As Researchers Watch, Deforestation Closes In on Panamanian Enclave : Rain Forest: In this Garden of Eden for researchers, trees commit suicide, ants scream and moths become addicted to drugs.

BARRO COLORADO ISLAND, Panama —  For scientists who want to unravel the mysteries of the tropical rain forest, this island in the middle of the Panama Canal is an irresistible place of double-dealing and intrigue, where animals pursue hidden agendas and plants live shady and often disingenuous lives.

In this dark and humid world populated by giant rodents and see-through frogs, nothing is as it seems. Here, trees commit suicide and ants scream. Bees consume rotting flesh. Leaves are colored red, white or blue. And pretty moths become hopeless drug addicts.

But just as researchers begin to tease apart the tangled plot of one of the greatest mysteries in nature, the forests are disappearing around them.

Sitting on the porch of their research station at dusk, drinking cold Panamanian beer and watching the bats come out, the scientists at Barro Colorado Island can look across Gatun Lake and witness the deforestation creeping down the once verdant slopes, a microcosm of what is happening throughout much of the tropics.

Even at Barro Colorado Island, in the middle of what ecologists consider a Garden of Eden for tropical research, the feeling is never far away that within a generation there may be little forest left to study.

The only likely exceptions would be protected reserves like Barro Colorado, a six-square-mile world center for tropical studies located in the middle of the Panama Canal and operated by the Smithsonian Tropical Research Institute.

Scientists have learned that as the forests shrink, so does the climate that favors their survival. Recent work in Brazil by Eneas Salati of the University of Sao Paulo has shown that as much as half of the water that falls on the Amazon rain forest comes from evaporation arising from the same forest.

New computer simulations just published by the United Kingdom Meteorological Office indicate that if the Amazon forests were converted into cattle pasture, rainfall could decline as much as 20%.

In Panama scientists have documented an unmistakable decrease in rain over the last 50 years. In 1925 annual rainfall was almost 10 feet on Barro Colorado Island. Today it is about eight feet a year, a reduction researchers consider dramatic.

Smithsonian scientist Donald Windsor and his partner, Robert Stallard of the U.S. Geological Survey, initially suspected the decreased rainfall was directly due to extreme deforestation in the Panama Canal watershed.

But Stallard speculated in December that the trend may be an early signal of something worse: that global climate is changing, perhaps as a result of the “greenhouse effect.”

But change may not be new to the forests. Far from being consistently pristine places, they appear to have been subject to enormous change. Sifting through the tiny microfossils of pollen grains and plant remains found at the bottom of lakes, Smithsonian paleobotanist Dolores Piperno and colleagues have shown that the effects of the last Ice Age about 11,000 years ago had a profound impact on the tropical forests, where the climate may have been 9 degrees cooler and much drier.

Piperno said she thinks prehistoric people also substantially altered the face of the tropics. Paleoindians who crossed over the Siberian land bridge and moved into South America hunted the mastodons and giant ground sloths that once roamed the region. They may have even caused the animals’ extinction.

Today only smaller mammals remain in the forests. On Barro Colorado, they survive because game wardens patrol constantly, keeping poachers at bay.

Large troops of white-faced, howler and spider monkeys still crash through the forest canopy, a still largely unexplored place where bizarre animals and parasitic and symbiotic plants spend their lives.

There are also large populations of agoutis and pacas on the island. These huge rodents can reach 22 pounds. The pacas are nocturnal, asocial and carry sharp incisors that they use to disembowel other pacas. When disturbed, they growl like dogs.

Smithsonian mammalogist Nicholas Smythe has launched a project to tame pacas and imprint social behavior on the young so they can be raised together in cages for food. So far, it seems a success: Smythe can handle caged animals that normally would have attacked.

“I really don’t like to kill them, though they are delicious,” said Smythe, who serves paca barbecued or in a stew to his colleagues.

Although prehistoric hunters and gatherers ate pacas too, they also cleared vast tracts of forest, according to archeological studies. Beginning about 11,000 years ago, signs of forest-burning appear across the Pacific Coast of Panama, according to Piperno. By 4,000 years ago, vast tracts of forest had been cleared for agriculture.

“We have evidence that the forests here have been disturbed for a very long time,” Smithsonian anthropologist Richard Cooke said. “The Spanish recorded riding horseback over a treeless landscape. They wrote of riding for hours without seeing a tree.”

