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,
the Atitlán grebe and the Omilteme cottontail rabbit. Only the last
three vanished after the Second World War - and for all three there
is some debate as to whether they were full species or sub-species.
Not a single one of the nine went extinct because of forest loss or
climate change. Most succumbed to hunting, or, in the case of
grebes, introduced predatory fish.
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."
There are plenty more species that are threatened, endangered
and of concern, some of which are probably irretrievable. A
desperate attempt is under way to take eggs from Siberian nests of
the last 100 or so pairs of spoon-billed sandpipers and rear chicks
in captivity. Yet, remarkably, we have doubled the human population
since 1965 while reducing, rather than increasing, the extinction
rate of wild species, especially in the most industrialised
countries. It is now 167 years since a bird native to Europe went
globally extinct (the great auk), though the slender-billed curlew
is almost certainly now gone. Of course the extinction rate of
lesser creatures than birds and mammals may be accelerating, but
there is no hard evidence either way.
I am not denigrating the efforts of those who try to prevent
extinctions. I have three times worked on projects to avert the
extirpation of birds - the western tragopan, the cheer pheasant and
the lesser florican, all still extant but rare. But the constant
repetition of the baseless meme that we are causing a mass
extinction 100 or 1,000 times as fast as the natural background
extinction rate is counter-productive.
Rather, let us build on recent improvements. We now know that
even the tiniest fragment of forest can be a refuge from which to
rebuild an ecosystem. During this century, let us see if we can get
the extinction rate not just low, but lower than it would naturally
be, by saving species that might be going extinct naturally - and
by resurrecting extinct species.
