Extinction, the permanent disappearance of species of organisms from the earth. Extinction is usually due to more than one cause. These causes (some of which are interrelated) include:
Climate changes over time. Warm or temperate periods change into cold periods (or ice ages), and back again. Droughts sometimes occur. Changes in climate cause changes in the types of food available. Species that cannot adapt to meet such changes are likely to become extinct.
Some animal species come to depend upon one source of food, such as a certain species of plant. If this food source should suddenly die out, so would the animal species unless it could quickly adapt to eating something else. Certain plant species require particular species of insects to help them carry out their reproductive processes. If the insects were to become extinct, so would the plants.
If two species come into direct competition for limited necessities (such as water or food) in the same place, one species may eliminate the other.
Predators normally have a healthy effect on their prey populations by preventing those populations from outgrowing their food supplies. However, a dramatic increase in the numbers of predators could lead to the extinction of prey species.
When organisms engage in sexual reproduction with their close relatives, the offspring tend to be weak, have hereditary defects, and reproduce poorly.
A species changes over time by adapting to its environment through a process called natural selection. Eventually a species is changed to such an extent that it can be considered a new species, and the ancestral species is thus extinct.
The study of fossils shows that there have been periodic episodes of mass extinctions. In each episode, a large proportion of all life on earth became extinct. There have been five such episodes in the last 500 million years; the most recent of these occurred 65 million years ago, when the dinosaurs became extinct. Many scientists suspect extraterrestrial causes, such as meteorite impacts. Such impacts may have caused extensive fires and raised clouds of smoke and dust, blocking out sunlight for months at a time. This loss of sunlight would have caused global cooling and the deaths of plants. Animals would have died from the cold and lack of food. An alternate theory proposes that smoke and dust from intense volcanic activity blocked out sunlight, leading to the deaths of organisms.
Through hunting and trapping, and through the destruction of natural areas by clearing land for agriculture and construction projects, humans have caused the extinction of many species. Most extinctions today are caused mainly by human activity.
Among the species that have become extinct due to human activity are the passenger pigeon, dodo, British wild boar, Steller's sea cow, great auk, and Labrador duck. Species that are in danger of becoming extinct include the gorilla, California condor, ivory-billed woodpecker, green pitcher plant, persistent trillium, and elfin tree fern. Species that have been saved from extinction by conservation measures include the American bison, southern elephant seal, wild turkey, and trumpeter swan.