Alfred Russel Wallace, the nineteenth-century evolutionary theorist and Amazon explorer, described the subtle complexity of the Amazon forests in the book Tropical Nature: “If the traveller notices a particular species and wishes to find more like it, he may often turn his eyes in vain in every direction. Trees of varied forms, dimensions and colors are around him, but he rarely sees any of them repeated. Time after time he goes towards a tree which looks like the one he seeks, but a closer examination proves it to be distinct. He may at length, perhaps, meet with a second specimen a half mile off, or he may fail altogether, till on another occasion he stumbles on one by accident.”
Wallace’s point has since been borne out scientifically: although there is an abundance of species, there is usually a paucity of individuals of each species. Moreover, a species may inhabit only a tiny part of the total forest mosaic. In a study of rain forest plots around Manaus, for example, forest tracts that were 50 miles from each other shared only 1 percent of their species. It is this characteristic of the rain forest that has biologists so convinced that species are becoming extinct each day. The felling of a few acres may destroy all the individuals of a species, obliterating forever that bit of Earth’s archive of genetic possibilities.
Theories attempting to explain the origins of the biodiversity of the Amazon have changed dramatically in the past couple of decades. Twenty years ago, it was common to see texts authoritatively state that the richness of species in the tropical rain forest was due primarily to the unchanging nature of the region. For 65 million years, climatic conditions remained remarkably constant, the story went, allowing organisms to evolve extremely specialized adaptations to tight ecological niches that all fit together seamlessly, like the pieces in a jigsaw puzzle. The Amazon rain forest was considered a classic “climax” community, a system that had reached a pinnacle of maturity, apportioning every resource, with not a leaf out of place. Now, though, this concept has lost ground to a competing theory that is almost its opposite: the species of the Amazon are numerous and diverse because the ecosystem is constantly in upheaval—changing all the time and at every scale of time and geography.
On the greatest scale—that at which global geology changes—there may well have been a long stretch of time during the Cretaceous period, between 144 million and 65 million years ago, when the region now called the Amazon had a stable tropical climate. In fact, much of the planet’s exposed land was covered by tropical forests during that period. Fossilized pollen grains have shown that tropical forest plants once thrived in Alaska, for example. That period also saw the evolution of the major families of plants. Then South America, North America, Africa, and other continental chunks—which had been joined into a single landmass—began to drift apart, causing the evolutionary paths of organisms on different continents to diverge as their environments diverged. The three major regions of rain forest today—the Amazon, equatorial Africa, and equatorial Asia—share many families and genera of plants and animals, possibly because those general types of organisms evolved before the continents moved away from one another. But only a few species of rain forest organisms exist on more than one continent. Thus, the explosion of species in the Amazon and other rain forests seems to have occurred fairly recently in geological time.
In the Amazon, the burst of species formation may have been the result of changes that started to take place late in the Miocene period, some 25 million years ago. The earth began to slide into a climatic cycle of ice ages and brief warming periods, a cycle that dramatically altered the face of the Northern Hemisphere. (Right now, we are at the end of a 9,000-year warm interlude.) During the ice ages, mile-thick glaciers scraped across the continents of the Northern Hemisphere. Even now, vestiges of the last ice age, which peaked just 22,000 years ago, persist; among them are the Great Lakes and Long Island, which is simply a big moraine, a great heap of dirt and rock that was pushed ahead of a glacier. Conditions changed in the tropics as well—not as drastically, but enough to have a substantial impact on life forms there. Affected by the glaciers to the north, the climate of the Amazon basin periodically became cooler and drier, perhaps twenty times or more, over the past 3 million years.
Some scientists believe that with the onset of each ice age, the cooling and drying of the Amazon caused savanna-like conditions to spread and the rain forests to retreat. But one theory holds that there were always some wet pockets, called refugia, in which the basic rain forest ecosystem was able to persist. Wherever there was sufficient rain and warmth, the churning biological engine of the tropical forest was sustained, idling, ready to break out as soon as favorable conditions expanded. Dispersed across the region, these refugia contained similar stocks of plants and animals. Because there was no genetic mixing among refugia, over time each group of animals and plants evolved along an independent pathway. Much of the support for the refugia theory comes from studies of the variations in wing patterns of passion-flower butterflies and the variations in color schemes in toucans, the resplendent family of fruit-eating birds with oversize beaks. As warm, moist conditions returned to the entire basin, all these types of insect, plant, and animal could colonize new terrain, evolving further to suit new niches, then bumping up against their now-different counterparts from other refugia. The result is a mosaic of variations on themes.
