When the Otters Vanished, Everything Else Started to Crumble

In 1970, Jim Estes made his first trek up to Alaska’s Aleutian Islands. He was greeted by an ocean filled with furry faces.

Everywhere the young biologist looked, there were sea otters — lollygagging on kelp beds, shelling sea urchins, exchanging their signature squeals. Back then, crowds of these charismatic creatures shrouded the sprawling archipelago, congregating in “rafts and bunches, as many as 500 at once,” said Dr. Estes, an ecologist at the University of California, Santa Cruz. “There were so many of them, we couldn’t keep track.”

Now, Dr. Estes said, more than 90 percent of those otters are gone. In just a few decades, this bustling civilization has withered into a ghost town. “You can travel down 10 miles of coastline and never see an animal,” he said.

The loss is more than cosmetic. In the Aleutian’s delicate seascape, otters hold the entire ecosystem together. As they have disappeared, the rest of the local food web has started to crumble — a process that’s been accelerated and compounded by climate change, Dr. Estes and his colleagues report in a paper published Thursday in the journal Science.

Without otters to keep them in check, populations of sea urchins have boomed, carpeting the sea floor in spiny spheres that mow down entire forests of kelp. Now, even the living, red-algae reefs on which the swirling stands of kelp once stood are in peril.

“These long-lived reefs are disappearing before our eyes,” said Doug Rasher, a marine ecologist at the Bigelow Laboratory for Ocean Sciences in Maine and the study’s first author.

Softened by warming and acidifying waters, the coral-like structures have quickly succumbed to the urchins’ tiny teeth, which can annihilate years of fragile algae in a single bite.

The findings point to the importance of otters in the Aleutians, where the marine mammals act not just as predators, but protectors, maintaining biological balance through their voracious appetites. A single sea otter can scarf down nearly 1,000 sea urchins a day. “They eat them like popcorn,” Dr. Estes said.

“The amount of things they control in this ecosystem is pretty astonishing,” said Anjali Boyd, a marine ecologist at Duke University who wasn’t involved in the study. “For their size and how cute they are, they are aggressive eaters.”

Aleutian sea otters have been in flux before. Fur traders in the 18th and 19th centuries hunted the animals to the brink of extinction, allowing sea urchin numbers to skyrocket, Dr. Rasher said. Although the urchins eagerly descended upon the local smorgasbord of kelp, the bubblegum-pink reef beneath them seems to have persisted — in part because healthy algae produce a protective limestone layer that can thwart even the most determined grazers. When otter populations recovered after trapping was restricted, the reef rebounded, too.

But against the backdrop of climate change, Dr. Rasher said, the reef’s safety net is gone. In the past several decades, a glut of carbon dioxide in the atmosphere has acidified ocean waters, making it harder for algae to armor themselves. “The reefs are producing less dense skeletons,” Dr. Rasher said. “And temperature exacerbates that issue.”

To quantify the damage, Dr. Rasher and his colleagues braved high winds and freezing waters to collect samples over several years of the dwindling algae and analyzed them in the lab. When the oceans had been healthy, the team found, nips from urchins had barely scuffed the algae’s surface. But met with weakened reef layers, urchins excavated chasms several millimeters deep — the equivalent of up to seven years of growth.

From 2014 through 2017, some reefs shrank by up to 64 percent. Where algae had once coated the Aleutian sea floor like a swath of pink pavement, only patches remained.

Warmer temperatures also speed animal metabolism, driving urchins to eat even more enthusiastically than usual. “Given those two things happening simultaneously, it’s really getting hit from both sides,” said Alyssa Griffin, an ocean biogeochemist at the University of California, Davis, who wasn’t involved in the study.

The algae’s decline also seems to be speeding up. When the researchers grew urchins and algae under conditions that simulated the preindustrial past, the present and a projected future in the lab, they found that contemporary circumstances spurred urchins to gnaw away at algae up to 60 percent faster. Changes yet to come will likely prompt the grazers to pick up the pace even more, the team’s analysis showed, barring sweeping change in carbon emissions.

“Just seeing that trend is staggering,” Ms. Boyd said.

The findings add yet another example to the list of ecosystems being ravaged by an ever-warming world, and underscore how food chain alterations and climate change can disastrously collide. “Predator loss can impact the environment in ways we haven’t even thought of,” Dr. Griffin said.

But these hidden relationships might contain hints of remedies. Repatriating otters could help reefs in the near-term, Dr. Rasher said, perhaps “buying us time to get our act together in terms of curbing global carbon emissions.”

That could be a difficult task, given the probable cause of the Aleutian Islands’ stunning vanishing of otters. Dr. Estes suspects that starving orcas — perhaps deprived of their preferred gray whale prey by industrial whaling — have turned in desperation to the little mammals, which they can gulp down by the hundreds or thousands a year. That could make it hard to sustain larger otter populations: Once introduced, they might just disappear all over again.

Dr. Estes, who is 74, hasn’t visited the Aleutians since 2015.

He doubts he will live to see the otters return. But he holds out hope that the islands will someday boomerang back to the breathtaking ecosystem he witnessed as a young man. “There was this incredible diversity,” he said. “It was spectacularly beautiful.”