Are we causing a mass extinction?

Are we really living through a mass extinction? If so, we ought to take care. The lesson of the past is that the biggest and most dominant creatures are most likely to vanish when such a mass extinction gets going, while the more modest ones keep plugging on. If humans are using up a large percentage of the Earth's resources, they are more vulnerable, not less, to big changes in the Earth system.

There have been estimates that the rate of extinction is now as much as 100-1000 times as high as it would “normally”be. But is that the same as a mass extinction?There is room for doubt about the figures. We know a lot about past extinction from the fossil record but our knowledge of today's living species is in some ways less complete.

The most famous mass extinction took place at the end of the Cretaceous. Although it is mainly remembered for the demise of the dinosaurs, the destruction extended to fish and many other sorts of plant and animal. Before the asteroid impact theory of their extinction became popular, there was a whole industry devoted to thinking about what killed the dinosaurs, with some spectacular ideas such as mass constipation. But none of these theories ever explained how the same cause killed off all the other species as well. Even the ammonites, a hardy family of swimming marine animals that had survived previous mass extinctions, died out.

The extinction at the end of the Cretaceous involved the destruction of about 85 percent of species in the fossil record. On the basis of this event, we are certainly not living through a mass extinction today. There is little chance that human activity as we see it now will wipe out 85 percent of species. If it did. Homo sapiens would probably be among them, which would solve the problem. But one thing we know about these extinctions is that they are not simple events in which species curl up and die en masse. As geology has got cleverer, we have been able to see that some species die out many thousands of years after others.

This seems to make sense because of everything we know about ecosystems. We are now aware that plants, animals and other types of life form subtle networks in which members depend upon each other.

Although a meteorite strike that chills the Earth overnight and kills most of the life it contains is one image of a mass extinction, there are others. At the end of the Permian, 95 percent of the shallow marine species went extinct, but there is no sign of a meteorite impact to explain what happened. It may have had more to do with plate tectonics and sudden changes in the size and shape of the oceans.

So although green propagandists are wrong to say that we are living through a mass extinction as the movies portray them, maybe we are at the start of something like a mass extinction as they more usually happen. Here, props are slowly knocked away which will eventually lead to the structure of life on Earth falling in.