CANTERBURY, U.K.--The fungus thought to be responsible for worldwide amphibian declines may have been spread by human trade, according to new genetic evidence presented here 15 July at the annual meeting of the Society of Conservation Biology.
The rapid decline of amphibian species around the globe has long puzzled and worried herpetologists. Habitat destruction and pollution can account for some of the extinctions, but a host of mysterious cases remains. Even in relatively undisturbed, remote areas, species have gone extinct within a few years. In 1998, parasitologist Peter Daszak, now director of the Consortium for Conservation Medicine in Palisades, New York, fingered a fungal disease, chytridiomycosis, as a possible cause. But since then conservation biologists have wanted to know whether the fungus was a new pathogen racing around the globe or an old pathogen whose deadly effects were exacerbated by recent environmental changes.
In a preliminary attempt to answer this question, Daszak and colleagues studied a rapidly evolving stretch of DNA isolated from 30 samples of the fungus collected from amphibian populations in Australia and North, Central, and South America. The sequences were highly conserved and in some cases even identical. A tree-diagram summarizing the relatedness of fungus specimens from different continents suggests that the pathogen has hopped frequently between populations. Although Daszak declines to estimate when the fungus spread--a crucial indication of whether humans are to blame--he says that large shipments of bullfrogs for food and the pet trade could be ferrying the fungus and causing the extinctions.
Jim Collins, an ecologist at Arizona State University in Tempe who uses similar methods to study viral diseases in salamanders, says that Daszak's phylogenetic tree “could well be consistent with a recent spreading of the disease.” The next step, he says, must be to construct and compare more elaborate phylogenetic trees of the fungus and its amphibian victims. This could reveal whether humans are to blame or whether the pathogen and amphibians have a long history of co-evolution.