3057. Could Restoring Large Mammals Help Combat Climate Change?

By Science, October 23, 2018




Restoring reindeer, rhinoceroses, and other large mammals could help protect grasslands, forests, and tundra from catastrophic wildfires and other threats associated with global warming, new studies suggest. The findings give advocates of so-called trophic rewilding—reintroducing lost species to reestablish healthy food webs—a new rationale for bringing back the big grazers.

Rewilding offers “solutions to some of the important problems arising from climate change,” says ecologist Jens-Christian Svenning of Aarhus University in Denmark, an editor of a special issue on the topic published this week in the Philosophical Transactions of the Royal Society B. “The scope for … beneficial spin-offs for human society is tremendous,” adds co-editor Elisabeth Bakker, an ecologist at the Netherlands Institute of Ecology in Wageningen.

Rewilding is often associated with an ambitious proposal to restore large mammals, including even ice age mammoths, to a huge park in Russia. But mammoth resurrection is still just a dream, and most rewilders are focused on restoring animals including giant tortoises, dam-building beavers, or herds of grazers.

Now, it seems rewilding could offer a climate bonus. As the planet has warmed, fire seasons have become 25% longer than they were 30 years ago, and more areas are experiencing severe blazes, notes ecologist Christopher Johnson of the University of Tasmania in Hobart, Australia. He and his colleagues combed the literature back to 1945 for data on habitats around the world that have lost or gained large grazers, to see whether they experienced change in fire frequency or intensity. They found studies of 14 ancient landscapes up to 43,000 years old that used fungal spores associated with dung as a proxy for herbivore abundance, charcoal as a proxy for fire frequency, and pollen to reveal past vegetation. In about half of the landscapes, fires increased and the vegetation changed after the herbivores disappeared, they report. The other studies saw no clear effect.

The researchers also examined records from three modern landscapes, including the 100-year-old Hluhluwe Imfolozi Park in South Africa. There, data show larger and more frequent fires occurred after managers culled or moved large grazers, including white rhinos, wildebeests, zebras, buffalos, and impalas. (Rangers moved the animals as part of tsetse fly eradication efforts or to reduce overgrazing.) In the case of white rhinos, fires averaged just 10 hectares when the animals were present—because they kept plants closely cropped, and their paths created fire breaks—but increased to an average of 500 hectares after the rhinos vanished.

Similarly, in Australian grasslands, Johnson’s team found that herds of feral swamp buffalos have helped control wildfires by similar means. When the researchers examined the herbivore and fire history of the southwestern United States, other grazers—including pronghorn antelopes, desert bighorn sheep, bison, and even domestic cattle—seemed to have helped starve fires by eating the grasses that serve as fuel. Modeling studies, meanwhile, suggest some grazers reduce fire risks by disturbing the soil, which buries leaf litter and other flammable material. The bottom line, Johnson and his colleagues write, is that “vertebrates can have strong effects on fire regimes,” and restoring large grazers could be a fairly effective control measure at a time when risks are growing.

Other studies suggest grazers can also help maintain tundra—the semifrozen, treeless ecosystem found in the Arctic and on high mountains. In the Arctic, rapidly warming temperatures are enabling trees and shrubs to invade the tundra. The woody plants amplify Arctic warming by absorbing heat and trapping a layer of snow that insulates the ground, keeping it warmer. The result is that the soil thaws and releases even more stored carbon and other warming gases.

Reintroducing large numbers of herbivores to browse on the shrubs could help stop this vicious cycle, write ecologists Johan Olofsson at Umeå University in Sweden and Eric Post at the University of California, Davis. The more species—muskoxen, moose,  bison, or caribou (also known as reindeer)—the better. But they say the biggest benefit might come from rethinking existing hunting and development rules to create the densest and most diverse herds possible. Rewilding might be “one of the few ways humans in the Arctic can mitigate global warming, or at least its consequences,” Olofsson says.

Others are skeptical. Like any ecosystem re-engineering effort, the long-term effects of rewilding are hard to anticipate, warns Joseph Bump, an ecologist at the University of Minnesota in St. Paul. Some modeling, for example, suggests increased Arctic grazing will lead to greater carbon release, not less. And creating Arctic herds big enough to make a difference could be difficult, says ecologist Andrew Tanentzap of the University of Cambridge in the United Kingdom. They could become “a drop in the bucket in the sea of melting permafrost.”

Even the strongest rewilding advocates concede its limits. “It would be overly optimistic to claim rewilding as ‘the solution’ to climate change,” Svenning says. “But [it] clearly can play a role.”