How did one fungus push hundreds of frog species toward extinction?

The chytrid fungus Batrachochytrium dendrobatidis, known as Bd, attacks the skin that amphibians rely on, and it has been linked to some of the worst wildlife declines on record. It is a fungus that causes the disease chytridiomycosis in amphibians. It was discovered in 1998 by Lee Berger and described as a species in 1999 by Joyce E. Longcore, and the disease has devastated amphibian populations around the world.

The fungus infects the keratinized skin of amphibians. It has enzymes that help it digest amphibian cells and use the skin as food. Frogs and toads need their skin to keep their balance of water and salt, and the damage disrupts that balance, eventually causing heart failure. Bd spreads with swimming spores called zoospores, active only briefly and able to travel one to two centimetres.

How big is the toll? A 2019 Science review assessed that chytridiomycosis was a factor in the decline of at least 501 amphibian species over 50 years, of which 90 were confirmed or presumed extinct in the wild and another 124 had declined by more than 90 percent. It called this the greatest recorded loss of biodiversity attributable to a disease. But a follow-up study in Science said the review lacked the evidence for its claims, so it remains unclear how many and which species have been affected.

The fungus can cause 100 percent mortality in some populations, and no effective measure is known for controlling it in the wild. Some amphibians seem able to withstand it, partly through a symbiosis with the bacterium Janthinobacterium lividum.