People used to hunt sea otters for their thick and warm fur. Many otters were killed. They almost went extinct.

The fur trade didn't just harm the hunted animals. It also hurt entire food chains. A food chain is a group of organisms. They are linked together in the order of the food they eat. Food chains are not always simple. They can connect to form a food web. A food web is made of all the food chains in an ecosystem.

Sea otters (Enhydra lutris) sit at the top of their food chain. Sea otters are a top predator or apex predator. Top predators hunts other animals for food. When many sea otters disappeared, it affected the whole ecosystem. The effects trickled down to other animals. Due to the changes in their ecosystem, sea otters have had a hard time bouncing back.

Without Top Predators, Entire Ecosystems Change

In the 1700s, there were more than 10,000 sea otters off the coast of California in the United States. By the early 1900s, fewer than 100 were left. The ecosystem changed a lot when the otters were gone. The food web changed too.

A sea otter floats on its back. Due to overhunting, the sea otter population off the California coast dropped from more than 10,000 in the 1700s to less than 100 by the early 1900s. When the otter population dropped during the fur-trade era, it initiated a cascade of events that altered the region's food web.

Otters eat small animals like sea urchins. Without otters, there were too many sea urchins. These creatures ate kelp. This is a type of seaweed. Many fish relied on kelp forests. Without the kelp, the fish had to leave the ecosystem.

Many changes happen when a top predator is taken away. The changes trickle all the way down the food chain. These changes are called a trophic cascade.

Something similar happened with gray wolves (Canis lupus) in the 1900s. Settlers began moving to the U.S. northwest. Then the settlers began hunting wolves. Wolves hunt elk (Cervus canadensis). Without wolves around, the elk populations exploded. The elk ate too many plants, including young trees. Fewer trees and plants meant fewer beavers and birds.

Not Always Successful

Scientists have tried bringing some top predators back. They brought wolves back into Yellowstone National Park in the 1990s. The wolves had a big effect on the ecosystem and food web. The wolves ate the elk. With fewer elk, there was more plant growth. There were more small animals too.

In Yellowstone, bringing back predators has been successful. But in other cases, top predators have caused problems. Brown bears (Ursus arctos) were brought back to the Pyrenees mountains in Europe. They ended up attacking livestock. Bringing back predators can be tricky. It's hard to predict what will happen.