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Save the planet and you will save its creatures. That point seems obvious. It turns out, however, that the reverse may be true, too.
A growing body of research suggests that the decline of many of Earth's largest and most majestic animals—such as elephants, wolves and whales—could actually speed global warming because of the underappreciated role they play in mopping up carbon dioxide emissions.
Plants and microbes have gotten most of the attention for their ability to store carbon. But a small group of scientists is showing that some animals can influence the types—and numbers—of plants that populate their environments.
With more than half of the world's largest land animals already either threatened or endangered, the goal is to highlight a hidden climate benefit of species conservation before it's too late.
"What we're finding is, when we start looking, that animals have an incredible potential to shape how much carbon is taken up by ecosystems," said Oswald J. Schmitz, a professor of ecology at the Yale School of Forestry and Environmental Studies.
In an opinion article to be published soon in Ecosphere, Schmitz and lead author Christopher C. Wilmers look at gray wolves in North America. They found that a healthy population of wolves may enhance boreal forests' ability to soak up carbon, while in the grasslands of Yellowstone National Park, an abundance of wolves may limit that ability.
Schmitz, Wilmers and several colleagues published the first major synthesis of work on the topic in 2014. They searched for studies in which scientists were able to quantify an animal's effect on the carbon cycle over decades, and where the animal was removed or reintroduced. The result was an analysis of six ecosystems, including the African savanna, where the wildebeest, a wide-ranging herbivore that weighs up to 550 pounds, roams the grasslands in vast herds.
After disease and poaching decimated the wildebeest's numbers in East Africa early in the 20th century, dead grass and other vegetation began accumulating across the savanna. The detritus ignited in wildfires, emitting 5 tons of carbon dioxide per year, as much as 1 million cars. When conservation programs helped restore the herbivore to its historical numbers in the 1960s, the wildfires abated. More trees grew, and the savanna began sucking up nearly 1 million tons of carbon annually.
"Once the wildebeest recovered, you can compensate for all the CO2 emissions of East Africa," Schmitz said.
Another study they reviewed looked at the sea otter, that cuddly master of the backfloat. Fur traders nearly eliminated the carnivore centuries ago, but its return along the Pacific coast helped reveal that their link in the food chain played a significant role in pulling carbon out of the air.
"Sea otters have a very strong effect on the abundance of kelp through the influence they have on sea urchins," said James A. Estes, a professor of ecology and evolutionary biology at the University of California, Santa Cruz who was a co-author on Schmitz's 2014 review and on the original sea otter study. Otters eat urchins. Urchins eat kelp. So in ocean patches where otters are abundant, urchin populations plummet and kelp tends to flourish.
The study, led by Wilmers and published in 2012, found that across the otter's range the animal could be indirectly responsible for sequestering as much as 8.7 million tons of carbon dioxide, more than twice what an average coal-fired power plant emits in a year.
Based on the 2012 price in Europe's carbon market, the otters' appetites were worth $200 to $400 million, the study said.
"It's an emerging, important area that the science hasn't addressed thoroughly enough," said Gregory P. Asner, an ecologist at the Carnegie Institution for Science and who studies land use and climate change. "I think the unknowns are the magnitude of the effects of different animal species."
Untangling these relationships is a huge challenge, but Schmitz is careful to note that he'd never advocate tinkering too much with nature.
"You would never go to the Serengeti and annihilate lions and leopards so you can protect wildebeest," he said. "The message is let's restore biodiversity where it has dropped off, because then we will restore some of these natural ecological functions that have great benefits."
Estes agrees, adding that while conservation won't be a panacea for the climate crisis, it could be a piece of a solution.
"How important this can be to climate change or to attenuating the slow in the increase of carbon dioxide? I don't know," he said. "If our sea otter work is representative, it's a big deal."