Environment

Want to slow global warming? Save a sea otter. So says Chris Wilmers at the University of California, Santa Cruz, whose team has calculated that the animals remove at least 0.18 kilograms of carbon from the atmosphere for every square metre of occupied coastal waters.

That means that if sea otters were restored to healthy populations along the coasts of North America they could collectively lock up a mammoth 10¹⁰ kg of carbon – currently worth more than $700 million on the European carbon-trading market. Wilmers explained this at the annual meeting of the Society for Conservation Biology in Edmonton, Canada, this month.

The figures are part of a growing realisation that predators play a crucial ecological role, promoting the growth of vegetation by controlling herbivore populations. Just as wolves benefit trees and shrubs by killing deer, sea otters allow the luxuriant growth of kelp by consuming sea urchins.

In former kelp forests that have lost their otters, Wilmers says, "all you are left with is piles of urchins and very little else".

Sink sizing

To estimate the minimum size of the carbon sink that could be provided by sea otters in North America, Wilmers and his colleagues determined the total available habitat for kelp – rocky reefs in up to 20 metres of water – and summed the amount of carbon that would be locked up in kelp either if no otters were present, or if the animals were present throughout in populations sufficient to control sea-urchin numbers.

The true size of the sink is likely to be larger than the calculated 10¹⁰ kg, Wilmers suggests, as some carbon drawn from the atmosphere by kelp forests may find its way into the deep ocean and be sequestered for long periods.

The exact size of historical sea otter populations, before they were nearly wiped out by hunting for fur in the 18^th and 19^th centuries, is uncertain. But after bouncing back from the brink, the animals are now in decline once again in parts of their range. In Alaska, for example, populations have dropped from up to 125,000 in the 1970s to around 70,000 today – possibly due to a rise in killer whale populations.

The new calculations provide an added incentive to protect sea otters, but do terrestrial predators provide a comparable carbon sink? No calculations have yet been done, but Wilmers believes the numbers could be similarly impressive. As a result of the loss of wolves across most of their former habitat, he points out, deer populations in parts of North America are currently around five times as high as historical levels, dramatically changing vegetation.

Dense woods

Predators are not the only large animals that help create carbon sinks, suggests Jedediah Brodie at the University of Montana in Missoula. In October 2009 in Science (vol 326, p 364), he argued that hunting in forests for bushmeat removes fruit-eating animals, reducing the numbers of trees that rely on them to disperse their seeds. Because these trees tend to have denser wood than those with smaller seeds that rely on wind for dispersal, the net result will be a decrease in the amount of carbon stored by a forest.

Some ecologists remain to be convinced by his argument (Science, vol 327, p 30), but Brodie is now crunching the numbers using measurements of the growth of saplings in two Peruvian forests – one heavily hunted and one with an intact fauna – plus data on the density of wood from different species.

Merely protecting a forest from logging isn't enough to ensure that it functions properly as a carbon sink, Brodie says. "You've also got to be concerned about the large animals."

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