Tropical Rain Forest

About 5% of the earth's surface, representing nearly half of the forested areas of the earth, are included in the tropical rain forest biome (Fig. 26.9). The biome is distributed throughout those areas of the tropics where annual rainfall amounts normally range between 200 and 400 centimeters (79 and 157 inches) and where temperatures range between 25°C and 35°C (77°F and 95°F). Although monthly rainfall amounts vary, there is no dry season, and some precipitation

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516 Chapter 26

Figure 26.9 A tropical rain forest scene. (Courtesy Mary Lane Powell)

occurs throughout all 12 months of the year, frequently in the form of afternoon cloudbursts. The humidity seldom drops below 80%.

Such climatic conditions support a diversity of flora and fauna so great that the number of species exceeds those of all the other biomes combined. The forests are dominated by broad-leaved evergreen trees, whose trunks are often unbranched for as much as 40 or more meters (160 feet), with luxuriant crowns that form a beautiful dark green and several-layered canopy. The root systems are shallow and the tree trunks often buttressed (buttress roots are shown in Fig. 5.14). There are literally hundreds of species of such trees, each usually represented by widely scattered specimens.

Most of the plants of the rain forests are woody, although not all of them are evergreen. Several of the deciduous tree species shed their leaves from some branches, retain the leaves on others, and flower on yet other branches all at the same time, while branches of adjacent trees of the same species are losing their leaves or flowering at different times. Many hanging woody vines and even more numerous epiphytes—especially orchids and bromeliads—can be seen on or attached to tree branches. The epiphyte roots are not parasitic and have no contact with the ground. The plants are sustained entirely by rain water that accumulates in their leaf bases and by their own photosynthesis. Traces of minerals, also necessary to the growth of the epiphytes, accumulate in the rain water as it trickles over decaying bark and dust.

The multilayered canopy is so dense that very little light penetrates to the floor, and the few herbaceous plants that survive are generally confined to openings in the forest. Despite the lush growth, there is little accumulation of litter or humus, and the soil is relatively poor. Decomposers rapidly break down any leaves or other organic material on the forest floor, and the nutrients released by decomposition are quickly recycled or leached by the heavy rains.

A few larger animals, with adaptations for moving through the mesh of branches in the rain forest, are found on the forest floor. Such animals include peccaries, tapirs, and

Biomes, which are biotic communities viewed on global or continental scales, are associated with particular climates determined mainly by latitude, altitude, and distance from coastal areas. While these large-scale communities include all groups of organisms, biomes are distinguished primarily by their predominant plants and plant growth forms. The biomes of North America and their characteristic plant growth forms include tundra (few trees, lichens, mosses and grasses), taiga or boreal forest (coniferous trees, deciduous trees in wetter areas), temperate deciduous forest (deciduous trees, diverse understory herbaceous plants, conifers at geographical margins), grassland (perennial and annual grasses), deserts (drought-adapted plants), mountain forests (coniferous trees), and tropical rain forests (high diversity of trees and vines). The biomes, which encompass the earth's terrestrial biodiversity, are an excellent focus for conservation efforts. Saving large tracts of biomes, such as the tropical rain forests, is a practical approach to preserving biodiversity for future generations.

anteaters. Most of the great numbers of animals, however, live out their lives in the canopies. Tree frogs, with adhesive pads on their toes, are common in tropical rain forests, as are various monkeys, bats (especially fruit bats), sloths, opossums, tree snakes, and lizards. Ants abound, and many of the extraordinary variety of other insects are adapted to their environment through excellent camouflages. Large flocks of parrots and other birds feed on the abundance of insects as well as the available fruits.

The Future of the Tropical Rain Forest Biome

In the 1960s, major plans were developed to convert the Amazon rain forest into large farms, hydroelectric plants, and mines. During the 1990s and early 2000s, gold-mining activities were filling the rivers with silt, and, as indicated in the discussion of the greenhouse effect in Chapter 25, the tropical rain forests were being destroyed or damaged for commercial purposes at the rate of more than 100 acres per minute. In one study of Amazon rain-forest birds conducted in 1989, it was found that a population of low-flying species in a 10-hectare (25-acre) section of the forest fell by 75% within just 6 weeks after adjacent land had been cleared, and 10 of the 48 species apparently disappeared completely.

This situation has been repeated on numerous occasions and confirmed by a project cosponsored by the Smithsonian Institution and Brazil's Institute for Research in the Amazon.

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During 10 years of study, this project demonstrated that when large expanses of forest are cut into smaller pieces separated by as little as 10 meters (33 feet) of cleared land, it can have disastrous effects on the ecology of the entire forest, with most of the bird and other animal species originally present disappearing permanently. Very little of this biome that is the home of more than 50% of all living species of organisms is presently protected from commercial development. Many of the organisms are doomed to extinction, often before they have been described for the first time, and the biome will essentially have vanished within 20 years if governments and individuals do not take definitive action to halt the large-scale destruction.

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