The frogs were critical elements of a 10-year global investigation by 38 research institutions of a pathogenic fungus, Batrachochytrium dendrobatidis (Bd), that is decimating amphibian populations around the world. The fungus, called a chytrid, causes an often-fatal skin disease, chytridiomycosis. Fisher and his colleagues cultured samples of the fungus, ran genetic tests and sought to understand when and where the pathogen emerged and how it spread around the planet.
Their results, published Thursday in the journal Science, indicate that the Bd fungus pandemic did not begin 23,000 years ago, as one earlier hypothesis suggested, but rather sometime in the 1900s. Global trade and the marketing of exotic pets likely propelled it.
The genetic signals point to a common ancestor in East Asia, possibly on the Korean Peninsula. Fisher said the surge in activity in East Asia during World War II and the Korean War, and the increased movement of people and cargo, could have played a role in distributing fungus-infected frogs and toads to other parts of the world.
“When you globalize trade, you globalize unexpected secondary consequences of trade,” Fisher says.
Karen Lips, a University of Maryland biologist who has documented declines in amphibian populations in Central America, said the report shows that the chytrid fungi are more widespread and diversified than previously known. Such fungi are an invasive species and have a devastating impact on biodiversity, yet they don't get the kind of attention given to more charismatic alien species — such as Burmese pythons, brown tree snakes, Northern snakeheads, feral hogs and rabbits.
Lips, who was not involved in the new study, said it shows there are many lineages of fungus, and “these things can hybridize. ... The impacts are global and much larger than we've seen before.”
In fact, not just frogs and toads are threatened by the chytrid fungi. Salamanders in Europe have been slammed by a chytrid fungus, Batrachochytrium salamandrivorans (Bsal), that emerged in Vietnam. Its arrival in the United States would imperil this country's huge salamander population.
It was in the 1980s that field researchers noted a global decline in amphibian populations. No one could explain it. One hypothesis pointed to the thinning ozone layer. Only in the 1990s did the fungus explanation surface.
Some frog species have already gone extinct in the wild, including the mountain chicken frog, said Simon O'Hanlon, a research associate at Imperial College London and the lead author of the new report. There are roughly 7,800 species of frogs and toads globally, and only about 1,400 have been tested for the fungus. About half showed signs of infection.
Frogs and toads are marketed globally as exotic pets, a food source and for use in scientific research. They can also be stowaways. Asian toads, for example, recently invaded Madagascar by remaining hidden within imported mining equipment. The new report notes that it's probably not a coincidence that the estimated dates for the emergence of the chytrid pandemic is roughly the same period as the “big bang” in international trade.
“We’re shipping whatever around the world with no regulation, moving things from one environmental niche to another,” O'Hanlon said. “It seems that we’re putting our global biodiversity recklessly at risk.”
The researchers said they need more data, meaning more captured frogs and toads from which to glean genetic information about the pandemic fungus.
Fisher ultimately nabbed 600 poison dart frogs in French Guiana, taking small tissue samples before releasing them back into the rain forest. He stressed the importance of wearing gloves in this work.
“These frogs are absolutely lethal,” he said. “If you lick them, it's curtains.”