Leapfrogging to doom

Leapfrogging to doom The planet's amphibians are under threat. Declining populations of frogs, salamanders and toads have been reported from all over the globe and some of the amphibian groups are disappearing completely from their natural habitats.

Frogs are in close communion with their surroundings - both water and soil - at different times in their life history. Besides, they have different food habits - adult frags are carnivores, while tadpoles are herbivores. This makes them suitable indicators of the state of the planet's health. Scientists argue that what happens to the f rags is a reflection of what can happen to humans, as they occupy the same living area (Scientific American, Vol 274, No 4).

Several causes have been suggested for the decline in amphibian populations - first recognised as a global phenomenon in 1990. Andrew Blaustein, professor of ecology at the Oregon State University says that the depletion of stratospheric ozone may well be harming the amphibian species in some parts of the world. The ozone layer shields the earth from the Sun's harmful rays. The ultraviolet rays, particularly ultraviolet-B (UV-B) radiation - a specific wave-length of light with a range of 280 to 320 nanometers - is harmful to both plant and animal life.

Ultraviolet radiation also affects the aquatic insects population - the food of the amphibians, and so influences the amphibian population. Amphibian eggs, which are normally laid in open, shallow waters, are not only being exposed to UV-B radiation but are also being attacked by a fungus - Saprolegnia - which infects fish like salmon and trout.

Pollution is another factor that affects the population of amphibians. Frogs are particularly sensitive to acid rain, insetticides, herbicides and industrial chemicals, which adversely affect the reproduction and development of amphibians. Frog populations have also suffered because of the popularity of frog legs, particularly in France.

One of the biggest human follies has been the introduction of imported species into an ecosystem. The introduction of salmon and trout into streams in California has resulted in the disappearance of the mountain yellow-legged frog.

David Wake, professor of biology at the University of California. Berkeley says that no single explanation fits everycase of declining amphibian populations, but all seem to be important to one degree or another.