Question: What happens to an ecosystem when its top predator is removed?

Answer: Removing top predators can cause a wide variety of ecological disruptions that ripple throughout the food web. Save the predators!

Outside of my columns here at Scuba Diving magazine, I’m probably best known as a shark conservation ecologist and the author of Why Sharks Matter. The book focuses heavily on the ecological impacts of shark population declines, which my research has shown is the most common argument that conservation nonprofits use to persuade people to save sharks.

I’ll summarize some key points here, noting that this is not just true for sharks but for any kind of predator. Ecology is complicated and gets math-y very quickly, so please forgive the hand-wavy simplification of some of these points.

Top-Down Control

In ecological theory, there’s a concept termed “top-down control,” which suggests that predators eating prey has a very strong impact on the overall structure of the ecosystem—and losing that control can unravel the system very quickly. This can sometimes lead to what’s called a “trophic cascade” (a trophic level is a step on the food chain).

In a healthy kelp forest ecosystem, for example, hundreds of fascinating species live in the complex 3D habitat of the kelp, which is attached to the seafloor via a root-like structure called a holdfast. Sea urchins crawl along the seafloor and graze on those holdfasts.

Sea otters eat sea urchins—you may have seen them doing this, looking adorable as they float on their backs holding a rock to smash open their shellfish dinner. But when we lose sea otters, sea urchin populations grow out of control, and they overgraze the kelp forest, replacing it with something called an “urchin barren.”

In turn, the countless marine species who inhabited that kelp forest have nowhere to live. Even though these species may never have interacted with the sea otter predator or their sea urchin prey directly, the loss of otters had a huge impact on their homes.

Related Reading: Divers Cull the Tide of Urchins to Help Save Kelp

Similar ecological interactions have been studied on coral reefs, and it’s been proposed that a loss of sharks leads to increases in their grouper prey. Those grouper then eat all the parrotfish, which used to graze on algae, preventing the algae from overgrowing and killing coral. It’s more complicated than that, because humans aren’t just overfishing sharks. They’re often also overfishing grouper and even sometimes parrotfish. But you can see another example of how the whole food chain might destabilize from messing with the top predator.

Top Predators Keep the Ocean Healthy

Along with keeping prey populations in check in general, predators eat the sick, the weak and the dying, helping to keep the gene pool of prey animals healthy. You’ve heard of survival of the fittest? Predators are an important reason why the not-fit don’t survive!

Related Reading: What Are the Best Schools to Study Marine Biology?

Additionally, predators often scavenge dead prey animals, preventing them from spreading disease. And there’s also the fascinating concept of “fear ecology,” which posits that the mere presence (or possible presence) of predators influences prey behavior, with prey animals avoiding going to a particular habitat due to the risk of being eaten. The loss of fear ecology effects leads to prey animals moving into areas that were once protected from their overgrazing.

Predators, including sharks, are very important for healthy functional marine and coastal ecosystems. When we lose predators, the impacts are difficult to predict, but they can be extremely disruptive. Let’s save the predators—then we don’t have to find out what goes wrong when we lose them.