They Released 15 Sea Otters Into a Crab-Dominated Marsh — 20 Years Later, Nobody Saw This Coming

[00:00:07]Brent Hughes had walked these creeks a hundred times. He was a marine ecologist and for years his field site was a muddy tidal marsh on the coast of central California called Elhorn Slow. It was not a famous place. Most people driving Highway 1 never knew it was there. A maze of salt marsh and quiet water tucked behind the dunes of Mterrey Bay.

[00:00:28]But Hughes kept coming back because Elorn's slow was falling apart. The banks of its creeks were collapsing into the water.

[00:00:37]Every year, the marsh lost more ground.

[00:00:39]The green edges crumbling, sliding, dissolving into the brown tidal channels. Between 1956 and 2003, this one estuary had lost more than half of its marsh. Engineers had studied it.

[00:00:53]Money had been spent. And still the coastline kept eating itself foot by foot, season after season. Then Hughes noticed something that did not make sense. In a few of the creeks, the collapse [music] had slowed. The banks were holding. Nobody had built anything there. Nobody had touched them. And the only difference he could find, the one thing those creeks had that the others didn't, was floating on its back in the water, calmly cracking open a crab.

[00:01:31]This is the story of how a marsh that engineers could not save was rescued by an animal most people think of as a pool toy and of the hidden machine underwater that no one saw until they went looking for it.

[00:01:56]To understand what was happening at Elhorn Slow, you first have to understand what was killing it. The Slow sits at the mouth of the Selenus Valley, one of the most intensively farmed regions in the United States.

[00:02:09]The place they call the Salad Bowl of America. [music] For decades, the runoff from those fields, the fertilizer, the nitrogen, everything that makes lettuce grow, drained downhill and settled here in the still water of the estuary.

[00:02:24]The result was a slow poisoning. The nutrients fed explosions of algae. The algae smothered the eel grass, the underwater meadows that hold an estuary [music] together, the nursery where crabs and fish and young sharks begin their lives, the anchor for tens of thousands of migrating birds.

[00:02:43]The water turned murky. The seaggrass thinned and then it vanished. What had once been one of the richest nurseries on the California coast became, in the words of the people who studied it, a place where there was very little seaggrass and algolmats were everywhere.

[00:02:58][music] An estuary is supposed to be one of the most productive ecosystems on Earth, richer acre for acre than a rainforest.

[00:03:07]This one was quietly starving, and the marsh itself was retreating. The pickle weed, the low salt tolerant plant whose roots knit the banks together, was losing its grip. The creek edges slumped into the channels. The estuary was very literally dissolving into the sea.

[00:03:27]For an ecosystem, this is a death spiral, [music] and nothing people tried seemed able to stop it.

[00:03:42]To find the thing that would change everything, you have to go back a hundred years and a few hundred miles up the coast.

[00:03:49]The sea otter was once one of the most abundant predators of the North Pacific.

[00:03:54]Before humans came for them, an estimated 250,000 sea otter ranged along the coast from California up into Alaska.

[00:04:09]And then came the fur trade. Sea otterters have the densest fur of any animal on Earth. Up to a million hairs in a single square in. And that fur very nearly killed the entire species.

[00:04:22]Hunters took them by the hundreds of thousands.

[00:04:34]By around 1910, there were fewer than 2,000 sea otterters left alive in the world. Along most of the California coast, there were none at all. People assumed they were gone for good. They were wrong. A tiny population had survived. Perhaps only 50 animals hidden along a remote, roadless stretch of the Big Su coastline in water too rough for the hunters to reach. From that one thread, the species began impossibly to come back. Slowly over decades they crept north and south cove by cove.

[00:05:16]And there was a reason scientists paid attention wherever they went because the sea otter is what ecologists call a keystone species. An animal whose presence holds an entire system in place. The most famous example is written across the North Pacific. Where otter live, they eat the sea urchins that graze on kelp and the great underwater kelp forests flourish.

[00:05:44]Where otter disappear, the urchins explode, mow the kelp down to bare rock, and the forest becomes a desert. One animal deciding whether an entire seascape is a forest or a wasteland.

[00:05:56]That is what a keystone predator does.

[00:06:11]So when in 1984 a small group of male otter did something no one had recorded in living memory when they swam up into a muddy estuary called Elorn Slow, a few researchers quietly wondered what these animals might set in motion this time.

[00:06:26]At first, there were only a handful. The number fell at one point to around 15.

[00:06:33]Then, the Mterrey Bay Aquarium stepped in with one of the strangest and most tender programs in American conservation.

