Invasive species are the greatest cause of extinction

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.