My Times column on the causes of extinction:
Human beings have been causing other species to go extinct at an unnatural rate over the past five centuries, a new study has confirmed. Whether this constitutes a “sixth mass extinction” comparable to that of the dinosaurs is more debatable, but bringing the surge in extinctions to an end is indeed an urgent priority in conservation.
So it is vital to understand how we cause extinctions. And here the study is dangerously wrong. It says that “habitat loss, overexploitation for economic gain, and climate change” are the main factors and that “all of these are related to human population size and growth, which increases consumption (especially among the rich)”.
Inexplicably, they have left out the main cause of extinctions over the past five centuries: invasive species. The introduction by people of predators, parasites and pests, especially to islands, has been and continues to be far and away the greatest cause of local and global extinction of native fauna. In his green encyclical, Pope Francis likewise never once refers to this problem. It is the Cinderella of the environmental movement.
Over the past 500 years, we know of 77 mammal species (out of about 5,000) and 140 bird species (out of about 10,000) that have gone totally extinct. There may be a handful more we do not know about, and there are plenty more on the brink. Nonetheless, these are the official total species extinctions for the two groups of animal we know best, as compiled by the International Union for Conservation of Nature.
Of those 217 species of bird and mammal, almost all lived on islands — if you count Australia as an island — and just nine on continents: Bluebuck antelope, Algerian gazelle, Omilteme cottontail rabbit,Labrador duck, Carolina parakeet, slender-billed grackle, passenger pigeon, Colombian grebe and Atitlan grebe.
Were it not for the efforts of conservationists there would be more, of course. And this is not counting subspecies, or those in extinction’s waiting room — ones that have not been seen for years, but have yet to be officially declared extinct, like the slender-billed curlew. Nonetheless, the extinction rate of bird and mammal species on continents is a few hundredths of a per cent per century.
This is far short of the apocalyptic predictions being made in the 1970s. Paul Ehrlich, one of the authors of the new paper, himself forecast in 1975 that half of all the species in tropical rainforests would be gone by 2005. Yet not a single bird or mammal that we know of has gone extinct in a tropical rainforest.
My point is not to say extinction does not matter, but to try to get at the real cause of the extinction surge, and it is clearly not the growth of human population and consumption, which has mostly happened on continents. Europe has lost just one breeding bird in 500 years, for example — the island-breeding great auk in 1844.
It is true that there was a surge in extinctions of mammals in the American continent about 12,000 years ago, but that was caused by hunter-gatherers with stone-tipped spears, not modern people with cars. Indeed, there is a pretty spectacular revival of wildlife today in rich continents like North America and Europe. Modern prosperity is plainly not the cause of animal extinctions.
So what is? By far the greatest cause is invasive species, especially on islands. Hawaii has lost about 70 species of bird since contact with Captain Cook: ten times as many as all the world’s continents combined. The cause is man-made, all right, but it’s not because we killed them or destroyed their habitat.
It’s the rats, cats, goats, pigs, mosquitoes and avian malaria we brought with us that did the damage on Hawaii and throughout the Caribbean, the south Atlantic, the Indian ocean and the rest of the Pacific. The dodo disappeared from Mauritius not because sailors ate them (though they did) but because of predation by monkeys, pigs, rats and the like. The Tristan albatross is in trouble on Gough Island because its chicks are eaten alive by introduced mice.
Closer to home, it’s invasive species that are the main cause of conservation problems and local extinctions: grey squirrels, mink and signal crayfish have recently all but extinguished red squirrels, water voles and native crayfish respectively near where I live. Ash dieback, zebra mussels, harlequin ladybirds, Chinese mitten crabs, New Zealand flatworms and muntjac are all causing declines in native British animals.
Misdiagnosing the cause of extinction leads to mistaken policies. Here’s an example. Two decades ago, scientists began to notice alarming declines and disappearances among frogs and toads all over the world but especially in central America. At the time, the hole in the ozone layer was topical, so environmentalists blamed the amphibian declines on ultraviolet rays getting through the supposedly thinner ozone layer.
When sceptics pointed out that the ozone was not thinning over the tropics, many environmentalists fell back on blaming climate change, and for a while the extinction of the golden toad in Costa Rica’s cloud forest was confidently blamed on a changing climate: the first of many extinctions brought about by climate change.
This too proved wrong, and scientists are now agreed that the golden toad’s demise, and that of up to 30 other amphibians in central America, was caused by a chytrid fungus, originating in Africa, to which frogs on other continents are especially vulnerable. How did the fungus reach the Americas? Through the use by scientists of the African clawed toad as a popular laboratory animal. The clawed toad carries the fungus but does not die from it, and has escaped into the wild in many places. Conservation efforts had been misdirected.
So it is not just dishonest to pretend that the way to prevent species extinctions in the future is to throttle back on economic activity and to spend a fortune fighting climate change. It is also dangerous. It has already slowed and hindered the conservation movement from focusing on the real, growing and urgent threat to native wildlife all over the world, and especially on islands. We have to get serious about bio-security, about regulating the trade in live animals and plants, which results in more and more alien species jumping into ecosystems where they run amok.