“A century ago,” Yong writes, “a strain of pandemic flu killed up to 100 million people—5 percent of the world’s population. In 2013, a new mystery illness swept the western coast of North America, causing starfish to disintegrate. In 2015, a big-nosed Asian antelope known as the saiga lost two-thirds of its population—some 200,000 individuals—to what now looks to be a bacterial infection. But none of these devastating infections comes close to the destructive power of Bd—a singularly apocalyptic fungus that’s unrivaled in its ability not only to kill animals, but to delete entire species from existence.”
Amphibians, which include approximately 8,000 species of which 90% are frogs, are ancient survivors that have been diversifying for 370 million years, and in just five decades, one disease has nearly decimated their ranks. “Imagine.” writes Yong, “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.”
“It’s a terrifying summary,” says Jodi Rowley from the Australian Museum about Bd—Batrachochytrium dendrobatidis — that causes the often lethal disease, Chytridiomycosis, killing frogs and other amphibians by eating away at their skin and triggering fatal heart attacks, causing an estimated decline or extinction of 200 amphibian species, a figure is almost two decades out-of-date.
New data, Yong reports, compiled by a team led by Ben Scheele from the Australian National University, are much worse: estimating that the fungus has caused the decline of 501 amphibian species—about 6.5 percent of the known total. Of these, 90 have been wiped out entirely. Another 124 have fallen by more than 90 percent, and their odds of recovery are slim. “It rewrote our understanding of what disease could do to wildlife,” Scheele says.
Scheele’s team compares the modern world to Pangaea—the ancient 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.”
Ancient Trade routes have shaped human history by connecting distant civilizations, allowing the exchange of materials, technology, and people, but also diseases that have been frequent hitchhikers, bringing them in contact with new hosts that provide the fuel for epidemic outbreaks of disease, summarizes the journal Nature. Humans are not the only victims of trade-driven diseases, but scientists have only recently begun to appreciate the risk to biodiversity of inadvertently introducing new pathogens to naïve evolutionary arenas by reconstructing the hidden history of disease-driven declines and extinctions for hundreds of amphibian species.
Sapiens have been Bd’s 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.
Scheele said the team found that chytridiomycosis is responsible for the greatest loss of biodiversity due to a disease. “Highly virulent wildlife diseases, including chytridiomycosis, are contributing to the Earth’s sixth mass extinction,” Dr. Scheele said. “Globalization and wildlife trade are the main causes of this global pandemic and are enabling the spread of disease to continue.”
Image Credit: Top of Page, with thanks to Joel Sartore, National Geographic Photo Ark