Can an abundance of sea otters help reverse a principal cause of global warming?



A new study by two UC Santa Cruz researchers suggest that a thriving sea otter population that keeps sea urchins in check will in turn allow kelp forests to prosper. The spreading kelp can absorb as much as 12 times the amount of CO2 from the atmosphere than if it were subject to ravenous sea urchins, the study finds.



The theory is outlined in a paper released online today (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, associate 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."



The paper, "Do trophic cascades affect the storage of flux of atmospheric carbon? An analysis of sea otters and kelp forests," will be published in the October issue of Frontiers in Ecology and the Environment. Wilmers and Estes were joined by Matthew Edwards, San Diego State University; Kristin L. Laidre, University of Washington; and Brenda Konar, University of Alaska.



Funding for the research came from the National Science Foundation and the National Oceanic and Atmospheric Administration.



