Amphibians are known for their resilience. They have bounced out of mass extinctions, survived vegetation changes, and outlived the dinosaur by evolving distinctive behaviours and morphologies and adapting their lifecycles. But a study from the US reveals these very attributes have placed the future of nature’s sentinels in peril (BioScience, Vol 57, No 5, May 2007).
Amphibian decline has been documented in the Americas, Europe, Asia and Africa. Of the 5,743 known species, 43 per cent is declining, 32 per cent threatened, and 168 considered extinct. Their state came into sharp focus in 1989, when scientists compared notes at the first International Conference on Herpetology; many amphibian species were declining mysteriously.
Indian scenario
In India, a 1998 survey by the Conservation Assessment and Management Plan Workshop for Amphibians of India used techniques developed by the Conservation Breeding Specialist Group of IUCN (World Conservation Union). It concluded that about 48 per cent of all amphibian species and 59 per cent of endemic ones were threatened.
The California red-legged frog (Rana aurora draytoni, made famous by the character of Daniel Webster in Mark Twain’s Jumping frog of Calaveras County), has disappeared from 75 per cent of its range. The noisy natterjack toad (Buffo calamita) in England is almost extinct in lowland heaths.
The causes are several: exposure to ultraviolet (UV) radiation, habitat loss, pesticide contamination, global warming, acid rain and infectious diseases. What works against amphibians are the very characteristics acquired over their evolution to survive a variety of environmental regimes.
Most frog, toad, and salamander species lead double lives; starting out in water in the egg and larva forms, they switch to a terrestrial adulthood. They deal with two kinds of habitats, with varying dangers of pollution. Their eggs have no shells; their permeable skin is little protection from fungal infection and UV rays.
In the early 1990s, Andrew Blaustein led a team of researchers from the Oregon State University in USA to the range of the Cascades frog (Rana cascadae) in the Oregon mountains, where the young ones of this species regulated their body temperatures by basking in sunlight. Something was killing the creatures. The possible cause emerged a few years later: increasing exposure to UV rays due to a depleting ozone layer.
UV rays
Later, field experiments proved UV rays decrease the hatching success of several species. In the Pacific Northwest of the US, for example, the hatching success of Cascades frogs, western toads, long-toed salamanders, and northwestern salamanders was lower when exposed to ambient U V radiation. In Europe, the hatching success of common toads (Bufo bufo) was lower for UV exposed eggs than for those shielded from the rays.
Amphibian eggs face aquatic predators and a variety of chemical contaminants. For amphibians, extremely low concentrations of widely used pesticides are harmful. These suppress their immune systems and affect their growth.
A new, serious threat is a fungal disease called chytridiomycosis. The fungus kills amphibians by attacking their sensitive skins. Studies show changes in climate in the American tropics have led to more outbreaks of chytridiomycosis.
Scientists talk of a complex link between climate change, radiation exposure and amphibian morbidity. When water levels decrease, U V exposure increases, and eggs become infected with Saprolegnia ferax—a freshwater mould that spreads across the surface of its host as a cotton-like film.
Amphibians evolved and survived for millions of years through many climatic regimes. But they are unable to keep step with the increasing pace of climate  change.
CSE/Down to Earth
Feature Service