We can tell from the fossil record that extinctions are a normal part of evolution. However, the current rate of extinction is the cause of our concern. The fossil record shows that five major extinction episodes have taken place over the past 500 million years, but the fear is that:

… humanity has initiated the sixth great extinction spasm, rushing to eternity a large fraction of our fellow species in a single generation.

(E. O. Wilson, 1992)

You may feel that this is alarmist, given that the Earth recovered from the other five episodes, but consider the time-scales. Previous mass extinctions (the most recent of which put paid to the dinosaurs some 66 million years ago), are presumed to have taken place over several million years, with the recovery to similar levels of diversity taking up to 20 million years. At the present time, extinctions are occurring most rapidly in the tropics, yet, even in Britain it is estimated that around 6 per cent of our species have become extinct during the 20th century.

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