Why Are Sea Otters a Keystone Species? Otters and Kelp

What is a keystone species?

A keystone species is an animal or plant whose effect on its ecosystem is far larger than its numbers or body size would suggest. The name comes from architecture: the keystone is the wedge-shaped stone at the top of an arch, and if it is removed, the arch falls down.

The ecologist Robert Paine introduced the idea in 1969. On rocky shores in Washington State he removed the ochre sea star, a predator of mussels, from test areas. Without the sea stars, mussels took over the rocks and crowded out many other species, so the number of species fell sharply. Soon after, the sea otter became one of the most famous examples of the same principle.

How sea otters protect kelp forests

Kelp forests are made of giant kelp, bull kelp and other large brown seaweeds that can grow tens of meters tall from a root-like holdfast on the seabed. They provide food and shelter for hundreds of species, from rockfish and crabs to seals and seabirds, and they are among the most productive habitats on Earth.

Sea urchins, including the purple sea urchin and the larger red sea urchin, are the main grazers of kelp. When they are kept in check, they mostly hide in crevices and feed on pieces of drifting kelp. When there are too many of them, they move into the open and chew through the stems and holdfasts, and whole stands of kelp break loose and drift away.

Sea otters love urchins. An otter eats about a quarter of its body weight a day, and in many areas urchins make up a large part of that. By keeping urchin numbers down and making the survivors hide, otters allow kelp to grow back. Scientists call this chain of effects, from predator to grazer to plant, a trophic cascade.

What happened when sea otters disappeared

Between 1741 and 1911 the fur trade wiped sea otters out of most of their range, from Japan to Mexico. Urchins were freed from their main predator in many places, and kelp forests were replaced by urchin barrens: seabeds covered in urchins and pink crusts of coralline algae, with little else.

The classic evidence came from the Aleutian Islands of Alaska. In 1974 the ecologists James Estes and John Palmisano compared Amchitka Island, where otters had recovered, with Shemya Island, where otters were still missing. Around Amchitka the shallow seabed was covered in dense kelp; around Shemya it was dominated by sea urchins, with very little kelp.

Nature then ran the experiment in reverse. In the 1990s, sea otters in the Aleutians collapsed, falling from an estimated 55,000–100,000 in the 1980s to about 6,000 by 2000. The leading explanation, though still debated, is increased hunting by orcas. As otters vanished, urchin numbers rose and kelp forests declined again.

Bringing sea otters back

Between 1965 and 1972, wildlife agencies moved hundreds of sea otters from Alaska to places where they had been wiped out, including Southeast Alaska, the west coast of Vancouver Island and the outer coast of Washington. About 400 otters released in Southeast Alaska grew into a population of many thousands, and 89 otters released in British Columbia started a population that still keeps expanding.

As the otters spread, kelp returned. On the coast of British Columbia, scientists have documented rocky reefs switching from urchin barrens to kelp forests after otters moved in, with knock-on effects for fish and other kelp-forest life. Similar changes have been seen around the Commander Islands in Russia and along California’s Big Sur coast.

There is a cost for some people. Sea otters also eat abalone, crabs, clams and sea urchins that humans harvest, and where otters return, these shellfish fisheries often shrink. Many coastal communities still argue about where sea otters should be allowed to spread.

Beyond kelp: seagrass and salt marshes

Sea otters shape other habitats too. In Elkhorn Slough, a California estuary, otters eat many crabs. With fewer crabs, more small grazing animals such as sea slugs survive, and they clean algae off eelgrass leaves, helping the seagrass meadows grow. A 2024 study in the same estuary found that, as otters returned and ate more burrowing shore crabs, the erosion of salt marsh banks slowed.

Because kelp forests store carbon as they grow, some researchers have also argued that sea otters help keep carbon locked up along the coasts they protect. That benefit is real but local, and it depends on the kelp staying healthy.

Is it always that simple?

Not quite. Kelp forests are shaped by more than otters. In southern California, kelp forests survived without sea otters for a century, partly because other predators such as California sheephead fish and spiny lobsters eat urchins, and because people harvest urchins. Storms, warm water and nutrients also affect kelp. Sea otters matter most on open, exposed coasts where they are the main urchin predator.

Northern California shows what can happen when several problems strike at once. Sea otters have been missing there since the fur trade. From 2013, a disease outbreak killed most of the sunflower sea stars, another key urchin predator; see What is sea star wasting disease? A marine heat wave followed, and purple urchin numbers exploded. More than 90% of the bull kelp canopy along roughly 350 km (220 mi) of coast in Sonoma and Mendocino counties was lost.

This has renewed interest in returning sea otters to northern California and Oregon. The U.S. Fish and Wildlife Service has studied whether a reintroduction would be feasible, weighing the benefits for kelp against the effects on fisheries. The idea shows how much a single small predator can matter to an entire coast.