Piperno and Cooke said they believe much of today’s forests appear to have grown up since the Spanish conquest in the 1500s.

“We wonder whether these forests are largely artificial creations,” said Piperno. “We wonder how much of the forest we see today is really natural.”

In fact, the human hand and changes in rainfall may help explain a central enigma of the tropical forest: Why are there so many kinds of plants?

Stephen Hubbell of Princeton University and Robin Foster of the Field Museum in Chicago, working on Barro Colorado, have accumulated a vast databank on tropical-tree diversity to study the shifting dynamics of the forests, a study that should help tropical foresters know which trees to plant that would be both commercially and biologically viable.

In a single 125-acre plot, Hubbell, Foster and their assistants have mapped the exact location of 250,000 individual trees and shrubs with trunks at least one-half-inch in diameter. More than 300 species are represented.

A sister plot in Malaysia tallied 320,000 trees and saplings representing 835 species, the highest number ever recorded. Ecologists suspect that a similar plot in Western Amazonia may be even more diverse. In even the most robust woods around Washington, there are probably no more than 40 species in a similar plot.

This diversity of life has enthralled and perplexed visitors to the tropics ever since Charles Darwin stepped off the Beagle and waded into a South American “jungle” more than 150 years ago.

Smithsonian ecologist Egbert Leigh, a longtime researcher on Barro Colorado Island whose whiskey has fueled evening discussions on tropical biology for almost 20 years, thinks the diversity of tree species is the single most important and unanswered riddle of the forests.

For, he said, if one can know why and how so many different trees thrive in the tropics, one can attack with new insight other questions, such as why there are so many insects, bats, frogs and snakes.

The elaborate relationships between plants and animals are at the heart of the mystery of the forests. On Barro Colorado, most of the 1,400 plants somehow rely on animals to pollinate and disperse their seeds, according to Smithsonian botanist Kevin Hogan.

Over millions of years, plants and animals have evolved elaborate schemes to use and abuse each other. Plants have focused particular energy in manipulating and protecting themselves from insects. Termites are a special threat. Indeed, in the middle of the forest, all is usually still, except for the movement of insects. Termites can be heard chewing trees to death.

Among other denizens are the giant tropical ant, the largest in Central America, which releases a heavy musky odor when disturbed and utters a shrill scream before it stings.

Masses of stingless bees, perhaps the most productive creatures in the forest, patrol for nectar and pollen. But Smithsonian entomologist David Roubik has discovered at least one species of stingless bee that feeds exclusively on the carcasses of dead animals.

Against such competitors, plants in the tropics have devised ingenious ways to protect themselves or their offspring. One tree grows for at least 40 or 50 years without reproducing. The tree flowers and releases a shower of seeds only once in its life. Then it dies, killing itself for the sake of its offspring, and opening a hole in the dense canopy for sunlight to reach its seedlings.

Other trees cope by putting forth new leaves that look sickly or dead, since new leaves are a delicacy in the forest. Some new leaves are red, white or blue, a color scheme that may turn off insects. More important, the leaves do not make an investment in developing the machinery for photosynthesis until they are tough enough to resist insect jaws.

Untangling these interactions and beginning to understand how a rain forest works can take years, but may produce surprising results.

Smithsonian biologist Neal Smith, for example, wanted to know more about the spectacular mass migrations of a pretty black and green, swallow-tailed moth named Urania fulgens.

Caterpillars of the moth will eat only one species of plant, a vine from the genus Omphalea. The vine defends itself by producing an “antifeedant,” a complex alkaloid called DMDP that keeps most insects from eating the plant. But the U. fulgens moths become addicted to DMDP.

“They’re hooked on the stuff,” Smith said. “They crave it.”

But the vine has another trick. If its leaves are being grazed too heavily by the moth, it stops producing the alkaloid. Smith believes that withholding the drug from the moth sets off the mass migrations.

There is more. Several years ago, chemists at the Royal Botanic Gardens at Kew, England, along with colleagues elsewhere found that DMDP may have some remarkable characteristics.

To date, Smith said, scientists are testing the substance as a drug to fight AIDS, diabetes and cancer. It may also be made into a diet pill.

“You tell me what has a better spinoff than this. It is a classic case of a scientist interested in an obscure little moth who finds something totally new and unexpected,” Smith said. “But I still don’t know where the moths come from and where they go. We’re still working on that.”