The latest news on the missing frogs.

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, this was the first recorded case of their tackling a vertebrate organism. The fungus at Fortuna appears to have made this switch with devastating effectiveness; in barely a year's time since it was first detected, the fungus had caused the local extirpation of at least eight species of frogs and toads.

These population crashes at Fortuna follow on the heels of similar frog disappearances at the Monteverde cloud forest preserve in northwest Costa Rica in 1987-88, and at Las Tablas, a site on the Costa Rica-Panama border, in 1994. All three sites are intact, mid- to high-elevation tropical forest where frogs that dwell or breed in streams suddenly underwent catastrophic declines (frogs that breed in other habitats, such as tank bromeliad plants, have so far been unaffected). Together the three sites have seen at least 21 local species vanish. At least one endemic species, the famed golden toad of Monteverde, is now almost certainly extinct. Even though the chytrid fungus was not identified as a causal factor until Fortuna, the similarities of the declines strongly suggest the fungus is responsible at all three sites. Moreover, in Australia, which has had its own puzzling frog declines, scientists also have found chytrid fungal infections in stream-dwelling, upland rainforest frogs.

The question now facing Karen Lips and other herpetologists is why and how a formerly innocuous rainforest fungus has suddenly become a virulent frog pathogen on a global scale. One possibility is that rising temperatures, declining rainfall, or other environmental changes are stressing frogs enough that they are now susceptible to a fungus they could previously ward off with ease. Scientists may have found the smoking gun, but the question still remains as to who is pulling the trigger.