Spring has become more silent in the past 10 years and scientists have started wondering where all the frogs have gone.
It used to be, George Harrison writes in an article in Sports Afield magazine, that one of the harbingers of spring was the tinkling call of the spring peeper, to be followed a few weeks later by the cricket frogs, then the American toads, the wood, leopard and pickerel frogs.In 1989, attendees of the first World Conference of Herpetology in Canterbury, England, compared notes and realized that frogs, toads and salamanders were disappearing on a global scale.
Since that conference, scientists have been searching for answers, as amphibians continue to disappear - some have even become extinct.
There was immediate speculation that the amphibians were victims of acid rain, disease, decreases in the ozone layer, pollution and habitat destruction. There was no hard evidence to show that any one factor was responsible.
In 1993, a team of Oregon State University researchers uncovered strong evidence that an increase in the amount of ultraviolet radiation reaching the Earth was responsible for the demise of some populations of amphibians.
The study was conducted by a team led by Dr. Andrew Blaustein and Dr. John Hays. It found that UV-B, a type of ultraviolet radiation that can cause skin cancer, cataracts and the weakening of the immune system in humans, is also destroying the eggs of frogs and toads that breed in Oregon's Cascade Mountains by altering the structure of their DNA.
Because the gelatinous strings of eggs amphibians lay don't have the protective shells of birds' eggs, they are exposed to the sun's rays as they lie in water for several weeks before hatching.
In their experiments, the researchers collected the eggs of Cascades frogs, western toads and northwestern salamanders, placed them into enclosures in the natural habitat, and used filters to expose them to varying degrees of UV rays. The eggs given the most UV protection had the highest hatching rates; those with the least protection had the lowest.
"More than 40 percent of the western toad and Cascades frog eggs exposed to UV-B radiation died, compared with 10 to 20 percent of those that were shielded," Blaustein said.
Last year, the researchers conducted the experiments again, and the results were even more convincing.
"More than 90 percent of the northwestern salamander eggs exposed to the UV-B died," Blaustein said.
Surprisingly, the Pacific tree frog eggs, also exposed to UV-B, were unscathed. However, the researchers found that Pacific tree frogs carry in their cells a defense mechanism - photolyase, an enzyme that removes defects caused by UV-B.
Blaustein also believes that if UV-B alone doesn't kill the eggs, the growing presence of a pathogenic fungus, introduced with stocked fish, weakens the eggs' defenses and makes them even more vulnerable.
"If ultraviolet radiation continues to increase in the world as is projected, it bodes very badly for animals such as frogs, toads and salamanders - and humans, too," Blaustein told Sports Afield, adding:
"If the frogs disappeared, an important link in the food chain would be broken and there would be a large increase in insects, and perhaps a significant drop in the numbers of frog-eating fish and wildlife."