In 1967, South Africa’s National Parks Board made a fateful decision: The elephant population in Kruger National Park, which had been rising steeply, should stay stable in order to preserve the other species living there. Each year, wildlife managers would choose a number of elephants to cull—usually somewhere from 350 to 500. The animals were shot, their carcasses necropsied, and their meat salted and dried for food.
After international uproar and a change in management practices that separated the park into different zones, Kruger stopped culling elephants in 1994. As a result, the park’s elephant numbers swelled from more than 7,800 to 12,500 in about a decade, and its landscape changed dramatically. More elephants dispersed seeds across the park, giving life to more types of plants. They used their tusks to dig for water in the dry season, creating water holes used by many species. And most of all, they knocked down trees, especially tall ones, to get access to their tasty roots and leaves. According to a 2016 study, parts of Kruger consequently stopped absorbing carbon from the atmosphere, and started producing excess carbon instead.
Although researchers disagree on the precise effect that elephants have on carbon storage, the study makes clear an important fact: Animals, especially huge ones, have the power to change how much carbon is moving in and out of an ecosystem, and not always for the better. That means that the growth and decline of animal populations have important implications for climate change. So, too, does human stewardship of these big animals, with their great carbon-moving power, many of which are the ones we love best.
The various elements that make up an ecosystem—plants, animals, soils, fungi, bodies of water, even rocks—store or release carbon depending on how the system is changing at any time. When plants grow, they suck in carbon dioxide and store it in their leaves and roots, and the soil they inhabit. When fires, animals, or deforestation kills plants, that carbon is usually released. Animals, including humans, are part of the carbon cycle too: We ingest, digest, and breathe carbon, and eventually return carbon to the soil when we die and decompose. Usually, we breathe out the same amount of carbon we breathe in, minus the amount that goes to build our body. But an animal’s activities, whether tumbling trees or burning fossil fuels, can start to upset the balance.
Historically, wild animals have been considered negligible to carbon calculations, especially compared with dramatic events such as fire and the overwhelming biomass of plants, says Andrew Davies, a biologist at Harvard. “But the thing about animals is, they move: They’re moving nutrients, and they’re moving seeds, and they’re knocking over trees and eating and going everywhere,” he told me.
In some places, big wild animals seem to be significant movers of carbon. In 2019, Davies co-published a study of Kruger Park that compared elephant densities with aboveground woody biomass, a good proxy for carbon storage in a particular ecosystem. He and his co-author found that male bull elephants were the biggest driver of changes in stored carbon. Giant ocean animals can have outsize effects on carbon storage as well. Whales feed at depths but poop and rest at the surface, where their waste stimulates the growth of carbon-storing phytoplankton. When they die, their massive carcasses fall to the ocean floor; the carbon contained in their bodies can be stored for decades. According to one recent study in Nature, if five whale species were returned close to their pre-whaling population levels, they could add 600,000 tons of CO2 storage to the oceans each year. That’s equivalent to the carbon stored in 3.6 million trees.
Wild fauna’s effects on the climate are generally harder to assess than humans’, or even livestock’s. We know that livestock are responsible for about 14.5 percent of all greenhouse-gas emissions, because their effects are usually concentrated in small geographic areas and on only a few plant species, and because they’re usually corralled, so they don’t transport nutrients and seeds across the landscape.
For wild animals, researchers often rely on natural experiments. The elephants of Kruger are one example. Another occurred 2,000 miles to the north in the 1960s, when a viral disease known as rinderpest was eradicated from the Serengeti Plain. As a result, the wildebeest population climbed from about 300,000 to 1.3 million. All of those extra wildebeest mouths ate up extra grass from the savanna, which constrained fires and increased tree cover—and caused the Serengeti to flip from emitting carbon to sequestering it, according to a 2009 study.
The results of these natural experiments aren’t always consistent. In a 2019 paper, a group of ecologists examined two forests in the Congo Basin rainforest: one with a population of forest elephants—a smaller, elusive cousin of the African-savanna elephant—and one where they had been almost completely wiped out by ivory poachers decades ago. They found that the forest without elephants had 7 percent less aboveground biomass—and stored less carbon. The authors wrote that the elephants tended to eat and trample smaller trees, and thus promoted the growth and survival of the larger trees that store more carbon.
These findings seem to fly in the face of the research on elephants in the savanna. One possible explanation, according to Fabio Berzaghi, a researcher at the World Maritime University who co-wrote the study on forest elephants, is that the mixed results are artifacts of ecosystems where too many elephants have been squished into too small a space. Another is simply that elephants have different effects on different parts of a given ecosystem. They probably contribute to carbon emissions by knocking down trees—something they do to showcase their strength, in addition to getting easier access to leaves and roots. But they also trample the ground with their serving-platter-size feet, which helps integrate leaf litter and biomass into the soil matrix, storing it as organic carbon. Elephants’ weighty steps also condense existing carbon, packing it into a sturdier form and allowing more carbon to be stored in an ecosystem. Their dung contributes to carbon storage. Some evidence also suggests that elephant feet help plants shed their roots more frequently, which means even more carbon stored in the soil.
Many estimates of carbon storage focus exclusively on trees—the biggest, most visible way that carbon is stored on land. “But that’s a bad assumption,” Carla Staver, an ecologist at Yale, told me. A lot of stored carbon—even the majority, in some places—can be underground in roots and soils. Measuring subsurface carbon can be difficult and costly. Partially as a result, researchers have yet to pin down whether elephants, or any other large herbivore, are, strictly speaking, eco-friendly.
As scientists learn more about the carbon footprints of various species, the question, of course, will be what to do with that knowledge. Some start-ups are trying to harness animals’ carbon-moving potential for good. One, a group called Rebalance Earth, is launching a pilot study in Liberia, trying to finance conservation by selling ecosystem tokens that represent the carbon captured by each elephant. Berzaghi, who also serves as the company’s lead science researcher, suspects it might work: His studies estimate that if the forest-elephant population were restored to its former size, they would sequester the same amount of carbon that 250,000 trees can capture.
Other experts are less optimistic about large animals’ potential as a climate solution. “People are so desperate for anything that we can do about climate change, and anything that doesn’t involve a lot of social-level hard work,” Staver said. But “most of these nature-based solutions just don’t work nearly as well as people want them to,” in part because they don’t scale. Robert Pringle, an ecologist at Princeton, told me that the results of carbon-storing-animal projects are likely to depend on the ecosystems and species involved. “When those win-wins are real, that’s awesome,” he said. But until the data are in, he cautioned, wishful thinking shouldn’t be allowed to drive carbon-policy decisions.
And carbon storage, in turn, shouldn’t be the only thing that drives animal-stewardship policies. Davies, at Harvard, told me that elephants’ tree-knocking is an ecological boon, because trees and shrubs intruding into savannas is a global problem for the diversity of plant vegetation. Preserving elephants could mean a healthier savanna, which could mean more room for biodiversity. “We shouldn’t sacrifice everything on the altar of carbon,” Davies said.
The more we understand about animals as carbon sources and sinks, the harder the decisions we will have to make about stewardship of these species. Even if elephants were an ecological disaster, humans wouldn’t be justified in simply eliminating them from their ecosystems. Elephants are long-lived, social, intelligent beings that have historically suffered at human hands. In their lives—and in the lives of whales, gorillas, and other charismatic megafauna—humans can see our own existence. All of that might mean that we owe them protection, regardless of their potentially weighty carbon footprint.