At one point, it was nearly impossible to see sea otters along the west coast of the United States.

The animals were hunted nearly to extinction in the 1800s. People paid for their fur. At one point, only about 2,000 remained. Most lived in the waters off Alaska.

But by the 1980s, the animals started to recover. Three programs helped increase their population in northern California. First, hunters were banned from killing the otters. Next, an environmental program restored the wetlands where the otters once lived. And finally, California's Monterey Bay Aquarium began to raise otters and release them into the wild.

For about 40 years, the animals have been back in the seaside area called Elkhorn Slough. It is between Monterey and San Francisco.

Brent Hughes is studying how the otters' return affected the environment. He is a marine ecologist at nearby Sonoma State University.

He recently published a report in the science journal Nature. Hughes and his team looked at a wetland area called a tidal estuary. The researchers found that the otters perform a service that keeps the area from eroding -- or wearing away.

Hughes said the otters feed on a destructive shellfish called the striped shore crab. The crabs dig into the mud and sand and bite the roots of a marsh grass known as pickleweed. Pickleweed holds the soil in place during coastal flooding.

The holes the crabs make cause the area to look like "Swiss cheese," Hughes said. Swiss cheese is a kind of cheese with many holes.

For the new study, researchers examined erosion rates dating back to the 1930s. The historical data help the researchers understand the effects of sea otters' return to the area.

The researchers also set up fences that prevented the otters from entering certain areas for three years. In the fenced-off section of the estuary, the scientists say the soil eroded much more quickly.

Hughes said the otters "don't completely reverse erosion but slow it down to natural levels."

The otter study is one of several research projects that follow what happens to an area when a top predator returns after a long period of time.

A famous past study looked at the return of gray wolves to Yellowstone National Park. The wolves hunted elk and moose that ate small trees along riverbanks. With fewer elk and moose, the trees started growing again and the riverbanks did not erode as quickly.

Something similar happened when the otters returned.

Johan Eklof is a Swedish marine biologist from Stockholm University who reviewed the study. Other studies, he noted, depended only on observation. But this new study leaves no doubt that the otters limit erosion.

Other studies show that otters can be helpful for plants. One study said the otters eat sea creatures called urchins that feed on kelp. The return of the otters permitted the kelp to grow again.

Brian Silliman is a coastal ecologist from Duke University in North Carolina. He co-wrote the new study. He described the otters as "amazing finders and eaters."

I'm Dan Friedell.

Dan Friedell adapted this story for Learning English based on a report by the Associated Press.

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extinction –n. the state or situation that results when something (such as a plant or animal species) has died out completely

affect –v. to produce an effect on something

certain –n. a known place or area

predator –n. an animal that lives by killing and eating other animals

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