If the world is going to meet its goal of limiting planetary warming to less than 2 degrees Celsius above pre-industrial levels, we’ll have to burn dramatically fewer fossil fuels — but that’s not enough. We’ll also need nature on our side, and that means dramatically halting the rate of deforestation so that tropical forests around the world stop losing their carbon to the atmosphere (currently contributing between 7 to 17 percent of global emissions) and start pulling it in again.

A new study in Science Advances, though, suggests that that already-daunting task may be even tougher than expected — because humans are thwarting what’s called forests’ “carbon storage” in far more ways than merely by chopping down trees to provide land for agriculture and other uses. By hunting and poaching many large forest animals whose diets depend on eating fruit (scientists call them “frugivores”), the research finds, humans are also undermining carbon storage and thus worsening the climate problem.

“Defaunation of large frugivores, which limits the recruitment of large-seeded species and induces compositional changes, can alter the community-aggregated values of wood density and height and eventually result in a markedly limited carbon storage capacity,” notes the study by Carolina Bello, an ecologist at the Universidade Estadual Paulista in Sao Paulo, Brazil, and a list of colleagues from a number of other universities in Brazil as well as the UK, Spain and Finland.

These large forest creatures, such as tapirs, spider monkeys, toucans and agoutis (a large rodent), spread seeds that are critical to the growth of large, hardwood trees. But precisely because they are large and eat fruits that contain bigger seeds, they are also more attractive targets for those seeking trophies or simply bushmeat.

In other words, it’s not just deforestation — it’s also defaunation that’s at the heart of the tropical forest carbon problem.

The study was based on an ecosystem model that, itself, drew upon extensive studies of the Atlantic Forest of Brazil, the second largest rainforest in South America and a region that has seen even worse deforestation than the Amazon.

“When you land in Rio de Janeiro, for instance, you are landing in the Atlantic forest,” says Mauro Galetti, one of the study’s authors and also based at the Universidade Estadual Paulista. “But only 12 percent of this forest is left, it was destroyed in the last 500 years.”

The model suggested that smaller fruit eating species, like bats, birds and small marsupials, tend not to be hunted as much and also tend to disperse smaller seeds — which grow into smaller trees. But larger fruit eating species consume fruits with the larger seeds that, when dispersed, lead to the growth of huge hardwood trees that store the most carbon as they grow.

And it’s the loss of these species — the ones targeted by hunters — that in turn stunts large tree growth and carbon sequestration.

“There’s a direct link between the animals, the trees, and the carbon storage,” says Galetti.

The results should be applicable not just to the Atlantic forest but to the Amazon, Galetti said, as it also features this strong relationship between large fruit-eating vertebrate animals and the tallest trees. Indeed, he thinks the principle also would apply to tropical forests in Africa, where the large frugivores threatened by humans include gorillas, chimps and elephants.

“Halting the ongoing, fast-paced defaunation of tropical forests will not only save large charismatic animals and the plants they disperse but also have effects on climate change, carbon markets, and reforestation processes,” the authors conclude.

“While I am the first to say that carbon is actually the lowest value of a tropical forest and that valuing a forest for its carbon is like valuing a computer chip for its silicon, this demonstrates clearly that loss of large fruit eating species selectively reduces the presence of large trees and their important carbon reservoirs,” says Thomas Lovejoy, an ecologist at George Mason University who has conducted extensive research on the Amazon rainforest. Lovejoy was not involved in the research.

For Galetti, the results suggest that society needs to broaden the discussion about what needs to be done to combat deforestation and its contribution to climate change.

“People don’t discuss this very much in all the roundtables and climate change discussions,” he says. “They always talk about trees, and carbon, and climate, but what keeps these trees alive, or keeps these forests functioning, is the animals.”