Local extinctions more
critical and rapid than species loss, scientists say
BY JANET BASU
The cod is not in
immediate danger of extinction, yet populations of cod in
the Atlantic have been so badly depleted that fishing
communities in North America and Europe have lost a
traditional source of livelihood for generations to come.
The blue spruce is not
likely to be wiped off the face of the earth soon, yet
every time a large forest is clear-cut, the loss affects
not only the animals and other species that lived there,
but communities damaged by downstream erosion and
landslides, and the greenhouse gases that the trees would
have helped to absorb.
That is why the loss of
populations of plants, animals and other species may be
as, or more, significant than the extinction of an entire
species, three Stanford scientists say in an article in
the Oct. 24 issue of the journal Science. While
species are being lost globally with alarming speed
the highest rate since the mass extinction that included
the dinosaurs the researchers calculate in their study
that separate populations that make up various species
are going extinct at a rate three to eight times faster.
"You could destroy
all of a species' populations but one, and the species
still exists," said Jennifer Hughes, lead author of
the study. "However, you would have lost the
benefits supplied by those populations. This is a
tremendously important dimension of biodiversity which is
often ignored."
Hughes is an ecologist at
Stanford's Center for Conservation Biology. She conducted
the study with Gretchen Daily, The Bing Interdisciplinary
Research Scientist, and Paul R. Ehrlich, the Bing
Professor of Population Studies, at Stanford.
Hughes and Daily spoke on
Oct. 28 and 29 in Washington, D.C., at the National
Academy of Sciences' second forum on biodiversity,
"Nature and Human Society: The Quest for a
Sustainable World."
Their paper in Science is
linked to an article on species extinction by Sean Nee
and Robert May of Oxford University, and a
"Perspectives" essay by British ecologist
Norman Myers. "[A] mass extinction is now overtaking
the world's biodiversity," Myers writes.
Biodiversity is the
variety of life found at all levels of biological
organization, ranging from individuals and populations to
species, communities and ecosystems. "Much of the
current public and scientific concern over the extinction
of biodiversity emphasizes the loss of species, but
species are only one aspect of biodiversity," Hughes
said.
In their study, Hughes,
Daily and Ehrlich looked at the loss of biodiversity on a
different scale that of localized populations that
make up a species. "While species are important,
many of the benefits that biodiversity confers upon
humanity are delivered through populations. This means
that species extinction rates do not accurately represent
the loss of the benefits of biodiversity," Hughes
said.
In the study, the Stanford
scientists estimated the current rate of population
extinction. A population is a group of individuals of the
same species in a given location a group that is
genetically different from other such groups. Species are
made up of one or more populations.
The scientists calculated
that there were one billion to six billion populations on
Earth. They then estimated that, by a conservative
calculation based on known rates of habitat loss,
populations are going extinct at a rate of 0.8 percent
per year, or 1,800 populations per hour in tropical
forests.
They contrasted that with
studies by other scientists of species loss in tropical
forests that predict an extinction rate between 0.1 and
0.3 percent each year. (Since the largest numbers of
species live in tropical forests, this is a conservative
rate compared to other climates, but an alarming one --
an estimated two to five species per hour.)
By the Stanford group's
calculations, the loss of populations therefore is
occurring three to eight times faster than species loss.
Population loss is
significant because most of the benefits provided by
individual species, or species working together in an
ecosystem, are local and regional. For example, each
population of a seafood or timber species that survives
is available as a stock to be harvested. Each population
of a particular species of plant has a slightly different
genetic makeup genetic material that may make a
difference in the development of pharmaceuticals or the
improvement of agricultural crops.
"About half of the
annual increase in crop production comes from the
incorporation of new genes from populations of wild
relatives that confer enhanced resistance to pests,
disease, soil salinity and so on," Hughes said.
Perhaps the most important
benefits that populations provide are in the form of
ecosystem services, the authors write. "Natural
ecosystems supply a wide array of services to society
whose full value is enormous, but often ignored,"
Daily said. These include purification of air and water,
stabilization of climate, detoxification of waste,
generation and maintenance of soil fertility, and
pollination of crops.
In her presentation at the
biodiversity forum, Daily discussed society's dependence
on natural ecosystems, and how to factor that dependence
into decision-making. One method for doing this, she
said, is to factor into planning programs the cost to
society when ecosystem services are lost.
Local populations also
deliver global ecosystem services, the Stanford
scientists said. "Although populations operate at a
local scale, they are responsible for producing services
that are far-reaching," Hughes said. The blue spruce
is an example: "The large-scale destruction of tree
populations from a Canadian coniferous forest would
influence the global balance of greenhouse gases in the
atmosphere even if no species were exterminated."
The authors conclude that
current conservation strategies, which focus on
individual species, will not be sufficient to protect the
benefits that humanity derives from biodiversity.
"It is critical to go beyond saving certain species.
Habitats must be conserved for the preservation of
biodiversity and the life support systems that maintain
human civilization," Hughes said.
In his
"Perspectives" essay, Myers states that Hughes'
findings about population loss raise critical questions
for the foreseeable future. "If we lose, say, half
of all species plus 90 percent of the populations of
surviving species, which will be more detrimental for the
biggest [ecosystem] service of all, environmental
maintenance of the biosphere?" SR
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