Why Sea Otters Matter: Kelp Forests and Keystone Species

Sea otters are a keystone species because, by eating sea urchins, they keep urchins from mowing down kelp forests, and those forests shelter a huge variety of ocean life. A small number of otters can change a whole coastline. Below we walk through the otter-urchin-kelp chain, what happens when otters vanish, the surprising seagrass story from Elkhorn Slough, and the real conflicts otters create for shellfish harvesters.

What is a keystone species?

The name comes from the wedge-shaped stone at the top of an arch: pull it out and the whole thing wobbles. In ecology, the U.S. Fish and Wildlife Service explains, a keystone species affects its environment out of proportion to how many of them there are, so just a few individuals can have a big impact.

The agency calls the sea otter a textbook example. Monterey Bay Aquarium describes otters the same way and adds that their health is a window into the health of the ecosystems around them.

How do sea otters help kelp forests?

It comes down to a three-link food chain, often called a trophic cascade, where a predator at the top shapes what happens at the bottom.

1. Kelp grows in cool, shallow, nutrient-rich water near the shore. NOAA notes kelp can grow up to 18 inches a day in ideal conditions.
2. Sea urchins are voracious kelp grazers. Left unchecked, they can chew through a forest.
3. Sea otters eat urchins. By keeping urchin numbers in check, they give kelp the room to thrive.

The USFWS points to research going back to Estes and Palmisano in 1974 that documented this link. When otters are around, the cascade tips toward kelp.

What happens when otters disappear? Urchin barrens

When urchins overgraze and wipe out kelp in an area, the result is called an urchin barren. Monterey Bay Aquarium notes that without sea otters, urchin populations can leave exactly these barrens behind.

The story is not only about otters, and the USFWS is careful to say so. Off northern California and Oregon, sea stars kept urchins in check even without otters until sea star wasting disease began in 2013 and hit the sunflower star, a key urchin predator, especially hard. The agency cites research estimating that northern California has lost up to 90 percent of its kelp forests, with several factors pushing those reefs toward barrens. In southern Oregon, state biologists reported purple urchin densities rising well over 10,000 percent in just five years.

Why kelp forests matter to other ocean life

Kelp forests work a lot like forests on land, with a tall canopy that provides food and shelter. NOAA says thousands of species of fish, invertebrates and marine mammals depend on them, and that kelp forests hold a greater variety of plants and animals than almost any other ocean community.

• Nurseries: many animals tuck their young among the thick kelp blades to hide them from predators and storms.
• Feeding and shelter: seals, sea lions, whales, sea otters and seabirds such as gulls, terns, egrets, herons and cormorants all use kelp forests.
• Fish diversity: the USFWS cites research linking healthy kelp forests to greater species diversity, with rockfish as one example.

So when otters protect kelp, they are really protecting a whole neighborhood.

Do sea otters help fight climate change?

Indirectly, they may. The USFWS describes kelp forests as highly efficient at storing carbon, which can help ease ocean warming and acidification, and it backs that up with peer-reviewed work, including research by Wilmers and colleagues.

We are keeping this one qualitative on purpose. Carbon figures vary a lot between studies and places, and the sources we used don't give a single number we could stand behind. The fair summary: by protecting kelp, otters help protect a habitat that stores carbon.

The Elkhorn Slough seagrass surprise

Elkhorn Slough, a coastal estuary in central California, is home to more than 100 sea otters, according to NOAA. There is no towering kelp there. Instead, otters help eelgrass, an underwater seagrass, through a different chain reaction.

Monterey Bay Aquarium explains it like this: otters in the slough eat crabs. Fewer crabs means more sea slugs survive. Those sea slugs eat the algae that would otherwise smother eelgrass, so the eelgrass beds can thrive.

A 2013 study in the Proceedings of the National Academy of Sciences by Brent Hughes and colleagues, summarized by the U.S. Geological Survey, found that this four-level cascade helps eelgrass grow and expand even though the estuary receives heavy nutrient runoff from farms. The researchers combined a 50-year record with field and tank experiments, and concluded that recovering top predators like otters may be important for protecting seagrass. Many of those otters, by the way, descend from rescued pups raised by surrogate otter moms.

Why otter range recovery matters for kelp

If otters protect kelp only where they live, then where they live matters a lot. Monterey Bay Aquarium notes that the southern sea otter's historic range once stretched from Baja California up the coast to Russia and Japan, and today it covers only a small slice of that, roughly from Half Moon Bay to near Point Conception. The U.S. Fish and Wildlife Service puts it at about 13 percent of the historical range, including San Nicolas Island.

Spreading back out is not easy. The USFWS says shark bites at the edges of the range are blocking natural expansion, and Monterey Bay Aquarium reports that scientists believe population growth has leveled off, most likely because of that shark-bite risk. That is part of why the USFWS has been assessing whether reintroducing otters to more of their old coastline makes sense, weighing the possible benefits for kelp and seagrass against the costs.

The other side: otters and shellfish harvests

Otters are not only kelp heroes. They are also very good at eating the same shellfish people like to catch. The USFWS acknowledges that commercial fishers may worry about competing with otters for shellfish, and recreational harvesters may worry about their crab or clam catch.

In southern Southeast Alaska, where otters have been recolonizing, Alaska Sea Grant set up research on how otters affect four commercially important species: sea cucumbers, red sea urchins, Dungeness crab and geoduck clams, along with outreach in local communities.

The USFWS also lists potential benefits of otters, including ecosystem services, local wildlife tourism and cultural significance for coastal Native peoples. Both sides are real, which is why decisions about otters and fisheries involve weighing costs and benefits rather than picking a winner. For more on how otters are doing overall, see are sea otters endangered?

seaotter.cam is an independent fan site and is not affiliated with any aquarium, zoo, rescue group or agency mentioned here. Facts are drawn from the sources above; check them for the latest information.