Study Details How Animals Are Key To Restoring the World’s Forests

December 2, 2022
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Aerial view of lush trees and a body of water

An aerial view of regenerating secondary tropical forest in the Barro Colorado Nature Monument, Panama. Credit: Christian Ziegler, Max Planck Institute of Animal Behavior

The world faces a climate crisis paired with a record loss of biodiversity in every ecosystem. Increasingly, attention turns to forest restoration as a solution to these twin calamities. Forests soak up atmospheric carbon dioxide and create habitat for organisms. Scientists interested in helping forests bounce back from deforestation typically focus on one thing—planting trees. But a new study at the Smithsonian Tropical Research Institute (STRI) underscores a powerful, yet largely overlooked, driver of forest recovery: the animals. 

Led by an international team from the Max Planck Institute of Animal Behavior, STRI, the Yale School of the Environment and the New York Botanical Garden examined a series of regenerating forests in central Panama 20 to 100 years after they were abandoned. Their unique, long-term data set revealed that by carrying a wide variety of seeds into deforested areas, animals are key to the recovery of tree species’ richness and abundance to old-growth levels after only 40–70 years of regrowth. The article, published in Philosophical Transactions of the Royal Society B, is part of an issue focused on forest landscape restoration as part of the UN Decade on Ecosystem Restoration.

“Animals are our greatest allies in reforestation,” said Daisy Dent, tropical ecologist from the Max Planck Institute of Animal Behavior, research associate at STRI and the study’s senior author. “Our study prompts a rethink of reforestation efforts to be about more than just establishing plant communities.”

The report also notes that situating regenerating forests near patches of old growth and reducing hunting encourage animals to colonize and establish. 

“We show that considering the wider ecosystem, as well as features of the landscape, improves restoration efforts,” said Sergio Estrada-Villegas, a biologist now at Universidad del Rosario in Bogotá, Colombia, and the study’s first author.

Animals that eat fruit and drop their seeds elsewhere are key to forest expansion. In the tropics, over 80% of tree species can be dispersed by animals. Despite this, forest restoration efforts continue to focus on increasing tree cover rather than reestablishing the animal-plant interactions that underpin ecosystem function. 

“Figuring out how animals contribute to reforestation is prohibitively hard because you need detailed information about which animals eat which plants,” Estrada-Villegas said.

Data collected from the forest at Barro Colorado Island in the Panama Canal offers a unique solution to this problem. In one of the best-studied tropical forests in the world, generations of scientists at have documented plant-animal interactions to understand which groups of animals disperse which tree species. 

In the current study, the team led by Estrada-Villegas and Dent examined this unique long-term dataset to determine the proportion of plants dispersed by four groups of animals—flightless mammals, large birds, small birds and bats—and how this proportion changed over a century of natural restoration.

Their results offer the most detailed data of animal seed dispersal recovery across the longest timeframe of natural restoration. 
“Most studies examine the first 30 years of succession, but our data spanning 100 years gives us a rare glimpse into what happens in the late phase of restoration,” Dent said.

The study found that young regenerating forests were made up mostly of trees dispersed by small birds. But as the forest aged, trees dispersed by larger birds increased. Surprisingly, however, across all forest ages—from 20 years old to old growth—most plants were dispersed by terrestrial mammals. 

“This result is quite unusual for post-agricultural regenerating forests,” Dent said. “It is likely that the presence of large tracts of preserved forests near our secondary stands, coupled with low hunting, has allowed the mammal populations to thrive and to bring an influx of seeds from neighboring patches.”

“We hope this information helps practitioners to structure their restoration practices by enabling animals that disperse seeds to help the restoration process and speed up forest recovery,” Estrada-Villegas said.

STRI, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems. Watch our promotional video and visit us on our website, Facebook, Twitter and Instagram for updates.

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Adapted from original release from the Max Planck Institute of Animal Behavior

Reference: Estrada-Villegas, S, Stevenson, P.R., López, O. et al. Animal seed dispersal recovery during passive restoration in a forested landscape. Phil. Trans. R. Soc. B. 2022.

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