An invasive species had taken over ecosystems across the West Coast and was threatening to dominate another one in Monterey County, California, when ecologist Rikke Jeppesen began her research two decades ago.
Jeppesen was seeking solutions for the havoc green crabs had caused – damaging seagrass beds in multiple states, eating small prey crucial to other species’ survival and persisting despite frequent efforts to remove them. One state even spent millions of dollars to protect its waters from the crabs, which are native to Europe.
But to her surprise, Jeppesen learned there was a much simpler way to remove the crabs in the slough she studied: furry and hungry sea otters.
Sea otters are rare in most ecosystems after they were hunted to near extinction in the 18th and 19th centuries. But at California’s Elkhorn Slough, a reserve where about 120 southern sea otters live, the cute apex predators have led to green crabs’ demise by eating up to 120,000 of the invasive species as a group per year.
The researchers said their findings, published in the Biological Invasions journal this month, show the importance of protecting otters and other native predators at a time when there are about 1 million plant and animal species at risk of extinction.
“Sea otters are the assistant managers of the slough in helping us keep invaders in check,” Jeppesen, who now works for the reserve, told The Washington Post.
Green crabs became a major threat to coastal ecosystems on the West Coast in the late 1980s after they were unintentionally introduced in the San Francisco Bay – possibly by trade ships, the researchers said.
Washington state said in September that it plans to spend about $12 million to manage its green crab population. Oregon has encouraged harvesters to remove 35 green crabs per day from its waters. Scientists said in 2021 that they failed to eradicate green crabs from an estuary in Stinson Beach, California, after years of efforts.
Jeppesen was a graduate student at the University of California at Santa Cruz in the early 2000s when she first set fish traps – baited with raw anchovy or sardine – across Elkhorn Slough to study green crabs.
She sometimes captured up to 100 green crabs in a single trap. But in the following years, Jeppesen said, she nabbed fewer crabs.
So instead of studying why green crabs were successful – like they had been at other estuaries – she wondered why they were no longer thriving in Elkhorn Slough. She knew one of the largest shifts at the reserve over those years was the sea otter population.
Sea otters, which had long been hunted for their thick and soft fur, have slowly rebounded on the West Coast since the early 1900s. They were believed to be extinct until a small population was discovered off the rocky coast of Big Sur in 1914. They received federal protections in the 1970s.
The first male sea otters arrived in Elkhorn Slough in the 1990s, said Kerstin Wasson, a research coordinator for the reserve. When female otters came in the early 2000s, Wasson said, they began having babies. Plus, the Monterey Bay Aquarium released some of its sea otters into the slough.
Some improvements in the ecosystem came quickly. Sea otters ate multiple crab species, allowing snails, which crabs eat, to thrive. The snails then ate more algae that had blocked sunlight from reaching the sea grass. More sea grass provided protection for young fish and food for migratory birds.
The otters’ impacts on the green crab population became apparent around 2010, Jeppesen said. In 2014, a researcher was grabbing crabs from traps when he witnessed a sea otter consume about 30 green crabs in an hour.
The researchers said they worried the green crabs’ population decline could’ve been a fluke, but the trend continued over the following decade. While there are other animals in Elkhorn Slough that eat green crabs, such as sharks, rays, wading birds and larger crabs, green crabs only decreased when the otter population increased.
“We sort of think of restoration usually more from a bottom-up perspective – we need to plant the trees that have been lost or … plant the salt marshes,” Wasson said. “But at the same time, that’s not enough, and we also need to restore the food webs.”
Unlike other marine mammals that keep warm with blubber, such as whales and seals, sea otters stay warm by eating up to 25 percent of their body weight each day, according to the Monterey Bay Aquarium. Otters eat a variety of animals, such as clams, sea urchins and mussels, but they still fit green crabs into their diets in Elkhorn Slough.
There are about 3,000 southern sea otters in California, down from the tens of thousands that once ranged down to Mexico. The U.S. Fish and Wildlife Service has considered an otter reintroduction plan on the Pacific Coast, which the federal agency said would cost between $26 million and $43 million over 13 years.
The California researchers said their study proves otters can provide unexpected benefits when returned to their natural habitats. Now when researchers set traps, Jeppesen said, they usually catch fewer than 10 green crabs in each.
“That is really a win-win if you can help protect those native species,” said Jeppesen. “It may benefit your ecosystem in multiple ways, including protecting against invaders. No one loses out in that case.”