TROPICAL RAIN FOREST
The tropical rain forests are the richest biome in terms of number of species,
probably containing at least half of the species of terrestrial organisms-
more than 2 million species, and possibly as much as ten times that many.
In such rain forests neither water nor temperature is a limiting factor, and
an evergreen loresi of giant trees supports a rich and diverse assemblage
of plants and animals on its branches, high above the forest floor. Plants
that grow on the branches of other plains are called epiphytes and, along
with the numerous vines [hai occur in tropical forests, they reach the light
high up in the foresi canopy. Hemiepiphytes, such as strangler figs, begin
their life as epiphytes, but eventually their roots reach down to the soil,
and they become rooted in the forest floor The communities that make up tropical
forests are rich in species and are diverse, ••n that each kind
of plant, animal, or microorganism is often represented in a given 31-1,1
by few individuals. There are seldom fewer than 40 species of tree per hectare,
four or five times as many as are typical in temperate forests. In a single
square mileoi tropical forest in Peru or Brazil there may be 1500 or more
species of butterflies- twice the total number found in the United States
and Canada combined. The ways of life of tropical organisms are often specialized
and highly unusual
some were discussed. This pattern of narrow ecological specialization enables
many species to occur together in the tropical rain forests.
The rainfall in areas where tropical rain forests occur is generally 200 to
450 centimeters per year, with little difference in its distribution from
season to season, There are, of course, substantial changes with elevation.
About two thirds of the soils of the tropics are acidic and deficient in phosphorus,
potassium, calcium, magnesium, and other nutrients. In addition, the phosphorus
in them tends to combine with iron or aluminum to form insoluble compounds
that are not available to plants. Such soils also tend to have toxic levels-of
aluminum. Most of the roots of the trees spread out in a thin layer of soil,
often no more than a few centimeters thick. These roots transfer the nutrients
from the leaves and other fallen organic debris quickly and efficiently back
to the trees themselves. Below the thin layer of topsoil, there is virtually
no organic matter, and nutrients are scarce; most of the nutrients that exist
in the system are concentrated in the trees themselves.
Tropical rain forests are highly productive, even though they exist mainly
on infertile soils. Most of the nutrients are held within the plants themselves
and are rapidly recycled when the plants die or when parts, such as leaves,
are lost.
Tropical rain forests are widespread in South America, particularly in and
around the Amazon Basin; in Africa, particularly in Central and West Africa;
and in South-east Asia. Their ecological properties are so unusual that we
simply do not know, in. must cases, how to cultivate them in such a way as
to maintain the agricultural productivity of these areas year after year once
their forest cover has been removed. Even~ so. these forests are being cut
down at an ever-increasing rate, mainly by people living ji the edge of starvation.
(The human population of the tropics and subtropics now constitutes more than
half of the world total, and their numbers are growing rapidly, hiremely poor
people constitute more than a third of the population of most of these countries.
The small fields that these people clear by what are called slash-and-burn
methods can generally be cultivated for only a few years, after which they
are worthless, unless they are given many decades to recover. When wide areas
are cleared, the Irrests will probably never recover. The thin soils erode
rapidly, and the minerals are ;arried away with the trees and the crops that
are harvested from the cultivated plants. Consequently, few tropical forests
are likely to be left in an undisturbed condition anywhere in the world by
the first part of the next century.
The destruction and disturbance of all tropical rain forests will be accompanied
b'. the extinction of a major proportion of all plant, animal, and microorganism
species on earth-perhaps one fourth-during the lifetime of many of us. Studying
the :Unts and animals of these forests-which are the most poorly known, as
well as the Hast numerous in terms of numbers of species on earth at present-is
a matter of passing importance from both a scientific point of view and from
the point of view of improving the conditions of life for human beings- Such
study would undoubtedly uncover many species of organisms of great scientific
interest and of potential importance in terms of contributions to the quality
of human life.
