Biodiversity

About 1.7 million of Earth's species have been identified and designated with a scientific name. About 6% of the identified species live in boreal or polar latitudes, 59% in the temperate zones, and the remaining 35% in the tropics. However, the knowledge of Earth's species is highly incomplete, especially for tropical countries. According to some estimates, there could be as many as 30-50 million species on Earth, 90% of them occurring in tropical ecosystems. Tropical ecosystems are much richer in species than are those at higher latitudes.

Most of the described species on Earth are invertebrates, particularly insects, and most of the insects are beetles (order Coleoptera). A famous scientist, J.B.S. Haldane, was once asked by a theologian to briefly explain what his knowledge of biology told of God's purpose. Haldane reputedly said that God has "an inordinate fondness of beetles," reflecting the fact these insects are so much richer in species than any other group of creatures on Earth. Some biologists believe that beetles account for most of the undescribed tropical insects.

The suggestion of enormous numbers of undescribed insects in tropical forests initially emerged from the research of Terry Erwin. This entomologist performed experiments in which tropical-forest canopies in Amazonia were treated with an insecticide, and the subsequent "rain" of dead arthropods was collected using ground-level sampling devices. This innovative sampling procedure indicated that: (1) a large fraction of the insect species of tropical forests is unknown to science; (2) most insect species are confined to a single type of tropical forest, or even to a particular tree species, which may itself have only a local distribution; and (3) most species of tropical-forest insects have little ability to disperse very far. Erwin's studies of tropical rainforest found that beetles accounted for most of the insect species and that most of the beetles are narrowly endemic, that is, they have a local distribution and are found nowhere else. For example, the tree Luehea seemanii has more than 1,100 species of beetle in its canopy, of which 15% are specific to that plant. The emerging conclusion from this and other descriptive research is that there is an enormous abundance of undescribed species of insects and other invertebrates living in tropical forests.

Compared with invertebrates, the numbers of species (that is, the species richness) of other groups of tropical-forest organisms are better known. Although it is extremely difficult to do so, the numbers of species of vascular plants have been described for a few tropical forests. For example: a plot of only 0.0004 sq mi (0.1 ha) in a moist forest in Ecuador had 365 species of vascular plants; there were 98 species of large trees in 0.006 sq mi (1.5 ha) of forest in Sarawak, Malaysia; there were 90 tree species in 0.0032 sq mi (0.8 ha) of forest in Papua New Guinea; 742 woody species occurred in 0.012 sq mi (3 ha) of forest in Sarawak, with 50% of the species recorded as single individuals; and more than 300 species of woody plants were discovered on a 2-sq mi (50-ha) area of forest in Panama. These tropical forests are much richer than temperate forests, stands of which typically support fewer than 12-15 species of trees. The Great Smokey Mountains of the eastern United States have some of the richest temperate forests in the world, and they typically contain 30-35 species, far fewer than tropical rainforests.

It is extraordinarily difficult to determine the numbers of birds in tropical forest because the dense foliage and darkness of the understory make it inconvenient to see small animals, even if they are brightly colored. As a result, few studies have been made of the birds of tropical rainforest. However, one study in Peru discovered 245 resident and 74 transient species in 0.4 sq mi (97 ha) of Amazonian forest. Another study of rainforest in French Guiana recorded 239 species of birds, and another found 151 species in a forest in Sumatra. In comparison, stands of temperate forest in North America typically support only 15-30 species of birds.

Almost no systematic surveys have been made of all of the species of tropical ecosystems. In one case, a 42 sq mi (108 sq km) reserve of dry forest in Costa Rica was estimated to support about 700 plant species, 400 vertebrate species, and 13,000 species of insects, including 3,140 species of moths and butterflies.