“It’s a terrifying summary,” says Jodi Rowley from the Australian Museum. “We knew it was bad, but this really confirms how bad. And these are just the declines we know about.”
The scale of these losses can be hard to appreciate, especially if you think that a frog is a frog is a frog. But amphibians are ancient survivors that have been diversifying for 370 million years, and in just five decades, one disease has nearly decimated their ranks. Imagine if a new disease started wiping out 6.5 percent of all mammal species—that would be roughly everything with hooves and everything with flippers. The world would freak out.
And amphibian experts “have been freaking out a long time,” says Karen Lips from the University of Maryland, who was involved in the new study. “Despite all the attention, I don’t think we fully appreciate what was lost.”
In the 1970s and ’80s, amphibian experts began sharing ominous anecdotes about once-plentiful populations that had mysteriously disappeared. Streams once full of eggs were clear. Nights once resonant with ribbits were silent. Nothing about the habitats had changed, save for their sudden, inexplicable froglessness. No one knew what the problem was, let alone the culprit. “It was more than a search for a needle in a haystack—we were still debating the existence of the haystack,” Lips wrote recently. Steele’s analysis shows that by the point the fungus was finally identified, in 1998, it had already done the brunt of its lethal work. At least 60 species were already extinct, and hundreds more were going south.
Bd is perhaps the perfect frog killer. It kills with gusto and without fuss. While some diseases affect only specific hosts, Bd covets nutrients found across amphibian skins, and so targets the entire group indiscriminately. It spreads easily through the water, and it persists outside its hosts.
The fungus hasn’t acted alone; humans have been its unwitting accomplice. A genetic study led by Matthew Fisher from Imperial College London suggested that Bd had originated somewhere in Asia. From there, one especially virulent and transmissible strain spread around the world in the early 20th century—a time when international trade was booming. Infected animals could have stowed away aboard ships, or been deliberately transported as food, pets, or pregnancy tests. Either way, the killer strain eventually spread to five other continents.
In the new study, Scheele’s team compares the modern world to Pangaea—the single, epic supercontinent that existed at the dawn of the dinosaurs. It has long split up, but humans have effectively re-created it. For wildlife diseases, all the world is once again a single connected mass, easily traversed. For that reason, new fungal diseases seem to be emerging at an ever-increasing pace, affecting bats, snakes, salamanders, and more. “These fungi would normally have fried on a sailing craft across the Atlantic, but now they’re viable,” Scheele says. “We’re just able to move things around at higher speed and volume than we used to.”