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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