Tina Maiie.Lyons, age 15, of Baltimore, Md., for her question:
WHAT CAUSES A SPECIES TO BECOME ENDANGERED?
Endangerid species are plant and animal species that are in immediate danger of extinction. Species become extinct or endangered for a number reasons, but the primary cause is the destruction of habitat.
Drainage of wetlands, conversion of shrub lands to grazing lands, cutting and clearing of forests (especially in the tropics), urbanization and suburbanization and highway and dam construction have seriously reduced available habitats.
As the various habitats become fragmented into "islands," the remaining animal populations crowd into smaller areas, causing further habitat destruction. Species in these small islands lose contact with other populations of their own kind, thereby reducing their genetic variation and making them less adaptable to environmental change.
These small populations are highly vulnerable to extinction. For some species, the fragmented habitats become too small to support a viabie, population.
Since the 1600s, commercial exploitation of animals for food and other products has caused many species to become extinct or endangered. As an example the slaughter of great whales for oil and meat has brought them to the brink of extinction.
Introduced diseases, parasites and predators against which native flora and fauna have no defenses have also exterminated or greatly reduced some species. As an example of this, the accidental introduction of a blight eliminated the chestnut tree from North American hardwood forests. Predator and pest control also have adverse effects. Excessive control of prairie dogs has nearly eliminated one of their natural predators, the black footed ferret.
Pollution is another important cause of extinctions. Toxic chemical sprays can interfere with the calcium metabolism of birds, causing soft shelled eggs and malformed young. Water pollution and increased water temperatures have wiped out endemic races of fish.
Extinction is actually a normal process in the course of evolution. Throughout geological time, many more species have become extinct than exist today. These species slowly disappeared because of climatic changes and the inability to adapt to such conditions as competition and predation.
However, since the 1600s, the process of extinction has accelerated rapidly through the impact of both human population growth and technological advances on natural ecosystems.
Today the majority of the world's environments are changing faster than the ability of most species to adapt to such changes through natural selection.
Laws were enacted in the United States in the early 1900s to protect wildlife from commercial trade and overhunting.
In 1973 the Endangered Species Act provided mechanisms for the conservation of ecosystems on which endangered species depend. It also discouraged the exploitation of endangered species in other countries by banning the importation and trade of any product made from such species.to