A frog-killing fungus is responsible for the decline of more species than any other pathogen on record, a global analysis has found.
The study, published on 28 March in Science1, reveals that chytrid fungi have caused the decline of at least 501 amphibian species worldwide from 1965 to 2015, including 90 that have become extinct.
Other well-known pathogens such as the bat disease white-nose syndrome or West Nile virus, which kills birds, have affected just a fraction of the number of species. Chytrid fungi’s impact on biodiversity is comparable to those of the most destructive invasive species, such as cats and rodents, says the study.
Seeing the global effects of the fungus summed up is striking, says Stephen Price, a disease ecologist at University College London. “It’s almost a bit Hollywood,” he says. “We love these movies about pathogens that emerge suddenly and wipe out humanity, and we’ve really seen something like that with respect to frogs in the real world.”
Chytrid fungi normally live in soil and water, where they break down dead organic material. But in 1998, scientists identified the species Batrachochytrium dendrobatidis, which causes a deadly disease called chytridiomycosis in amphibians. The fungus attacks the skin of toads, frogs and related animals, preventing them from properly regulating their salt and water levels; this eventually causes the animals' hearts to stop.
Researchers realized that this fungus was responsible for mysterious declines in amphibian populations in South America, Central America and Australia, where it had been introduced in the second half of the twentieth century through the wildlife trade. But no one had added up the effects of the fungus on a global scale. So Benjamin Scheele, a population ecologist at the Australian National University in Canberra, put together a team of 41 ecologists and other experts from around the world to assess the extent of the problem.
Each researcher gathered evidence on which amphibian species had been affected by chytrid fungus in their region, by scouring the scientific literature and unpublished data, as well as interviewing other specialists.
The team found that the fungus was probably involved in the declines of at least 501 amphibian species between 1965 and 2015 (see ‘Killer fungus’). These ranged from relatively small losses of less than 20% of a population to complete extinctions. Atelopus, or stubfoot toads, were particularly badly affected, with 30 species in that genus presumed extinct.
The number of species experiencing declines peaked in the 1980s, coinciding with the spread of the fungus through Central America and Australia. A smaller peak in the 2000s seems to be related to the spread of the fungus in South America. But no species has been affected in Asia, where the fungus originated. The researchers suggest that amphibians that evolved alongside the fungus developed ways to deal with infection.
Only a few species in Europe have experienced declines, the study found. It’s possible that the chytrid fungus reached those regions earlier in the twentieth century, says Scheele: the areas saw mass declines in amphibians in the 1950s and 1960s. These have been attributed to the intensification of farming, but the fungus could also have been involved.
Protecting the frogs
Scheele says the threat to species from pathogens — and the wildlife trade that spreads them — is underappreciated. “We need to really think much more thoroughly about biosecurity,” he says. And he hopes that his data can help. The team found that the species that experienced the worst declines have certain characteristics in common: they tend to have a small range and live in particularly wet regions, for example. Knowledge of these risk factors could direct attempts to protect other species.
Price sees a ray of hope in the otherwise gloomy data. The researchers found that 20% of the surviving species for which data are available are showing small signs of recovery — suggesting that other species might also bounce back.