Hungry Sea Otters Help Prevent Erosion on California’s Coast

The marine mammals, which were once hunted nearly to extinction, feed on crabs that would make the land more susceptible to erosion by digging holes in the soil and eating roots

An otter floats on its back in the water with kelp draped over its stomach
A sea otter basks in the water with some kelp. Sea otter populations plummeted as they were killed for their pelts in the 18th and 19th centuries. Ricardo DeAratanha / Los Angeles Times via Getty Images

Sea otters have been returning to California’s Monterey Bay after humans drove them out—and as they repopulate the surrounding marshes, the aquatic mammals are helping prevent erosion.

Voracious eaters, the otters munch on crabs, which would otherwise chew through the roots of marsh grass and burrow into the soil, destabilizing the ground. Now, in places where the habitat hosts fewer crabs—thanks to the otters—erosion has declined, researchers report Wednesday in the journal Nature.

“It’s remarkable when you think about it,” Jane Watson, a community ecologist at Vancouver Island University in Canada who did not contribute to the findings, tells Nature News’ Jude Coleman. “You can have a single animal, the sea otter, come in and through predation actually mitigate the effects of erosion.”

The results suggest a lesson for conservation efforts—that restoring the top predator in an ecosystem can help with habitat recovery, the study authors write.

“We think they should be incorporated in plans moving forward, when in the past they haven’t,” Brent Hughes, lead author of the study and a biologist at Sonoma State University, tells Courthouse News’ Sam Ribakoff.

Southern sea otters, also known as California sea otters, live along the state’s Central Coast, while their northern counterparts live off of Washington, Alaska and Canada’s British Columbia, as well as the Pacific coasts of Japan and Russia. They spend most of their time in the water and like to eat invertebrates such as urchins and crabs. Since urchins eat kelp, otters’ predation also plays a role in helping kelp forests grow.

But sea otters are endangered. The fur trade in the 19th century drove their population sizes down. Human activities like agriculture and development also played a role, according to a statement from Duke University. By the early 20th century, only 1,000 to 2,000 otters were left.

Without as many otters around, crabs started to run rampant in the salt marshes of Elkhorn Slough, an estuary on California’s Monterey Bay. In time, the crustaceans turned the marsh’s banks “into Swiss cheese,” making them susceptible to collapse, Hughes tells Christina Larson of the Associated Press (AP).

Eventually, people became interested in sea otter conservation. The Marine Mammal Protection Act of 1972 and the Endangered Species Act in 1973 helped protect the creatures from extinction, writes ABC News’ Julia Jacobo.

In the mid-1980s, sea otters recolonized Elkhorn Slough and brought their appetites with them—adult otters eat 25 percent of their body weight every day, per the statement.

Monitoring over the past couple of decades has suggested that as sea otters have returned to the area, erosion has slowed. For the new study, the researchers conducted surveys at 13 tidal creeks in Elkhorn Slough, comparing historical rates of erosion and the otters’ population.

They also carried out field experiments at five marsh creeks for almost ten years. To test otters’ impact on the environment, they fenced off some areas so the mammals couldn’t get at the crabs in those places.

Both the exclusion experiments and the surveys found that higher otter populations appeared linked to reduced crab numbers and less erosion. Erosion had declined from 30 centimeters per year to 10 centimeters per year in areas repopulated by otters, according to Nature News.

“They don’t completely reverse erosion, but slow it down to natural levels,” Hughes says to the AP. But to have a long-term impact, people will still need to prevent other threats to erosion, such as rising seas, pollution and hydrology changes, Johan Eklöf, a marine ecologist at Stockholm University who did not contribute to the findings but wrote an editorial accompanying the paper, tells ABC News.

As sea otters return to the coast of California, their presence could also limit erosion in other areas, like the marshes of San Francisco Bay, so long as their populations can survive, Watson tells Nature News.

“We made a prediction recently in 2018 that the San Francisco Bay could probably currently support about 6,000 sea otters,” Hughes tells ABC News. “The entire state of California has 3,000. And so you can put sea otters, in theory, in San Francisco Bay and triple the population.”

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