Changing climate such as that experienced during the Ice Age (which eliminated many plant species from very large areas of North America and Europe) and other natural events such as volcanic activity have caused localized plant extinctions. The slow movement of the continents (most notably Antarctica and Australia) into unsuitable climate zones caused many organisms to become extinct. Far more widespread and devastating natural extinctions have been caused by the rare impacts of asteroids and comets on the Earth. Some impacts have caused the extinction of even common species on a global scale.
The danger to plants and animals today is most often a direct result of human activities and human population increase. These activities have taken the form of habitat alteration, economic exploitation, the intentional eliminationof pests, the introduction of exotic (nonnative) organisms, the increase of invasive native grazers, and the effects of environmental pollution.
Habitat alteration is the main factor endangering species throughout the world, from the American Midwest where the prairies have been converted to cropland to the equator where the tropical rain forests are being cut and burned. Wetlands filling and draining, agricultural expansion, and residential housing development are all significant factors in habitat destruction.
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