Those you with access to today's edition of Science might like to have a look at
the following item - I have appended an news item from the Independent.
Mammal Population Losses and the Extinction Crisis
Gerardo Ceballos and Paul R. Ehrlich
Science May 3 2002: 904-907.
http://news.independent.co.uk/world/environment/story.jsp?story=291369
Species under threat as their habitats are cut in half
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.
.
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