Destruction of habitat.
When people build cities or cut down forests to obtain wood or to
clear land for farming, they destroy the habitats that animals need to
survive. For example, grizzly bears and mountain lions once roamed
freely where the city of San Francisco now stands. But a wild grizzly
bear or mountain lion could not survive in San Francisco today.
The habitats of animals in tropical forests are particularly threatened today.
People are rapidly cutting down these forests to obtain such valuable
hardwoods as mahogany and teak. They are also clearing the land to
plant crops. However, soils in such areas are not especially fertile,
and farms there produce crops for only a few years. To continue farming
in such areas, people have to keep cutting down more of the forests to
create new farmland. By the early 1990's, about two-fifths of the
world's tropical forests had already been destroyed.
Many scientists and other people are
especially concerned about the destruction of tropical forests. They
point out that these forests have more biodiversity-that is, a greater
variety of plant and animal species-than any other place. One square
mile (2.6 square kilometers) of forest in South America may have more
species of birds and insects than many countries do. In fact,
biologists discovered a single tree in a tropical forest in Peru that
supported 43 species of ants. That is as many species of ants as live
in the entire United Kingdom.
Even though many types of plant and animal
life can be found in one place in the tropics, the total range of many
tropical species is extremely small. As a result, when a large area of
forest is cleared, all the members of some species are killed.
Pollution.
Various types of pollution can also destroy animals and their habitats.
Agricultural chemicals and industrial wastes sometimes drain into
ponds and streams and kill the plants and animals there. Air pollution
produced by factories that burn such fossil fuels as coal and oil has
seriously damaged forests and wildlife. Acid rain-rainfall with a high
concentration of sulfuric and nitric acids due to air pollution-kills
fish and other animals.
An increase in carbon dioxide in the
atmosphere presents a long-term threat to animals and habitats. Many
factories-as well as automobiles and power plants-release carbon
dioxide into the air. Forest trees and plants help absorb this gas, but
as more of them are cut down, carbon dioxide levels rise. Many
scientists believe that higher amounts of carbon dioxide in the
atmosphere speed up global warming caused by the phenomenon known as
the greenhouse effect. A major global warmup could produce significant
changes in Earth's climate. Such changes could destroy many kinds of
plants and animals.
Introduction of new species into an area can
sometimes have unexpected consequences. In the mid-1800's, for example,
people introduced rabbits into the wild in Australia for sport.
However, the rabbits had no natural enemies there, and their population
grew quickly. Partly as a result of the rapid increase and spread of
rabbits, rabbit-eared bandicoots, which are native to Australia,
disappeared from some areas of the continent. The bandicoots had to
compete with the rabbits for burrow space. The traps and poisons people
set out for rabbits also killed bandicoots.
People may unintentionally cause new species
to enter an area. Zebra mussels are shellfish that are native to the
area around the Caspian Sea, which lies between Europe and Asia. They
were first found in North America in 1988. Their larvae had been
unintentionally released into the Great Lakes in ballast water, the
water kept in the hold of a ship to keep the vessel stable. Today, the
mussels are a major pest in North America. The explosive growth of
zebra mussel populations may threaten the food supply of many species
of fish and shellfish that are native to the Great Lakes.
Hunting.
Through the centuries, people have overhunted certain animals and
caused them to become extinct. For example, prehistoric hunters
probably helped make woolly mammoths and mastodons extinct.
Overhunting in the past 200 years has been
especially destructive of animal life. It contributed to the extinction
of such animals as the great auk, the passenger pigeon, and the
Steller's sea cow.
Human population growth. The
human population is growing rapidly. By the late 1990's, the world had
almost 6 billion people-about five times as many as it had in 1850.
Some experts predict that by 2050 the population will have about
doubled from what it is now-to more than 11 1/2 billion people.
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