[00:06:39]When a stranded otter pup washes up with no mother, the aquarium pairs it with an adult female otter in their care, a surrogate mom who grooms it and teaches it to dive and shows it how to pry a crab out of the mud exactly as she would her own.

[00:07:03]And then when the pup is ready, they release it into [music] the wild.

[00:07:13]A large share of the otter living in Elorn slow today are descended from pups raised this way. The population climbed toward 100. Today, the Slow holds one of the densest concentrations of sea otterters anywhere in California.

[00:07:32]But understand this, nobody brought them back to save the marsh. Nobody believed a two-foot animal that eats shellfish had anything to do with a collapsing coastline. They returned because the sloth was calm and safe and full of food. What they would do next, no model had predicted.

[00:07:57]And the crabs were only half of it.

[00:08:01]Years earlier, the same researchers had watched a second chain reaction unfold in the very same water. This time in the seaggrass. When the otter ate the crabs, they set free the tiny animals the crabs had been feeding on, sea slugs and small crustations that graze on algae. Those grazers swarmed over the eelgrass leaves and scrubbed them clean of the algae that had been smothering them.

[00:08:25]And the seaggrass, the seaggrass that pollution had all but wiped out, came roaring back.

[00:08:31]In the parts of the slough with the most otter, the eelgrass grew greener and thicker than in estuaries with no pollution at all. Over the course of the recovery, the seagrass in the slo increased roughly sixfold.

[00:08:46]Think about what that means. The water was still polluted. The runoff never stopped. And yet, the ecosystem healed, not because humans fixed the source, but because a predator at the top of the food web reached all the way down and pulled the whole system back into balance.

[00:09:07]This is a thing scientists call a trophic cascade. It is one of the most powerful and least visible forces in nature. Change the top of a food web and the effects ripple all the way to the bottom to the roots to the mud to the shape of the land itself. It is the same force that lets otter turn a rock desert back into a kelp forest a thousand miles away. Here in a California marsh, it was doing something no one had ever documented. It was holding a coastline in place. For a hundred years, the top of this food web had been empty. The cascade had been switched off. The otter switched it back on.

[00:09:48]Two studies came out of Elhorn's slow, one in the proceedings of the National Academy of Sciences and one in the journal Nature. And that second paper carried a conclusion that would have sounded absurd a generation ago. a top predator simply by recovering had reversed the physical collapse of an entire coastal ecosystem.

[00:10:13]Before we go under the water, it is worth saying plainly what people had already tried. For years, the effort to save estuaries like this one focused on the obvious enemy, the pollution.

[00:10:24]Clean up the runoff, the thinking went, and the ecosystem will heal itself. It was reasonable. It was expensive.

[00:10:32]And on its own, it was not enough. The nutrients kept coming and the marsh kept crumbling.

[00:10:38]Others treated the erosion as an engineering problem. If the banks are collapsing, then reinforce the banks.

[00:10:45]But you cannot armor an entire estuary.

[00:10:47]There's too much of it, too soft, too alive, shifting with every tide.

[00:10:54]A hardened shoreline can cost millions of dollars a mile to build and it still fails in soft mud because the tide simply undercuts it and moves on. Living coastlines do not sit still long enough to be bolted down. The estuary kept retreating no matter what was poured along its edges. What almost no one had seriously considered was the possibility that this marsh did not need a machine or a chemical or a seaw wall to save it.

[00:11:20]It needed a predator, one that had been missing for a hundred years.

[00:11:31]Here is what Hughes and his colleagues found when they finally looked beneath the surface. And it is the whole reason this story exists.

[00:11:40]The banks of the marsh are held together by the roots of the pickleweed. Strong roots make a strong bank. But those roots have an enemy, a small striped shore crab that burrows into the mud of the creek edges.

[00:11:54]The crabs dig. They tunnel through the sediment and they chew on the roots themselves.

[00:12:01]In small numbers, it is harmless. But when there is nothing to keep them in check, the crabs multiply >> and an army of them burrowing and grazing along a creek bank turns solid marsh into something like a sponge.

[00:12:16]Weak, undercut, ready to collapse at the next high tide.

[00:12:21]A >> and they had been left completely unchecked. A high tide would come in, soak into the burrowed, hollowedout bank, and pull a little more of it away as it went out.

[00:12:41]Then another tide, then another.

[00:12:44]Multiply that by every creek in the estuary, by every day for decades, and you get a marsh sliding into the sea in slow motion.

[00:12:56]Not because of one dramatic storm, but because of millions of small crabs eating the ground out from under it. For a hundred years, there was nothing to keep them in check. Because the animal that eats them was gone.

[00:13:22]A sea otter is a machine that runs on crabs. It has no blubber, none of the thick fat layer that keeps a seal or a whale warm in cold water.

