- The dietary habits of African forest elephants (Loxodonta cyclotis) promote the survival of large, high wood density trees better at storing carbon.
- New research finds that forest-dwelling elephants browse on trees with low wood density, making space for bigger, heftier carbon-rich trees.
- The elephants also act as seed dispersers for these larger trees.
- Cautioning that determining exactly what role local elephant extinctions have played in changing forest composition is tricky, the researchers argue that elephants may boost above-ground carbon storage in Central African forests by 6-9%.
Previous studies have shown that Africa’s tropical forests store more carbon per hectare than Amazonian forests. But how they do so is still a puzzle. It turns out that elephants play an outsized role in the upkeep of healthy forests. New research shows that without these giant gardeners, forests in West and Central Africa would be poorer in biodiversity and carbon.
“We talk a lot about trees and soil, but a functioning ecosystem and its services also depend on animals,” said Fabio Berzaghi, lead author of a study published in the journal Proceedings of the National Academy of Sciences. The presence of large herbivores, particularly elephants, could be making a difference, the new paper suggests.
Forest elephants are smaller than their savanna counterparts but can still weigh anywhere between 2,000 and 6,000 kilograms (4,400 and 13,200 pounds). They have cavernous appetites; an adult can eat 100-200 kgs (220-440 lbs) of food daily, and there are about 350 plant species on their jumbo menu.
These pachyderms live in tropical forests of West and Central Africa, with their last strongholds in Gabon and the Republic of Congo. In wider conservation circles, forest elephants were recognized as a distinct species from their savanna-dwelling cousins (Loxodonta africana) only in 2021. While both kinds of elephants are endangered, forest elephant numbers have fallen by more than 60% in the last decade, mainly due to poaching and habitat destruction. Today, they inhabit only about a quarter of their historical range.
“Forest elephants are dramatically understudied compared to other megaherbivores, and this study contributes to a growing understanding of their enormous impacts on forest ecosystems,” John Poulsen, an ecologist at Duke University, who was not involved with the study, said.
Berzaghi and his colleagues analyzed elephant browsing habits at the Nouabalé-Ndoki National Park in the Republic of Congo, combining their findings with data from seven other sites in Africa. Across their range, forest-dwelling elephants browse on trees with low wood density and munch on fruits of trees that are high in wood density, the researchers found.
As they move through forests, foraging on leaves, fruit and tree bark, elephant herds allow larger trees to outcompete other species. Thinner, shorter trees are within reach for elephants, and many grow readily digestible leaves. Elephant foraging grounds are thus more abundant in heftier, taller trees that store more carbon.
The research team also probed elephant dung heaps collected over three years at Nouabalé-Ndoki for clues. The dung pile searches uncovered another reason elephants are important for carbon sequestration. Their large size and affinity for fruits mean they can disperse the seeds of some of the grandest trees.
The Mukulungu (Autranella congolensis) is a critically endangered tree species in the Congo rainforest. It can rise to 50 meters (164 feet) in height with a trunk that can run 2 meters (6.5 feet) across. Scientists believe elephants are the main dispersers of this tree’s seeds, which they ingest intact and later poop out in a heap of nutrient-rich dung.
“This study does a tremendous amount of work collating tree traits like nutritional value of leaves and wood densities,” Poulsen said, adding that the researchers were able to relate these tree traits to data on elephant browsing and seed dispersal from several sites in West and Central Africa.
Elephant dung could also be acting as plant fertilizer, promoting germination and plant growth, Berzaghi said, but to what extent this contributes to carbon storage is as yet unknown.
Losing elephants would mean losing more than these majestic mammals; it would transform the forest structure. If forest elephants disappeared, low wood density tree species would proliferate to the detriment of bigger species. The study authors forecast that this could translate into a 6-9% loss in above-ground carbon storage.
Pinning down or explaining fluctuations in carbon stores is a tricky business. The estimate for how carbon stocks respond to disappearing elephants assumes that elephant-dispersed trees would disappear entirely along with the elephants.
Scientists can theoretically measure how carbon stocks have changed in areas where forest elephants no longer live. However, it is much harder to assess how much of this loss in carbon stores is due to local elephant extinctions. Some factors that drive out elephants, like logging and forest conversion, also deplete carbon stocks directly.
At LuiKotale in the Democratic Republic of the Congo, a team of researchers showed that more than three-quarters of the tree species dependent on elephants for dispersal failed to recruit enough young trees in their absence. Since changes in forest structure occur over generations, the researchers could only provide estimates for how much the carbon storage was likely to change as a result.
Despite this, the researchers argue that if African forest elephants reclaim their historical range, their contribution to reducing atmospheric carbon will be significant. “Protection of forest elephants, including in logging concessions and other exploited forests, is a critically important wildlife-driven mitigation response to climate change,” the study authors write.
Poulsen at Duke University who has investigated the effects of loss of elephants on Central African forests said that the data collected by Berzaghi’s team would be “critical” to gauging the full impact of elephants on forest species composition and carbon stocks.
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Banner image: Forest-dwelling elephants browse on trees with low wood density, making space for bigger, heftier carbon-rich trees. Image by João Aguiar via Pexels (Public domain).
Citations:
Cuni-Sanchez, A., Sullivan, M. J., Platts, P. J., Lewis, S. L., Marchant, R., Imani, G., … Zibera, E. (2021). High above-ground carbon stock of African tropical montane forests. Nature, 596(7873), 536-542. doi:10.1038/s41586-021-03728-4
Berzaghi, F., Bretagnolle, F., Durand-Bessart, C., & Blake, S. (2023). Megaherbivores modify forest structure and increase carbon stocks through multiple pathways. Proceedings of the National Academy of Sciences, 120(5). doi:10.1073/pnas.2201832120
Beaune, D., Fruth, B., Bollache, L., Hohmann, G., & Bretagnolle, F. (2013). Doom of the elephant-dependent trees in a Congo tropical forest. Forest Ecology and Management, 295, 109-117. doi:10.1016/j.foreco.2012.12.041