There are many estimates of the biological diversity (in terms of species richness) of this planet, although relatively few species have been scientifically described – about 1.5 – 1.8 million in all (mostly insects, followed by plants and vertebrates). Even considering the described species alone, we have careful studies of only 1%. And we have very little reliable biodiversity data, not even species descriptions, on other groups, such as most bacteria, fungi, non-vascular plants, and invertebrates. [In this discussion we will exclude bacteria and fungi because so little is known of their taxonomy]. There may well be ten million species (some estimate as many as 30 million) existing today. The tropics have the greatest biodiversity on the planet, and, within the tropics, the areas richest in species are the rainforests. It is estimated that tropical forests, comprising only 6% of the world’s surface area, contain one-half to three-quarters of the earth’s species of plants and animals. This is in part because the groups of organisms which contain the most species (arthropods and flowering plants) are found in high concentrations in tropical forests. These species, although numerous, tend to have smaller geographical ranges than temperate species, and there is considerable endemism (the restriction of a species to a circumscribed area or region). Europe north of the Alps has fifty species of trees; eastern North America, 171; but even a small area of tropical forest may have 100 or 200 species of trees of reasonable size (Whitmore, 1995). In Borneo, 3200 species of plants can be found in 100 hectares of rainforest. In fact, a land area of 0.5 km2 in some tropical forests contains more tree species than does the entire land mass of Europe and North America combined.
The tropical rainforests richest in species are those of Southeast Asia; the poorest, those in Africa. This may be because Africa has mainly seasonal forest with relatively low rainfall and a long history of human intervention. Here there are few palms – only about 100 species compared to 1400 in Australasia – and 403 known species of orchids, compared to more than 5000 in Malesia. Other species – epiphytes and lianas, are comparably fewer in number than in other tropical regions. Within Africa, west-central Africa n forests have the highest biodiversity Malesia (the region of Southeast Asia including Malaysia and the western part of Indonesia), which has many mountains and islands, has at least 30,000 species of plants. Within this area, Borneo and peninsular Malaysia have the greatest variety of species. Here the dominant trees are called dipterocarps, of which Borneo alone has 267 species. Indonesia has more species of flowering plants, amphibians, birds and reptiles than all of Africa. The Mekong river, which passes through Laos and Vietnam, has more than 110 species of snails, and Asia has more than 80 genera of freshwater crabs and many turtles. Sri Lanka, although it has only 750 km2 of forest (less than 5% of its original forest cover), has recently been discovered to have more than 140 species of frogs (Meegaskumbura, et al., 2002). Within the Neotropics, the upper Amazon is the richest in the number of species. Amazonia, which also has an extremely rich flora and fauna, is dominated by leguminous trees of many genera and species. Here one will find 2000 species of bromeliads (the pineapple family) and 837 species of palms. There are 1383 known species of fish in Brazil alone and 456 in Central America (as compared to 192 species in Europe). Colombia, which is not very large, has perhaps the third most diverse forest in the world. It has 1815 bird species, 142 of which are endemic; approximately 700 species of amphibia, 367 of which are found nowhere else; and between 45,000 and 51,000 plants species, one-third of which are endemic. It has 10%- 20% of the worlds orchids.
Old-growth forests have greater biodiversity than younger forests, although differences may not be very considerable under conditions of natural disturbance and recovery. Lowland forests in regions with evenly-distributed rainfall also tend to have greater diversity, while soil fertility appears to have a lesser effect than rainfall levels. In Southeast Asia, diversity declines where soils are rich in magnesium and phosphorus.
Most of the organisms in rainforests are undescribed and unknown. Even today it is quite common to read reports of new mammals found in tropical forests. Last year the black-capped dwarf marmoset, previously unknown and one of the world’s smallest monkeys, was found in the Brazilian Amazon. It is the seventh new monkey species found in Brazil during the past seven years. More undoubtedly await discovery. Horns of an unknown wild ox were found a few years ago in a market in Vietnam. Tiny newly-discovered rodents tumble into buckets sunk into the ground in Madagascar. A spiny mouse species, its only relatives 1000 miles away in the Andes, has been found in Brazil’s Amazon basin. Three new species of mouse lemurs, the world’s smallest primates, have just been discovered in Madagascar. Lawrence Heaney of the Field Museum has found 11 new species of mammals in the Philippines within the past few years; he estimates that the number of known mammal species will rise from the current 4600+ to 8000 (Morell, 1996). Most of these will be found in the tropics. And mammals are perhaps the best-known group (because of their relatively large sizes and our interest in our closer relatives). We know woefully little of other types of organisms in the rainforest, and much of what we do know is simply an artefact of availability. Species which seem to be limited in distribution (i.e., endemic) may appear to be so simply because no one has collected them elsewhere. Other species, which are in fact common, have only been recently described (such as Caryodaphnopsis fosteri, one of the commonest tree species in upper Amazonian Peru, not described until 1986). A major timber tree, “asceite caspi,” the source of most construction wood in this part of Peru, has recently been found to be a previously undescribed species of Caraipa (Gentry, 1992).