Batrachochytrium dendrobatidis, commonly known as the chytrid fungus, has cut a swathe through the world's amphibians, frogs in particular, over the last 40 years. Now the first comprehensive assessment of the toll has been published, revealing it may have contributed to 90 extinctions and caused declines in more than 400 other species, some of whose numbers are still falling. It appears to have outpaced rats and cats, previously regarded as the two most destructive invasive species on the planet, and represents a major contribution to the Earth's 6th great mass extinction.

In Science, an international team, including chytrid fungus discoverer Dr Lee Berger, provide an overview of the species known to be affected by B. dendrobatibis and the recently discovered related fungus B. salamandrivorans. The affected species are divided into five categories, from those that have lost less than 20 percent of their population to the ones considered extinct.

Almost half of the affected species lived in Mesoamerica, but survival rates there are much better than in South America. The fungus is thought to originally be from Asia, as frogs and salamanders there have evolved a high level of resistance to it. Larger species and those that live in permanently wet environments have suffered more.

Co-author Dr Claire Foster of the Australian National University told IFLScience that the team's biggest fear is that the fungus will make it to New Guinea, which she calls “a real hotspot for amphibian diversity,” as well as Colombia's Sierra Nevada de Santa Marta mountains, where many unusual species remain untouched.

To stop this, the world needs to step up biosecurity and improve quarantine, since Foster said many frogs are traveling around the world in shipping containers, with some of the more resistant species carrying the fungus with them. However, it is believed the major mode of transmission is the illegal wildlife trade, with fungus-carrying specimens collected from the wild and giving it to others in terrariums. When their owners wash the containers or dump dead frogs, B. dendrobatidis can get into the water supply, where it is able to survive without a host much longer than most pathogens.

Far less is known about B. salamandrivorans. Foster told IFLScience only one species is known to have been majorly affected by it, but the identification is so recent this could be a major underestimate. With amphibians being less well studied than other vertebrates, it is likely the true toll is larger even than those the paper covers.

The one good piece of news is that declines peaked in the 80s and 90s, and 20 percent of surviving species appear to be recovering. Unfortunately, in most cases, the rebound is slow and species that have evolved resistance can still give the fungus to others.