extinction
The takahe, native to New Zealand, was once thought to be extinct, but now its habitat is protected and numbers are gradually increasing. In 1995 there were approximately 180 birds.
In biology, the complete disappearance of a species from the planet. Extinctions occur when a species becomes unfit for survival in its natural habitat usually to be replaced by another, better-suited species. An organism becomes ill-suited for survival because its environment is changed or because its relationship to other organisms is altered. For example, a predator's fitness for survival depends upon the availability of its prey.
Past extinctions
Mass extinctions are episodes during which large numbers of species have become extinct virtually simultaneously in the distant past, the best known being that of the dinosaurs, other large reptiles, and various marine invertebrates about 65 million years ago between the end of the
Cretaceous period and the beginning of the Tertiary period, the latter known as the
K–T extinction.
There have been several others in the more distant past. There is disagreement about the causes, but one of several major catastrophes have been blamed, including meteorite impact, volcanic eruption, massive lava flows, and significant global warming. Another mass extinction occurred about 10,000 years ago when many giant species of mammal died out. This is known as the ‘Pleistocene overkill’ because their disappearance was probably hastened by the hunting activities of prehistoric humans. The greatest mass extinction occurred about 250 million years ago, marking the Permian–Triassic boundary (see geological time), when up to 96% of all living species became extinct.
| In 2000, Chinese palaeontologists examined fossils of marine species from the mass extinction at the end of the Permian period, and determined that the extinction took place over 160,000 years. What caused the mass extinction remains undecided, though most earth scientists believe it probably came about through increased volcanic activity or a meteor strike. Mass extinctions apparently occur at periodic intervals of approximately 26 million years. |
Current extinctions In the past 100 years species have become extinct at a rate as high as any thought to have occurred in the past. Where it is known, the causes of extinction in recent times are generally linked to human activities. This is either due to direct killing or, more often, due to loss of habitat. For example, it is believed that many unknown species are becoming extinct due to the loss of rainforests around the world. The number being lost is not known, but it may be large. The extinction of the flightless bird of the island of Mauritius, the dodo, was due to it being killed for food. It became extinct around 1681. The moas of New Zealand, and the passenger pigeon of North America were also exterminated by hunting. Australia has the worst record for extinction: 18 mammals have disappeared since Europeans settled there, and 40 more are threatened. Endangered species are close to extinction; conservation work aims at saving them. |
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