Tropical rainforests are distributed in equatorial regions of Central and South America (most extensively in Amazonia), west-central equatorial Africa, and South and Southeast Asia through to New Guinea and the northeastern coast of Australia. Tropical rainforests are the most complex of the world's ecosystems in terms of the physical structure that they develop, and also in their tremendous biodiversity of species and community types. Because of these characteristics, tropical rainforests represent the acme of ecosystem development on Earth.
Tropical rainforests have a very complex canopy, consisting of multiple, intermeshed layers of foliage. The area of this canopy can be equivalent to 12-13 sq yds (10-11 m2) of foliage per sq yd (m2) of ground surface. This is among the densest foliar surfaces maintained by any of Earth's ecosystems, a characteristic that allows a relatively great efficiency of capture of solar energy and its conversion into plant biomass. Of course,
the most important foliar layer of the tropical rainforest consists of the upper canopy of the largest trees, which extends to more than 330 ft (100 m) in height in some cases. However, there are also lower canopies associated with the foliage of shorter, subdominant trees, and with lianas (or vines), shrubs, and ground vegetation. These subordinate canopies are everywhere, but they are best developed where gaps in the overstory allow some sunlight to penetrate deeper into the forest.
Tropical rainforests also have a uniquely rich canopy of epiphytes, or plants that use other plants as a substrate upon which to grow. There are especially large numbers of epiphytic species in the orchid (Orchidaceae) and air-plant (Bromeliaceae) families, of ferns and their relatives (Pteridophytes), and of mosses, liverworts, and lichens. Some species of woody plants, known as strangler figs (Ficus spp.), begin their lives as epiphytes, but if they are successful they eventually turn into full-sized trees. The sticky, bird-dispersed seeds of strangler figs are adapted to finding appropriate nooks high in the canopy of a tall tree, where they germinate and live as an epiphyte, independent of the soil far below. However, as the seedling grows into an aerial shrub, it begins to send roots down towards the ground. If the ground is eventually reached, the strangler fig is no longer a true epiphyte, although it continues to rely on the host tree for mechanical support. Over time, the strangler fig sends more and more of these roots downwards, until their coalescing biomass eventually encircles the host tree and prevents it from growing radially, while the fig preempts the space occupied by its foliage. Eventually the host tree is killed, and its place in the forest canopy is assumed by the hollow-trunked strangler fig.
About 80% of the ecosystem biomass of tropical rainforests occurs as woody tissues of trees, while only about 15% of the organic matter occurs in soil and litter, and about 5% is foliage. (As with all forests, the biomass of animals is much less than 1% of that of the total ecosystem.) In contrast, temperate forests maintain much larger fractions of their total ecosystem biomass as organic matter of the soil and forest floor. The reason for this difference is the relatively rapid rate of decomposition of dead biomass in the warm and humid environmental conditions of tropical rainforests. Because most of the biomass and nutrient content of tropical rain-forests occurs in the biomass of living trees, and because their soils are usually highly infertile and extremely weathered, the fertility of this ecosystem is rapidly degraded after the forest is cleared. This is especially true if the site is converted to an agriculture land-use.
An enormous number of species of plants, animals, and microorganisms occurs in tropical rainforests, and this type of ecosystem accounts for a much larger fraction of Earth's biodiversity than any other category. Of the 1.7 million species that biologists have so far identified, about 35% occur in the tropics, although less than one-half of those are from tropical rainforests. However, this is actually a gross underestimate of the importance of tropical rainforests in this regard, because relatively few of the species of this ecosystem have been identified. In fact, some biologists have estimated that as many as 30-50 million species could occur on Earth, and that about 90% of them inhabit tropical ecosystems, the great majority of those in rainforests. Most of the undiscovered species are insects, especially beetles. However, tropical rainforests also harbor immense numbers of undiscovered species of other arthropods, as well as many new plants and microorganisms. Even new species of birds and mammals are being discovered in tropical rainforests, further highlighting the frontier nature of the biological and ecological explorations of that biodiverse natural ecosystem.
Clearly, tropical rainforests are enormously rich in species. For example, an area of only 0.25 acre (0.1 ha) in a rainforest in Ecuador had 365 species of vascular plants, while a 7.5 acre (3 ha) plot in Borneo had more than 700 species of woody plants alone. Such rainforests typically have hundreds of species of full-sized trees. In comparison, temperate rainforests typically have no more than 10-12 species of trees, and often fewer. Tropical rainforests also typically support more than 300-400 bird species, compared with fewer than about 40 in temperate forests. If we had access to accurate knowledge of the insect species of tropical rainforests, an even more enormous difference in species richness could be demonstrated, in comparison with temperate forests. The extraordinary biodiversity of tropical rainforests is probably the most critical, defining attribute of this ecosystem, and is a natural heritage that must be preserved for all time.