Destroying life

Biotic crises, which vary in severity, occur when regional or global species biodiversity falls to low levels. Mild crises involve an elevated turnover of species, while severe crises involve a loss of 20 per cent or more of all species. When such severe crises act globally, they are mass extinctions. Biotic crises may arise from a higher than normal extinction rate, from a lower than usual speciation rate, from species loss through net outward migration (if the extinction is not truly global), or from a combination of all these - they are not necessarily the outcome of a bout of concentrated extinction. The notion of biotic crises and mass extinctions has sparked several lively debates surround these ideas. At least three critical questions arise - What are mass extinctions. What causes mass extinctions. How fast do mass extinctions occur.

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