ARI SHAPIRO, HOST:
For decades, the number of California sea otters cratered, but since the 1980s, they've made a stunning comeback, a return that researchers now say helped restore their coastal habitat. Here's science reporter Ari Daniel.
ARI DANIEL, BYLINE: For years, Elkhorn Slough in Monterey Bay, Calif., was falling apart.
BRENT HUGHES: They opened up a new harbor in the 1940s, and so all this new tidal energy was eroding away the marshes.
DANIEL: Brent Hughes is a marine ecologist at Sonoma State University. Plus, he says there was sea level rise and an explosion of shore crabs.
HUGHES: They burrow. On top of it, they eat the roots. What this all does is destabilizes the banks, the shoreline, and it just causes this erosion.
DANIEL: The marsh was disintegrating, making the coastline and people living there more vulnerable to flooding and storm surge. And there was just no way of getting it back without an expensive intervention. But Hughes knew those shore crabs would be an all-you-can-eat buffet for sea otters.
HUGHES: They're really good at eating crab, and it's just easy pickings for them. They just eat them like popcorn - shell and all.
DANIEL: For a long while, though, sea otters were teetering on the edge of extinction, killed by the hundreds of thousands for their pelts, which over time let the shore crabs get the run of the place. But a small group of several dozen otters managed to survive near Big Sur and by the 1980s, with a bit of human help, began to recolonize the coastline. Today, Elkhorn Slough has the highest concentration of otters in all of California, about 100 in total.
HUGHES: Meaning the hotel is full - you can't stick one more otter in there if you tried.
DANIEL: So Hughes wondered, might the return of all these otters have had an impact on the crabs, maybe even on the marsh itself? But he couldn't figure it out alone.
CHRISTINE ANGELINI: I got a call saying, hey, do you want to go to California and set up some experiments?
DANIEL: Christine Angelini is a coastal ecologist, now at the University of Florida. She'd been studying wetlands and crabs up and down the East coast, but this was her first time in a California marsh.
ANGELINI: They're much uglier than the ones in the East Coast (laughter).
HUGHES: Yeah, I agree with that. We have out here on the West Coast what we call marsh envy.
DANIEL: Nevertheless, the two of them got to work. They set up an experiment involving two kinds of plots that had a key difference. In one set, the otters were allowed in.
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.
DANIEL: They let the experiment run for three years, and 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.
HUGHES: The marsh banks were becoming solidified to the point where they weren't eroding away.
DANIEL: Despite sea level rise and those stronger tides, the sea otters helped restore the entire marsh ecosystem naturally. By the study's end, they were preventing 10 or so inches of salt marsh loss per year.
ANGELINI: The conservation of a top predator can really enhance the health and the resilience of a system that's otherwise under a large portfolio of stress.
DANIEL: The findings are published in the journal Nature.
LEKELIA JENKINS: My honest reaction was this could become a classic in the literature.
DANIEL: Lekelia Jenkins wasn't involved in the study. She's a marine sustainability scientist at Arizona State University and says marsh restoration - it helps people, too.
JENKINS: Sea otters suddenly goes from just a cute thing we like to have around to something that can protect our livelihoods and our properties.
DANIEL: Everyone wins, including all those sea otters, their bellies stuffed with shore crabs.
For NPR News, I'm Ari Daniel.
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