But in recent decades, disease seems to have taken a more prominent role in causing mortality. Because of multiple stresses, many induced by humans, amphibians now succumb to diseases they may historically have been better able to resist or tolerate.
"There's more and more evidence of the role of disease in the biodiversity crisis, in both amphibians and other types of animals," said Andrew Blaustein, a distinguished professor of zoology at Oregon State University and author of the recent analysis.
"It's normal for animals to deal with infectious organisms, often many of them simultaneously," he said. "But in the face of pollution, a reduced immune response, climate change, evolving pathogens and many other stresses in such a short period of time, many species now simply can't survive."
Infectious disease around the world is increasing at an unprecedented rate.
Natural stresses such as competition and predation have been joined by human-induced stresses, ranging from pollution to global warming.
These forces can reduce immune competence in amphibians, even as climate change, invasive species and other factors increase pathogen spread, persistence, growth and mortality.
Some amphibians deal with stress by hormonal changes such as increased production of glucocorticoids, but on a sustained basis, that approach can further suppress their immune system.
Warmer winters and night-time temperatures may reduce the cycle of pathogen die-offs that would naturally occur in colder regions.
Q in the survival of the fittest hoaxsters.