The California sea otter, once hunted to the edge of extinction, has staged a thrilling comeback in the last century. Now, a team of scientists has discovered that the otters' success story has led to something just as remarkable: the restoration of their declining coastal marsh habitat.
"To me, it's quite an optimistic message," says Christine Angelini, a coastal ecologist at the University of Florida and one of the authors of the study published in the journal Nature.
It's a demonstration, she says, "that 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."
The demise of Elkhorn Slough
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.
Salt marshes are important. They protect the coastline and people living there from flooding and storm surge. They sequester carbon and filter contaminants from the water. But with the marsh of Elkhorn Slough disintegrating, there was no way of getting it back without an expensive intervention.
The otter part of the equation
Hughes knew, however, that all those shore crabs represented an all-you-can-eat buffet for the sea otters. "They're really good at eating crab," he says. "It's just easy pickings for them. They can actually grab about 10 of these at a time. They just eat 'em like popcorn, shell and all."
Sea otters devour about a quarter of their weight every day, Hughes says. They use most of those calories to heat their bodies since they're the only marine mammal without a layer of insulating blubber. The other way they keep warm is through their dense mat of fur. "They have about a million hair follicles per square inch of fur," says Hughes.
The lushness and softness of that fur is what got the otters into trouble. Fur traders killed hundreds of thousands of animals for their pelts, nearly obliterating the population, driving them from the salt marshes and allowing shore crabs to get the run of the place.
A small group of several dozen otters did manage to survive near Big Sur. And by the 1980's, with a little bit of human help, the sea otters began their steady march towards recolonizing the coast.
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.
This was her first time encountering a California marsh. "They're much uglier than the ones on the east coast," Angelini says with a laugh. "The vegetation [out east] is just lush and vast and they're beautiful systems. And you get to a California salt marsh and it's kind of scrubby."
Hughes agrees. "We have out here on the West Coast what we call marsh envy," he says. "But that doesn't mean our California ones aren't important and [are] not worth studying."
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."
Otters helped bring the marsh back
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.
"My honest reaction was — this could become a classic in the literature," says Lekelia Jenkins, a marine sustainability scientist at Arizona State University who wasn't involved in the study. She says marsh restoration also helps people by reducing flooding. "Suddenly, [sea otters go] from just a cute thing we like to have around to something that can protect our livelihoods and our properties."
Andrew Solow, Senior Scientist Emeritus at the Woods Hole Oceanographic Institution who wasn't part of the research, was similarly convinced by the findings.
"A significant challenge is that factors other than otter abundance that affect erosion are also varying through time," he says. "The authors, who are well aware of this problem, reach their conclusion by multiple lines of analysis."
The many years of field work and analysis paid off, says Angelini.
"The marsh looks like a system that has very little hope," she explains. But the "efforts that have been made over the last several decades to conserve these top predators are paying the dividends for the health of the ecosystem."
It's a situation, she says, where everyone wins — including all those sea otters, their bellies stuffed full of shore crabs.