On the regional scale, variation in species in the Amazon was probably promoted by the disastrous floods that occasionally seem to have swept over vast areas of the basin. Kenneth Campbell, at the Natural History Museum of Los Angeles County, has been studying sediments in western Acre; he posits that this part of the Amazon was hit by an enormous mass of water that may have broken through some temporary glacial dike in the Andes. By scouring large areas, such events would have created a clean slate upon which species would be jogged into competing and adapting to new conditions.
On the local scale, rivers are constantly changing their course, eating their way crabwise through the forest, destroying established biological communities ahead of them and creating new opportunities as shallow silty beaches are deposited in their wake. Boats navigating on Amazonian rivers constantly scrape against the submerged trunks of massive rain forest trees, whose shallow roots have been undermined by the water’s steady advance. On the upper reaches of the Juruá River, a traveler is likely to meet ribeirinhos—the farmers who live by the river and plant their corn and beans on the rich silt exposed during the dry season. One such farmer is Manuel de Caro de Souza. He had to move his entire house and manioc mill twice in ten years to stay near the river, which was shifting away from his tract 30 feet each year.
Another phenomenon that can disrupt a biological community at the local scale is the occasional windstorm. The Amazon almost never sees the tornadoes and other violent storm conditions that sweep North America when a cold front clashes with warm air. But every few years a strong friagem, cold front, makes it all the way up to the tropics from Antarctica. When such a weather front barrels through, it can topple trees and rend the normally tightly knit forest canopy. Again, animals and plants are thrown into an unbalanced situation that favors change. In August 1989, Xapuri lost much of its tin roofing in a sudden blast that was stronger than any wind the townsfolk could remember. Such gusts can reshape a forest, even if they occur only once a century.
Finally, there is the scale of the reach of an individual tree. When a tree falls, it can create in microcosm the same kind of disruption caused by a strong storm. Such tree falls may be a crucial element in shaping the mix of species in the forest. A tropical forestry scientist named Gary Hartshorn studied tree-fall rates around the tropics and found that in many areas, the time it takes for a section of forest to be completely replaced can be as little as eighty years. The overall effect is that the forest is perpetually off kilter, in a continual state of recovery but never quite returning to some inanimate state—a condition that opens up opportunities and lets no organism settle too comfortably into a static niche. And it creates a kind of biolo
gical disarray that is bound to mystify scientists.
There is ultimately something about the elaborate, evanescent kaleidoscope of life in the tropical rain forest that almost taunts biologists who attempt to master it. Generations of scientists have been lured by the biological marvels of the Amazon. Those with the hubris to go home convinced that they have solved the region’s riddles almost invariably end up humbled. The better ones go away with more questions than they had when they arrived. The closer one gets, the more intertwined things become. Brush against a slim plant on the forest floor and a flood of biting red ants streams from the plant’s hollow stems, ready to kill anything that might attack. The plant’s defenders are rewarded with a specialized home and feeding center that sustains them.
When one comes across a three-toed sloth, a slow-moving vegetarian mammal of the canopy, one is struck by its greenish cast, which is imparted by the algae that live in its matted fur, along with several dozen species of mite, moth, and other organisms. The sloth has an odd habit: rather than defecating randomly from its perch —like monkeys and birds—once a week or so, it descends to the ground and deposits its feces in a hole it digs near the base of the tree in which it spends the most time. Perhaps this strategy has evolved as a way of returning some of the nutrients it took from the tree by eating its leaves. Whatever the reason, it must be pretty important; venturing onto the forest floor is a risky business for an animal that can barely drag itself across a flat surface. Thus, the sloth itself is an almost incomprehensibly intricate biological system.
