Rain forests are also potent climatic and environmental stabilizers. A fifth of the earth's fresh water cycles through the Amazonian filter and back to the Atlantic each year.
Time is running out for the most valuable bioregion on the planet, our tropical rain forests.
The World's Tropical Rain Forests
Imagine a place where it's dusk at noon, where the temperature seldom varies from 80 degrees Fahrenheit and the humidity is always close to 100%. You stand on a spongy mat of decaying leaves that few plants penetrate. Massive roots lying on the ground gather to buttress tree trunks that vanish into a leafy canopy at least 30 feet above. As open as it is at ground level, the foliage layer overhead is so dense that less than 3% of the sunlight falling on the treetops 150 feet above filters to the forest floor. But for the occasional piercing calls of birds, there's scant evidence of animal life. Little would you guess that this is home to half the species on our planet.
Misconceptions abound about what a rain forest is and what value it has. To people of temperate climates, the tropical rain forest is mysterious, alien. Hollywood images are threatening: thick, viper-and vermin-ridden undergrowth accessible only to sweating, machete-wielding persons of limited (and often diminishing) sanity. As a result, tropical rain forests too often is taken to mean jungle.
The Earth's Greenbelt
Tropical forests are no more monolithic than they are jungles. Botanists subdivide them into 30 or 40 categories, but we can probably get by if we stretch the term tropical rain forest to include woodlands that get over 80 inches of rainfall per year and are close enough to the equator to be so unaffected by seasons that all the trees are evergreen.
Virgin tropical rain forests wrap around the equator, covering about 1,500,000 square miles. Almost 60% are located in the Amazonian regions of Brazil, Peru, Colombia, Bolivia and Venezuela, but there are other significant stands in Indonesia, Zaire, Papau-New Guinea and Burma.
For hundreds of millions of years, tropical rain forests have protected life-forms against the vagaries of climate—serving as Earth's safe-deposit boxes of biological diversity. They have grown and shrunk with the eons, they have been divided by the parting of the continents, and different areas have evolved separately. But threads can be tied from current residents to species of the early Cretaceous period, 140 million years ago.
Perhaps as a result of their long-term stability, rain forests contain an incredible variety of different plants and animals. While covering only about 2% of the world's land area, they harbor over half our planet's roughly 10 million species. In the Amazon alone, there are more than 50,000 species of plants and 3,000 offish. The whole of North America has only about 17,000 plant species and Europe has only 300 different types of fish. Costa Rica alone is home to 103 species of bats; all of North America, 40.
Even more impressive than the raw totals is the geographical diversity. Imagine 12,500 different types of beetles on 2½ acres in Panama. In Brazil, botanist Ghillean Prance has identified 236 species of trees greater than two inches in diameter in a similar area. In a typical New England woods, you might find five or six species. Ten square feet of rain-forest floor leaf litter has turned up 50 types of ants.
Two-thirds of the species and as much as 80% of the nutrients in the rain forest are in the canopy. Trees, for example, efficiently and rapidly cycle nutrients and water from the leaf litter into their biomass. Root fungi called mycorrhizae dramatically enhance this process. Because they are so old and receive so little of the nutrients from above, rainforest soils are typically infertile and acidic. They offer little more than a physical base for the rich ecosystem operating dozens of feet above.
The life-forms of this cornucopia have coevolved in ways that drive biologists to dizzy rapture. The ant acacia tree, for example, is completely dependent on its ant residents for defense against other insects and plants. The workers fight off intruders, clean leaves and kill anything that grows within 30 inches of the acacia. In return, the acacia supplies the ants with their entire diet. A successful ant acacia will have fewer than 2% of its shoots occupied by insects other than its ant friends. One without a colony will do poorly, if it survives at all.
Tropical Rain Forests
The rain forest's abundance and complexity have evolved many species that live only in very restricted areas. Of Ecuador's 20,000 different plants, a fifth are endemic. Isolation can make endemism more extreme. Ninety-eight percent of Hawaii's species live nowhere else. Likewise, their density may be very low—perhaps only one individual per acre. Symbiosis, complexity, endemism and rarity all contribute to make the tropical rain forest a fragile ecosystem.
