16.11: Human activities can damage the environment: 4. Deforestation of rain forests causes loss of species and the release of carbon.

Towering trees, colorful birds and butterflies, maybe a glimpse of Tarzan swinging past on a hanging vine—that is the popular image of a tropical rain forest. And it is a reasonably accurate picture (minus the Tarzan part). But it is also a picture that is rapidly fading, as agriculture, logging, gold mines, and oil wells destroy tropical forests (FIGURE 16-27).

Tropical rain forests grow in a region extending just a bit north and south of the equator, in South America, Africa, Asia, and Australia. As recently as a few centuries ago, that belt of rain forest covered about 6 million square miles (15.5 million square kilometers). More than half of that forest has already been destroyed, most of it in just the past 200 years (FIGURE 16-28).

The destruction of a tropical rain forest means an enormous loss of species, because these forests are the most diverse terrestrial habitats on earth. More than 170,000 species of plants grow in tropical rain forests—more than 60% of the total number of living plant species in the world. A survey of a Brazilian rain forest found 487 different species of trees in just 2.5 acres (1 hectare), an area less than half the size of one city block in Manhattan. To put that number in perspective, consider that only 700 species of trees are found in the 5 billion acres (2 billion hectares) of the United States and Canada combined!

Agriculture is responsible for the greatest loss of tropical forests, and it takes a variety of forms. Sometimes, a relatively small area is affected. In “slash-and-burn agriculture,” trees are cut and burned, and crops are planted in the newly opened area. Usually, just a few acres of forest are cleared, but the land is fertile for only two or three years after it has been cleared. Then that plot is abandoned and more forest is cleared. So the cumulative effect of slash-and-burn agriculture is substantial. At the other end of the size scale, multinational corporations clear hundreds of acres of forest to plant bananas, coffee, or oil palms, and clear thousands of acres of forest at a time to create pastures for cattle.

Pollution from oil wells and mining in tropical rain forests has an impact that can extend far beyond the areas that are actually cleared. Leaking oil contaminates streams and groundwater, and acidic water drains from mines. Even worse, gold miners use mercury to extract gold, and some of the mercury enters streams, where it is converted to methyl mercury—a toxic compound that accumulates in plant and animal tissues as it moves up the food chain.

Agriculture and mining both require roads to bring equipment to the sites and take crops or minerals to market. Roads have an impact on forests that greatly exceeds the relatively small area they occupy, because they make access to the forest easy. Traveling through a virgin tropical rain forest is difficult—bogs and natural tree falls often prevent travel in a straight line; slopes are often steep and the wet soil is slippery; and the ground-level vegetation can be both dense and thorny. No wheeled vehicle larger than a motorcycle can penetrate most rain forests, and traveling by motorcycle is typically slower than walking. These difficulties protect rain forests, but as soon as a road is bulldozed through a forest, people flock to it. They and their activities then spread into the forest on both sides of the road, creating new clearings that spread farther and farther into the forest.

Destruction of tropical rain forests has two serious environmental impacts: reducing the earth’s biodiversity and increasing the concentration of greenhouse gases in the atmosphere (which, in turn, affects global climate change).

1. Reducing biodiversity. Tropical rain forests of Africa, Asia, the Pacific region, and Central and South America, as we’ve seen, contain an unusually large number of species of plants and animals and probably other groups of organisms. Half of the world’s biodiversity hotspots are in tropical rain forests.

2. Increasing greenhouse gases. Photosynthesis in tropical rain forests removes an estimated 610 billion tons (550 trillion kg) of carbon dioxide from the atmosphere each year. As we learned earlier, accumulation of carbon dioxide in the atmosphere is the major cause of global warming. Thus, the continued photosynthetic activity of tropical rain forests is important in slowing the rate of warming. The huge quantity of carbon stored in rain forests can have a downside, however. When forests are cleared and burned, that carbon is released into the atmosphere. And tropical forests are being cleared at a frightening rate—more than 50,000 square miles (an area larger than the state of New York!) each year between 2000 and 2010.

Because deforestation alters the characteristics of land cover, changing the proportion of solar radiation reflected and absorbed, destruction of the rain forests can have additional significant effects on climate—including rainfall amounts and seasonality. These changes can further accelerate climate change, with potentially significant impacts on ecosystems.

The problem of tropical deforestation is one of the most difficult environmental problems to solve, and solving it will rely on international cooperation and involve multiple strategies: (1) identifying and protecting the most diverse areas; (2) addressing the poverty that drives the need to destroy rain forests for human activities; (3) developing alternative sources of food and income; (4) reducing population growth; and (5) making education about the value of biodiversity a central part of these solutions.

In spite of these varied and complex difficulties, numerous programs around the world are beginning to show some success (FIGURE 16-29). In the state of Pará, in the Brazilian Amazon, for example, encouraging progress has been made. As of 2012, this region, which is three times the size of California, has nearly eliminated illegal deforestation by adopting several new forest management practices.

• Accountability and protection: establishing a local system requiring landowners to register their holdings and an environmental police force to identify and punish illegal deforestation.
• Education: creating training programs for ranchers and farmers to help them make more efficient use of already-deforested land.
• Better forest management: developing environmentally sound infrastructure, such as reducing the building of new roads, long associated with increasing deforestation, in favor of improving existing roads and thus enhancing the ability to bring renewable forest products to market.

The initial transition is difficult, particularly because it can be accompanied by job losses. But efforts are under way to offset the loss of jobs associated with deforestation with increased economic development in non-deforestation-related areas. These areas include ecosystem services such as water management and non-wood forest products, including medicinal plants. The governor of Pará is realistic about the challenge, but still hopeful. In a 2012 speech about his state’s goal of “zero net deforestation” by 2020, he identified an essential element of the program, saying, “Additional reduction in deforestation is going to be very difficult. Locals have to really feel they are part of the solution.”