Fifty miles from Manaus, scientists have taken 250 acres of rain forest and imposed a Cartesian grid on it. Every 65 feet they planted a plastic pipe in the ground. Each pipe is identified by a letter and a number, instantly telling visitors where they are. As various teams of botanists, mammalogists, entomologists, and other specialists come through, they are mapping the location of individual organisms. One palm specialist, Andrew Henderson, from the New York Botanical Garden, spent three weeks counting 6,476 palms, of 29 species, in just 25 acres. His goal was to understand the distribution of the species in light of underlying conditions, such as soil type and elevation. One could only wish him luck and endurance. Even if he came to understand that tiny slice of Amazonia, he might not know much about the next slice, let alone a piece of forest on the far side of the basin. When Henderson later traveled up the Moa River in the westernmost part of the state of Acre, in a matter of days he counted 69 species of palm. Just a few hundred miles away, he was in an entirely different biological galaxy.
Until the murder of Chico Mendes, Acre was off the beaten track for most scientists. Research had generally been concentrated along the main branches of the Amazon, which were relatively accessible. When the killing drew attention to the rubber tappers’ proposal for extractive reserves, the remote state suddenly became something of a Mecca for botanists and graduate students studying everything from palm tree diversity to the woman’s role in rubber tapper society. In the summer of 1989, there were so many foreign researchers in the area that disgruntled ranchers began to spread rumors that female American graduate students were prancing naked through the forest and having sex with tappers.
Douglas Daly’s students were doing no such thing. On a hot morning in the dry season eight months after Chico Mendes’s death, Daly—a colleague of Andrew Henderson’s from the New York Botanical Garden—led a group of graduate students and government technicians into the rain forest a few miles up the road from Xapuri. Daly’s group was part of a larger, twenty-day course on tropical ecology funded by American organizations ranging from the U.S. Fish and Wildlife Service to the Jessie Smith Noyes Foundation, a private philanthropic organization that supports environmental activities in Latin America. Like Daly, most of the other teachers were American. Oddly, Americans and Europeans have been teaching Brazilians about Amazonian biology for many years. Until recently, Brazil focused most of its funding for scientific research on applied sciences such as agronomy and electrical engineering; tropical biology was merely an incidental pursuit, supported only to the extent that it furthered research in cocoa or rubber cultivation. It is only the wealthy, developed nations that have had the luxury of funding pure research.
Daly and his students were dropped off by bus at the entrance to a 25,000-acre rubber tapping estate called Seringal Triunfo, about 40 miles from the network of trails where Chico Mendes once tapped rubber trees and organized the fight for the forest. It was forested tracts such as this one that Mendes had hoped to convert to protected extractive reserves. The other students in the course had been split into small teams. One group went by boat up the Abunã River, which is part of the Bolivian border in eastern Acre, to observe birds; another headed off in search of monkeys and marmosets; a third took several dozen small cage traps into the forest to survey the population of small, ground-dwelling mammals. The plan was for all of the students and teachers to go over their findings that evening back at a bar in the border town of Plácido de Castro.
Daly and his team would spend the day identifying plants, particularly those that were useful as food or had possible medicinal properties. They began by hiking to the barracão, the central compound of the rubber estate. They were met by two tappers who, for $2 apiece, would act as guides for the day. The compound seemed rundown, and there was little rubber in sight. The group was told that the owner was not concerned about falling rubber production; his main goal these days was to let time pass. The value of the property was steadily rising, and soon he could sell it at a huge profit to a rancher or investor who would cut down the forest. This barracão was something of a zoological garden, very much of the inhumane kind. One wooden cage held several monkeys. Two adult jaguars, caught as cubs but now well over 100 pounds each, paced in a cramped stockade built of heavy wooden posts and shaded by a thatch roof. One of the spotted cats scowled and screamed and raked a broad paw at the air between two posts. Despite the rush of roads and deforestation, jaguars were still fairly common in eastern Acre, although the only ones you were likely to see were in captivity.
