Sea Otters Rescue Coastal Habitat

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That predators play vital roles in shaping ecosystems, as keystone species, is not a new idea. Sea Otters have already been credited with helping to restore kelp forests along the northern CA coast. A recent collaborative study by UC Santa Barbara and east-coast wetlands researchers has quantified just how profound this effect is with sea otters in Elkhorn Slough in Monterey bay. I am including some excerpts from the npr article (not paywalled) and also a link to an article in a UC Santa Barbara newsletter that has a couple of nice pictures, one of the otters and one of the crabs mentioned in the article.



Elkhorn Slough in Monterey Bay, California is the second-largest estuary in the U.S. For decades, it was falling apart.

"They opened up a new harbor in the 1940s that created this full permanent opening to the ocean," says Brent Hughes, a marine ecologist at Sonoma State University. "And so all this new tidal energy was eroding away the marshes."

Nutrient runoff led to massive algal blooms that smothered the marshes. And sea level rise was slowly drowning them.

"It's weird, right? Plants in the ocean, drowning," says Hughes. "But that's what salt marshes do if they can't accrete their sediments and build up."

Finally, there was a dramatic surge in the numbers of shore crabs populating the marsh.

"When they're not super abundant, they can be positive," he says. "They're burrowing, aerating the soils. But when they're left unchecked by predators, then they can just explode. And that's when they start doing damage."

In addition to burrowing, the crabs eat the roots and underground stems of the marsh plants. "What this does is [it] destabilizes the banks — the shoreline," says Hughes, "and it just causes this erosion." Large chunks of marsh — plants, dirt, and all — began to calve off steadily into the water. The ecosystem was facing death by a thousand cuts.
Then came the sea otters, which have been steadily recolonizing the Northern CA coast since the 1980s.

Today, Elkhorn Slough has the highest concentration of otters in all of California — about 100 in total. Hughes says this means that "the hotel is full. The no vacancy light's on. You can't stick one more otter in there if you tried."

That got Hughes wondering: Might the return of all those otters have had an impact on the crabs, and maybe even on the marsh itself? This led him to get in touch with Christine Angelini, who'd been studying wetlands up and down the East Coast for about a decade at that point, examining how crabs destroy their ecosystem through all their nibbling and burrowing.

[snip]

The two of them, along with their colleagues, got to work. They set up an experiment involving two kinds of plots that were identical in every way — the amount of sunshine they received, the tides flowing in and out, the crabs ... But there was one key difference.

In one set of plots, the otters were allowed in. "We had plots where sea otters were allowed to forage on the marsh and interact with the crabs and the vegetation," says Angelini. "And then we had experimental cages that would keep the sea otters out, but would allow for movement of the crabs in and out of them. They were like a little otter fence, if you will."

They let the experiment run for three full years. The results couldn't be clearer. Without otters, there were more crabs, more of their burrows, and fewer plants — which all contributed to more erosion. But when otters were allowed in, they feasted on the crabs, allowing the marsh to revegetate. Birds and raccoons also ate crabs, but "overwhelmingly, it was the sea otters that were the ones responsible for taking out the crab," says Hughes.

The result, he says, is that "a lot of those burrows started filling in — the marsh banks were becoming solidified to the point where they weren't Swiss cheese and eroding away."

In other words, despite sea level rise, pollution, and stronger tides, the reintroduction of the sea otter helped restore the marsh ecosystem naturally. By the time the study wrapped, the otters were preventing 10 or so inches of salt marsh loss per year.

We can get incredible results if we just let nature do its thing.


Abstract directly from the Feb 1 issue of Nature. Unfortunately, the entire article is paywalled.