Habitat loss and species extinction

[h/t Matt Ridley Wall Street Journal]

"A recent paper in the journal Nature has found habitat loss as cause of species extinction to be happening less than half as fast as usually expected," Ridley writes in his opening sentence. The study concluded that while a larger patch of habitat does have more species in it, a shrinking habitat will not lead to a proportional rate of species loss. That's interesting. If habitat loss isn't doing the damage, then what is doing the damage? 

Ridley writes:

"In nearly all such cases, the damage was done not by habitat loss but by the introduction of predators, competitors or parasites: monkeys and pigs in Mauritius, rats and other birds in Hawaii, Nile perch in Lake Victoria. The species-extinction crisis on islands peaked around 1900, but it continues today. In the Galapagos and other places, newly arrived animals are driving endemic species to the brink.

By contrast—and so long as you count Australia as an island, because its rash of extinctions was caused mostly by introduced aliens—the rate at which continents are losing species is remarkably slow, despite huge changes in habitat wrought by human beings. According to the Red List of the International Union for the Conservation of Nature, 122 bird species and 58 mammals have gone extinct in the last 500 years. But of these, the independent scholar Willis Eschenbach has concluded, only six birds and three mammals were on continents—out of 8,971 and 4,428 continental species, respectively. None was exclusively a forest dweller, and none was extinguished exclusively by habitat loss.

Europe got through the 20th century without losing a single species of bird. (The Faroese pied raven was at most a subspecies.) The last European breeding bird to die out altogether was the great auk—an island species—in the 1840s. In a drastic and unusual case of habitat destruction, an underwater volcano off Iceland finally did in the flightless bird, after centuries of human persecution. The eruption sank the great auk's last breeding colony, an island called Geirfuglasker. A forlorn few pairs subsequently tried breeding on the much less suitable island of Eldey, but they were killed by a collector of rare birds." 

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