When trying to regrow a tropical forest filled with the soaring canopies of massive trees, the little things matter. Such as opossums.
The critical role of creatures such as opossums and monkeys in re-seeding logged land was a surprise discovery for scientists recently studying the rebirth of a Panamanian rainforest. Their research highlights the underappreciated importance of these furry mammals, as well as birds and bats, in restoring the world’s vanishing tropical forests. It also points to how seemingly unrelated polices such as hunting regulations might influence these success of these efforts.
“Animals are our greatest allies in reforestation,” said Daisy Dent, a tropical ecologist from Germany’s Max Planck Institute of Animal Behavior, who helped lead the research. “Our study prompts a rethink of reforestation efforts to be about more than just establishing plant communities.”
It has long been known that animals play an important role ferrying plant seeds from one place to another. Acorn-carrying mice can help oak trees in New England keep pace with a changing climate. Fruit-eating bats and birds deposit seeds in mountainous Philippine rainforests. But the constellation of critters that live in an around logged areas usually isn’t factored into predictions of how reforestation will proceed.
To fill that gap, Dent and colleagues turned their attention to Panama’s Barro Colorado National Monument. Composed of an island and five nearby peninsulas created by flooding for the Panama Canal, the area has become a giant laboratory hailed by the Smithsonian as “the most intensively studied tropical forest in the world.”
In this case, scientists tapped into 28 years of records tracking the revival of forests that had been logged at different points in time. The oldest forest dated back 100 years, another 70, then 40 and 20 years old, as well as an unscathed old growth forest.
In each forest, researchers had counted every tree growing within an area roughly half the size of a World Cup soccer field in 1994 , 2001 and 2011. The scientists then assigned each tree to the kinds of animals known to move that species’ seeds from place to place. For example, the seeds of the toquilla palm, Carludovica palmata, are dispersed by both bats and non-flying mammals. All told, the scientists found as many as 319 species of slender-stemmed plants and 227 tree species in a given year.
This combination of plant species and animal seed-carriers enabled the scientists to see which creatures played the biggest roles, and whether that changed as forests aged.
The results held some surprises. For starters, flightless mammals such as those opossums, played the leading role regardless of how old the forests were, dispersing seeds for more than 75% of all tree species, the scientists reported on November 14 in the Royal Society’s Philosophical Transactions B.
That differs from previous studies, where such animals were found in low numbers in regenerating forests due to habitat loss and hunting. By contrast, hunting is forbidden in the national monument and surrounding forests are relatively intact.
“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,” said Dent.
By contrast, bats, which often get a high ranking in studies of tropical seed spreading, had a relatively small role, with fewer than 20% of the trees in all forests except the 100-year old stand. Smaller birds played a bigger role in younger forests but were replaced in older forests by larger birds such as toucans, many of which feed on fruit.
The study also showed that many seeds had more than one type of animal acting as a delivery service. That underscored the benefits of biodiversity, in which some animals can play redundant roles. For instance, birds might be able to move seeds that would otherwise be carried by ground-dwelling mammals.
While not all forest restoration projects enjoy the same restrictions on hunting or nearby logging, the new research shows that plans to regrow forests could benefit from knowing what animals are living there. And, perhaps, find ways to tap into the feeding habits of creatures such as opossums and toucans.
“We hope this information can help practitioners to structure their restoration practices by enabling frugivorous (fruit eating) species to help the restoration process and speed up forest recovery,” said Sergio Estrada-Villegas, a biologist now at Universidad del Rosario (Bogotá, Colombia) and the study’s first author.
Estrada-Villegas, et. al. “Animal seed dispersal recovery during passive restoration in a forested landscape.” Philosophical Transactions of the Royal Society B. Nov, 14, 2022.
Photo: Christian Ziegler, Max Planck Institute of Animal Behavior