The richest and most productive biological communities in the world are in the tropical forests. These forests have been reduced to less than half of their former extent by human activities and now cover only about 7% of Earth's land area. In this limited area, however, is about two-thirds of the vegetation mass and about half of all living species in the world.

The largest, lushest, and most biologically diverse of the remaining tropical moist forests are in the Amazon Basin of South America, the Congo River basin of central Africa, and the large islands of southeast Asia (Sumatra, Borneo, and Papua, New Guinea). Whereas the forests of mainland southeast Asia, western Africa, and Central America are strongly seasonal, with wet and dry seasons, the South American and central African forests are true rain forests. Rainfall is generally over 160 in (406 cm) per year and falls...