Species under threat as their habitats are
cut in half
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By Steve Connor, Science Editor
03 May 2002
Life on Earth is facing an extinction crisis that could be
far worse than previously thought, according to two leading ecologists who have studied
the rate at which animal populations are being lost.
The scientists have found that the geographical ranges of
173 species of mammals have declined, collectively, by more than 50 per cent over several
decades, indicating a severe constriction of the animals' breeding territories.
Paul Ehrlich of Stanford University in California and
Gerardo Ceballos of Mexico University believe that the loss of viable breeding populations
is a critical factor that has often been overlooked.
"The loss of species diversity has correctly attracted
much attention from the general public and decision makers. It is now the job of the
community of environmental scientists to give equal prominence to the issue of the loss of
population diversity," Professor Ehrlich said. "We are talking about nothing
less than the preservation of human life-support systems. We neglect the issue at our
peril."
Studies of biodiversity should take into account the number
of endangered populations of breeding animals in a species, and not rely on identifying
extinct species, the scientists said. In the journal Science they write: "Most
analyses of the current loss of biodiversity emphasise species extinctions and patterns of
species decline and do not convey the true extent of the depletion of humanity's natural
capital ... We need to analyse extinctions of both populations and species," they
say.
Professor Ehrlich and Dr Ceballos compared the territories
of existing breeding populations of 173 mammals living on five continents with historical
records of the known distribution of the same species. They estimated, conservatively,
that about 2 per cent of all populations of mammals had been lost compared with a global
extinction rate of about 1.8 per cent. "But according to our data, the loss of mammal
populations actually may be much more severe, perhaps 10 per cent or higher," said
Professor Ehrlich. "While distribution maps often showed species occurring over wide
areas, it turns out that many of them actually have lost most of their populations in
these areas and have been reduced to scattered remnant groups."
The North American brown bear and grey wolf, and the Asian
tiger are examples of animals whose ranges may be far smaller than suggested by official
maps. "We suspect that many less prominent species ... have lost portions of their
ranges but without detection because they have not been subject to intensive mapping
attempts," the scientists said.
The study reveals wide differences between how species
resist human interference. All wild populations of Pere David's deer from China, for
instance, have gone extinct, whereas the spotted hyena has lost only 14 per cent of its
populations despite large loss of its natural habitat caused by man. There were striking
differences in the demise of breeding populations of wild animals between continents, with
Africa and South-east Asia suffering some of the largest losses.
"Population extinctions today seem to be concentrated
either where there are high human population densities, or where other human impacts ...
have been severe," the scientists said. "Australia, which is the continent with
the largest number of mammal species extinctions, is also a continent showing a widespread
severe reduction of populations."
Some species are threatened further because most or all of
their breeding populations are in one country and are, therefore, vulnerable to the
vagaries of a single political system. "A combination of political endemism and
political instability has certainly made the fates of the black and Sumatran rhinos much
more uncertain. In both of these conservation cases, a high priority would be to
re-establish populations not only over a broader geographic range but also within a
greater variety of countries," they said.
At risk: How the world's rare animals have declined
Black rhinoceros
One of the most endangered animals on the planet, numbers
have fallen from about 65,000 individuals 20 years ago to less than 2,500 today. Widely
hunted and now poached in sub-Saharan Africa for its ivory.
Tiger
The largest predator in Asia needs extensive territories to
maintain a viable breeding population. Habitat loss, forest logging and hunting have
reduced its numbers to precariously low levels.
Gorilla
Hunted for its meat and threatened by habitat encroachment,
the largest of the great apes is suffering a gradual decline, made worse by the isolation
of once fully interbreeding populations.
Spotted hyena
Even this resourceful predator-cum-scavenger has suffered at
the hands of man. Scientists estimate that its wild populations have diminished by 14 per
cent over the past few decades.
Pere David's deer
Hunted to extinction in the wild but preserved for centuries
in the huge deer parks built by the Chinese emperors. Brought to Europe to grace the deer
parks of the aristocracy.
END OF REPORT
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