Tropical rainforests have the greatest biodiversity. These are classified as forests that have year-round high temperatures with plentiful rainfall, averaging 50 to 260 inches per year. The humidity in a tropical rainforest averages between 77 to 88 percent.
While only covering 6 percent of the ground surface, tropical rainforests contain an estimated 50 percent of the world's terrestrial animal species and up to 80 percent of the world's plant species. In one hectare in South America, scientists found 300 different species of trees. Because of the high biodiversity and amount of plant growth, tropical rainforests produce 40 percent of the world's oxygen. At one time, tropical rainforests covered more than 15 percent of the ground surface, but humans converted much of the land to farmland and pastures.