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In 1970, Jim Estes made his first trek to Alaska’s Aleutian Islands.

Everywhere he looked, there were sea otters, congregating in “rafts and bunches, as many as 500 at once,” said Estes, an ecologist. “There were so many of them, we couldn’t keep track.”

Now, Estes said, more than 90% 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 Aleutians’ 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, Estes and his colleagues report in a paper published 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 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 and the study’s first author.

Softened by warming and acidifying waters, the coral-like structures have succumbed to the urchins, 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 a biological balance. A single sea otter can scarf down nearly 1,000 sea urchins a day.

“The amount of things they control in this ecosystem is pretty astonishing,” said Anjali Boyd, a marine ecologist at Duke University.