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.