The first signs that something was wrong with the amphibians came in 1989 at the First World Congress of Herpetology in Canterbury. Scientists from many countries told the same story of decline and extinction. Perhaps, they thought, this was a truly global problem with a single underlying cause.
Last year, the Species Survival Commission of the International Union for the Conservation of Nature set up an international task force of zoologists to study the problem. Since then, 30 groups in 40 countries have tried to assess the true extent of the amphibian decline. Jim Vial, head of the task force and professor of fisheries and wildlife at Oregon State University, says that what little information has been collected so far confirms that the decline is genuine and global. 'We've only had one breeding season so far in our study, but everything suggests that there is a major die-off.'
Stories of catastrophe within certain groups of amphibians abound. The gastric-brooding frog in Australia, which keeps its tadpoles in its stomach until the froglets are old enough to emerge from their mother's mouth, has not been seen in its native Queensland rainforests for more than 10 years. In the Seventies the frog was common.
In the mid-Eighties, the spectacular golden toad and the harlequin frog could be counted in their hundreds in the Monteverde Cloud Forest Reserve in Costa Rica. Now they appear to have disappeared completely. Scientists at another nature reserve in Brazil cannot find more than half of the 13 species of frogs that were common there in 1981. Among those that have disappeared are unusual frogs whose bird-like calls were distinctive to the region.
It is not only exotic species in pristine and remote environments that appear to be suffering. American researchers report dramatic declines of native frogs, toads and salamanders. In the Cascade Range of Oregon, they have documented the unexplained dying-off of millions of toad eggs in alpine lakes, and the once- common western spotted frog has become extinct in western Oregon. In the Rockies, boreal toads that used to clog trails are now scarce.
'Some of what's been most disturbing is the disappearances we're seeing in nature reserves, areas in which any obvious impact by humans is quite low,' Professor Vial says. 'Isolated populations at higher altitudes appear to be the most vulnerable, but the problems haven't been confined to those areas.'
In Britain, the crested newt and natterjack toad have suffered serious declines over the past 30 years. But at least biologists believe they know what has gone wrong here. The watery habitats - notably the farmland pond - that these animals need for breeding have become scarcer as land use has changed radically in post-war Britain.
Trevor Beebee, an amphibian authority at Sussex University, is not, therefore, wholly convinced that there is a problem peculiar to amphibians. Destruction of habitats is something other species - notably the reptiles - have had to suffer. 'There is no doubt that amphibians have declined in Britain. The question is whether they've declined more than any other comparable group. Wildlife of all kinds is in decline,' he says.
Nevertheless, there are two possible reasons why amphibians are particularly susceptible to environmental change. First, they breathe through the skin, which, because it is so exposed, could make them more susceptible than other small animals to toxic pollutants in the environment. Secondly, amphibians exploit not one habitat but two: they live on land but need water for breeding. Any changes in global climate, for instance, could affect the availability of breeding sites, even in areas remote from direct human influence.
Another theory is that amphibians have a built-in tendency for boom-bust growth and decline in their numbers. This is primarily an adaptation they developed because the ecological niche they exploit - the temporary pond - may in some years dry out too quickly and, in years of plenty, surviving adults need to breed fast and furiously in order to compensate.
Part of the job of the international task force is to decide whether the declines are part of natural fluctuations in amphibian populations or a permanent die-off. 'One of our primary goals is a database to quantify scientifically amphibian populations and changes in them, both from now on and with a historical perspective,' Professor Vial says. 'The problem is serious, but it's important to determine whether it's a natural phenomenon or an indicator of some other environmental impact.'
Professor Vial suggests that amphibians could be the 'canary in the coal-mine'. By dying off in the way they are, they could be indicating something more serious that could end up affecting other animals, including humans.
Andrew Blaustein, professor of zoology at Oregon State University, is investigating one such doomsday scenario. He has found that amphibian eggs and embryos are peculiarly sensistive to damaging ultraviolet light of the type usually screened out by the ozone layer, high in the atmosphere. An atmospheric phenomenon, such as increased penetration of UV-B radiation from sunlight, is one of the few possibilities that could explain such widespread extinctions, Professor Blaustein says. 'There are particular instances where we can link amphibian declines or extinctions to certain causes, such as localised predators, pollution, habitat destruction or acid rain. But those causes don't explain all the serious problems that we're seeing.'
Increased levels of UV-B radiation resulting from ozone depletion, which affects higher altitudes and latitudes first, could explain why amphibians living in these parts of the world appear to be suffering most.
All this, however, remains largely speculative until the results of the global amphibian census by the international task force have been gathered over many years - some biologists say perhaps a decade or more, because of the risk of short- term natural fluctuations skewing the data.
One of the most detailed and long-term studies of amphibian populations to date illustrates just how difficult is the problem of distinguishing between natural fluctuations in numbers and genuine, more serious die-offs. Since 1979, biologists have been monitoring three species of salamander and one species of frog at a breeding-ground in South Carolina. They found substantial fluctuations in population sizes in all four types of amphibians - judged by the overall numbers of breeding adults and young that survived the rigours of life as a tadpole.
But although sizes varied dramatically year by year, the researchers found no overall trend in three of the species, and an actual increase in numbers in the fourth. Drought was the single biggest influence on their numbers - which is what would be expected for animals that have to lay their eggs and develop their young in water.
The researchers, led by Joseph Pechmann, of the University of Georgia, concluded: 'Year to year variation and short-term trends make it difficult to discern long-term trends . . . Our data illustrate some cautionary tales for evaluating declines in amphibian populations.'
Caution, however, should not prevent scientists from determining whether the alleged crisis among the 4,000 species of frogs, toads, salamander and newts alive today is real or anecdotal, Professor Vial believes. He says that in some woodland habitats, for instance, frogs and toads exercise considerable control over insect pests. A decline of amphibian predators - which all indications suggest is real, he says - could upset the delicate ecological balance that has evolved over millions of years.
'Amphibians have survived every mass extinction so far, including that of the dinosaurs,' he says. We now have to look hard and long into whether they are also the canary in the coal-mine.-
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