We Can’t Save the Rainforest Without Rainforest Animal Poop

Large, intact forests are crucial for stabilizing the climate. And the fruit-eating, seed-dispersing animals that live there are crucial too.

An agouti chewing food.
An agouti in Panama.
Brian Gatwicke/Wikimedia Commons

In January and February of this year, the Amazon lost 70 percent more ground than it did in the first two months of 2019. Under Brazilian President Jair Bolsonaro, laxer environmental regulations that benefit ranchers and prospectors have allowed the forest to disappear at ever-increasing rates.

The shrinking of the Amazon translates into more than vanishing habitats and the destruction of natural beauty. The planet’s climate patterns depend on large, intact stretches of forest, which affect global water and carbon cycles. The Amazon, for example, “may hold about 80 billion tons of carbon above ground,” Thomas Lovejoy, a world-renowned conservation biologist, professor at Yale University, and senior fellow at the United Nations Foundation, wrote via email. “It would be disastrous for climate if released to the atmosphere.”

At first, the loss of trees is just that, but eventually deforestation can hit a tipping point, Lovejoy said, where there are no longer enough trees to produce fruit to sustain native animals, or enough animals left to eat it. Self-regeneration will grind to a tragic halt. If that nightmare scenario were to happen in the Amazon, for example, the moist, weather-creating forest from Brazil to Peru could give way to dry grasslands, and the effects would be far from localized. Lovejoy said: “There would be a tremendous loss of carbon, huge loss of biodiversity unique to those regions, and a major impact on the quality of life of people living there. But it will also … impact all the countries which benefit from the hydrological cycle.”

Lovejoy thinks we should be “balancing each new increment of deforestation with at least three times as much reforestation (because early stages of forest recovery contribute little moisture to the hydrological cycle in comparison to the way mature forest does).” Like many conservationists, he sees reforestation as a crucial component of preventing a disastrous outcome.

Reforestation is generally done with seeds and clippings stored long-term in facilities like seed banks and botanical gardens, which presently number 350 in 74 countries. The Global Strategy for Plant Conservation (a program overseen by the international Convention on Biological Diversity) calls for the collection of at least 75 percent of all threatened plant species in seed banks and botanical gardens. Scientists argue that the collection and banking of seeds “has a major role to play” in ecosystem restoration. At some (hopeful) future date, the thinking goes, someone can plant those seeds and let nature take it from there.

But there’s at least one big catch with this plan as far as the tropics are concerned: The only seeds suitable for seed banks are ones that go dormant, and more than 36 percent of critically endangered plant species have seeds that do not go through the dormant phase. They stay metabolically active, ready to germinate quickly before they get chewed up or decomposed. Scientists can sometimes preserve such species by cryopreservation of plant tissue, but it is an extremely expensive and labor-intensive last-ditch effort. This is why forest regeneration through natural means—in situ—is best.

Sarah Wyse, a postdoctoral fellow at Lincoln University in Canterbury, New Zealand, argued for in-forest conservation in a 2018 study. “These are highly complex, species-rich ecosystems,” she wrote via email. It’s no easy feat to march in, identify key species, and find their seeds. Even when they are found, “preserving intact ecosystems [where they are located] protects the entire ecosystem (all flora and fauna), not to mention carbon sequestration, etc.” She says we should look at seed-bank intervention only as a backup—in case scientists later decide to restore an area.

This means that the best seed bank for more than one-third of Amazon plants is, essentially, the Amazon. But while these tropical seeds can’t survive a stay in a seed bank and do well left to their usual conditions, they do fare best when they’ve been moved away from the parent tree before they sprout. (Plant disease is a common threat in the tropics, and the nearer young trees of the same species are to one another, the greater the chance of contagion.) Many tropical trees spread their seeds with the help of fruit-eating birds, mammals, and even fish. The animal eats the attractive, sweet fruit, digests the pulp, and excretes at least a few of the seeds intact—at a useful distance from the fruit’s origin—into a handy heap of fertilizer.

Zoochory—the dispersal of seeds by animals—is a major ecological function. The production of colorful, fragrant, sweet fruits, and animals’ ability to find and eat them, evolved as a mutualism, a long-established give-and-take arrangement between two or more species. In this case, an animal gets food and a plant’s future offspring get a ride to their new homes.

The loss of animal species—such as through overhunting—has far-reaching effects on the biosphere. According to Jedediah Brodie, an ecologist at the University of Montana, “Without fruit-eating mammals and birds, we can lose some of the biggest and most iconic tree species in the tropical rainforests.”

Conservation’s tendency to focus on large, charismatic mammals can be controversial—after all, less-attractive animals also count—but in this case megafauna may be key. Larger trees tend to put out larger seeds; larger seeds (and therefore the fruits that enclose them) require larger mouths to swallow them without damaging seed coats. So larger mammals, such as tapirs in South America and elephants in Asia and Africa, cannot be replaced, not by human seed bankers, nor even whole squadrons of smaller fruit eaters like monkeys or midsize rodents like agoutis. And the value of size doesn’t only apply to mammals. Aquatic biologists in the Amazon have found that commercial overfishing in the river affects seed dispersal performed by fish: Humans favor bigger individuals and species when they hunt, leaving smaller species—who tend to damage seeds more with their teeth—to eat fallen fruit.

Also at play is a lesser-known mechanism, diploendozoochory, the transportation of seeds inside two animals at once. Sometimes a predator will consume another animal with seeds already in the prey’s gut. This seed “double-bagging” provides an important function for the plant in question, since jaguars and other large predators roam far in their forest ranges. Having eaten an agouti, say, the cat will deposit the seeds that are sitting in the agouti’s stomach much farther away than the seeds would have gone on rodent power alone. Thus, forest conservation efforts must target the tree, the agouti, and the jaguar.

Forests are complex systems, so simple measures like collecting and planting seeds will not be enough to save them. We need a multitiered approach. Other measures to protect tropical forests include: the implementation of a carbon tax on tropical deforestation, pouring funds into seed banks that specialize in recalcitrant seeds, training new teams of taxonomists to classify or verify species, and cracking down on the trade in exotic woods, charismatic plant species (like orchids), and wildlife meant for pet and traditional medicine industries.

The dire state of tropical ecosystems, including rainforests, has set off a kind of slow-motion panic hoarding, by way of stocking seed banks and botanical gardens. By and large, this is prudent: Many species can be stored in some way, while trees and other plants fall to the chain saw. At the same time, relying on seed banks, even as they fill with desirable specimens, would be too reductive an approach for an enormous ecological, political, and social problem. Keeping as many components of a vibrant ecosystem as possible—from small and large animal seed dispersers to the trees themselves—will increase the odds that rainforests will continue to exist all over the tropical world. Our own backup or remedial methods will always lag behind nature’s already magnificent powers of regeneration, so policies that embrace both are most likely to succeed.

Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.