“Animals are our greatest allies in reforestation,” says Daisy Dent, a tropical ecologist who was the senior author of the study. “Our study prompts a rethink of reforestation efforts to be about more than just establishing plant communities.”

The scientists say that placing regenerating forests near patches of old growth while reducing hunting encourages animals to colonize these new habitats.

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

However, most forest restoration efforts continue to focus on increasing tree cover rather than reestablishing the animal-plant interactions that underpin ecosystem function, the scientists note.

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

To obtain that information, the scientists examined data on the forest at the Barro Colorado Nature Monument in the Panama Canal, one of the world’s best studied tropical forests where generations of scientists have been documenting frugivore interactions to understand which animals disperse which tree species.

Estrada-Villegas and Dent mined this long-term dataset to determine the proportion of plants dispersed by four groups of animals: flightless mammals, large birds, small birds, and bats. Their aim was to see how this proportion changed over a century of natural restoration.

This approach has yielded highly detailed data on 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 says.

Young regenerating forests, it was found, were made up mostly of trees dispersed by small birds. However, as the forest aged, the number of trees dispersed by larger birds increased.

In a surprising discovery the majority of plants were dispersed not by birds but by terrestrial mammals across all forest ages from 20 years old to old growth.

“This result is quite unusual for post-agricultural regenerating forests,” Dent observes. “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.”

The experts say this newly learned insight should inform forest restoration practices as we can co-opt frugivorous species to help the restoration process and speed up forest recovery.