On a spring night in the marsh behind the cottage, a chorus of tiny frogs called spring peepers, Pseudacris crucifer, produces a deafening peal. The same is true of toads, Bufo americanus, that converge on the Eramosa River by the thousands to breed on certain May evenings. Their trills combined become a roar. (The linked pages include embedded sound files.)
But in many places around the world, amphibious voices are disappearing. Just as Rachel Carson gathered studies from the 1950s and early 1960s documenting the mysterious deaths of songbirds and other species, scientists are now uncovering evidence that another silent spring may be imminent. This time toxic chemicals in the environment cannot account for the problem. The cause appears to be multi-faceted, and once again it bodes ill for human health. By studying the decline of amphibian species, scientists are gathering clues about the increased incidence of infectious diseases in humans. Let's take a look at why this decline is so important.
Amphibians emerged from Devonian seas about 350 million years ago and became the first vertebrates to widely colonize the land. 5620 modern species have been identified, including 4932 Anura (frogs and toads), 520 Caudata (newts and salamanders), and 168 Gymnophiona (limbless caecilians). After emerging from their soft eggs, most species go through an aquatic larval stage, then metamorphose into an adult that lives on land.
This complex life cycle is a survival strategy. It allows the creature to adapt to prevailing environmental conditions. The young can live in ephemeral pools, eating algae and maturing quickly, then turn into four-legged, air-breathing, carnivorous animals when the water dries up. Adults live several years, so they can even survive years of drought with inadequate environments for reproduction, however this adaptability does not equip them for long-term climate change.
Amphibians are particularly sensitive to the environment because, unlike other vertebrates, they lack a protective epidermis with scales, hair, skin or feathers. In fact they breathe through their skin. This feature allows them to move easily between air and water, because they can always absorb oxygen from their surroundings. The disadvantage is it makes them susceptible to pathogens. Parasites easily invade their bodies, and toxic chemicals are readily absorbed. Amphibians are also vulnerable to ultraviolet radiation. This problem affects all life stages, even the eggs, which are not protected by a hard shell as in reptiles.
Despite their small size and often innocuous appearance, amphibians play a vital role in food webs and ecosystems. They are important consumers of algae and plankton, in turn being consumed by fish, birds and other predators.
