Over the past decade there have been dramatic declines among some amphibian populations in many parts of the world, including a number of apparent species extinctions1. These events have caused particular concern because many have occurred in protected areas such as nature reserves and national parks2,3. Moreover, until recently they have been detected only after they have happened. The Declining Amphibian Populations Task Force — a worldwide network of scientists, of which I am an international director — has been investigating the declines since 1991, and we have reached several conclusions. First, amphibian declines are occurring throughout the world, although some regions are not affected. Second, at most affected sites some species are declining whereas others are not. Finally, there is no single cause for these declines.
This last point is now addressed by Berger et al.4, reporting in Proceedings of the National Academy of Sciences. They have identified a pathogen that links declines among frog populations in two such geographically distinct parts of the world — Panama and Queensland, Australia. Population declines have been well documented at both of these localities5,6 and, in an exemplary example of international scientific collaboration, biologists from Australia, the United Kingdom and the United States have come together to examine the material collected from the two areas.
Berger and colleagues studied many dead and dying frogs; this allowed them to identify the cause of population declines as they occurred rather than after the event. They found that the same pathogen, a chytrid fungus, is the cause of mortality at the two localities. The fungus invades the skin of adult frogs and probably causes death by interfering with their cutaneous respiration and water uptake. It particularly infects the pelvic patch, an important site of water absorption in many frogs.
There are a number of remarkable features about this discovery. First, the chytrid fungus is a newly discovered genus. Rather than having a typical branched, filamentous structure, it resembles a protozoan and was originally misidentified as such. Second, although chytrids are widespread in soil and are known to attack plants and insects, this is the first to be identified as a vertebrate pathogen. Third, this fungus has now been found in two distant natural localities, as well as among captive amphibians in zoos and aquaria in Australia and the United States. Finally, the chytrid seems to attack just adult amphibians (Fig. 1), having no harmful effects on their tadpoles. This is probably because it only attacks skin that contains the protein keratin, which does not occur in amphibians until metamorphosis. This feature differentiates Berger and colleagues' study from many previous reports of amphibian population declines, a characteristic of which has been reproductive failure owing to mortality among eggs and larvae.
Figure 1: Decline and fall — this tree frog (Litoria caerulea) is one of the amphibian species that Berger et al.4 have found to be infected with the chytrid fungus.
![Figure 1 : Decline and fall |[mdash]| this tree frog (Litoria caerulea) is one of the amphibian species that Berger et al. have found to be infected with the chytrid fungus. Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com](/nature/journal/v394/n6692/images/394418aa.eps.0.gif)
High resolution image and legend (2K)
Although this is a very exciting discovery, claims in the popular media7 that the answer to the declining amphibian conundrum has been found are premature. It does not solve the puzzle, but it does raise further questions. For example, is the chytrid fungus ubiquitous, or has it only recently found its way to Australia and Panama? If it is widespread, why have amphibians only just become susceptible to it? If it has been introduced, where has it come from and how is it being spread? Until we can answer these questions, we will not be able to assess the significance of the new findings in the context of amphibian declines. Of immediate concern is the alarming possibility that herpetologists, seeking evidence for causes of declines in natural populations, may be unwittingly helping to spread disease on footwear or collecting gear7.
This is not the first time that disease has been implicated in amphibian population declines. The ubiquitous bacterium Aeromonas hydrophila, which causes the condition known as 'red leg', has been a contributory factor in some cases8,9, and iridoviruses have been implicated in mass frog mortalities in the United Kingdom10. All of these events may be linked not by the specific pathogen, but by the possibility that the immune systems of many amphibian species are being compromised by environmental factors such as climate change, chemical contamination or increased levels of ultraviolet radiation8. Amphibian declines seem to have many causes, and it is vital that scientists do not relax their attempts to investigate these other factors — for which there is mounting evidence — in the mistaken belief that this new study has solved the puzzle.
The significance of Berger and colleagues' discovery4 may go beyond the mystery of amphibian declines — this is the first time that an infectious disease pandemic has been implicated in the decline and possible extinction of animals. Moreover, if the chytrid pathogen was accidentally introduced into previously naive populations, as measles was to South America, we may be seeing a new kind of anthropogenic insult to the environment. This threat may be more insidious, and difficult to control, than existing environmental problems such as chemical pollution or habitat destruction.
