Sea Otters Have Helped Bolster California’s Kelp Forest

A study that looks back more than 100 years shows that where the animals have thrived, underwater forests have, too

Sea Otters Float Amid Kelp
Sea otters float amid a forest of kelp off California’s Central Coast. Mario Tama / Getty Images

Sea otters are not just cute and lovable—they also help a crucial habitat flourish.

These critters have a well-known influence on kelp forest ecosystems. In Central California, the mammals mostly eat sea urchins, which feed on kelp. If sea urchin populations grow too much, they eat kelp to the point of destroying underwater forests. But otters control the sea urchin population, allowing kelp to thrive.

Otters’ ecological impacts can all be tied back to their appetite, says Gena Bentall, a sea otter biologist and the director of the outreach organization Sea Otter Savvy. Otters have a fast metabolism, meaning they have to eat about a quarter of their body weight every day. “They have to eat so much that their foraging has a strong impact one way or another,” says Bentall.

Now, new research published this winter in PLOS Climate shows that otters have helped kelp forests survive environmental threats in Central California. Scientists at Monterey Bay Aquarium looked at a century of kelp maps and found the growth of sea otter populations over the last century improved vital habitat. The findings emphasize the role of sea otters in their ecosystems and support nature-based solutions for restoring kelp forests into the future.

Kelp forests are living structures, rapidly growing brown algae that provide a home for fish, invertebrates and marine mammals. Even if surrounded by strong currents, the forest provides a calmer environment, a refuge for these species. Kelp also buffers waves, absorbing power from storms and protecting the shore from erosion. And they are a great source of carbon storage: Some estimates suggest kelp and other macroalgae can sequester roughly 190 million tons of carbon dioxide per year.

But human activity on land directly impacts kelp forests off the coast, explains Kyle Van Houtan, lead researcher on the study. “And that can have an impact which cascades through the ecosystem,” says Van Houtan. “If you’re taking out a kelp forest, you’re taking out so much more than just kelp.”

Agricultural runoff, like pesticides and other pollutants, can end up in the ocean and damage kelp forests. Erosion has converted parts of the coastal ocean floor from a rocky to a sandy surface. That’s bad news for kelp, which needs to hold on to rock to form a strong forest base. Increasing ocean temperatures from climate change also threaten kelp, which prefers colder water.

To protect kelp forests and the ecosystems they support, scientists need to understand how they change over time. That’s why the research team looked at maps of kelp along the entire California coast over about 100 years.

Historical documents with detailed measurements of kelp date back to the early 1900s—at the time, government scientists tracked kelp because it was burned to make gunpowder during World War I.

Comparing these historic maps from 1910 through 1912 with satellite imagery from 1989 through 2016 created the most comprehensive map to date of changes to California kelp forests.

What the researchers found was surprising. From 1910 until recent years, kelp has experienced a roughly 7 percent decline along the entire California coast. But dividing the state up by region revealed an interesting story. While the Northern California coast saw a 63 percent decline and the Southern California coast saw a 52 percent decline in kelp, Central California actually saw a 58 percent increase in the seaweed.

The research team found the biggest factor that drove Central California’s kelp survival was the density of sea otters. California’s sea otter population today is mostly limited to the Central Coast as a result of the international fur trade. At the start of the research period in 1911, otter populations were at a low point, having been hunted almost to extinction for their fur. Yet a small, precarious population survived off California’s Central Coast. In 1913, California declared sea otters a fully protected mammal, and in 1977 they became protected under the Endangered Species Act, allowing populations to rebound.

“Across the last century we found a dramatic realignment, or shifting, of California’s kelp forests to the Central Coast,” says Teri Nicholson, a sea otter biologist at Monterey Bay Aquarium and another lead researcher on the study. “Which is the only place in the state where sea otter populations have survived. … So where sea otters are thriving, kelp forests in California are more resilient.”

The researchers created a statistical model to identify what changes in the environment explained kelp forest growth. Other main factors that drove kelp changes in the study were shifts in ocean floor substrate and periods of extreme heat. “There is more extreme heat in the ocean now than we’ve ever seen before,” says Van Houtan. “So how do we set these ecosystems up for success given the challenge of extreme heat?”

The new study shows that otters naturally promote kelp conservation.

“Over the last 100 years, where otters are present, in spite of dramatic warming that has occurred, in spite of rapid population growth, in spite of a lot of near-shore pollution … in spite of all those things, kelp has grown almost 60 percent in regions where otters are,” says Van Houtan.

Humans need to protect sea otters, which provide a service in kelp forests, says Bentall, who was not involved in the study. Direct human disturbance to sea otters—for example, getting close to an otter to take a picture—can be deadly for them. Close encounters leave otters stressed and exhausted, making them susceptible to health problems or predators.

Bentall explains that as the only marine mammal without a blubber layer, otters don’t store energy, so they’re living “paycheck to paycheck.” If an otter needs to suddenly dive away from a kayaker coming too close, that uses a lot of energy.

In places where these disturbances happen repeatedly, it creates a cumulative cost. Since otters don’t “budget” their energy for extra expenses, they become more vulnerable to poor body condition and illness.

But a lot of people just don’t know what sea otter disturbance looks like, explains Bentall. “Their one approach for a photo for Instagram could be one incident in death by a thousand kayaks,” she says. “They’re not going to be the first or the last person to approach the otter that day.”

Otters disturbed by humans a lot can also habituate, making them less cautious not just around humans, but also around other predators like sharks.

“The average person recreating in the sea otter’s home can just do one thing: give them space,” says Bentall. “That is a profound, immediate act of conservation that anybody can do.”

Sea Otter and Kayak
Kayakers should keep their distance from sea otters. Melina Mara / The Washington Post via Getty Images

On a larger scale, reintroducing otters to places where they formerly thrived, for example in Northern and Southern California and Oregon, could help kelp, making those coastal ecosystems more resilient to marine heat waves and other threats. The U.S. Fish and Wildlife Service has conducted a feasibility assessment reviewing possible impacts of reintroduction, but there is not currently a plan to move forward.

Otters don’t just have a dramatic positive impact on coastal habitats; they also create economic benefits. California sea otters generate money and jobs through recreation and tourism. Some skeptics of reintroduction question whether otters could threaten Dungeness crab fisheries, one of the most economically valuable in California, but research has found their populations do not harm these fisheries.

Important cultural reasons also support reintroducing sea otters to their historic range. Northern California’s Kashia Band of Pomo Indians have backed reintroduction, with tribal leaders speaking at a sea otter summit about otter and human coexistence for thousands of years before the fur trade. The Confederated Tribes of Siletz Indians in Oregon has also shown support for reintroduction, in the hope of restoring this historical relationship with sea otters.

Nicholson says that the study provides support for the recovery of otters throughout their historical range. “It also provides strong evidence that we should adopt policies that tap into the immense value of protecting and conserving wildlife along our coastline,” she says. “Because a healthy ocean with a diversity of life can be one of our best defenses against climate change.”

Get the latest Science stories in your inbox.