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ALL around the world, amphibians are in decline. A recent meeting of
biologists organised by the National Re search Council in Irvine, California,
was told that frogs, toads, salamanders and other amphibians are disappearing,
but that the reasons remain unknown.

Å®ÉúСÊÓƵs admit that they lack hard data. Much of the evidence of declining
amphibians comes from personal experience, rather than from rigorous studies
of populations. ‘Frogs around the world are in decline,’ says David Wake,
director of the Museum of Vertebrate Zoology at the University of California,
Berkeley, ‘but that doesn’t mean that all frog populations are in decline,
nor that all areas are equally affected.’

‘What’s really caused the greatest alarm,’ says Wake, ‘was the finding
that amphibian populations are in decline in our great national parks in
the US and in nature reserves in the tropics.’ In one national park in the
US, 39 ponds contained frogs in the late 1970s, but by 1988 only one still
did so.

The loss of amphibians could have important implications for the stability
of ecosystems. Frogs and salamanders often dominate the animal and plant
communities in which they live. Tadpoles are major consumers of plant life,
removing large quantities of algae from streams and ponds. Once they have
metamorphosed into adults, they become voracious carnivores that prey on
a wide range of insects and also other invertebrates.

Wake believes that the dual ecological role of amphibians makes them
excellent ‘bioindicators’ – species whose fortunes reflect the general health
of the environment. The lifestyle and physiology of amphibians combine to
expose them to a vast array of environmental pollutants. For example, their
eggs, which receive little physical protection, are highly sensitive to
chemicals in water. This sensitivity persists into later life, because a
frog’s skin is permeable, designed to absorb and release respiratory gases.

Some scientists believe that amphibians can be viewed collectively as
a kind of ‘global warning system’, so that their decline is a serious matter
for the planet. Yet it is very difficult to pin the blame for their decline
on specific factors, chemicals or otherwise.

Wake is cautious about suggestions that any single effect – such as
climatic change – might be responsible for the worldwide decline. There
may be some such trigger, he says, but he has been unable to indentify one:
‘We have no smoking gun.’ But the possibility must be borne in mind, he
says. ‘To me, it’s inconceivable that we can go on having an increase in
carbon dioxide levels and not have some other effects.’

At least partly to blame must be the destruction of habitats suitable
for amphibians. Wetlands are becoming scarcer throughout the world. In specific
cases, researchers can often point the finger of blame at local conditions.
In Britain, for example, a major factor affecting the decline of amphibians
is the loss of suitable habitat. Mary Swan of Leicester Polytechnic has
organised a national survey of British amphibians for the Nature Conservancy
Council. ‘During the last 30 years or so,’ she says, ‘we’ve lost about 20
per cent of small water bodies, which amounts to around 182 000 ponds.’

Yet this is not the whole story, because some of the prime sites in
Britain now have no amphibians living in them. In some areas, says Swan,
nearly half of the suitable ponds remain unoccupied. One possible explanation
for this decline is that there is a lack of suitable habitat on land – principally
decent cover. Another theory is that acid rain is contributing to the decline,
but, once again, the situation is complex. To find out, Clive Cummins of
the Institute of Terrestrial Ecology at Monks Wood exposed frogspawn to
acidified water. He found that water did indeed kill the spawn, but that
in some circumstances a modest degree of acidity actually benefited the
surviving tadpoles. ‘Having been thinned out, those tadpoles that survived
apparently could acclimatise to those conditions,’ says Cummins. ‘Because
of their low density, they grew faster than tadpoles in non-acid pools.’

Acid water may not necessarily threaten the survival of a population
until the pH – a measure of how acidic it is – drops below a threshhold
value of about 4. ‘Once you get below that,’ says Cummins, ‘you get so much
mortality in the egg stage that no amount of compensation can make up for
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