Living organisms have been disappearing from the face of Earth since the beginning of life on the planet. Most of the species that have ever lived on Earth are now extinct. Extinction and endangerment can occur naturally as a normal process in the course of evolution. It can be the result of a catastrophic event, such as the collision of an asteroid with Earth. Scientists believe an asteroid stuck the planet off Mexico's Yucatán Peninsula some 65,000,000 years ago, bringing about the extinction of almost 50 percent of the plant species and 75 percent of the animal species then living on Earth, including the dinosaurs. Wide-spread climate changes, disease, and competition among species can also result in natural extinction. To date, scientists believe there have been five great natural extinction episodes in Earth's history.
Since humans became the dominant species on the planet, however, the rate at which other species have become extinct has increased dramatically. Especially since the seventeenth century, technological advances and an ever-expanding human population have changed the natural world as never before. At present, scientists believe extinctions caused by humans are taking place at 100 to 1,000 times nature's normal rate between great extinction episodes. Species are disappearing faster than they can be created through evolution.
It is impossible to measure the total number of species endangered or going extinct because scientists have described and named only a small percentage of Earth's species. Just 1,400,000 species—out an estimated 10,000,000 to 100,000,000—have been described to date.
Scientists do know that humans are endangering species and the natural world primarily in three ways: habitat destruction, commercial exploitation of animals and plants, and the transplantation of species from one part of the world to another.
The destruction of habitats all over the world is the primary reason species are becoming extinct or endangered. Houses, highways, dams, industrial buildings, and ever-spreading farms now dominate landscapes formerly occupied by forests, prairies, deserts, scrublands, and wetlands. Since the beginning of European settlement in America, over 65,000,000 acres of wetlands have been drained. One million acres alone vanished between 1985 and 1995.
Habitat destruction can be obvious or it can be subtle, occurring over a long period of time without being noticed. Pollution, such as sewage from cities and chemical runoff from farms, can change the quality and quantity of water in streams and rivers. To species living in a delicately balanced habitat, this disturbance can be as fatal as the clear-cutting of a rain forest.
As remaining habitats are carved into smaller and smaller pockets or islands, remaining species are forced to exist in these crowded areas, which causes further habitat destruction. These species become less adaptable to environmental change; they become more vulnerable to extinction. Scientists believe that when a habitat is cut by 90 percent, one-half of its plants, animals, insects, and microscopic life-forms will become extinct.
Animals have been hunted by humans not only for their meat but for parts of their bodies that are used to create medicines, love potions, and trinkets. Overhunting has caused the extinction of many species and brought a great many others to the brink of extinction. Examples include species of whales, slaughtered for their oil and baleen. The rhinoceroses of Africa are critically endangered, having been killed mainly for their horns.
International treaties outlaw the capture and trade of many endangered or threatened species. These laws, however, are difficult to enforce. The smuggling of endangered species is a huge business. In 1996, between $10,000,000,000 and $20,000,000,000 in plants and animals were traded illegally around the world.
Native species are those that have inhabited a given biological landscape for a long period of time. They have adapted to the environment, climate, and other species in that locale. Introduced or exotic species are those that have been brought into that landscape by humans, either accidentally or intentionally.
In some cases, these introduced species may not cause any harm. They may, over time, adapt to their new surroundings and fellow species, becoming "native." Most often, however, introduced species seriously disrupt ecological balances. They compete with native species for food and shelter. Often, they prey on the native species, who lack natural defenses against the intruders. In the last 500 years, introduced insects, cats, pigs, rats, and others have caused the endangerment or outright extinction of hundreds of native species.
Source: Endangered Species, U·X·L. 2003.