Although we’re wiping them out with little consideration of it, big herbivores matter. Hippo poop helps Africa’s largest lake flourish; bison wallowing can enhance insect diversity in the prairie. And Africa’s imperiled forest elephants might just have a role to play in the fight against climate change.
That, at least, is the takeaway of a new modeling study published Monday in Nature Geoscience, which looks at the role that forest elephants—officially a subspecies of African elephant, but with distinctive physical characteristics and habitat compared with their savannah-dwelling cousins—play in shaping the rainforests of the Congo Basin. The research suggests the recent and precipitous decline in forest elephant numbers might have already caused the world’s second largest rainforest to lose billions of tons of sequestered carbon.
“[C]entral African forests have on average higher carbon stocks than Amazonian forests, and one big difference between the two continents is the presence of large herbivores, such as elephants, in Africa,” Fabio Berzaghi, a researcher at the Laboratory of Climate and Environment Sciences in France, told Earther in an email.
Berzaghi and his colleagues wondered if the larger amounts of carbon stored in African rainforests compared with their Amazonian counterparts had something to do with those elephants, which snack on small trees and help to thin out the understory. And so, the researchers used ecosystem models to see what a forest would look like given different levels of elephant grazing. They then compared their results with real-world field data from two lowland rainforest sites in the Congo, one that sees elephant grazing and another that doesn’t.
The models showed that grazing by elephants “transforms the structure of tropical forests.” By weeding out some of those younger trees, elephants seem to create a forest with fewer, bigger trees—and up to 25 tons more tree “biomass” per acre of land. Extrapolating their results to the entire 2.2 million square kilometer (850,000 square mile) central African forest, the models showed 7 percent more biomass, or nearly 3 billion tons of additional sequestered carbon, when elephants are present versus when they’re not.
Unfortunately, that carbon might already be lost, because it assumes levels of elephant grazing the forests saw in the 19th century, before humans began systematically exterminating these animals. It’s estimated there were once a million elephants spread across the rainforests of central Africa; by 2011 the population had dropped to about 100,000, thanks largely to poaching fueled by the ivory trade.
“We were not able to estimate how much of the 7 percent of carbon has already been lost or how long it will take to lose it completely,” Berzaghi said.
But Marjin Bauters, an ecologist at Ghent University who studies nutrient cycling in the Congo Basin, cautioned against taking these estimated impacts too definitively. “They hypothesis of megafauna [like elephants] having such a big effect on forest structure... is a hypothesis which has been standing for a long time,” Bauters told Earther in a voice message. “Experimentally, it’s very hard to validate, or try to check a hypothesis like this.”
Indeed, the study itself relied on just two field sites to check its model results, and while the field data supported what the models showed—fewer and larger trees in forests that elephants had grazed—more data would certainly help bolster the significance of that relationship across the entire Congo Basin. Monitoring these gentle giants and doing field work in central Africa is complicated and difficult, Berzaghi said, adding that if the study generates enough interest he’s hopeful he can team up with other researchers to collect more field data. And as Bauters indicated, real-world experiments in addition to models would be ideal to confirm that elephants are actually causing the forest changes researchers see.
Data limitation aside, Bauters felt that the authors were “very brave” for trying to disentangle the role of elephant grazing and something as complex as the carbon cycle. Berzaghi added that the study didn’t look at how forest elephants’ well-documented role as seed dispersers and sources of fertilizer—elephants eat a lot of fruit and poop a lot of seeds and nutrients—could affect forest carbon storage, so there’s clearly plenty more research to be done.
At the same time, there’s now at least one data point suggesting an additional reason to protect the remaining forest elephants.
“These results add more evidence of the fundamental ecological role of forest elephants in African rainforests,” Berzaghi said. “Now forest elephants also have a connection with climate, you could say that they help us fight climate change, which is a complex problem that requires a portfolio of solutions. This is one of those solutions, which is free and would benefit also biodiversity, it seems a win-win opportunity to me.”