Of all species
that have existed on Earth, 99.9 percent are now extinct. Many of them perished
in five cataclysmic events.
According to a recent poll, seven out of ten biologists think we are
currently in the throes of a sixth mass extinction. Some say it could
wipe out as many as 90 percent of all species living today. Yet other scientists
dispute such dire projections.
As our panelists debate the issue, they also consider how one species
-- Homo sapiens -- may be triggering a modern mass extinction.
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Daniel
Simberloff is professor of environmental studies
and director of the Institute for Biological Invasions
at the University of Tennessee. He has conducted
extensive research on the rates and causes of species
extinction, on the nature of ecological communities,
and on the impacts and management of introduced
species (those who migrate to a new area).
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 of biodiversity.
Daniel
Simberloff is professor of environmental studies
and director of the Institute for Biological Invasions
at the University of Tennessee. He has conducted
extensive research on the rates and causes of species
extinction, on the nature of ecological communities,
and on the impacts and management of introduced
species (those who migrate to a new area). 
