extinction (biology)

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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.

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