THE ORIGIN OF ANIMALS

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.