A plethora of products come to us from tropical rain forests. Among the obvious are timber (half the world's annual hardwood harvest), raw materials for latex, Brazil nuts ($16 million worth to the U.S. each year), fruits, oils, spices and even shade-loving houseplants such as the philodendron.
Less well known is the fact that about 25% of all prescription pharmaceuticals are derived from plants growing in tropical rain forests. Alkaloidal drugs from rosy periwinkle found on Madagascar have revolutionized the treatment of lymphocytic leukemia in children. Scientists at the National Cancer Institute state that 70% of the anticancer drugs with promise come from the tropical rain forest. Without curare, heart surgery would be impossible. The medicinal potential in tropical rain-forest plants has barely been touched. Only one in 10 has been examined for medicinal use.
Rain forests are also potent climatic and environmental stabilizers. At least 50%, and in some cases as much as 80%, of the rain that falls on them is evaporated or transpired by plants back into the air—cleansed—within a week. A fifth of the earth's fresh water cycles through the Amazonian filter and back to the Atlantic each year. On the order of 200 billion tons of carbon is bound up in the plants of the rain forest, carbon that might otherwise be in the form of carbon dioxide and contribute to the greenhouse effect.
As few as 3,000 years ago, tropical rain forests covered more than 3 million square miles of Earth's surface—twice what they do now. They are currently being converted to nonproductive forest and agricultural lands at a rate of about 25,000 square miles, an area about the size of West Virginia, each year. Two-thirds of the rain forest in Central America is already gone. Madagascar and the Ivory Coast will be completely stripped by 2000—sooner if cutting rates increase. Worldwide, 20% of the present rain forest will be gone by 2000. Even these ominous figures may be optimistic, since deforestation rates are increasing nearly everywhere and in some places (Brazil's Rondonia and Acre states, for example) at exponential rates.
Even the selective cutting of trees won't serve as an effective preservative. Beyond a threshold level of thinning—about 30% removal of the canopy—rain forest ceases to be rain forest. Sunlight dries the forest floor, disrupting the fragile system. The effect on animals may be even more profound. A loss of 10% of habitat can result in a 50% loss of species. In some areas, over 90% of the birds are so accustomed to the relative darkness of the rain forest that they won't cross a clearing to breed or to find food.
Rain forests don't regenerate quickly, either. As long as the logged area isn't burned (as most are) and isn't too large, vegetation may approach the original state in 50 to 120 years. In massive slash-and-burn operations, however, the rain forest may never return to full verdancy. Endemic species may be entirely lost, or the cut may be too big to reseed naturally. After all, it isn't surprising that something that took eons to reach its present state won't regenerate within decades.
Cutting large areas of rain forest also may affect regional climate enough to harm untouched rain forest (not to mention area agriculture). Because a rain forest returns so much rain to the atmosphere, moisture is recycled many times before it finally flows back to the ocean. But when the trees are removed, rain runs off at as much as 20 times the rate as before. Water flowing away down silt-laden rivers will be unavailable for trees downwind. If, as is now feared, 8% of Brazil's tropical rain forest is cut by 2000, average rainfall in the Amazon may drop by 25 inches annually.
Tropical Rain Forest Destruction
The causes of rain-forest destruction are various but mostly arise from a combination of social, economic and political problems. Worldwide, the most significant contributors have been land reform, cattle ranching, logging and hydroelectricity.
In 1970, Brazil's President Medici devised a plan to solve two problems at once: squalid conditions for peasants in northeast Brazil and concern about the country's sovereignty in the largely uninhabited western and northern states. He initiated construction of the Transamazon highway in hopes of moving a million settlers west—it would have seemed to be a brilliant scheme in a country where 2% of the people own half the land.
But initially, at least, it was a miserable failure. Fewer than 20,000 people settled permanently in the first years. The land just wouldn't support the monocultures the government was advocating. Slash-and-burn clearing gives the soil a temporary boost— from the nutrients bound in the vegetation— that is quickly burned up. Most of the immigrant farmers found that after one good year, crops failed. They abandoned the devastated land and moved back east.
In an attempt to salvage the program, the next president, Ernesto Geisel, instituted tax incentives for cattle ranching in the Amazon—a purpose for which the rain-forest soils are also uniquely unsuited. Monoculture grasses do poorly on infertile soils, where the nights are warm (requiring calories to be burned in respiration) and there's no cold season to control pests. It generally takes from five to 20 acres of converted rain forest to support a cow. Dedicate 55 square feet over an eight-year period, and you get one hamburger.
