Nancy Marie Brown
June 01, 1987

"Perhaps one in 10 to one in 20 species in the tropics are known to science. How can conservation work in the face of this degree of ignorance? How can economic development be guided in a more benign direction?"

—Thomas E. Lovejoy of the World Wildlife Fund (1986)

An unbroken string of ants plies the leaf-cutters' foraging trail, a well-beaten highway through the forest. Each large ant cuts a round piece of leaf and carries it home on its back; a small ant rides shotgun on the leaf to defend it from the fly that would lay an egg on it, an egg that would hatch into a parasitic maggot by the time it reached the ants' underground nest.

Inside the nest, the two ants defecate on their leaf, depositing enzymes that tenderize it, and feed it, shredded, to a fungus—Rhozites gongylophora—that is found only in leaf-cutter ants' nests and has no spores with which to propagate itself. The ants eat the fungus.

Leaf-cutter ants refuse to cut leaves that contain chemicals toxic to themselves. They also refuse those with chemicals toxic to their fungus. The amount of light and nutrients an individual plant receives affects the kind and quantity of toxic chemicals it can produce.

Entomologist Jack Schultz returned from Costa Rica last January with four colonies of leaf-cutter ants. A graduate student, Colin Nichols-Orians, remained in the field until May, then came back to test his ideas with the laboratory ants. Nichols-Orians wants to learn exactly how, from the point of view of an ant, light and nutrients affect a plant's chemical defenses. That is, how do ants choose which leaves to cut?

At a Penn State lecture last year, Ghillean T. Prance of the New York Botanical Garden, a man the fliers said is "generally acknowledged as the foremost North American authority on the Brazilian tropical rainforest," summarized the quandary rainforest scientists find themselves in. Receiving question after question about the interactions in the rainforest, Prance threw up his hands: "Everything is related to everything else," he said.

Schultz and Nichols-Orians work at La Selva Biological Station, 3,300 acres of Costa Rican rainforest owned by the Organization for Tropical Studies, of which Penn State, along with 36 other American and two Costa Rican universities, is a member. "La Selva," says Schultz, an assistant professor in Penn State's College of Agriculture, "just means 'the forest.' " It is roughly the size of a Pennsylvania state park and holds over 450 species of trees; in all of Penn's Woods, there are about 100 species and only 30 to 40 of them are tall enough to be "canopy" trees. "In La Selva, it's not unusual to have to walk a third of a mile between any two individuals of even the commonest plants." Schultz and two others spent two weeks last January trying to collect 10 leaf samples each from 10 species; they had time to find only 60 samples.

La Selva also has 388 species of birds, 143 species of butterflies, and 63 species of bats. On an average walk outside the 5,000-square-foot, air-conditioned laboratory ("It's really tough to answer the questions people have about tropical rainforests without more and more sophisticated laboratories right there," says Schultz), an average, inattentive visitor might see three species of monkeys, a two-toed sloth, agoutis (guinea-pig-sized rodents), river otters, a cayman on the beach, a three-wattled bell bird—"its call sounds rather like someone hitting an anvil with a hammer"—and electric-blue morpho butterflies "like flying blue plates." In the middle of the Jaguar Trail might be a large, steaming cat pile, filled with bones and hair and agouti teeth.

The rainforests of the tropics, covering seven percent of the earth's land surface, contain more than half the earth's species of living things. No one knows the total number of species on Earth. Until a scientist from the Smithsonian found 41,000 species of insects in a two-and-a-half acre plot of Peruvian rainforest, the estimate for the planet was five million. It has now risen to 30, even 50 million.

The home of these 15 to 25 million species is rapidly disappearing. About 40 percent of the world's rainforest is already gone, says Harvard University biologist Edward O. Wilson; an area the size of West Virginia is cleared each year. At this rate, rainforest will be extinct (if that term can be applied to an ecosystem) sometime in the 21st century. With it will go what one scientist has called the "wonder plants of the future": plants like those that in the past have given us peanuts, chocolate, rice, coffee, eggplants, oranges, tea, bananas, cashews, coconuts, papayas, and pineapples, as well as quinine, curare, the birth control pill, ipecac, and 70 percent of the 3,000 plants the U.S. National Cancer Institute lists as having anti-cancer properties—not to mention rubber.

