Sea otters are one of the best examples of what happens when an ecosystem loses its “keystone” species (the species that keeps other animals and plants in balance, the way a “key stone” holds an arch in a building together). Once, sea otters were common all along the Pacific coast. Then the fur trade came along. For hundreds of years, humans hunted them. By the early 1900s, they were “extirpated” in Canada. That means that even though they survived in some places, none were left on our shores.

Without its top predator, the coastal ecosystem changed. Suddenly, sea urchins were not being eaten as much, and their populations exploded. As a result, they overgrazed the underwater kelp forests. Those important habitats had been full of biodiversity. Now the many species they supported began to struggle.

But the story did not end there. Between 1969 and 1972, biologists moved 89 sea otters from Alaska to British Columbia. This reintroduction project was a success: the otters thrived and spread. Most importantly, they ate lots of sea urchins. Finally, the kelp forests began to recover. The species that call them home, like the copper rockfish, also grew in number.

Here’s some extra good news: the renewal of the kelp forests is good for more than biodiversity. These ecosystems also play a role in storing carbon. That makes restoring them an important action to help our climate, too.

Today, sea otters are considered a species of Special Concern. That means they are not Threatened or Endangered now, but we need to be careful so that doesn’t change. Their range is still limited — just a third to a half of what it used to be — and it’s crossed by major oil tanker routes. A single oil spill could cause them big problems. For that reason, scientists will be watching them closely.