|The current mass extinction has been unfolding for
millennia, and unlike the greenhouse effect, global warming, or the hole
in the ozone, it is visible without sophisticated imagery or complex
computer modeling. It is real, and it is happening to a greater or
lesser degree all over the globe; it is most apparent, however, in the
tropics. It will not eliminate life from the Earth; no mass extinction
does that. But enough species will die that the nature of life on the
Earth will be forever changed.
Many scientists dispute whether an extinction is currently taking
place at all, or suggest that we are facing the prospect but have not
yet begun the experience. Others agree that we are indeed in a period of
increased extinction, but that the net result will little change the
Earth's flora and fauna. I do not share such a sanguine view. I believe
that the current extinction is well under way, having started with the
dawn of the Ice Age, about 2.5 million years ago, and since then
accelerating in its rate of species destruction. In some ways it is very
much like the dinosaur-killing event of 65 million years ago, when a
biosphere already stressed by rapid changes in climate and sea level was
knocked into mass extinction by the impact of asteroids, striking,
according to new evidence, simultaneously in North and Central America.
A very similar scenario is currently unfolding. Over 2 million years
ago, giant glaciers began to cover large portions of the planet,
changing climate and sea level on a global scale in the process. And
then, 100,000 years ago, another great asteroid hit Earth, this time in
Africa. That asteroid is named Homo sapiens.
We are surely in the midst of a mass extinction. Even though it's
hard to compare past extinction rates with that of the present, given
missing data from the past, we do know how to identify extinction
periods: the elevation of extinction rates in those periods are at least
a hundred-fold over the slow "background" rate of "normal" extinction.
Of about 6 to 10 million currently existing species, we have still
only identified 1 million; we know more about vertebrate species than we
do about plants and insects. But for groups that we know well, knowledge
of very recent species extinctions -- and for current species, their
ranges and the threats to them -- allows us to be certain that
extinction rates are comparable to those of the great past extinctions.
For example, for birds, of about 10,000 species worldwide, at least 128
have disappeared in the last 500 years, about 1,200 are currently
seriously threatened with extinction (all but three from human
activities); there is a real prospect of the loss of 500 bird species
within this century.
For less well-known groups, we must use inference. We know there is a
rough relationship between the area of a patch of habitat and the number
of species it will contain. Since habitat destruction is the leading
cause of endangerment and extinction, and we have data on the rate of
habitat destruction, we can estimate rates of extinction in some cases.
Introduced species -- those who migrate to a new area -- are the second
leading cause of endangerment and extinction. Information on the rate of
species introduction and the nature of the impacts of introduced species
on native species and ecosystems allows inferences about extinction
rates. The evidence all points to a global tragedy with a profound loss