Why Are Frogs Disappearing? A Fungus, a Trade and the Animals That Breathe Through Their Skin
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In the late 1980s herpetologists at a conference in Canterbury compared notes and found that each of them had a story of frogs vanishing from places where they had been abundant, in Costa Rica, Australia, California and the Andes, in pristine reserves as much as in cleared land. The golden toad of Monteverde, bright orange and photographed by the thousand in 1987, was last seen in 1989. Amphibians, which survived the extinction that took the dinosaurs, turned out to be disappearing faster than birds or mammals, and the largest single cause was a fungus nobody had described.
An animal with no armour
Frogs and their relatives, the salamanders and the legless caecilians, live in two worlds and are exposed in both. They breathe partly through a skin that must stay moist and that absorbs whatever is in the water, from pesticides to pathogens; they lay jelly-coated eggs in water with no shell; their young are aquatic and their adults are terrestrial, so they need both habitats intact and connected; and they are cold-blooded, so their immune systems slow when they do. An animal built like that is a sensor for the state of a landscape, which is why ecologists watched them, and why, when the readings went wrong everywhere at once, the alarm was general. Around 8,000 species are known, and the assessment of 2023 found 41 percent of them threatened with extinction, against 27 percent of mammals and 13 percent of birds.
The fungus
The killer was identified in 1998: a chytrid fungus, Batrachochytrium dendrobatidis, Bd, that infects the keratin in amphibian skin, thickens it, and stops the animal regulating the water and salts that pass through it until its heart stops. It spreads by swimming spores, survives in water and mud, and kills some species within weeks of arrival while leaving others as carriers. Its origin was traced by genetics in 2018 to the Korean peninsula, and its spread around the world in the twentieth century to the trade in amphibians, above all the African clawed frog, exported by the hundred thousand from the 1930s for use in pregnancy tests, and the American bullfrog, farmed for food. Bd has been recorded in over 500 species, has driven declines in at least 500 and extinctions in about 90, the greatest loss of biodiversity attributed to a single pathogen ever documented, and a second species, Bsal, found in 2013, is doing the same to European salamanders.
Everything else
The fungus lands on populations already under pressure from several directions:
- •Habitat loss: wetlands drained, forests cleared and the ponds and streams that link breeding sites to the rest of a frog's range cut by roads
- •Pollution: agricultural runoff, and the herbicide atrazine, which at trace concentrations feminises male frogs
- •Climate change: drier and more variable seasons that strand eggs and tadpoles, and warmer nights that favour the fungus
- •Introduced species: trout stocked in mountain lakes that eat the tadpoles, and bullfrogs and cane toads that eat everything
- •Collection: for the pet trade, for food, and for traditional medicine
- •Disease interactions: the fungus, ranaviruses and pollution weakening animals for each other
What is being tried
There is no cure for a fungus loose in every stream on five continents, and the responses are holding actions. Zoos and field stations have built biosecure arks, taking the last individuals of the most threatened species into captivity ahead of the fungus's arrival, as was done for dozens of Panamanian species and for the Kihansi spray toad of Tanzania, which was extinct in the wild for a decade before being returned. Some populations have been found to be recovering on their own, with animals whose skin bacteria or immune genes resist the fungus surviving and breeding, and researchers are selecting for those traits and trialling probiotic baths. Trade controls, clean-boot rules for field workers and hikers, and the removal of trout from mountain lakes have local effects. And the Bsal salamander fungus, unlike Bd, was caught early: Europe and the United States restricted the import of salamanders within three years of its discovery.
Why it matters beyond frogs
Amphibians eat insects by the ton, including the mosquitoes that carry human disease, and feed everything from snakes to storks; in the Central American forests where the fungus emptied the streams, snake populations crashed and the algae the tadpoles had grazed took over the water. Their skins are a pharmacy, having supplied painkillers, antibiotics and the compounds under study for their ability to block viruses. And they are the group in which the sixth mass extinction, if that is what is under way, is furthest advanced, which makes them the measure of how it might be stopped. The golden toad is gone; the Panamanian golden frog survives in a few hundred tanks; and the frogs that still call from a pond on a spring night are the ones that have not yet met what killed the others.
The takeaway
Frogs and other amphibians are declining faster than any other animal group, with two fifths of species threatened, because their permeable skins, shell-less eggs and double lives expose them to everything, and because a chytrid fungus spread by the amphibian trade from Asia in the twentieth century has killed hundreds of populations and about ninety species outright. Habitat loss, pollution, climate and introduced predators compound it, and the responses are captive arks, resistance breeding, trade controls and the early containment of a second fungus.