‘Negative feedback’ boosts diversity of tropical forests

In this photo of the tropical rain forest canopy in Panama, Handroanthus guayacan, the focus of the new study, blooms in yellow while Jacaranda copaia has blue flowers and Cavanillesia plantanifolia has pink fruit. Taking advantage of regular annual changes, like flowering and fruiting, allowed ecologist Jim Kellner to track individual trees through time and map distributions of some species throughout a large area. (Credit: Jonathan Dandois and Helene Muller-Landau/Smithsonian Tropical Research Institute)

The population of a common tropical tree species increases mainly where it is rare—rather than in locations where it is common, according to a 10-year study that used high-resolution satellite imaging.

“There are more tree species living in an area not much larger than a few football fields in Panama than in all of North America north of Mexico combined,” says Jim Kellner, an assistant professor of ecology and evolutionary biology at Brown University and first author of the paper, which appears in the Proceedings of the National Academy of Sciences.

“How this diversity originated and why it persists over time is a paradox that has challenged naturalists for more than a century.”

Until now.

“The take-home of the study is that there is a ‘negative feedback’ on population growth,” Kellner says, which puts the brakes on population growth in locations where the species is common.

Natural enemies

The findings confirm a prediction from the 1970s, which posited that tropical forests are diverse because natural enemies keep populations in check. An enemy could be a seed eater, an herbivore, or a pathogen, Kellner says.

For example, consider an oak tree and a squirrel. The squirrel eats acorns and prefers to forage where oak trees are abundant. A squirrel isn’t likely to notice a lone acorn in the middle of a grove of maples, whereas squirrels will eat many acorns in an oak grove.

If this kind of behavior is widespread in tropical rainforests, it could keep species from becoming too common, he explains.

“…for this species, you would have to search about 250 acres to find one new adult tree every year.”

Earlier studies have shown that this negative feedback phenomenon holds true among young trees—seeds, seedlings, and saplings—but ecologists hadn’t been able to determine whether it influences adult trees, the reproductive portion of populations, Kellner says.

“It takes decades for trees to become reproductive in tropical forests, and the problem is compounded by how rare each species is,” Kellner says. “We found that for this species, you would have to search about 250 acres to find one new adult tree every year.”

That challenge isn’t feasible on foot, but remote sensing can overcome the challenges of observing large areas.

Kellner and coauthor Stephen Hubbell, an ecology professor emeritus at the University of California, Los Angeles, used high-resolution satellite images to track individuals on Barro Colorado Island, a six-square-mile island in the middle of the Panama Canal, over 10 years. They looked for Handroanthus guayacan, a tropical rainforest tree that produces bright yellow flowers for a few days a year.

“By timing the satellite image acquisition with seasonal flowering, we were able to identify most of the adults for this species on the island,” Kellner says.

They found 1,006 adult trees. Starting in 2012 and looking backward over the 10-year study period, the researchers were able to identify when new trees joined the adult population for the first time. They used advanced statistical methods to make sure that they were in fact identifying new adults and not just trees that had skipped a year of flowering or had flowered early or late.

Population dynamics

Negative feedback affected the abundance of new adult trees and can influence the population of new adult trees in an area of almost 100 football fields. This contrasts with prior studies of juvenile trees, which found the effects of host-specific enemies are usually restricted to small areas, Kellner says.

To confirm the locations of trees from the satellite data, they went to the island and independently found 123 adult trees of the same species. Of these, 89 percent had been detected in the high-resolution images, suggesting that their data are a nearly complete census of the species.

The implications could be broad, Kellner says.

“I can’t think of any idea in ecology that is more important than population dynamics. It’s important for everything from fishing licenses to forecasting disease outbreaks,” he says.

“This study is the first demonstration on such a large scale that escaping from the area of one’s parent greatly increases the chance that a seed will survive to become a parent tree itself,” says Doug Levey, a program officer at the National Science Foundation, which supported the study.

Source: Brown University