Decline of a tropical montane amphibian fauna

On the basis of surveys conducted between 1991 and 1996, I report a decline of the amphibian fauna at Las Tablas, Puntarenas Province, Costa Rica. I propose that the reduction in the abundance of Atelopus chiriquiensis and Hyla calypsa, the presence of dead and dying individuals of six species of frogs and salamanders, and changes in population sex ratios of A. chiriquiensis and H. calypsa are evidence for "atypical" population fluctuations. Species with both aquatic eggs and aquatic larvae were most affected (e.g., Rana vibicaria, Hyla rivularis), whereas species with direct development or those that lack tadpoles, such as rainfrogs (Eleutherodactylus spp.) and some salamanders (e.g., Bolitoglossa minutula), do not seem to have declined in numbers. In light of this evidence and in comparison with other declines in tropical upland Australia, Brazil and Costa Rica, I conclude that environmental contamination (biotic pathogens or chemicals) or a combination of factors (environmental contamination plus climate change) may be responsible for declines in the amphibian populations at this protected site.