What happens to an ocean when its sharks disappear?

When an ocean loses its top predators, the effects can ripple down the food web in what ecologists call a trophic cascade. A trophic cascade is a powerful indirect interaction that can control an entire ecosystem, occurring when one level of a food web is suppressed. In a top-down cascade, predators reduce the numbers or change the behaviour of their prey, which releases the next level down from being eaten.

Several shark species are apex predators, at the top of their food chain, including the bull, tiger, great white and hammerhead sharks. Sharks are considered a vital part of the ocean ecosystem.

The clearest examples are not all sharks. In Pacific kelp forests, sea otters eat sea urchins, and where otters were hunted to extinction the urchins increased and kelp was reduced. In the northwest Atlantic, overfishing of cod was followed by rises in their prey, such as northern snow crab and shrimp.

For sharks, one Caribbean study is described as finding that removing sharks let the carnivorous fish they usually ate thrive, and those fish preyed on parrotfish that kept the corals clean, so the reef shifted from coral to algae. Other reef work disagrees: a 2021 study on the northern Great Barrier Reef found that reef shark numbers had only a small influence on their prey. Ecologists agree that cascades exist but have long debated how common they are.