Colonization of the rain forests has thus followed a consistent cycle. Peasants cleared the land, failed within a few years and turned the property over to large cattle operations (for next to nothing), which got by on tax breaks and land speculation. Cattle ranching receives 70% of the tax credits offered by the government in Brazil, but it still wouldn't be a going concern without inflation of property values.
The real acceleration of Brazilian deforestation began in the mid-'70s, when the roads reached Rondonia, where soils are at least marginal for food crops. Since 1968, Rondonia's population has grown by 1,000%. Movement to the western states now amounts to a half-million people per year—30 busloads per day down the Transamazon Highway.
Cattle ranching has been the indirect (but major) cause of rain-forest clearing in Central America. Peasants displaced from more fertile lands by rapid conversion of fields to pasture have had little choice but to move into rain forests. Two-thirds of Central American agricultural land is used to grow beef, and the number of cattle in Central America doubled between 1959 and 1979. During the same period, per capita consumption declined. Most of the beef went to the U.S. to make fast-food hamburgers.
Logging accounts for a quarter of the destruction in Brazilian rain forest and is the main cause in Southeast Asia. Indonesia, Papua-New Guinea, Burma and the Philippines have cut more than half of their primary rain forest and shipped it to Japan and Western nations. The U.S. alone receives $2.2 billion worth each year.
Flooding of rain forest for hydroelectric dam reservoirs is a growing problem. Brazil's 8,000-megawatt Tucurui project is the most ambitious yet, covering nearly 800 square miles of forest. But Brazil has much bigger plans for the 100,000-megawatt potential of the Amazon and its tributaries. With help from the World Bank, it hopes to begin 100 new dams by 2000. Besides the destruction wrought by the dams themselves, industry attracted by the inexpensive power will take its toll. Furthermore, many of these reservoirs are expected to silt up and become unusable before the projects ever make a profit.
How to Help the Tropical Rain Forests
Just as the causes of tropical rain-forest devastation are diverse, the solutions are hard to pinpoint. At home, we can exercise restraint in using products we know are contributing to the destruction. Rainforest Action Network (San Francisco, CA) is organizing a fast-food boycott in the U.S., and Great Britain's division of Friends of the Earth is working on the Tropical Hardwood Product List, a guide to rain-forest wood products.
Likewise, we can lobby our legislators to restrain government agencies and lenders who fund tropical boondoggles. Congress has already put some reins on the U.S. Agency for International Development (AID). In 1985, the agency was told to weigh the effects on biodiversity of dams, roads and other projects it supports. Bills are pending to set aside AID funds to foster sound environmental practices and hire consultants.
No amount of external coercion, however, will relieve the growing problems of the dispossessed poor—the major cause of tropical rain-forest loss. Researchers are working on ways to improve the productivity of marginal land to relieve the pressure to clear new land. Many of the techniques are familiar: succession planting, intercropping, drip irrigation, natural pest controls. Others are new and exciting.
In the nick of time, we are beginning to tap the wealth of knowledge that indigenous peoples have about working with the rain forest. In Brazil, the Kayapo, of whom only about 10 remain, practice an amazingly sophisticated form of sustainable agriculture. They clear small plots, planted with a range of species in careful patterns, and have more than a dozen classifications for different soils. For a month or two each year, they move about the Amazon planting food along the trails. Their annual sabbatical may encompass an area the size of France.
Much is also being learned from the last remnants of Mayan culture in southern Mexico, the Lacandon. These people work their two-to-three-acre plots (milpas) on rotations of 20 to 30 years. A wide variety of species are interplanted, leaving no area for weeds to prosper. Annual burnings control pests. The productivity of a milpas is comparable to a North American agribiz operation—as much as 2½ tons of maize per acre per year.
Rain forests and their inhabitants are a resource the Western world has barely begun to understand. Today we merely mine the forests, a practice whose end is in sight. Unless the devastation is slowed, there may not be an opportunity for sustainable uses. As Aldo Leopold said, "The first rule of intelligent tinkering is to save all the parts."