Amphibians in Peril

 

    One of the most pressing issues facing Earth's biodiversity is the devastating losses seen among the world's amphibians. While there are many threats, including pollution, the pet trade, and habitat loss. The most devastating has been the spread of disease. 

"The most deadly of these has been an introduced fungal pathogen, a type of chytrid fungus, called Batrachochytrium dendrobatidis, or Bd for short. After years of unexplained amphibian die-offs, Bd was eventually discovered by researchers in 1998. It has been responsible for many of the devastating declines in amphibian populations around the world, especially in South America, Central America, and Australia. This highly contagious fungus infects the keratinized portions of amphibian skin. The resulting disease, called chytridiomycosis, thickens and hardens the epidermis of the infected animal, eventually shutting down the transport of oxygen and water through the skin and killing its victims.

Bd is thought to have originated in Asia and was likely spread to disease vulnerable groups of amphibians via introduced carriers. Not all amphibians are negatively affected by Bd, and several notable species are likely vectors for the spread of the disease into new environments. One significant species not affected is the American bullfrog (Lithobates catesbeianus). While this is good news for native populations of bullfrogs, the species is widely farmed around the world for food. Research has shown that these farming operations can harbor high amounts of Bd and, according to researchers, these “bullfrog farms constantly release substantial quantities of Bd zoospores into the surrounding natural environment.” Other species indicated in the spread are African clawed frogs (Xenopus laevis), which were once widely transported across the world to be used in pregnancy tests, and the introduced cane toad (Bufo marinus), which is likely the reservoir for the disease in Australia. How and exactly when Bd first left Asia is unknown, but it is clear that the increase in the international trade of amphibians for the pet trade, food, and medical purposes has fueled the spread of the deadly pathogen. 

Photo: Mikołaj Kołodziejczyk

While Bd is thought to potentially affect all orders of amphibians, a new threat has emerged for salamanders. A related fungus, Batrachochytrium salamandrivorans, was first identified in 2013 among wild and captive fire salamanders (Salamandra salamandra) in Belgium and the Netherlands. Bsal, like its cousin Bd, causes a similar disease and can have the same devastating effects. The once-thriving populations of fire salamanders in the Netherlands have now been reduced by 96%. Thought to also have originated in Asia, Bsal is believed to have been imported into Europe through the pet trade. Of great concern is the potential for Bsal to reach North America and the diverse population of salamanders there. To try to prevent the potentially devastating effects of an introduction, the United States has blocked the transfer of some 200 species of salamander that may harbor the fungus. Thankfully, there is still no evidence of Bsal in North America, but the risk of introduction remains extremely high, with dire consequences. 

Amphibians are one of the most threatened groups of animals on the planet. Currently, 40%–50% of all amphibian species could be at risk of extinction. The IUCN Red List of Threatened Species lists over 1,400 species of frog, 279 species of salamander, and 11 species of caecilian as endangered or critically endangered. Bd alone has been implicated in the decline of at least 501 species and the likely extinction of 90 in the last 50 years. Unfortunately, these numbers are probably underestimates. With new species being discovered every year, it is likely, with the often devastating and rapid declines observed in Bd infected amphibian populations, that there are many species unknown to science that have already been lost. The majority of amphibian declines have been in the tropics of Australia, Mesoamerica, and South America, but declines in amphibian populations around the world have been recorded for decades. Thankfully, there have been some recorded recoveries in recent years, but these recoveries come nowhere close to the presumed initial populations. While cautiously optimistic, scientists still harbor serious concerns. Many species are still extremely vulnerable, and any significant disruption could end in their demise. As the threats to the world’s biodiversity continue to increase, the pressures on these vulnerable species will continue to grow. Quick action to change the status quo is needed if humans are to avoid losing thousands of species of amphibians."  --Excerpt from Extinction in a Human World