It is often said that we are living through the sixth mass extinction, this one induced by human activity. The point is well made: the present biodiversity crisis appears to be comparable in scale to many of the biotic crises of the past.
There can be no doubt that many species have gone extinct on our watch. We know, for example, that the last great auk was killed by collectors in 1844, the dodo was last seen in 1662 and the last passenger pigeon died in a zoo in 1914. Hunters shot the last quagga, a zebra-like wild horse, in the 1870s and the last thylacine – or Tasmanian tiger – died in captivity in 1936.
These examples, however, tell us little about the scale of the crisis. For that we have to aggregate known historical extinctions. Unfortunately the records are not good, but we do know that 130 species of bird were driven to extinction by hunting between 1500 and 2000. This gives us a starting point.
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There are currently some 10,000 bird species, so these extinctions represent a loss of 1.3 per cent of species in 500 years, or 26 extinctions per million species per year – much greater than the background rate of extinction (see page ii).
Even this could be an underestimate because many other bird species might have become extinct in that time without being recorded. What is more, extinction rates have arguably risen in recent years due to habitat destruction. Taking these factors into account has yielded an alternative figure of about 100 extinctions per million species per year.
If we assume this applies to all of the estimated 10 million species on Earth, total losses might now be 1000 species per year, or three species every day. This is a very rough estimate but it suggests claims of a sixth mass extinction are not exaggerated.
It could of course be objected that this rate of loss cannot proceed inexorably. The optimist might argue, for example, that most of the species so far driven to extinction were already rare or vulnerable, and that they were hunted without mercy in less enlightened times. There is surely some truth in these assertions: it is unlikely that globally distributed species such as sparrows, rats or mice would be so easy to exterminate as the dodo. Further, no nation would allow hunters to slaughter animals as systematically as was done by Victorian-age hunting parties.
However, despite tighter controls on hunting and increasing conservation efforts, pressure on natural habitats has never been more extreme.
While it is frustratingly hard to put precise figures on current rates of species loss, uncertainties should not be seen as a reason for complacency. The fossil record shows how devastating mass extinctions are and that, although life does recover, it takes millions of years to do so. The study of mass extinctions, and comparisons with the modern world, show that we are almost certainly responsible for another mass extinction, and the living world could soon be a much-diminished place.