I have a piece in today's Times newspaper on extinction of species. Here it is, with added links:

The suitably named Dr Boris Worm, of Halifax, Nova Scotia, led the team that this week estimated the number of species on the planet at 8.7 million, plus or minus 1.3 million. That sounds about right. We human beings have described almost all the mammals, birds, butterflies and other conspicuous creatures, but new beetles, wasps, moths, flies and worms abound in every acre of tropical forest.

Some patterns are clear. Most species are on land; marine life, though just as abundant, is slightly less diverse. Most are in the humid tropics; the rest of the globe is an ecological footnote to the rainforest. Most are animals - though plants, fungi and microbes vastly outweigh us beasts, they tend to come in fewer kinds, perhaps because plants hybridise and bacteria swap genes, blurring the boundaries of species. Most are insects: spiders/mites and molluscs take silver and bronze, but if Planet Earth had a mascot, it would be a ground beetle.

Species come and go, lasting on average about a million years, scientists reckon. Islands are especially good species factories, evolution generating peculiar forms in isolation, as Charles Darwin spotted in the Galápagos. Lakes, being islands of water, are species factories for fish. Lake Victoria seems to have spawned some 500 species of cichlid fish from just a few ancestral species since it was last dry just 14,000 years ago. Some of these have now died out, after the introduction of predatory Nile perch.

Nobody quite knows what human beings are doing to the speciation rate, but in his paper Dr Worm makes the now routine claim that extinction rates are running at 100 or 1,000 times their normal rates, because of human interference. We are often told we are causing a "sixth mass extinction" similar to that wrought by the asteroid that wiped out the dinosaurs. So what is the evidence for this claim?

One estimate of the species extinction rate - 27,000 a year - came from the biologist E. O. Wilson, of Harvard University, based on an assumption that habitat loss leads to predictable species loss through a mathematical relationship called the species-area curve. The trouble is, the theory is flawed.

A recent study by Stephen Hubbell and Fangliang He, of the University of California at Los Angeles, found that these "estimated" extinction rates are "almost always much higher than those actually observed" because destruction of forest habitat simply does not lead to proportionate species loss as predicted by the theory. In eastern America, in Puerto Rico and in the Atlantic rainforests of Brazil, more than 90 per cent of forest was extirpated, but the number of birds that died out locally were one, seven and zero respectively.

Another widely used estimate for the extinction rate - 40,000 species a year - came from Norman Myers, a British conservationist. Though often cited as if it were a scientific estimate, this number was more of an assumption. This is what Myers wrote in 1979: "Let us suppose that, as a consequence of this man-handling of natural environments, the final one quarter of this century witnesses the elimination of one million species - a far from unlikely prospect. This would work out, during the course of 25 years, at an average extinction rate of 40,000 species per year." For more on Myers, see here.

There is no doubt that humans have caused a pulse of extinction, especially by introducing rats, bugs and weeds to oceanic islands at the expense of endemic species. Island species are often vulnerable to parasites, predators and competitors that continental species have evolved to cope with. Mauritius's dodos, New Zealand's moas, Madagascar's elephant birds and many of Hawaii's honeycreepers all succumbed to the introduction of rats, pigs, monkeys - and humans.

But now that most of these accidental introductions to islands have happened, the rate of extinctions is dropping, not rising, at least among birds and mammals. Bird and mammal extinctions peaked at 1.6 a year around 1900 and have since dropped to about 0.2 a year. Wilson's 27,000 a year should be producing (pro rata) 26 bird and 13 mammal extinctions a year. Myers would predict even more.

Moreover, according to an analysis by the scholar Willis Eschenbach, of the 190 bird and mammal species that have gone extinct globally in the past 500 years, as recorded on the comprehensive list kept by the American Museum of Natural History, just nine were continental species (if you count Australia as an island, which in ecological terms it is).

They were, in chronological order: the bluebuck, the Labrador duck, the Algerian gazelle, the Carolina parakeet, the slender-billed grackle, the passenger pigeon, the Colombian grebe