Crabs turned this marshland into ‘Swiss cheese’ - until sea otters arrived to keep them in check

A sea otter is seen in Morro Bay, 15 January 2010. Bringing otters back to a California estuary has helped restore the ecosystem.
A sea otter is seen in Morro Bay, 15 January 2010. Bringing otters back to a California estuary has helped restore the ecosystem. Copyright AP Photo/Reed Saxon, file
By Christina Larson with AP
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Hunting bans and habitat restoration have brought otters back to save this California marshland.

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California's collapsing marshlands have found an unlikely saviour in hungry sea otters.

The return of otters and their voracious appetites could halt one of the biggest causes of erosion, a new study shows.

Sea otters eat constantly and one of their favourite snacks is the striped shore crab. These crabs dig burrows and also nibble away roots of the marsh grass pickleweed that holds dirt in place.

Left unchecked, the crabs turn the marsh banks “into Swiss cheese”, which can collapse when big waves or storms hit, explains Brent Hughes, a Sonoma State University marine ecologist and co-author of the new study published on Wednesday in the journal Nature.

Researchers found that the return of the crab-eating sea otters to a tidal estuary near Monterey, California, helped curb erosion.

"They don’t completely reverse erosion, but slow it down to natural levels," says Hughes.

A sea otter is seen in the estuarine water of Elkhorn Slough, 19 May 2019.
A sea otter is seen in the estuarine water of Elkhorn Slough, 19 May 2019.Emma Levy via AP

What's behind the return of otters to California's marshland?

For many years, there weren't any sea otters in Elkhorn Slough's coastal wetland.

The 19th century fur trade decimated their global population which once stretched from Alaska to California, as well as into Russia and Japan. At one point, as few as 2,000 animals remained, mostly in Alaska.

Hunting bans and habitat restoration efforts helped sea otters recover some of their former range. The first returnees were spotted in Elkhorn Slough in 1984. The Monterey Bay Aquarium's programme for raising and releasing orphaned sea otters also boosted the estuary's population.

Sea otters loll in the water at the Monterey Bay Aquarium in Monterey, 26 March 2018.
Sea otters loll in the water at the Monterey Bay Aquarium in Monterey, 26 March 2018.AP Photo/Eric Risberg, File

How are otters slowing erosion in California's marshland?

For the new study, researchers analysed historic erosion rates dating back to the 1930s to assess the impact of sea otters' return. They also set up fenced areas to keep otters away from some creek sections for three years - those creek banks eroded much faster.

Past studies about the return of top predators to various habitats - most famously, the reintroduction of grey wolves to Yellowstone National Park - show how such species maintain ecosystem stability. Wolves curbed the number of elk and moose that ate saplings and slowed riverbank erosion.

Many past studies relied on observations, but the design of the latest research left no doubt as to the sea otters' impact, says Johan Eklöf, a Stockholm University marine biologist who was not involved in the new study.

Other research has shown that sea otters help kelp forests regrow by controlling the number of sea urchins that munch kelp.

Sea otters “are amazing finders and eaters,” says Brian Silliman, a Duke University coastal ecologist and co-author of the latest study.

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