A sea otter eating a crab in Monterey Bay, California. Killiii Yuyan
California sea otters were nearly hunted to extinction in the 19th century, with only a small number surviving along the central coast. As otters rebounded, a natural experiment unfolded. Scientists could see the ways that otters shape California’s kelp forests and marshland. Two new studies reveal how otters are safeguarding these coastal areas against worsening climate change.
Over the course of the 20th century, severe heat devastated underwater kelp forests in both northern and southern California. Between 1910 and 2016, canopy cover in these regions dropped by more than half, according to a new study published in PLOS Climate. But along the central coast, as sea otters multiplied, canopy grew by 56 percent. That’s because otters eat sea urchins, which, left unchecked, decimate kelp forests.
“Our study showed that kelp forests are more extensive and resilient to climate change where sea otters have reoccupied the California coastline during the last century,” said lead author Teri Nicholson, a biologist at the Monterey Bay Aquarium.
A separate study, published this week in Nature, found that the return of otters has also been a boon to Elkhorn Slough on Monterey Bay. Since otters recolonized the area in the 1980s, erosion has slowed by as much as 90 percent, helping to protect the marsh against rising seas.
Here too credit goes to the otters’ dining habits — in this case their preference for marsh crabs. Crabs eat plant roots that hold marshland in place, leaving it vulnerable to erosion. By eating the crabs, otters have given plants a chance to lay down more densely matted roots that can stand up to increasingly intense floods.
“It would cost millions of dollars for humans to rebuild these creekbanks and restore these marshes,” said coauthor Brian Silliman, a marine biologist at Duke University. “The sea otters are stabilizing them for free in exchange for an all-you-can-eat crab feast.”
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