Ross A. Alford

School of Tropical Biology, James Cook University, Townsville, Queensland 4811

Email: ross.alford@jcu.edu.au

It is now apparent that diseases have been the proximate cause of death of at least some amphibians in declining populations. These deaths may have been caused by outbreaks of pathogens that are normally present at low levels, but have been potentiated by some environmental factor. They may also have been caused by a pathogen that has only recently become established in the affected amphibian populations (the "novel pathogen hypothesis"). It is important to distinguish between these possibilities, because they should strongly affect the focus of disease-related research on amphibian declines. If environmental factors are causing outbreaks of a normally-present pathogen or pathogens, research needs to be focused on understanding the interactions of amphibians and their pathogens and on identifying and eliminating potentiating factors.

If a novel pathogen is responsible, research should focus on determining the mechanisms by which this agent is spreading and controlling or eliminating its spread. Fortunately, the novel pathogen hypothesis leads to testable predictions. If it is correct, the causative pathogen should be absent from historical samples of declining species at sites of declines and it should form a relatively genetically homogeneous population within all areas in which it has recently invaded, with any differentiation correlated with the timing and pace of invasions. I suggest a program of research that will test the novel pathogen hypothesis.

1. Experimental studies should be used to evaluate the accuracy of techniques for determining whether pathogens are present in animals.
2. Searches for evidence of pathogen presence in historical collections should be focused on those specimens in which the presence of a pathogen would refute the "novel pathogen" hypothesis.
3. Genetic studies are needed to determine whether the genotypes of pathogens conform to the patterns expected from recent spread.
4. Finally, simple statistical criteria need to be used to place objective confidence intervals on the presence, absence, or prevalence of pathogens in amphibian populations.