- Large animals, especially herbivores such as elephants, are often seen as being destructive of vegetation, so are not thought of as a nature-based climate solution. Scientists are proving otherwise.
- By removing living and dead plants, large animals dispose of material that may fuel wildfires, which can add large amounts of carbon to the atmosphere; by consuming vegetation and excreting dung, large animals may improve the availability of nutrients to plants and support the storage of carbon in vegetation and soil.
- By creating gaps in the vegetation and dispersing seeds, large animals create diverse ecosystems with plenty of opportunities for a variety of plants to grow, making ecosystems more resilient and better able to deal with climate change.
- By nibbling down polar region shrubs and trampling snow, large animals help maintain permafrost, helping prevent the release of carbon to the atmosphere.
In the battle to combat climate change, there is no single silver bullet, though we will clearly need nature’s help to reverse Earth’s human-induced overdose of atmospheric greenhouse gases. Trees are often used to exemplify the most promising organisms for this work, as they can absorb lots of CO2 and store carbon in their woody tissues for decades, or even centuries.
In contrast, large herbivores like elephants — ambling about, stripping branches, chomping on tree seedlings or even uprooting full-grown trees — appear to be the enemy.
But in a 2022 paper published in the journal Current Biology, a team of ecologists begs to differ.
The total amount of carbon stored in all the bodies of all the mammals and birds on Earth is negligible, they admit, totaling only about 9 million metric tons, the carbon storage equivalent of just eight hours of human fossil fuel emissions. But, the researchers note, the big wild herbivores roaming natural areas have often been shown to do things that may reduce global warming, through a variety of mechanisms.
“They help prevent fires, decrease the amount of solar heat absorbed by the Earth’s surface, and contribute a lot to the long-term storage of carbon in soil,” explains University of Oxford ecosystem scientist Yadvinder Malhi, the paper’s lead author. That means elephants, wildebeests and other big plant eaters may be helping, not hindering, our carbon storage efforts.
Helping move carbon underground
At first glance, forest trees look like a robust place to store carbon. But as heat waves and droughts become more common, that vegetation is increasingly vulnerable to fires that quickly release plant-sequestered carbon back into the atmosphere.
Large herbivores can blunt that danger by suppressing fires. “By consuming plant material that might otherwise serve as fuel, large animals make fires less likely and less severe, even in places like savannas where fire occurs naturally,” says ecologist and study co-author Jens-Christian Svenning of Denmark’s Aarhus University.
In addition, much of the carbon stored in vegetation eaten by large wild herbivores is quickly returned to the soil as dung, which will soon decay. Stored safely below ground, that carbon stays put even in the face of fires that sweep the vegetation away.
“In addition, this carbon may increase the productivity of the soil, allowing many new plants to grow and absorb more CO2,” notes Svenning. “Recent research also suggests that grazing [by wild herbivores] may stimulate the release of substances from roots that stabilize soil carbon.”
This process can be observed in East Africa’s Serengeti. Scientists calculated that when wildebeest populations bounced back there after a 1960s virus epidemic, the avoided emissions from fire, along with extra carbon stored underground, allowed the vast grassland to absorb more carbon than it emitted — enough, in fact, to just about equal East Africa’s annual carbon emissions from the use of fossil fuels.
Unfortunately, duplicating this Serengeti carbon storage success story across Africa may prove challenging, as many areas have lost their large wild animals. They’re estimated to have declined by 59% in protected areas across Africa between 1970 and 2005, and are unlikely to have recovered much since. Most African savanna elephant populations, for example, currently exist at estimated densities of less than a quarter of what their ecosystems could sustain. The situation in Asian grasslands, where epidemics have decimated hundreds of thousands of saiga antelopes, is likely more dire.
Not all big mammals are equal when it comes to carbon storage or emissions, however. That’s because the way large herbivores digest their food matters. The replacement of large wild savanna animals like elephants, rhinos and zebras (all hindgut fermenters with a simple stomach) with ruminant livestock (especially cattle, which emit much more methane from their multichambered stomachs) may have more than doubled methane emissions in sub-Saharan Africa from around 3.4 million to 8.9 million metric tons annually. Over the short term, methane is a much more powerful greenhouse gas than CO2; in the first 20 years after being emitted, it traps about 80 times more heat as CO2. (Over 100 years, it’s about 25 times as much.)
This fact alone provides a strong argument for a vigorous program to restore large wild herbivore populations to their ecosystems.
In the tropics, large browsing animals, such as elephants, may knock over trees and create clearings, making them appear destructive of forests in the short term. But those same herbivores eat fruit from large trees, and leave the seeds behind in rich mounds of dung, allowing new seedlings to grow in the openings.
