High Diversity

The phenomenal diversity of plants and animals in the tropics was first brought to the attention of Europeans by the missionaries and explorers in the 16th and 17th centuries. Eighteenth and 19th century scientists began documenting the biological riches of the tropics and proclaimed them to be extraordinary (Chazdon and Earl of Cranbrook 2002). Although it was not until the 20th century that the concept of species diversity became generally understood, Alfred Russell Wallace in his 1878 book, Tropical Nature and Other Essays, recognized high diversity when he recounted his difficulty in finding two individuals of the same species in tropical forests.

The primeval forests of the equatorial zone are grand and overwhelming by their vastness, and by the display of a force of development and vigour of growth rarely or never witnessed in temperate climates. Among their best distinguishing features are the variety of forms and species which everywhere meet and grow side by side, and the extent to which parasites, epiphytes, and creepers fill up every available station with peculiar modes of life. If the traveler notices a particular species and wishes to find more like it, he may often turn his eyes in vain in every direction. Trees of varied forms, dimensions, and colours are around him, but he rarely sees any one of them repeated. Time after time he goes towards a tree which looks like the one he seeks, but a closer examination proves it to be distinct. He may at length, perhaps, meet with a second specimen half a mile off, or may fail altogether, till on another occasion he stumbles on one by accident.

For almost all types of organisms, the number of species that existed in the tropics was far higher than the number these scientists were accustomed to seeing in their homelands. H.W. Bates (1864) in his book, The Naturalist on the River Amazonas, wrote:

I found about 550 distinct species of butterflies at Ega. Those who know a little of Entomology will be able to form some idea of the riches of the place in this department, when I mention that eighteen species of true Papilio (the swallow-tail genus) were found within ten minutes' walk of my house. No fact could speak more plainly of the surpassing exuberance of the vegetation, the varied nature of the land, the perennial warmth and humidity of the climate.

Perhaps the most famous commentator on tropical species diversity was Charles Darwin, who in his 1855 classic Journal of Researches into the Natural History and Geology of the Countries Visited During the Voyage of H.M.S. Beagle Round the World wrote as follows about the Galápagos Islands:

I have not as yet noticed by far the most remarkable feature in the natural history of this archipelago; it is, that the different islands to a considerable extent are inhabited by a different set of beings. My attention was first called to this fact by the Vice-Governor, Mr. Lawson, declaring that the tortoises differed from the different islands, and that he could with certainty tell from which island any one was brought. I did not for some time pay sufficient attention to this statement, and I had already partially mingled together the collections from two of the islands. I never dreamed that islands, about 50 or 60 miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted; but we shall soon see that this is the case.

In addition, the many finch species he encountered in the Galápagos Islands, many more than in his native England, stimulated his thinking on evolution.

Tree species diversity in tropical forests is perhaps better documented than diversity of other species, simply because trees are easier to see and count. Some tree species in rain forests are common, but most are rare: in the richest rain forests, every second tree is a different species (Whitmore 1998). It is quite common to find just a few individuals of each species per hectare. For example, a 40-m, or even a 70- to 80-m average distance between trees of the same species may be common. In eastern Sarawak, the density of three tree species averaged 3.6 trees/ha, and 4-5 trees/ha was considered to be high density (Jacobs 1988).

Even the best-represented tree species comprise a low proportion of the total number of species, perhaps a maximum of 15%. Some tropical forests are called by the name of a species that is characteristic of that forest, but is not necessarily dominant. For example, Richards (1996) describes the Mora forest in Trinidad and the Eperua falcata forest in Surinam, concluding that their presence is due to some limiting environmental conditions, mainly soils of poor drainage. A similar pattern is found in the Carapa guianensis forests in the Caribbean lowlands of Costa Rica, that are found on swamps, and with the Pentaclethra macroloba forests at La Selva Biological Station in Costa Rica (Hartshorn and Hammel 1994).

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