[00:13:34]To survive without it, an otter has to burn energy at a furious rate, which means it has to eat almost constantly.

[00:13:44]A single sea otter eats up to a quarter of its own body weight every single day.

[00:13:49]crabs, clams, urchins, whatever it can pull from the mud, crack open in those paws, and crush with those teeth.

[00:13:58]An otter is not a gentle grazer.

[00:14:01]Poundforound, it is one of the hungriest predators in the ocean.

[00:14:09]>> I got him.

[00:14:14]So when the otter returned to Elhorn Slow, they did the only thing they know how to do. They ate. And what they ate in enormous numbers were the burrowing crabs.

[00:14:27]The crab population crashed. The roots recovered. The banks firmed up.

[00:14:36]And the erosion, the erosion that engineers could not stop, slowed in the creeks with the most otter by an average of 69%.

[00:14:52]And the researchers did not just assume it was the otter. They tested it. They fenced off sections of marsh, keeping crabs in and otter out, and watched those banks weaken and crumble while the open otter patrolled creeks nearby held firm.

[00:15:09]They compared creek after creek, some with otter, some without, and the pattern held every time. This was not a hopeful story told over a number. It was a controlled experiment run across an entire estuary over decades.

[00:15:27]So, the otter are not the caretakers of this marsh. Their hunger is the machine that holds it together. Every crab cracked open on one of those furry chests is a piece of American coastline that does not fall into the sea.

[00:15:57]>> There is a fair question buried in all of this and it deserves an honest answer.

[00:16:03]Sea otterters are not a magic fix. They are still an endangered animal clinging to a fraction of their [music] former range. They cannot live everywhere and they do not solve the pollution that poisoned the sloth in the first place.

[00:16:24]>> [music] >> Bringing otter back is not a substitute for cleaning up what we put into the water. And the people who study them will tell you exactly that. But the lesson under the mud is bigger than one marsh.

[00:17:03]For a century, we tried to manage this coastline the way we manage almost everything with structures, with chemistry, with money and engineering.

[00:17:12]And the coastline kept dissolving.

[00:17:34]What finally held it together was not something we designed. It was something we had removed [music] and then almost by accident allowed to return.

[00:17:59]a predator we once hunted nearly to extinction for the fur [music] on its back turned out to be part of the machinery that kept an American coast standing.

[00:18:11]>> [music] >> Go to elhorn slout today and you can watch them do it. Floating on their backs in the channels wrapped in strands of eelgrass so they don't drift away while they sleep. Cracking crabs on their chest as though it were the easiest job in the world.

[00:18:36]>> They have no idea they're holding a marsh together. They are simply hungry.

[00:18:50]>> And that may be the strangest part of the whole story. We spent a 100 years certain that saving a coastline was a job for engineers. It turned out to be a job for an animal that weighs 40 lb, floats on its back, and eats crabs all day. an animal that had been here all along and only needed us to stop and let it come home.

[00:19:17]>> Scientists have a careful phrase for what happened at Elhorn Slow. They call it a nature-based solution. The idea that a living thing put back where it belongs. Can do the work of concrete and steel and do it for free and keep doing it long after the budget runs out.

[00:19:47]And up and down the coast, people are starting to ask the obvious next question.

[00:19:57]If a hundred otter could hold one California marsh together, what else have we torn out of these systems without ever knowing what it was holding up?

[00:20:15]>> Which brings us back to Brent Hughes standing in the mud looking at two kinds of creek.

[00:20:22]In the creeks with few otter, the banks were still falling apart. In the creeks crowded with otter, the erosion had slowed to a crawl. It was too consistent to be luck.

[00:20:34]Somehow, the presence of the otter was holding the marsh together.

[00:20:40]But here was the problem. Sea otterters do not eat marsh grass. They do not move sediment. They do not build anything.

[00:20:49]They float. They dive. They crack open shellfish and they sleep wrapped in kelp.

[00:20:55]On paper, an otter has no business affecting the shape of a coastline at all.

[00:21:04]So, how could a predator that never touches a single blade of pickle be the difference between a marsh that survives and a marsh that dissolves into the ocean? It was the kind of correlation that scientists are trained to distrust.

[00:21:20]Two things happening together, otter here, stable banks here, is not proof that one causes the other. Maybe the otter simply preferred the healthier creeks. Maybe something else was going on.

[00:21:35]To make the claim that a sea otter could hold a coastline together, Hughes and his colleagues would have to find the exact machinery connecting the two. And that machinery was not on the surface.

[00:21:45]It was underneath in the mud, in the roots, and in a small overlooked animal that the otter happened to love to eat.