The African forest elephant (Loxodonta cyclotis), an endangered species, is often referred to as a "gardener" in recognition of its role as a disperser of fruit seeds from the large diversity of trees it feeds on as it moves through the continent's tropical forests, contributing to the germination of over 100 species that supply food or shelter to primates, birds, and insects.
However, its importance is even greater than that, according to a new international study. With contributions from Brazilian researchers affiliated with the University of Campinas (UNICAMP) and the Agricultural Informatics unit of the Brazilian Agricultural Research Corporation (EMBRAPA), the study concluded that in addition to its seed dispersal role, the African forest elephant changes the structure of its forest habitat and helps increase carbon storage.
The elephants browse in, push over or scratch themselves against trees located on the forest trails they use for foraging. The chronic thinning of small trees by elephants alleviates competition for water, light and space in the low canopy strata, allowing the surviving trees to attain large diameters and a high wood density.
This increase in biomass entails a corresponding increase in carbon storage. Elephants change the structure of African tropical forests, influencing their composition in terms of tree species and increasing the aboveground biomass over the long term, according to the study.
"We found that at a typical density of 0.5 to 1 animals per square kilometer, elephant disturbances increase aboveground biomass by 26 to 60 tons per hectare," Simone Aparecida Vieira, one of the authors of the study, told.
Vieira is a researcher at UNICAMP's Environmental Research Center (NEPAM). She was a member of the organizing committee for the São Paulo School of Advanced Science on Scenarios and Modeling in Biodiversity and Ecosystem Services to Support Human Well-Being, held on July 1-14, 2019, in São Pedro, São Paulo State (Brazil). The event was supported by FAPESP via its São Paulo School of Advanced Science (SPSAS) program and was attended by 87 students from 20 countries.
The other Brazilian participants in the study were Marcos Longo and Marcos Augusto da Silva Scaranello. Longo is currently a postdoctoral fellow at NASA's Jet Propulsion Laboratory (JPL) in the United States. Previously, he conducted postdoctoral research at EMBRAPA Agricultural Informatics with a scholarship from FAPESP. Scaranello, currently a postdoctoral researcher at EMBRAPA, earned his PhD from UNICAMP, also with a scholarship from FAPESP.
According to the authors of the study, although large herbivores such as elephants are known to have profound effects on ecosystems and biogeochemical cycles by consuming biomass, transporting nutrients and affecting plant mortality, the influence of Africa's forest elephants on carbon stocks and on forest structure and productivity, in general, is poorly understood.
"The tropical forests of central Africa hold larger carbon stocks than the Amazon rainforest despite their similarities in climate and soil," Vieira said.
Central African forests have a lower average stem density, larger tree diameters and higher mean aboveground biomass than the Amazon rainforest. "The presence of elephants in central African forests helps explain how these differences emerged over long periods," she added.
The researchers tested this hypothesis using Ecosystem Demography 2 (ED2) software, which tracks down at a fine-scale the dynamics of a tropical lowland forest function and structure. The model simulates the horizontal and vertical heterogeneity of vegetation over long-term forest succession; competition among plants for resources, leading to mortality; and stochastic disturbance events, such as the presence of elephants, that influence forest structure over the short, medium and long term.
The simulations were compared with inventory data for two sites in the Congo Basin. Elephants still live in one but have been wiped out in the other.
In the simulations, the introduction of elephants caused a temporary reduction in aboveground mass lasting 125-250 years due to the increased mortality of small trees. This transient reduction was then reversed, and a new long-term equilibrium was reached between 250 and 1,000 years after elephants were introduced.
"The results support the hypothesis that elephants may have shaped the structure of Africa's tropical forests and probably played a key role in differentiating them from the tropical forests of Amazonia," Vieira said.
Extirpation of elephants
The researchers also simulated the effects of the extirpation of elephants on the aboveground biomass in the entirety of central Africa's forests, totaling some 2.2 million square kilometers. The simulation estimated a loss of approximately 7% in biomass and up to 3 billion tons of carbon if the population of African forest elephants was to disappear completely.
Conservation of the elephants could reverse this trend, representing carbon storage savings estimated by the authors of US$43 billion.
"Our simulations suggest that if elephant loss continues unabated, central African forests may release the equivalent of multiple years of fossil fuel CO2 emissions from most countries, thus potentially accelerating climate change. Therefore, their loss could have a drastic impact both locally and on global climate", said Christopher Doughty of Northern Arizona University (USA) and last author of the study. Doughty is quoted in a press release issued by France's Climate and Environmental Sciences Laboratory (LSCE-CEA), where lead author Fabio Berzaghi is a researcher.
Forest elephant populations have been in decline since the colonization of West Africa by Europeans owing to unsustainable hunting for ivory and habitat destruction and have collapsed to less than 10% of their original size.
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