Why Frog Species Are Being Wiped Out Across the World

Frogs and their amphibian cousins are being decimated by a deadly fungal infection contributing to the endangerment and extinction of hundreds of species around the world.

The disease, chytridiomycosis, is caused by the Batrachochytrium dendrobatidis fungus, which infects the keratin in the amphibians' skin. The infection has been found to cause nearly 100 percent mortality in some species, while only mildly affecting others.

The fungus had spread to populations across the world, but until now, had mostly not affected Africa. However, according to a new study published in thejournal Frontiers in Conservation Science, chytridiomycosis has steadily increased in occurrence across the continent since the year 2000.

"This is a microscopic fungus that infects the skin of amphibians and is fatal to many species," Vance T. Vredenburg, a chytridiomycosis researcher, co-author of the Frontiers in Conservation Science paper, and professor and associate chair of the Department of Biology at San Francisco State University told Newsweek.

"In fact, the disease caused by this fungus, called chytridiomycosis, represents the worst case in recorded history for a disease of vertebrates. Hundreds of species have been affected (over 500), and many went extinct in mass die-offs soon after fungal invasion."

Chytridiomycosis kills frogs by infecting their skin and causes skin shedding and other symptoms like ulcers. Frogs and other amphibians absorb oxygen through their skin and perform crucial ion transfer across their skin layers.

"The best evidence for how it causes death suggests that the fungus disrupts essential functions of the frog skin (uptake of essential ions, respiration)," Louise Rollins-Smith, an associate professor of pathology, microbiology and immunology at Vanderbilt University, told Newsweek.

Batrachochytrium dendrobatidis is associated with the extinction of over 200 species of amphibians, including the southern gastric-brooding frog, northern gastric-brooding frog, sharp-snouted day frog, and southern day frog in Queensland, Australia alone. The fungal infection is said in a 2019 study from the journal Science to be responsible for the decline in population of 39 percent of amphibian species worldwide.

It was first found in Queensland, Australia in the 1990s after several species of frog were found dead. It is present around the world but is most prevalent throughout South and Central America, Australia and North America. The fungus is thought to spread via spores released from amphibian skin into water.

"There appear to be multiple pathways (pet trade, food trade-where frogs are consumed by humans, moved by accident in trade, e.g., bananas) but humans are implicated in all of them. We don't yet know how it moves across a landscape, but we can track it," Vredenburg said.

"The fungus was first noticed in 1993 in Australia and was described (given a scientific name) in 1999. We have used collections of amphibians in natural history collections to trace the spread of the fungus around the world," he continued. "We wrote this paper because while there are now hundreds of scientific studies on this pathogen, few have been conducted in Africa even though there are thought to be over 1,200 species of amphibians that occur there."

The reason it took so long to fully invade Africa is still a source of confusion for scientists.

"We don't know [why Africa was spared until recently]. It could just be due to chance," Vredenburg said. "We know the fungus spread through Australia in the 1990s, in California in the 1970s, Mexico and Central America in the 1980s and further south in the 1990s. In Europe, it was also later, similar to Africa."

Alternatively, the lower numbers in Africa may have been due to a lack of data being collected.

"It has likely been in Africa for some time but unnoticed because there are fewer people looking for it there," Rollins-Smith said. "There have been earlier reports of its presence in South Africa and Madagascar."

Asia is the last stronghold in the global battle against the fungus, however, with the infection having only been found in Indonesia, South Korea, China and Japan so far, and of the species in these regions, the infection only affected around 2 percent of individuals tested.

"Asia appears different. There are no die-offs associated with the fungus, suggesting a longer evolutionary relationship between the pathogen and the host," Vredenburg said.

With no cure or vaccine available, there's not much that we can do to prevent the slow creep of this infection across the world's amphibians, unfortunately, but some frogs are bouncing back.

"In many areas where species were thought to go extinct after these fungal epidemics, some individuals are being rediscovered," he said. "In California where two species of mountain yellow-legged frog were decimated by the fungus, one, the Sierra Nevada yellow-legged frog (Rana sierrae) has bounced back. In Yosemite National Park, this frog whose populations collapsed in the 1980s continues to be lightly infected but the populations are increasing."

Climate change may prove to be advantageous to the fungus, however. One 2006 study published in the journal Nature found that increased cloud cover as a result of climate change may cause cooler day temperatures and warmer night temperatures, which would be more apt for the growth of the fungus, which thrives between 63 and 77 degrees Fahrenheit.

However, climate change may also negatively affect the fungus, as it could cause more hot, arid and dry conditions, where the fungus requires moist environments to spread via its spores, and cannot grow above temperatures of 86 degrees Fahrenheit.

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