KUSP Reports: Environment

Otters’ Effect on Kelp Offers Clues to Predators’ Link to CO2

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This otter, by controlling the urchin population, may be reducing atmospheric carbon dioxide. Source NOAA

By Melissae Fellet

At Steamer Lane, a popular surf spot in Santa Cruz, people lean against a metal railing, peering over the cliffs to watch surfers catch waves. You need binoculars to see the other aquatic attraction there: Sea otters rest in a kelp forest nearby.

“This is a real good feeding area for them,” says Dave Francis, a former Monterey Bay Marine Sanctuary volunteer from Santa Cruz. “It’s a fairly large kelp bed, fairly long, so they can move around and not deplete the food source.”

Otters eat sea urchins that would otherwise eat kelp. So kelp forests grow larger when otters are around. A 75 pound sea otter — that’s a little larger than an average male — can eat about 1500 urchins in one day. Any remaining urchins hide in the rocks and munch on kelp scraps that fall to the bottom.

Urchin barrens

The environment changes without otters living in an area. “If you get rid of sea otters, the sea urchins will come of the rocks,” says Chris Wilmers , a wildlife ecologist at the University of California in Santa Cruz. “They’ll increase in number and they’ll graze down the kelp until there’s almost none left.”

That creates areas called urchin barrens. For about 40 years, researchers at UC Santa Cruz have worked along the Pacific coast of North America studying this relationship between otters, urchins and kelp. Wilmers and his colleagues took a second look at some of that information to see if they could connect the relationship between predators, prey and plants to the amount of carbon dioxide in the atmosphere.

“For a long time, animals haven’t really been considered in how carbon cycles through ecosystems,” Wilmers says. “What this indicates is animals can actually have a big effect.”

The researchers compared the amount of kelp growing in areas where otters were present to the amount growing where otters were absent. They also calculated how much atmospheric carbon dioxide the kelp plants converted into plant matter as they grew. The result: Sea otters reduced carbon dioxide by six to eleven percent in the portion of the atmosphere over the kelp forests.

“I was actually quite stunned when all these numbers came together and so I think it’s a big deal,” says James Estes, a biologist at UCSC who pioneered the otter, urchin and kelp studies. These new results are published in the journal Frontiers in Ecology and the Environment.

Studying the link between predators, plants and atmospheric carbon

This removal of atmospheric carbon dioxide was evident in the region they studied, from the US-Canadian border to the tip of the Aleutian Islands. Sea otters don’t live in enough places around the world for their effect on kelp forests to influence global climate change, Wilmers says. The real question then, he adds, is whether this kind of activity is happening in other ecosystems.

If so, the collective presence of predators around the world might influence global atmospheric carbon dioxide concentrations. Scientists know that predators increase plant growth in ecosystems around the world. But in some ecosystems, the connections between predators, prey and plants actually decrease plant growth, which increases carbon dioxide in the atmosphere over those areas. That makes it hard to predict how animal predators around the world could influence global atmospheric carbon dioxide concentrations, Estes says.

“I think what we’ve seen in the otter system is that there’s reason to believe that this is going to have a pretty significant effect on the carbon cycle in one way or another,” he adds.

Recognizing how otters influence the carbon cycle gives them another role in nature beyond being a cute animal that people emotionally connect with or being a hungry animal that fisherman compete with for catch, Estes says. “They have an impact that very much relates to things that people are worried about on a global scale, that is global warming and atmospheric carbon.”

Otters’ effect on the atmosphere should also be considered when trying to figure out how best to manage otter populations, he says.

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