Extinction is the complete global disappearance of a species. During the long history of the planet there have been many extinctions caused by climate change, volcanism, floods and droughts. However, in recent years, the vast majority of extinctions of flora and fauna are due to the direct or indirect impact of human activities (see Crisis of Biodiversity).
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Disruptions caused by our activities diminish the areas of distribution of species and gradually reduce populations. When populations are small their risk of extinction is increased due to various factors. Small populations are more susceptible to extinction by natural events like fires, cyclones, drought, etc. In addition, they are more susceptible to loss of genetic variability, as members of the population increasingly become more closely related.
When populations of a species are lost, the size of the total population decreases and it loses its genetic variability.
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