Princeton Weekly Bulletin February 22, 1999
Tracking biodiversity
New finding overturns established wisdom about effect of light gaps on plant species
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Light gap created by a single fallen tree on Barro Colorado Island in Panama (photo: Marcos Guerra/Science Magazine)
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A painstaking effort to track every square inch of plant life in large patches of tropical forests has started to produce significant discoveries in ecology.
Stephen Hubbell, professor of ecology and evolutionary biology, is using this research to answer fundamental questions about what factors come into play in maintaining the diversity of life on Earth.
Hubbell's latest finding overturns one of the bedrock beliefs among ecologists about what allows tropical forests to maintain such a dazzling variety of tree species. The common thinking was that when a trees dies or is blown over in a storm, the resulting infusion of direct sunlight, called a light gap, allows new species to flourish and compete to fill the open slot in the forest. The frequency and size of light gaps was, therefore, thought to predict type and number of species present in the forest.
Hubbell has found that no such correlation exists. Using vast amounts of data generated from the tracking project, he showed that areas with many light gaps are no richer in species than areas with few pockets of sunlight. The mix of species also was not notably different. Although some species depend on light gaps to survive, they are such a small minority that they don't change the results.
The finding may cast doubt on one common idea for how to reconcile logging and conservation efforts in tropical forests, said John Terborgh, codirector of Duke University's Center for Tropical Conservation. There have been suggestions that if logging in forests mimicked the natural pattern of light gaps, it would promote the diversity of trees rather than harm it. That idea is based on a theory that Hubbell has now shown to be wrong, in the cover story of the January 22 issue of Science.
"I think it's a very exciting paper," Terborgh said. "This is doing a great service to ecology."
Hubbell's finding also may indirectly affect other conservation efforts. One reason that the old light gap theory failed to hold up is that the distribution of seeds in a forest is far from ideal. Light gaps create a variation in the local growing environment, which should, in turn, create a variation in the types of seeds that grow. As a practical matter, however, the same old varieties tend to grow in light gap areas, because there are not enough light-loving seeds that can take advantage of the new conditions. That lesson could have implications for conservation efforts around the world, where roads and development isolate one section of forest from another, exacerbating the effects of poor seed dispersal and possibly forcing the extinction of species that might otherwise be good competitors.
20-year diversity tracking project
The light gap question is just one of a series of issues being illuminated by a research effort that Hubbell helped start nearly 20 years ago, an ambitious project to track the diversity of species in tropical forests. The location is a 120-acre plot within a tropical forest on Barro Colorado Island in the Panama Canal. Starting in 1981, a team of researchers directed by Hubbell, who was then at the Smithsonian Tropical Research Institute, and colleague Robin Foster, a curator of the Field Museum in Chicago, began identifying and tagging every tree and sapling that was at least chest high and at least one centimeter in diameter. (According to Hubbell, that is a far more detailed level of sampling than had been done in any other study, most of which have looked at plants two or three times bigger and sampled just a few acres.) By the time the researchers finished the study two years later, they had tagged 300,000 trees. They have gone back every few years to repeat the process. It takes a team of 15 people nine months to comb through the plot and update the data.
From its very beginning, the project has yielded dramatic results, said Elizabeth Losos, director of the Center for Tropical Forest Science, an organization within the Smithsonian that was formed to manage the research. For example, the researchers discovered right away that tropical forests are not the stable, unchanging ecosystems they were assumed to be. In just two years, 40 percent of all the tree species had significantly changed their relative abundance, with some dropping to extinction in that plot and others becoming more dominant species.
The project's success has led to a series of collaborations that eventually resulted in the creation of 15 other 120-acre sites in 12 countries, involving scientists from three dozen institutions.
One long-term outgrowth of Hubbell's work is his discovery of what he calls "the E=mc2 of community ecology," a theory that for the first time links several other seemingly distinct theories about the abundance and distribution of species. Hubbell originally published the theory four years ago and is now finishing a book for Princeton University Press that presents the idea in expanded form. This theory, the "unified theory of biodiversity and biogeography," allows scientists to calculate a single number (the "fundamental biodiversity number") that describes a whole range of characteristics of plant and animal communities. For example, by figuring the biodiversity number for a particular forest, a scientist could estimate how many species the forest is likely to contain and whether some of those species are much more dominant than others, as well as a number of other factors.
Knowledge not keeping pace
Hubbell's work is inspired by a deep concern for the planet's future. He worries that scientific knowledge about ecological systems and biodiversity is not keeping pace with the speed at which humans are harming the planet. He cites a recent study which showed that humans consume 40 percent of all the energy that goes into producing the Earth's biomass -- a number far out of proportion to humans' relative abundance among species. At the same time, biologists don't even have a rough guess at how many species the Earth holds, and they are far from knowing how those species depend on and compete with each other.
"We're still in the Middle Ages in biodiversity research," Hubbell said. "We're still cutting bodies open to see what organs are inside. I've told myself I need to spend the rest of my life fighting for this. It may be a lost cause, but I at least want to be able to say I tried."
In the fall, Hubbell will be moving to the University of Georgia, where he will continue this work as a professor in the botany department. His wife, Patricia Gowaty, is a professor in the University of Georgia's Institute of Ecology. For his final semester at Princeton, he is teaching a senior seminar in Panama.