Frog fungus: 500 species face extinction

A fungus which kills frogs is among the most destructive invasive species ever recorded, pushing more than 500 amphibian species towards extinction, researchers reveal in a study published in the journal Science.

The virulent disease is caused by the fungus Batrachochytrium dendrobatidis (Bd) and is known as chytridiomycosis. The pathogen emerged from Asia in the 1980s and proliferated through the live animal trade, causing mass amphibian die-offs, even in uninhabited, pristine environments.

“Despite increasing understanding of the fungus, scientists have only been able to guess at the scale of damage caused by Bd to amphibian populations across the world,” largely due to a lack of an appropriate dataset, write Dan Greenberg and Wendy Palen, from Canada’s Simon Fraser University, in a related Perspective in the same journal.

The study represents the first global epidemiological analysis of the extent of amphibian biodiversity loss caused by this disease.

Led by Ben Scheele from the Australian National University, with colleagues from institutions worldwide, the research used a comprehensive dataset and expert interviews to reconstruct a detailed history of the extent and severity of the effect of Bd on amphibian populations.

Of 501 amphibian species, 90 are confirmed or presumed extinct in the wild, and a further 124 species are suffering abundance declines of more than 90%.

These grim findings are “the greatest documented loss of biodiversity attributable to a pathogen” the researchers write, placing Bd “among the most destructive invasive species, comparable to rodents (threatening 420 species) and cats (Felis catus) (threatening 430 species)”.

There are other wildlife pathogens such as white-nose syndrome (Pseudogymnoascus destructans) in bats which affects six species, and West Nile virus (Flavivirus sp) in 23 bird species, but the scale of Bd effects are unprecedented, with impacts on 6.5% of described amphibian species.

The most severe die-offs from the disease peaked globally in the 1980s, even before the Bd pathogen was identified, with another smaller peak in the 2000s.  Most amphibian declines have occurred in the tropics of Australia, central America and South America.

The study examined life history traits and environmental conditions to determine why some species declined more severely than others, and found that large-bodied, aquatic frogs with a restricted range are most affected by the Bd pathogen. This is because the fungus dies when desiccated, and thus thrives in more permanent water. Larger bodied, longer living frogs have less reproductive potential to offset mortality due to the disease.

Although the number of new declines has now eased, there remains potential for substantial amphibian loss in areas not yet exposed to Bd, most notably the highly biodiverse islands of Madagascar and Papua New Guinea.

Out of the 292 surviving amphibian species of which population trends are known, 12% have shown initial signs of recovery – however, not in the lineages most susceptible to the disease. The remaining 232 species in the analysis have shown no signs of recovery from Bd.

Along with habitat loss, exploitation and climate change, “Bd is but one more nail in the coffin for the state of amphibians globally”, Greenberg and Palen write.

Actions such as protecting habitat and limiting collection of wild caught populations may reduce these threats, they note, but in contrast “there appear to be few viable management actions one pathogenic Bd strains have established”, besides the evolutionary rescue of wild populations.

Therefore, limiting trade and the potential spread of the fungus is imperative.  This is challenging in the face of the current expansion of global trade routes.

“The unprecedented lethality of a single disease affecting an entire vertebrate class highlights the threat from the spread of pathogens in a global world,” the study warns. Effective biosecurity and an immediate reduction in wildlife trade are urgently needed to reduce the risk of further spread of this most virulent invasive species.