Sea otter wrapped in kelp

Sea otter wrapped in kelp (Photo credit: Linda Tanner)

Sea otters are considered a keystone species because of the crucial ecological role they play in maintaining the health and stability of the nearshore marine ecosystem. Without sea otters, sea urchins and other herbivorous invertebrates are left unchecked to graze through swathes of giant kelp forests, creating barren stretches of coastal habitat behind them that once served as nurseries for fish, seals and hordes of other sea life. Now, a new study published in the journal Frontiers in Ecology and the Environment demonstrates that the effects of otters stretch beyond the habitat that kelp forests provide, even affecting the amount of carbon in our atmosphere.

Despite their short lives by plant standards, kelps siphon huge amounts of carbon from the atmosphere—what scientists call “primary productivity.” They take carbon dioxide from the air and turn it into sugar fuel with the help of sunlight and water, releasing oxygen back into the air. “Their productivity is on par with a tropical rainforest in terms of the amount of carbon they turn over in a year,” says ecologist Chris Wilmers of the University of California, Santa Cruz, one of the study’s authors. Unhindered by a lack of sunlight, water or nutrients as many plants on land are, kelp “just produces like crazy,” says co-author and biologist Jim Estes.

That crazy production also makes coastal kelp forests excellent laboratories to study this type of predator-prey interaction because things happen much more quickly than they typically do on land. “Kelp can grow pretty big in a year, but for a tree, it might take 50 or 100 years to grow to maturity,” Estes explains. For example, a long-standing hypothesis in the ecology community says that, without wolves—keystone predators in the forests and plains of North America, much like otters along our coasts—trees will disappear. Unchecked by the predatory pressure that roving wolf packs apply, populations of herbivores such as deer and elk swell. As they overgraze grasslands, they also snip budding tree saplings, halting the natural reforestation process.

Scientists have long assumed that otters’ maintenance of kelp forests doesn’t make an appreciable dent in the amounts of carbon in Earth’s atmosphere. Although kelps are highly productive, they’re much less massive than the terrestrial trees, and therefore, they can’t hold onto as much of that carbon they suck from the atmosphere.

On the hunch that science had underestimated this food chain effect on carbon sequestration by kelp forests, Wilmers, Estes and their colleagues set out to figure out just how much carbon healthy kelp habitats could trap. To do that, they needed to compare areas with lots of otters to spots with just a few or none. Otters have been studied extensively for more than four decades along a stretch of their historical habitat on the west coast of North America, from the southern border of Canada up through Alaska’s Aleutian Islands, and there are portions of that 5400 km of coastal habitat that have otters and those that do not—a critical feature allowing the comparisons used in this study.

Kelp forest off the California coast

Kelp forest off the California coast

What they found was surprising. Based on their calculations, the presence of otters increased the carbon storage of kelp forests by 4.4 to 8.7 megatons—equivalent to the amount of carbon found in the annual carbon dioxide emissions from 3 to 6 million passenger cars. “I was pretty shocked,” Estes says. “I didn’t think it would be anywhere near that amount.” To further understand the economic impact of sea otters, the team calculated the value of this carbon on the European Carbon Exchange, which was set up to establish a ceiling for carbon emissions and require companies that breach the emission threshold to buy credits for the excess carbon they release. For the study area, the value of sea otters in maintaining carbon-storing kelp forests is anywhere from about $200 million to more than $400 million.

The degree of influence that otters have depends on where the carbon ultimately ends up. In general, there are two possibilities. The carbon is initially trapped in a one-off mechanism by the kelp in its tissues. This sequesters carbon for a time, but as pieces of kelp wash ashore and decompose, the carbon is released back into the atmosphere. The other possibility is that sloughed-off pieces of kelp may sink to the deep ocean where the trapped carbon can be stored for longer periods of time. This second prospect has the greatest potential to decrease atmospheric carbon levels, as carbon could remain trapped in plant tissues at the bottom of the sea for decades or centuries. Scientists know that some kelp does end up in the deep ocean; how much, though, is difficult to estimate. So the team looked at a range, from a conservative one percent up to 50 percent of shed kelp ending up in the depths.

An argument against encouraging the growth of otter populations has been that, as successful shellfishers themselves, otters diminish the economic return on shellfisheries. How to balance the interests of fishermen with the need to support the recovery of sea otters has been debated back and forth, often failing to take into account the full scope of the problem, Estes says. “One of my frustrations in the management community is that the argument is focused so much on shellfisheries, and it’s really a much bigger issue than that,” he adds. Monetizing the impact otters have on carbon sequestration reveals yet “another layer” of just how complex the issue is, and the study’s authors point out that the sale of carbon credits may provide an avenue for offsetting monetary losses to shellfisheries caused by otters.

After decades of sea otter research across much of the Western Pacific, Estes knows just how important otters are to the health of these coastal communities. “If you dive down in a reef habitat and you don’t see any herbivores around and there’s lots of kelp, you can be pretty sure there are otters around,” he says. It turns out that the health of our atmosphere may also depend on the survival of sea otters and other keystone predators.

Just another reason sea otters need to be protected… no sea otters, no kelp forests.


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