In La Selva, Schultz and Julie Denslow from Tulane University, Boyd Strain from Duke, and Peter Vitousek from Stanford, with a million-dollar National Science Foundation grant, are trying to describe not individual species, but the basic patterns of the rainforest itself. Does the shape of the forest, for instance, cause and maintain its diversity? "We hope we'll also learn how to reforest the tropics," says Schultz, "when people decide they want the rainforest to regrow." They are checking out what Schultz calls "the grey literature," an enormous body of personal recollections, tales, stories, and suppositions, on which much of what we "know" about the rainforest is based.

In the past few years, such work has led scientists to a striking realization: The rainforest's diversity and complex structure are intimately related to disturbance. "La Selva is undisturbed by humans," says Schultz, "but more than half of it is recovering from natural disturbances—from tree falls. At least two trees fell in the last two weeks I was there."

When a 150-foot tree, laced to its neighbors by an intricate web of woody vines, topples, it opens up a light gap the size of a football field. The "grey literature" says that certain species are adapted to take advantage of this sudden burst of light: their seeds disperse readily into light gaps or remain dormant until the light level increases; their seedlings grow tolerably well in the shade, always ready to shoot up when a tree falls nearby.

The adaptations assumed in the grey literature have been documented for temperate plants. Do they truly apply in the tropics? Not all. A tall canopy tree has large, heavy fruit and needs birds or animals to disperse its seeds. A seed cannot remain dormant on the wet forest floor. "If it doesn't germinate immediately," Schultz says, "it either rots or is eaten." But some species are always fruiting in the year-round tropical growing season. "What ends up here," says Schultz, peering down into the circle of his fingers, "may depend on what plants were reproducing when this gap appeared. If all trees fell in the same month—or if all plants fruited at the same time—the forest would be a different place."

But some plants are adapted to life in the dark. Four years ago, Schultz and the other scientists took thousands of cuttings from eight common species of shrubs in two genera; two shrubs in each genus were "sun" plants and two were "shade" plants. They transplanted some of the cuttings along the boles of new-fallen trees, some at the edges of light gaps, and some in deep forest. "The result was very simple and straightforward," says Schultz. "The shade species all responded well to the available light, while the sun species couldn't make it in the shade at all. It's hard to imagine why the shade species don't take over. They didn't outperform the sun species in the middle of the gap, but they did very well."

In the next four years, Schultz and his colleagues will extend their study to the tallest canopy trees, planting thousands of seedlings in the deep forest, and then, after a year, toppling six trees near the test sites and seeing how the seedlings respond. "The goal is to determine the value of being a little plant when a big tree falls. What characteristics must you have to be successful in the middle of a disturbance?"

One possibly crucial difference Schultz found between sun species and shade species has to do with the leaf-cutter ants. Trees make toxic chemicals—bitter phenolics and tannins, or alkaloids like caffeine—to defend themselves against ants aand other insects. Phenolics and tannins derive from carbon, which plants manufacture through photosynthesis. These chemicals are expensive make: They represent a significant carbon drain and constitute a large proportion of the plant's weight. Alkaloids are made from nitrogen, which comes from the soil; Denslow and Vitousek found that the top four inches of earth at La Selva—the dense mat of root hairs and detritus—had the highest nitrogen concentration of any "soil" anywhere in the world.

Sun species make phenolics and tannins. "Under high light," says Schultz, "these plants seem to make such a good income through photosynthesis that they can spend both on defense and on growth. They grow like incredibly well and they increase their chemical defenses. In the shade, they can't grow or make their defensive compounds, so when a light gap finally opens up nearby, they're too chewed up to benefit."

Shade species make alkaloids, which are toxic at very low doses. "In the shade, a plant can't fix too much carbon, but it has plenty of nitrogen. So a shade species defends itself with things that are relatively cheap, and puts most of its income into growth. In the shade, it grows slowly, but reasonably well, and it defends itself reasonably well. When it's put in the sun, it grows at the expense of defense, but it still does better than the sun species in the dark.

"It's probably rare for a plant to start growing and get going in the dark. It will be able to maintain itself, but it needs a gap to grow. We estimate that an individual tree has to be near at least two natural tree-falls over its lifetime in order to make it to the canopy."

Last Updated June 01, 1987