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II. EXTINCTION PATTERNS AND TRENDS
}} l2. Extinction has been a fact of life since life first emerged. The present few million species are the modern-day survivors of the estimated half-billion species that have ever existed. Almost all past extinctions have occurred by natural processes, but today human activities are overwhelmingly the main cause of extinctions.

13. The average duration of a species is some 5 million years. The best current estimates are that on average 900,000 species have become extinct every 1 million years during the last 200 million years, so the average 'background rate' of extinction has been very roughly one in every one and one-ninth years. The present human-caused rate is hundreds of times higher, and could easily be thousands of times higher. We do not know. We have no accurate figures on the current rates of extinctions, as most of the species vanishing are those least documented, such as insects in tropical forests.

14. Although tropical moist forests are by far the richest biological units in terms of genetic diversity and by far the most threatened through human activities, other major ecological zones are also under pressure. Arid and semi-arid lands harbour only a very small number of species compared with tropical forests. But because of the adaptations of these species to harsh living conditions, they feature many potentially valuable biochemicals, such as the liquid wax of the jojoba shrub and the natural rubber of the guayule bush. Many of these are threatened by, among other things, the expansion of livestock herding.

15. Coral reefs, with an estimated half-million species in their 400,000 square kilometres, are being depleted at rates that may leave little but degraded remnants by early next century. This would be a great loss, in that coral-reef organisms, by virtue of the 'biological warfare' they engage in to ensure living space in crowded habitats, have generated an unusual number and variety of toxins valuable in modern medicine. /…