Environmental Engineering Reference
In-Depth Information
between one and fifty species per day world-wide. Whatever the true number, we are
definitely increasing the rate of extinctions which would occur naturally.
LOSS OF NATURAL RESOURCES
The rain forests act as a reservoir of natural resources in the form of fruits, food, timber,
raw materials and medicines. Species extinction through clearance would lead to the loss
of any of these products. Greater concern has been expressed about the loss of resources
which have yet to be discovered. How many species living in the rain forest might be
commercially exploited? There may be much genetic diversity which is needed for
improved plant breeding to sustain our increasing population. An example of this
followed the blight which developed on the US maize crop in 1970 and halved
production in an area which acts as the main cereal surplus area of the world. Other
varieties of maize which were immune to that form of blight were sought and suitable
stock was found in Mexico. Later a new species of maize was found in the Mexican rain
forest which was immune to at least seven major diseases and which could be grown in
cooler, damper environments. World maize production should increase in areas
previously unsuitable. Ironically the maize was found in an area undergoing clearance
and only a few thousand stalks remained.
It is also believed that the rain forest has potential material for medicines. Almost a
quarter of the drugs prescribed in the United States are derived from tropical rain forest
plants, and at least 2000 rain forest plants have been identified by the US National Cancer
Institute as having anti-cancer properties.
ENVIRONMENTAL FACTORS
As well as their biological role, the rain forests interact with their environment and affect
the soils, hydrology and climate. Let us have a look at these aspects in turn.
Soils
. Clearance of trees removes the main source of nutrients to the forest soils and at
the same time allows rainwater to reach the floor unmodified by the canopy. As it is very
difficult to replace nutrients by the addition of fertilizer the soils will rapidly become
poorer through the leaching effects of heavy rainfall (Figure 28.9).
With complete forest cover the soils are protected from erosion by root mats, dense
ground and decomposing vegetation act as a sponge and the canopy provides a shield
from intense rainfall. Once it is removed, run-off is increased and erosion rates rise
dramatically, especially on slopes. Severe erosion can strip off top soil down to the
impermeable lateritic hardpan, which makes recolonization difficult. Additionally, the
run-off may become concentrated into gullies, which greatly increase sediment yield into
rivers. Logging activity often generates erosion along their access tracks. In Thailand in
1988 forty people were killed in mudslides which were blamed on illegal logging.
Run-off
. The more rapid run-off associated with deforestation can give rise to flooding
downstream of the affected area. In Venezuela mass movement caused by unseasonal
floods in December 1999 was accentuated by development and forest clearance on
unstable mountain slopes near Caracas. It was estimated that up to 30,000 people died
and up to 400,000 were made homeless as poorly constructed shanty dwellings were
swept down the hillsides into the roaring torrents that occupied the