“Many trees in tropical forests are adapted for animal seed dispersal,” notes Svenning. “There are species of trees that totally depend on rhinos, elephants or tapirs [for seed dispersal]. But large birds and monkeys, often lost due to overhunting, are also very important.”
Large animals can also help trees “migrate” — spreading seeds to places where a species didn’t exist before, he adds, but where conditions may be better for plant growth as the Earth gets warmer. In a recent study in Science, Svenning and colleagues estimated that dwindling numbers of wild, fruit-eating animals have globally reduced the ability of plants to move along with their climatic comfort zone by around 60%.
Even where large animals are just disturbing the vegetation by eating or uprooting plants, says Svenning, they can still help make ecosystems more resilient in the face of climate change. Their browsing, grazing, trampling and defecation can help a variety of plant species find their niche. This can turn monotonous vegetation patches into biodiverse landscapes, increasing the chance that at least some will thrive in a changing climate.
Bring back the herds, change the albedo
Temperate and tropical forests and grasslands aren’t the only examples of ecosystems in which carbon sequestration may be aided by large plant eaters.
A new study in Science reports that in drylands, where the evaporation of water tends to exceed rainfall, areas with a greater diversity of plant-eating mammals tend to store more carbon in the soil. When grazing pressure is high, areas with more grazer diversity also tend to have more aboveground plant biomass. At low grazing pressure, the quality of the forage is often higher.
In the Arctic, large herbivores help control woody plants, preventing them from steadily spreading northward as regional temperatures increase, helping to curb climate change.
This is because shrubs and trees are darker and absorb more solar heat than the snow-covered tundra, which reflects more sunlight back to space, Malhi explained. As the Arctic warms, vegetation expands its range and increases temperatures more, causing more permafrost to melt, and more stored carbon to be released to the atmosphere, further increasing warming. It’s estimated that the extinction of large plant-eating mammals, such as mammoths, may have increased temperatures in Siberia and Beringia by up to 1° Celsius (1.8° Fahrenheit).
Where native or reintroduced large browsing animals, like muskoxen, live today, they keep the shrubs clipped low, reducing solar energy absorption.
In addition, Malhi says, these animals have been shown to protect permafrost by compacting the snow with their hooves, reducing its insulating effect and increasing the likelihood that soil temperatures will remain below zero. This is important since the frozen soil contains plenty of carbon-rich material that, if it were to melt and decompose, would emit vast amounts of greenhouse gases.
In 2019, Malhi and colleagues sought to calculate the cost of significantly increasing bison, horse and reindeer populations in northern Siberia to study how it might increase carbon storage. But first COVID-19, then the Russian war in Ukraine, got in the way. “We had a project funded to do the science,” Malhi relates. But “we paused it because of the situation there. If and when the political circumstances are appropriate, we’ll try and get some of these measurements.”
Future science: Linking biodiversity and climate change
Malhi and Svenning argue persuasively that protecting large herbivores and other wild animals will be paramount to maintaining the healthy ecosystems we depend on to assuage the current biodiversity and climate crises. And certainly, it looks as if humanity’s efforts to deal with the first are likely to benefit the second as well, says Malhi.
“With just a few percent of the budget currently available for climate mitigation and adaptation, we could expand existing conservation efforts, and also restore large animals where they have disappeared,” Malhi argues.
Soil scientist Pete Smith of the University of Aberdeen in Scotland, who was not involved in any of the previously cited studies but co-authored a recent study showing that increased biodiversity could provide climate benefits, agrees that the two crises should be tackled together, and that large wild animals have an important role to play. “Reintroducing large animals might be an option in some areas, but surely preserving the remaining wild animals we still have should be the first priority. It is much better to prevent the destruction of vital habitats than to destroy them and then attempt to recreate them,” Smith said.
Importantly, says Malhi, nature-based climate solutions such as these can only be effective if they take local people into account. “It’s only a nature-based solution if it brings biodiversity benefits and also engages local communities and safeguards community rights. In the past, conservation has involved land grabs, it has involved people being excluded from their traditional areas … [T]he main thing overall is having community buy-in.”
Svenning agrees: “In the history of conservation, many poor people and minorities have been pushed away. If we implement nature-based solutions, we have to make sure we don’t do it in unfair ways. I totally believe that is possible.” Svenning’s experiences with sustainable development among the Maasai in Kenya convinced him that ecosystem protection needn’t remove people from the land, but should be “about finding ways where people can coexist with wildlife.”
Some of these conservation conflicts may eventually resolve themselves, Svenning believes: “One of the major trends in the world is urbanization, with many people voluntarily moving to cities,” which leaves more room for restoration. Similarly, he adds, “if we switch to more sustainable food production, and the consumption of many more plant-based foods, that will provide a lot of space for conservation and restoration as well.”