The chytrid fungus Batrachochytrium dendrobatidis caused over 90 species extinctions globally, according to a new analysis published on 29 March in Science (1). Moreover, 124 species experienced population declines of at least 90 per cent. The skin-eating fungus has caused more species losses than any other pathogen on record, and the tally is far higher than previously thought.
The usually harmless chytrid fungi typically live in soil and water, where they break down dead organic matter. But around 20 years ago, scientists discovered that on such fungus, B. dendrobatidis can cause something called chytridiomycosis, an infectious disease that disturbs the salt-water balance in the skin of amphibians and can lead to heart failure. Frogs and other species contract the disease by coming into contact with spores in soil or water.
Discovery of the infectious form of the fungus was first made in the 1980s when frog and toad species, such as harlequin toads, began to mysteriously disappear from the rainforests of Central America and Australia, even though their habits remained unmarred. However, until now, the effects of the deadly fungus have not been examined on a global scale.
The international team of ecologists and other experts, assembled by Dr Benjamin Scheele from the Australian National University in Canberra, to compile data on species affected by the chytrid fungus by searching through the scientific literature, as well as unpublished data, and interviewing the world’s top experts.
The findings show that the fungus has played a part in the decline of 501 species between 1965 and 2015, mostly in Central and South America and tropical Australia―the fungus is widespread in Australia and also present in Africa, the Americas, Europe, New Zealand, and Asia. The declines peaked the 1980s when the fungus was introduced into Central America and Australia through the wildlife trade. In Europe, few species have experienced declines as a result of the fungus. Furthermore, species that experienced the worst declines exhibit some common characteristics. For instance, they tend to be larger amphibians, live in wet regions, and inhabit small ranges.
Although the fungus originated in Asia, interestingly, no amphibian species have been affected on the Asian continent. The authors suggest that particular species may have slowly evolved to tolerate the fungus. Unfortunately, this also means that tolerant species are available to act as hosts to spread the disease.
The analysis does offer some hope. Of the species that have experienced a decline, around 20 per cent showed some evidence of recovery, suggesting other species may also be building up an evolutionary resistance to the deadly disease. However, the risk of further outbreaks in new areas remains a possibility.
Frogs and other amphibians are vital to ecosystems by providing food for many birds and animals and munching on insect pests, including mosquitos. Moreover, tadpoles eat algae, which keeps many waterways clean.
The authors call for more stringent biosecurity and an immediate reduction in wildlife trade to stem the spread of this deadly pathogen.
(1) Scheele, B.C. et al. Amphibian fungal panzootic causes catastrophic and ongoing loss of biodiversity. Science (2019). DOI: 10.1126/science.aav0379