nearly all cases, significant fishing activity targeting top predators is present, except in well-monitored Marine Protected Areas.
Both the disappearance of great white sharks in False Bay and the potential link between shark depletion and CoTS outbreaks on the Great Barrier Reef highlight the broader ecological consequences of predator loss. As Hammerschlag emphasizes, these findings underscore the necessity of global shark conservation efforts. Given the critical role healthy oceans play in sustaining food supplies, recreation, and ecosystem services, protecting large shark populations is essential for maintaining biodiversity and long-term marine stability.
