Extinction of large animals could make climate change worse

Black-fronted Piping Guan is a large fruit-eating and dispersing bird in the curassow family, which lives in, and helps perpetuate, the unique Atlantic forests of Brazil. Photo: Pedro Jordan.o
Black-fronted Piping Guan is a large fruit-eating and dispersing bird in the curassow family, which lives in, and helps perpetuate, the unique Atlantic forests of Brazil. Photo: Pedro Jordan.o
The extinction of large animals from tropical forests could make climate change worse, according to researchers at the University of East Anglia in Norfolk.

New research published today in the journal Science Advances reveals that a decline in fruit-eating animals such as large primates, tapirs and toucans could have a knock-on effect for tree species. This is because large animals often disperse the seeds of the large-seeded plant species often associated with big trees and high wood density; these habitats are more effective at capturing and storing carbon dioxide from the atmosphere than smaller trees.

Tree dispersal by large-bodied vertebrates happens via their ingestion of viable seeds that pass through the digestive tract intact. Removing large animals from the ecosystem upsets the natural balance and leads to a loss of heavy-wooded large trees, which means that less CO2 can be locked away.

Co-author Prof Carlos Peres from UEA’s School of Environmental Sciences said: “Large birds and mammals provide almost all the dispersal services for large-seeded plants. Several large vertebrates are threatened by hunting, illegal trade and habitat loss. [However], the steep decline of the megafauna in overhunted tropical forest ecosystems can bring about large unforeseen impacts.

“We show that the decline and extinction of large animals over time induces a decline in large hardwood trees. This in turn negatively affects the capacity of tropical forests to store carbon and therefore their potential to counter climate change.”

The research team studied data from more than 2,000 tree species in Brazil’s Atlantic Forest, and more than 800 animal species. They found that frugivores which are not targeted by hunters – such as small birds, bats and marsupials – are only able to disperse small seeds, which are associated with small trees. Meanwhile large heavy-wooded trees, which can capture and store greater amounts of carbon, are associated with larger seeds and these are only dispersed by large animals.

A simplified graphic of the carbon cycle in a tropical forest. Image: Mauro Galetti.

Co-author Prof Mauro Galetti from São Paulo State University, Brazil, said: "The big frugivores, such as large primates, tapirs and toucans ... are the only ones able to effectively disperse plants that have large seeds. Usually, the trees that have large seeds are also big trees with dense wood that store more carbon.

São Paulo State University PhD student Carolina Bello added: "When we lose large frugivores, we are losing the dispersal and recruitment functions of large-seeded trees and therefore, the composition of tropical forests changes. The result is a forest dominated by smaller trees with [less dense] woods which [store] less carbon.”

Prof Peres added: “Policies to reduce carbon emissions from tropical countries have primarily focused on deforestation, and to a lesser extent on forest degradation resulting from timber extraction and wildfires. But our research shows that a decline in large vertebrate populations and the loss of key ecological interactions also poses a serious risk for the maintenance of tropical forest carbon storage.

“We hope that our findings will encourage UN programmes on Reducing Emissions from Deforestation and Forest Degradation (REDD+) to consider faunally intact forests and their full functionality as a critical precondition of maintaining forest carbon stocks.”

Bello, C, Galetti, M, Pizo, M A, Magnago, L F, Rocha, M, Lima, R, Peres, C A, Ovaskainen, O T, and Jordano, P. 2015. Defaunation affects carbon storage in tropical forests. Science Advances in press.
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