Sea otters, once hunted to near extinction, are preventing coastal erosion as their populations grow, study finds

Sea otters are proving to be Mother Nature's solution to the prevention of coastal erosion, a recent discovery that further demonstrates how conservation efforts can help to restore an ecosystem as a whole.

Sea otters were nearly hunted to extinction in the 18th and 19th centuries for their pelts. But as populations begin to recover in California after decades of conservation efforts there, the marine mammals are helping to fortify the environment as they expand their range, according to a paper published in the journal Nature on Wednesday.

Known for their voracious appetite, sea otters are indulging in a buffet of the burrowing crabs that have been eroding the salt marshes in the central California coastal area of Elkhorn Slough, on Monterey Bay, according to Brent Hughes, a biologist at Sonoma State University and lead author of the study. In the process, they are having a "big impact" on the protection of sea grasses, researchers have learned after decades of monitoring the species, Hughes told ABC News.

In the absence of sea otters, the crabs have been living in the Elkhorn Slough estuary without its main predator around, Hughes said. As they burrow into the marsh banks, they destabilize the shoreline by creating a 'Swiss cheese' consistency in the soil, which then causes the marsh to slump off, or erode, into the water, Hughes said.

"When that happens, it's gone for good," Hughes said.

However, the sea otters have been able to consume enough of the crabs to slow the erosion almost to a halt, he said.

Conserving sea otter populations is considered a temporary solution to coastal erosion, Johan Eklöf, an ecologist at Stockholm University who peer-reviewed the paper, told ABC News. Prevention of threats such as sea level rise, pollution, and hydrology changes will take more time but have a larger, long-term impact.

"Most ecologists maintain that we need to reduce the stressors," he said.

The new study's results, which Eklöf described as "cool," also show the resilience of ecosystems despite all of the stressors they face, he added.

Prior to recent decades, it wasn't known that sea otters live amid the salt marshes, because they're typically seen in kelp forests farther out to sea, Hughes said. But because they took to the coastal environment as if they've been living there for "thousands of years," the researchers hypothesize that the otters could be returning to what may have been their natural habitat before they were hunted for their pelts.

"Now that they're protected, they can use a much wider breadth of habitats than what we previously knew," Hughes said. "And now they're using salt marshes. It's almost like the moving on to land."

The salt marshes also prove to be a more protective environment than kelp forests, Hughes added. In a salt marsh, sea otters only need to dive one or two feet to locate their prey, and they can safely leave their pups above water as they do so. But in the kelp forests, otters have to dive up to 50 feet, placing them at much greater risk of being eaten by a great white shark, Hughes said.

Sea otters were one of the many marine mammals hunted by European settlers in the 1700s and 1800s, Hughes said. Their pelts were considered to be especially valuable, and the demand for them caused the mammals to be hunted to the point that they were thought to be extinct for about two decades in the early 20th century, according to Hughes.

"They have around a million hair follicles per square inch, which means they're really, really, really soft," Hughes said. "If you ever touch a sea otter pelt, you can understand why it was a prized possession in the 18th and 19th centuries."

The species gained protections with the signing of the international Fur Seal Treaty of 1911 and later received additional safeguarding with the passage of the Marine Mammal Protection Act in 1972, and the Endangered Species Act one year later.

Since then, sea otter populations have steadily grown and expanded – a rarity for a top predator species, Hughes said.

Sea otters are a prime example of how conservation efforts for a particular species can have a domino effect on restoring and preserving the ecosystem as a whole, the research shows.

Russian and the Spanish traders who took "meticulous" records of the pelts they sold documented up to 10,000 sea otters in San Francisco Bay alone during the 18th and 19th centuries, Hughes told ABC News. If sea otter populations continue to grow, perhaps they too will return to the region, he said.

"We made a prediction recently in 2018 that the San Francisco Bay could probably currently support about 6,000 sea otters," said Hughes. "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."