The clearing was achingly bright and shimmered in the 90-degree heat, even though it was only eight o’clock in the morning. The blond grasses and red earth were bleached and desiccated. One tapper, a slim, dark man named Ilson, took the lead. Daly and the rest followed, and the second guide brought up the rear. The class left the scorching pasture and plunged into the forest. The effect was like diving into the sea after baking on a tropical beach. The temperature instantly dropped ten degrees and the abrupt change from light to shade hurt everyone’s eyes as tightly constricted pupils were suddenly forced to dilate. As the students’ eyes adjusted to the dimness, their minds too began to adjust, very slowly, to the landscape.
It was the dry season now. Leaves crackled underfoot as the group hiked along one of the estradas, rubber trails, that were hacked out of the forest by the tappers to get from one rubber tree to another. Fine red dust covered the foliage on each side of the trail, and unseen snakes and lizards scuttled loudly away from their basking spots as the group passed. The trail wound over humps and down into gullies. Each low spot was slightly damp, and dazzling butterflies congregated on the moist mud, lapping up leached mineral salts with their proboscises, which unrolled like tiny New Year’s Eve party favors. Every hundred yards or so, the students passed a rubber tree, whose bark bore the corrugated marks created by year after year of cuts from a special tapper’s knife, a faca de seringa. Periodically, Daly stopped to ask Ilson the tappers’ name for a certain plant or to make a point to the class.
Daly considered Ilson a true mateiro, a master woodsman. If he did not recognize a tree instantly, he sliced lightly into the bark with his machete and broke off a sliver. He sniffed the wood and examined any fluid that oozed out. One tree, limãozinho, had a distinctive lemony fragrance. When the papery bark was peeled from another tree, it smelled like garlic. Tropical cedar emitted the familiar pungence of its northern cousins, although it had a very d
ifferent shape, with sweeping buttresses—winglike masses of wood that jut out around the tree base—that are thought to provide structural support in lieu of deep roots. Ilson claimed that the forest contained pau brasil, brazilwood, which quickly blushes to a deep red color after it is cut. But there Daly disagreed. True pau brasil, he said, existed only in the coastal rain forests of Brazil, which once ran in an unbroken swath from São Paulo to Rio. It was this tree that gave the country its name. During the colonial era, the sixteenth through eighteenth centuries, brazilwood was heavily harvested for the valuable purple dye that could be extracted. Now the tree, and the coastal forests themselves, are all but extinct.
Daly was a slim sprite of a man and wore slightly crooked gold-framed glasses. His twelve years of experience in the Amazon showed in his wardrobe, which was perfect for the jungle: light cotton trousers, heavy boots, and a long-sleeve shirt, to keep the insects at bay. His pack seemed too large for his body; he carried a camera and a tripod as well. A student carried a press for botanical specimens—wooden racks held together by straps—and a pile of old Acre newspapers. On the top of the stack was a picture of a bikini-clad contestant in the upcoming beauty pageant sponsored by the state bank of Acre, Banacre. The newspapers would be placed between the samples of stems, flowers, and seeds that would be squeezed and later dried in the racks.
After an hour of hiking, it was time for the first break. Ilson took off his sweat-stained baseball cap and retrieved the crumpled pack of cigarettes tucked inside. He lit one, then crouched down on his haunches. A student asked him about some hard brown pods he had found in the leaf litter. Ilson said, “Aricuri.” He picked up one of the pods—it was about the size and shape of a small cigar —and set it on a half-buried tree root. It rattled slightly. “Delicious,” he said, smiling. Then he took out his machete and, steadying the nut with his left hand, began lopping at one end with the heavy blade, precisely missing his fingertips by a fraction of an inch despite his hammerlike strokes. The pod’s fibrous coat was remarkably tough. Once he had cut his way through, three cavities could be seen running the length of the pod, one held dense white nut meat; two were hollow but rattled. Ilson tapped the end of the pod with his palm and out popped two small, pale, leathery grubs. Called gongo, they have the texture of a roasted chestnut and take on the flavor of the nuts on which they subsist. Tappers prefer to pick out the slightly rotten nuts and eat the grubs; the nuts themselves are simply too hard to extract from the pods.
The Burning Season Page 5