What if sharks disappeared?

Sharks live in oceans all over the world. Many sharks catch and eat other sea animals.

What if all the sharks were gone? Would the other sea animals notice?

There are many kinds of sharks, from small reef sharks to great white sharks. NOAA says the great white shark is an apex predator: it eats at the top of the food chain and plays an important part in the ocean.

So what would happen if sharks disappeared? Would their prey simply grow in number, or would other things change too? In this thought experiment you can follow real studies from seagrass meadows and coral reefs, and see where scientists agree and where they are still arguing.

Large sharks are often described as apex predators, and NOAA calls the white shark one, with an important ecological role. Predators can affect ecosystems two ways: by eating prey (consumptive effects) and by changing prey behavior through risk (non-consumptive effects).

This thought experiment follows field research rather than a model. It asks what the evidence says about losing sharks in two settings, a subtropical seagrass bay and remote coral reefs, and notes where conclusions are firm, suggestive or disputed.

Sharks have been in trouble. People caught many of them.

Some kinds of sharks are now protected, and some are coming back.

Many sharks have become much rarer. A big study published in 2021 found that the number of ocean-roaming sharks and rays fell by 71 percent since 1970, mostly because fishing grew much heavier.

Many sharks grow slowly, which makes them vulnerable. NOAA says male great white sharks take about 26 years to grow up, and females about 33. But protection can help: NOAA reports that great white shark numbers in the northwest Atlantic have been rising since rules protecting them began in the 1990s.

Pacoureau and colleagues (Nature, 2021) combined 57 abundance time series for 18 oceanic shark and ray species and found a 71% decline in global abundance since 1970, driven by an 18-fold increase in relative fishing pressure; three-quarters of the 31 oceanic species are now threatened with extinction.

Low productivity makes many sharks vulnerable: NOAA lists the white shark's slow growth, with maturity at about 26 years for males and 33 for females. NOAA also reports rising abundance trends in the northwest Atlantic since protective regulations in the 1990s, and a northeastern Pacific population that appears to be increasing.

Without sharks, turtles and dugongs might eat seagrass anywhere they like.

That could change the underwater meadows where they graze.

In Shark Bay, Australia, scientists found that when tiger sharks are around, dugongs and green turtles avoid the risky shallow middle of seagrass banks and graze near the edges instead. Their experiments suggest that this changes the seagrass itself.

So without tiger sharks, the grazers might feed everywhere, and the meadows might look very different. On coral reefs the story is less clear: one study linked fewer sharks to more mid-sized fish hunters and fewer plant-eating fish, but other scientists say the evidence that reef sharks protect corals is weak.

Two lines of evidence frame the question. In Shark Bay, the Shark Bay Ecosystem Research Project found that dugongs, green turtles, dolphins and cormorants shift habitat use when tiger sharks are present, and exclosure experiments suggest tiger sharks influence seagrass by changing when, where and how grazers feed.

On reefs, Ruppert and colleagues (2013) found reef sharks about three times as abundant on protected atolls as on fished ones, with more mid-sized carnivores and, after disturbance, fewer herbivores on the fished reefs: evidence consistent with mesopredator release. Roff and colleagues (2016) countered that most reef sharks act as mesopredators and that support for coral-benefiting cascades is weak and equivocal.

Animals stay away from places where sharks hunt. So sharks change where other animals eat.

That can help plants and corals in some places.

A predator can change an ecosystem in two ways. It eats some animals, and it scares others into changing where they go. In Shark Bay, the fear of tiger sharks seems to matter a lot: grazers crowd into safer spots, leaving other parts of the meadow less grazed.

The chain can even reach animals sharks rarely eat. Dolphins in Shark Bay almost completely leave their best shallow feeding grounds when large tiger sharks arrive. Scientists call these chains of effects trophic cascades.

Risk effects can rival direct predation. When large tiger sharks (over 3.5 m) are present, Shark Bay dugongs and green turtles concentrate on bank edges; seagrass there has lower nutritional quality than in bank middles that grazers largely abandon. A second pathway runs through dolphins and cormorants that eat small seagrass-grazing fish, so tiger sharks may affect seagrass through both.

On reefs, many fish share the middle predator level with sharks, which may blunt any single species' effect. That is one reason scientists disagree, and why the outcome of losing sharks is uncertain rather than a single, predictable chain.

Scientists watch sharks, turtles and dugongs in Shark Bay, Australia. They learn how all the animals are linked.

They are still finding out what sharks do on coral reefs.

Researchers in Shark Bay built underwater fences that let fish in but kept dugongs and turtles out, then compared the seagrass inside and outside. They kept the experiment going for almost three years, cleaning the cages almost every week. Their results suggest that seagrass beds might be very different if the sharks were lost.

On coral reefs, scientists are still debating. That is how science works: different studies are compared until the evidence becomes clearer. Meanwhile, some shark populations, like great whites in the northwest Atlantic, have been increasing since they were protected.

The Shark Bay team maintained 2.5 × 3.0 m exclosures and paired controls for almost three years, then added fish-proof cages and transplanted seagrass to separate large and small grazers. They found fish to be important grazers in bank middles, and concluded that seagrass beds might be very different without sharks.

For reefs, the 2013 atoll comparison and the 2016 review reach different emphases; the authors' published exchange shows an active debate. Trends in some protected populations, such as northwest Atlantic white sharks, are rising, while the 2021 study found that oceanic sharks and rays overall declined by 71% since 1970.