Biologists consider the health of frogs to be indicative of the health of the biosphere as a whole.
Frogs have permeable skin that easily absorbs toxins. They require specific aquatic and terrestrial environments to survive and breed, making them highly susceptible to environmental disturbances. Because of this they are considered accurate indicators of environmental stress.
Frogs have lived on the Earth for 250 million years, surviving ice ages and other climate changes. Yet around the world, populations of amphibians, particularly frogs, are now in drastic decline.