My latest Mind and Matter column in the Wall
Street Journal:
A recent paper in the journal Nature concluded
that species extinction caused by habitat loss is happening less
than half as fast as usually estimated. The normal method for
calculating rates of extinction assumes that doomed species merely
cling temporarily to a shrunken patch of habitat, on their way to
disappearing (an idea called "extinction debt"). Apparently, this
isn't the case: Although a larger patch of habitat has more species
in it, shrinking a patch does not lead to a proportional rate of
species loss.
According to the authors of the study, the biologists Stephen
Hubbell and Fangliang He, estimates of extinction rates based on
the usual method are "almost always much higher than those actually
observed." Though you need a big patch of forest to attract a rare
species, you do not need such a big patch to retain it once it is
there. Mr. Hubbell added: "The method has got to be revised. It is
not right."
This is good news for conservationists and should cause them to
redouble their efforts to rebuild and reconnect the scraps of
wilderness that are left-as is happening in some parts of the
world, even as habitats are lost elsewhere. The new study tells us
that no patch of wild habitat, however small, is a hopeless cause,
and that the local disappearance of a species can often be
reversed. Consider, for example, the Brazilian coastal rain forest.
Ninety percent of it has been destroyed, but not a single bird
species has gone extinct as a result (though one survives only in
captivity).
It also suggests that in focusing on habitat loss we have been
neglecting the chief causes of species extinction. The
species-extinction crisis is largely-and always has been-about the
disappearance of unique and isolated creatures from islands, not
the vanishing of species on continents: for example, the dodo from
Mauritius, a host of birds from Hawaii and scores of unique fish
from Lake Victoria (lakes are water islands).
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. (Here's Eschenbach's
graph; his essay is well worth reading:)
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.
This month there was a swarm of earthquakes at the location of
Geirfuglasker, so perhaps the island will be reborn. Can genetic
engineers do the same for the great auk?
