How sea otters can reduce CO₂ in the atmosphere: Appetite for sea urchins allows kelp to thrive

The theory is outlined in a paper released online September 7, 2012  in Frontiers in Ecology and the Environment by lead authors UC Santa Cruz professors Chris Wilmers and James Estes.

"It is significant because it shows that animals can have a big influence on the carbon cycle," said Wilmers, assistant professor of environmental studies.

Wilmers, Estes, a professor of ecology and evolutionary biology, and their co-authors, combined 40 years of data on otters and kelp bloom from Vancouver Island to the western edge of Alaska's Aleutian Islands. They found that otters "undoubtedly have a strong influence" on the cycle of CO₂ storage.

Comparing kelp density with otters and kelp density without otters, they found that "sea otters have a positive indirect effect on kelp biomass by preying on sea urchins, a kelp grazer." When otters are around, sea urchins hide in crevices and eat kelp scraps. With no otters around, sea urchins graze voraciously on living kelp.

Kelp is particularly efficient at sequestering CO₂ from the atmosphere through photosynthesis. CO₂ concentration in the atmosphere has increased 40 percent since the beginning of the industrial revolution, causing global temperatures to rise, the authors write.

Wilmers and Estes acknowledge that a spreading otter population won't solve the problem of higher CO₂ in the atmosphere but argue that the restoration and protection of otters is an example how managing animal populations can affect ecosystems abilities to sequester carbon.

"Right now, all the climate change models and proposed methods of sequestering carbon ignore animals," Wilmers said. "But animals the world over, working in different ways to influence the carbon cycle, might actually have a large impact.

"If ecologists can get a better handle on what these impacts are, there might be opportunities for win-win conservation scenarios, whereby animal species are protected or enhanced, and carbon gets sequestered," he said.

Mitigating increased CO₂ in the atmosphere is a pressing issue in global environmental conservation with many obstacles and no easy solutions, the authors write. They note that markets have been established in Europe and the United States to trade carbon credits and thus inject an economic incentive into either reducing CO₂ output or increasing CO₂ sequestration.

They estimate that the CO₂ removed from the atmosphere via the otter-kelp link could be worth between $205 million and $408 million on the European Carbon Exchange. "An alluring idea," they write, would be to sell the carbon indirectly sequestered by the sea otter protected kelp forest "as a way to pay for their reintroduction and management or to compensate losses to shell fisheries from sea otter predation."