To Slow Climate Change, Save the Toucans and Tapirs

It’s well accepted that conserving forests is crucial to averting the worst impacts of climate change, because forests absorb heat-trapping carbon dioxide from the atmosphere and lock that carbon up in plant matter and soil. Globally, deforestation is second only to burning fossil fuels in generating the carbon emissions that are driving global warming.

Now, researchers have shown that to keep these essential carbon storehouses working, it’s crucial to conserve the large forest animals that consume the fruits of large, carbon-rich trees and spread their seeds, according to a study published Friday in the journal Science Advances.

Coming just days after the Paris agreement to cut carbon emissions by in part curbing deforestation, the finding has enormous implications for both forest and wildlife conservation efforts worldwide.

A team of scientists from Brazil, the United Kingdom, Finland, and Spain collected data in the field on more than 2,000 tree species and more than 800 wildlife species in South America’s Atlantic Forest, a biodiversity hot spot.

They analyzed the data to better understand wildlife-tree interactions and also modeled different scenarios for how the forest’s tree populations would change based on the abundance or scarcity of large fruit eaters.

Forests with a diverse mix of seed-and-fruit-eating animals store the most carbon, researchers found, because only larger animals can disperse the seeds from larger, more carbon-rich hardwood trees. If those animals vanish, softwoods with less carbon-storage capacity eventually take over the landscape. (Infographic: Carolina Bello/São Paulo State University)

“The Atlantic Forest used to be the second-largest rainforest in South America, after the Amazon,” said study coauthor Mauro Galetti, a biologist with the Universidade Estadual Paulista in São Paulo. “It used to cover from north of Brazil to Argentina and Paraguay, a huge area. But today only 12 percent of this forest is left, and most of it is fragmented forest,” lost to logging, agricultural development, fires, and other impacts.

The team found that when the populations of large fruit-eating animals such as spider monkeys, tapirs, and toucans declined, the density of the largest, heavy-wooded, most carbon-rich trees, such as mahogany, diminished as well. The forest’s capacity to store carbon fell as much as 20 percent, which effectively caused an increase in greenhouse gas emissions.

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“Most of the hardwood trees produce large fruits, which are eaten exclusively by large animals that are highly threatened everywhere—by poaching, by forest fragmentation, by other kinds of disease or fire,” said Galetti. “These trees are different from the other trees because they absorb more carbon than the other trees.”

When large fruit-eating animals become rare or disappear, these tree species cannot thrive and are gradually replaced with other species. “But it will be by softwoods, which store less carbon,” Galetti said, and whose small seeds are dispersed by smaller animals that are less threatened by overhunting.

The research applies to tropical forests in other parts of the world. “In Africa, there are several plants that rely on elephants for dispersal,” Galetti said. “Because elephants are being poached for the ivory trade, [the plants] are becoming ‘orphaned.’ They produce the fruit, but nothing comes along to eat the fruit. And if you don’t remove the pulp, the seeds don’t germinate.

“There’s a very intimate relationship between the plant and the animal,” he added.

For forests to thrive—and absorb the most carbon possible—wildlife officials must particularly prioritize the conservation of large animals, Galetti and his coauthors concluded.

“Most plans talk about forests and trees as entities without interaction,” Galetti said. “But the trees need the animals. We just had the Paris talks about climate change, and there was no mention about the role of animals in avoiding deforestation.”