Could The Species Extinction Rate Be Falling?

See Counting species out by Matt Ridely. Excerpts:

"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)."

"Eschenbach says: “This lack of even one continental forest bird or mammal extinction, in a record encompassing 500 years of massive cutting, burning, harvesting, inundating, clearing and general widespread destruction and fragmentation of forests on all the continents of the world, provides a final and clear proof that the species-area relationship simply does not work to predict extinctions.”"