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.
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 frog's demise
Frogs and toads, amongst nature's hardiest, are in serious trouble in many places. 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. Other animals groups face similar threats but amphibians are more vulnerable. What works against them 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.
Ultra dangerous
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.
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
uv 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 (see 'Feeling Jumpy',
Down To Earth, February 28, 2006). These suppress their immune systems and affect their growth.
Pathogens get lucky
A new, serious threat is a fungal disease called chytridiomycosis. The fungus kills amphibians by attacking their sensitive skins. The little-known pathogen was first described in 1998; there is no way to control it in the wild. The susceptibility to fungal disease might increase with climate change. 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,
uv 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. The increase in frequency of El Nio events, for example, could increase
uv exposure and infections.
Too fast, too soon
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. "We know that there are various causes for amphibian population declines; there have always been threats, and these have been some of the most adaptive and successful vertebrate animals on Earth. But the worldwide decline of amphibians is because of the fact that they are unable to keep pace with the current rate of global environmental changes," says Blaustein.
Contamination, radiation exposure and diseases coupled with habitat fragmentation are relatively rapid changes, given that amphibians have been laying their eggs in less-contaminated water for millions of years. Since 1976, at least 30 new diseases have emerged, many triggered by recent changes in the Earth's climate.
"Although relatively rapid evolution may occur within some amphibian populations when a novel threat arises, other threats--emerging infectious diseases, perhaps coupled with environmental changes--may be too intense and too new for amphibians to cope," Blaustein writes.
"Historically, amphibians were adept at evolving to deal with new conditions, now what they are doing is showing us just how rapid and unprecedented are the environmental changes underway. Many other species will also be unable to evolve fast enough to deal with these changes. Because of their unique characteristics, the amphibians are just the first to go," he adds.
