The infectious disease that has devastated the most biodiversity is a fungal killer of amphibians, researchers report today in Science. Around the world, 90 species are thought to have gone extinct because of the fungus. And at least another 491 species have declined because of it.
The culprit with this “unprecedented lethality” is Batrachochytrium dendrobatidis (Bd), a kind of chytrid fungus, whose relatives are harmless fungi found in soil and water. The researchers caution that international trade—the pet trade in particular—has spread the pathogen widely and could continue to do so.
The first signs of problems emerged from the rainforests of Central America and Australia in the late 1980s. Colorful harlequin toads and other species were disappearing, even though their habitat remained intact. Bd is highly infectious and deadly, destroying the skin and triggering heart attacks—adding to other biodiversity losses from habitat destruction. Subsequent work has shown that the chytrid fungus came from Asia and spread around the world over the past century, most likely via the wildlife trade. It seems impossible to eradicate the disease because some amphibian species tolerate it, acting as a natural reservoir, and keep spreading the pathogen.
The extent of the losses wasn’t known, so 41 researchers pulled together official records and scientific papers, and they interviewed other experts around the world. Australia and the Americas have been hit hardest, the team found. The worst losses were in the 1980s, particularly among frogs. Larger amphibians and those with small ranges seem to have suffered the most. In addition to the presumed extinctions, populations of 124 species have been reduced by 90% or more. Only a quarter of species have started to bounce back—unfortunately not the mossy red-eyed frog (pictured). And 39% are still declining.
The threat isn’t over. Bd could spread to the few places, such as Madagascar, that appear to have been spared so far. Other problems loom as well. Recently, a sister species of Bd emerged from Asia and is killing salamanders; although only one species in Europe has been hit so far, experiments have shown that many more are vulnerable if it spreads. The researchers call for regulation of the wildlife trade and tighter biosecurity at borders.