Biodiversity is the total number of species in the world, or within any limited place. Biodiversity is the product of evolution. Scientists are interested in how the patterns of biodiversity differ from place to place, and how they have changed over the history of the Earth, because this gives them insight into how the evolutionary process works.

The most general pattern of biodiversity is that the places on the Earth, and the times in Earth history, that have the most species are those with the highest temperature and greatest amount of moisture. Today this is represented by the tropical rain forests. Two acres (a hectare) of tropical rain forest can contain as many tree or bird species as the entire United States. The rain forests, though covering less than 5 percent of the Earth's surface, may contain half of its species. The equable climatic conditions of the tropics allow them to be a museum of old species. It has so many species interactions that it is also a prime nursery of new species.

Biodiversity has been repeatedly reduced by global catastrophes, most notably the five mass extinctions in Earth history. Biodiversity has increased after each mass extinction event. Sometimes it takes a long time for biodiversity to recover. After the Permian extinction, more than a hundred million years was required to restore the pre-disaster diversity levels.

Many scientists estimate that a sixth mass extinction, caused by human activity, is now under way. A combination of human activities is causing these extinctions. The most important is habitat destruction: Humans convert natural habitats into farmlands and cities that are unsuitable for wild species. Recovery will occur from this extinction event also. New species will eventually evolve suitable to the new, disturbed conditions created by humans. Science fiction writers invent wild scenarios, but even some serious scientists speculate that super rats and tree-sized weeds may evolve. This will occur on such a long timescale that during one's lifetime a human will see virtually nothing but extinctions. . .

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