It first became apparent in 1989, that species of frogs and toads had started to vanish, quite suddenly, in locations as far apart as South America, Australia and the Pacific northwest of the USA.
More worryingly, they were disappearing not only from areas where their habitats were under threat, but from nature reserves, national parks and other areas set up to protect wildlife.
Out of the experts' concern was born the Declining Amphibian Populations Task Force, a global organisation of scientists and volunteers, which since 1991 has been based at the Open University in Milton Keynes. Tim Halliday, Open University Professor in Biology, is its international director.
"The declining amphibian phenomenon seemed to be telling us two things. First, it cast serious doubt on the assumption that animals can be protected by setting up reserves. Second, it suggested that amphibians were subject to some adverse environmental process that affects them on a global scale," said Tim.
The second conclusion was highly disputed, with some experts claiming the declines were due simply to natural population fluctuations.
As the Task Force put together data collected from its 3000-plus members and 100 regional groups across the world, certain things became clear: the declines were occurring globally, but not in all regions of the world; and while many declines could be explained by human activities such as deforestation, draining of wetlands and changes in farming practices, some could not.
Attention began to focus on environmental factors, and disease. Amphibians - frogs, toads, newts and salamanders - are found on every continent except Antarctica. The American continent is particularly rich in species. In one reserve in Costa Rica alone, 20 out of 50 frog species have apparently become extinct within the last 20 years.
Tim Halliday calls amphibians an 'environmental barometer'. "Amphibians are particularly dependent on clean water. They have very thin skins, through which water passes easily, so they are extremely susceptible to pollutants such as herbicides and pesticides."
But the evidence from Task Force data was pointing to another factor. This came into the limelight in June this year with the publication of a paper which pinned responsibility for recent dramatic declines on a fungal infection, chytridiomycosis, which appears to be a new disease.
What surprised scientists, said Tim, was that dead and dying frogs found as far apart as Australia and Central America, were suffering from this same disease.
"One possibility is that this disease has been carried from one place to another by some agent - people, or fish, for example. The other is that it's one of those diseases that exist in the amphibian population worldwide, but amphibians have only just become susceptible to it. This would point to something in the environment which is reducing their resistance to the disease."
Parallels for this exist in patterns of human disease - some have suggested the rising incidence of asthma could be due to a reduction in people's level of resistance. On a more mundane level, we all carry the virus that causes cold sores around with us, but it is only when we're tired and "run down" that it strikes.
So why should amphibians be becoming more susceptible? More evidence is needed before this question can be answered, but the suspicion that environmental stress may play a part is strong.
Possibilities include chemical pollution but also global warming or the reduction of the earth's ozone layer. It is known from other work that some amphibian species are vulnerable to increased levels of ultraviolet radiation caused ozone reduction.
"People in different parts of the world are trying to raise money for a rapid reaction force, so that if there are reports of dead or dying frogs anywhere we can get someone there quickly to investigate," said Tim.
"At the moment we don't know how widespread it is, or what percentage of frogs are affected." Whether it could affect newts or salamanders is not known either, nor exactly how it is spread, although the Task Force has recently produced guidelines for anyone involved in amphibian research to try and prevent their inadvertently spreading the disease from one site to another.
The good news is that no cases of the disease have yet been found in Britain. As well as collecting data, the Task Force is concentrating on raising awareness - and money, to fund research into potential causes of decline. But its time is running out - collecting evidence must ultimately be translated into action, says Tim.
"We will probably produce a final report for the year 2001, which is going to be a big year for biodiversity, and then we are going to stop what we're doing. We'll hand over to other people what we have found, and let them do what they want to do with it.
"The hope is the Task Force will be followed by a global conservation organisation. In the meantime, perhaps we should all be grateful to the frogs."
Tim says: "Because of their great sensitivity to changes in habitats they may be giving us early warning of major and widespread environmental change. And the factors that have been shown to affect amphibians, like solar radiation and chemical contamination, affect all forms of life, including humans."
More information about the Declining Amphibian Populations Taskforce is available on: www.open.ac.uk/OU/Academic/Biology/J_Baker/JBtxt.htm.
The taskforce's quarterly newsletter FROGLOG is also available on the Web; or write to:
Professor Tim Halliday
Department of Biology, Open University, Walton Hall, Milton Keynes MK7 6AA, UK