U.S. researchers, in a study published in Annals of the New York Academy of Sciences, say no one single factor can explain all of the population declines occurring at an unprecedented rate.
Amphibian declines are linked to natural causes such as competition, predation, reproduction and disease along with human-induced impacts such as habitat destruction, environmental contamination, invasive species and climate change, the study said.
"An enormous rate of change has occurred in the last 100 years, and amphibians are not evolving fast enough to keep up with it," Oregon State University zoology Professor Andrew Blaustein said in an OSU release Monday.
"We're now realizing that it's not just one thing, it's a whole range of things. With a permeable skin and exposure to both aquatic and terrestrial problems, amphibians face a double whammy. Because of this, mammals, fish and birds have not experienced population impacts as severely as amphibians -- at least, not yet."
One estimate says amphibians are disappearing at more than 200 times the average extinction rate.
"Modern selection pressures, especially those associated with human activity, may be too severe and may have arisen too rapidly for amphibians to evolve adaptations to overcome them," the researchers said.