Why Are Amphibians Declining? Chytrid, Bsal and Habitat Loss

How bad is the amphibian decline?

The first Global Amphibian Assessment, published in 2004, was the first time scientists evaluated every known amphibian species against the IUCN Red List criteria. It found that about 32 percent were threatened and that many populations were shrinking even in protected areas.

The second assessment, led by the IUCN SSC Amphibian Specialist Group and published in the journal Nature in 2023, covered more than 8,000 species. It found that about 41 percent are now threatened, compared with an estimated 38 percent in 1980. Salamanders and newts are worst off, with roughly three in five species threatened. Hundreds of species have moved into higher threat categories since 2004, and several are thought to have gone extinct.

Behind these numbers are real losses. The golden toad of Monteverde, Costa Rica, was last seen in 1989 and is now listed as extinct, and many harlequin frogs of Central and South America vanished within a few years.

Chytrid fungus (Bd): the frog killer

The most dramatic declines have been caused by Batrachochytrium dendrobatidis, usually called Bd. This microscopic chytrid fungus infects the skin of amphibians and causes a disease called chytridiomycosis. Because amphibians use their skin to breathe and to control water and salts, a heavy infection disrupts their body chemistry and can stop the heart.

Genetic studies suggest that the most deadly strain of Bd originated in East Asia, and that it spread around the world during the 20th century through the international trade in amphibians. A study published in Science in 2019 estimated that Bd had contributed to the decline of at least 501 amphibian species, including about 90 presumed extinctions, the greatest recorded loss of biodiversity caused by a single disease.

Bd has struck hardest in the mountain rainforests of Central America, the Andes and eastern Australia. Some species, such as the American bullfrog, can carry the fungus without getting sick, which helps spread it to more vulnerable frogs.

Bsal: the fungus killing fire salamanders

A second, related fungus, Batrachochytrium salamandrivorans (Bsal), was described in 2013. Its name means “salamander-eating,” and it destroys the skin of salamanders and newts. It was discovered after the fire salamander almost vanished from the Netherlands: from about 2010, populations there fell by around 96 percent, and the largest dropped from 241 animals to just four in 2011.

Bsal probably came from East Asia, where local salamanders and newts tolerate it, and reached Europe through the pet trade in species such as fire-bellied newts. It has since spread through Belgium and western Germany, with outbreaks in other parts of Germany and a contained outbreak in Catalonia, Spain. Because of Bsal, the IUCN moved the fire salamander from Least Concern to Vulnerable in 2023.

North America has the greatest variety of salamanders on Earth, and laboratory studies show that many of them, including the eastern newt, would die if infected. To keep Bsal out, the United States restricted imports of 20 salamander genera in 2016 and added 16 more in 2025. Bsal has not been confirmed in wild North American salamanders. Read more in where fire salamanders live.

Habitat loss and fragmentation

Habitat loss affects more amphibian species than any other threat. Wetlands are drained, forests are cleared for farming and timber, and streams are dammed, polluted or channeled. Many amphibians need two habitats, water for breeding and land for living, plus safe routes between them, so even partial changes can cut a population off.

Roads are a special problem. On mild, wet nights in spring, huge numbers of common frogs, toads and newts migrate to their breeding ponds, and many are killed by traffic. In tropical countries, the clearing of rainforest destroys the tiny ranges of species found nowhere else, such as the golden poison frog of Colombia, which is listed as Endangered.

Climate change

The 2023 assessment found that climate change was the main driver of species moving into higher threat categories since 2004, followed closely by habitat loss. Amphibians are cold-blooded and depend on moisture, so they are very sensitive to changes in temperature and rainfall.

Droughts dry up breeding ponds before tadpoles can transform, hotter summers make forest floors too dry for salamanders, and extreme storms, floods and fires destroy habitat. Mountain species have nowhere higher to go as temperatures rise. Climate change can also change how diseases such as Bd spread and how badly they affect frogs.

Pollution, invasive species and other threats

Several other pressures add to the decline:

• Pollution and pesticides. Amphibians’ permeable skin and eggs absorb chemicals from water. Some pesticides, herbicides and fertilizers can kill tadpoles or affect their growth and development.
• Invasive predators. Fish stocked in mountain lakes, such as trout, eat tadpoles and larvae, and introduced species like the American bullfrog and the cane toad compete with or prey on native amphibians.
• Other diseases. Ranaviruses can cause mass deaths of frogs, including common frogs in Britain.
• Overharvesting. Millions of frogs are collected each year for food and the pet trade, and the trade can also move diseases around the world.

Why amphibians matter, and what helps

Amphibians are a key link in food webs: they eat vast numbers of insects and are eaten by birds, mammals, fish and snakes. Tadpoles graze algae and help keep ponds healthy. Because they react quickly to environmental change, scientists often call them indicators of the health of ecosystems.

Conservation can work. Protecting and restoring wetlands, ponds and forest streams, building tunnels under roads, controlling disease through strict hygiene and trade rules, and breeding the most threatened species in captivity have all saved populations. At home, a fish-free garden pond, fewer pesticides, and never releasing pet amphibians or aquarium water into the wild all make a difference. You can learn how frogs grow in how tadpoles turn into frogs.