Choir of croaks, chirps, whistles dying

Endangered

In 2004, the International Union for Conservation of Nature (IUCN) found 32.5 per cent of the amphibian species endangered. Frogs have been facing catastrophic declines over the past decades in what is being termed as the biggest extinction event in progress since the dinosaurs disappeared 65.5 million years ago. “This is a crisis situation. About a third of the amphibians are threatened or extinct,” said Jamie Voyles, a researcher at James Cook University in  Australia. Along with factors like habitat destruction and climate change, the drastic decline in frog populations is also attributed to the fungus Batrachochytrium dendrobatidis (Bd).

Voyles and her colleagues discovered the mechanism by which the Bd fungus operates. In a study published in a recent issue of Science, they identified the frog’s skin as the key target. Frogs breathe and drink through their skin. When the fungus colonises the skin, it chokes the frogs—the disease is chytridiomycosis. The fungus works by disrupting the electrolytic balance across the skin in green tree frogs (Litoria caerulea). The moisture-laden skin of amphibians is necessary for survival. A network of pumps allows the exchange of electrolytes such as sodium and potassium to maintain a chemical balance and help the skin breathe. When scientists measured the electrolyte balance in infected frogs, they found it had gone awry. The levels of sodium in blood and urine fell 50 per cent, and that of potassium went down 20 per cent. This led to cardiac arrest and subsequently the frogs died.

When Voyles placed infected frogs in an electrolyte-rich solution, she found the chemical balance got restored gradually. The sick frogs became more active even though the fungus was still there. Results suggested the disease was a skin infection that led to death. In Panama, the disease wiped out 90 per cent of the frogs in six months. They were saved from extinction because a team from Amphibian Ark (a collaboration of IUCN and World Association of Zoos and Aquaria) captured a few and brought them back to a zoo in Atlanta in USA. Removal of frogs—an essential part of the food chain—would spell doom for other animals such as snakes. As tadpoles eat algae, their disappearance would also lead to increased algal growth in water which would, in turn, choke other plant species.

Gayathri Vaidyanathan
Down To Earth Feature Service