The latest news on the missing frogs.

• Author: Tuxill, John

Dogged scientific detective work is beginning to shed light on the causes behind the puzzling global decline of frog populations, the scale of which was first realized by researchers in the late 1980s. The decline has captured widespread attention because many of the frogs have disappeared from remote, seemingly undisturbed habitats. It now appears that while their habitats may appear intact, frogs face plenty of environmental and ecological disturbances. In the northwestern United States, the Cascades frog and the western toad are suffering reduced breeding success because of increased levels of ultraviolet-B radiation - now penetrating a thinned ozone layer - which destroys their eggs in the species' high-altitude breeding pools. Other western U.S. frog species are declining because of competition and predation from introduced species. The mountain yellow-legged frog of California's Sierra Nevada range has seen many of its breeding streams and ponds rendered unfit by the introduction of predatory fish species, especially trout. Other native California amphibians in lowland areas have declined in the face of competition from the introduced bullfrog.

To explain the even more puzzling declines of frogs in upland tropical rainforests, where introduced species and excessive UV radiation are not major problems, scientists are closing in on an altogether different culprit. In 1997, herpetologist Karen Lips encountered large numbers of dead and dying frogs in the montane forest she was surveying at Fortuna Biological Station, in western Panama. Lips and her colleagues autopsied a number of dead frogs and found that all were harboring massive skin infections of a kind of chytrid fungus, a normal, nearly ubiquitous component of the streamside habitats favored by the afflicted frogs. Since frogs must breathe through their moist skin, the fungus essentially suffocated the frogs to death. Chytrid fungi - like all fungi - are decomposers, and while they have been known to attack certain invertebrates...