The Global Amphibian Decline:
Trends, Causes, and Challenges

Pacific treefrog (Hyla regilla)
photo by Glenn McCrea

Global Trends
North America
Central and South America
Australia
Europe

Causes
Habitat Modification
Exotic Species
Acidification and Toxic Contiminants
Ultraviolet Radiation
Climate Change
Pathogens

Challenges

Bibliography

Links

Potential Cause: Ultraviolet Radiation

Human-induced depletion of stratospheric ozone has resulted in seasonal increases in ultraviolet B (UV-B) radiation penetrating to the Earth's surface. Due to ozone-destroying chlorinated fluorocarbons present in the atmosphere, some researchers estimate that UV-B will increase by 7% per decade over the northern hemisphere.

Consequently, researchers are concerned that increased UV-B exposure could have a negative effect on amphibian populations by causing damage to relatively exposed eggs. The main effect of UV-B exposure is deleterious change in DNA, which can be repaired by photoreactivating enzymes such as photolylase. However, unrepaired DNA damage can result in deformed embryos and decreased hatching success.

Both field and lab experiments have shown that amphibian species have differential sensitivity to UV-B exposure, even at ambient levels. In some studies, sensitivity is correlated with differential levels of photolyase activity among different species. Even more compelling, some nondeclining species, such as the Pacific treefrog (Hyla regilla, above ), have significantly greater photolyase activity and demonstrate greater hatching success under UV-B exposure than do declining species such as the Western toad (Bufo boreas) and cascades frog (Rana cascadae). Similar results have also been found in Australia, where the declining green and golden grass frog (Litoria aurea) has lower photolyase activity than two sympatric nondeclining species, Peron's tree frog (L. peroni) and bleating tree frog (L. dentata). Other studies, however, suggest that UV-B is not always the causative agent for decline. For instance, the red-legged frog (Rana aurora) is a declining species with high levels of photolyase and experimental hatching success unaffected by UV-B exposure.

Some researchers suggest that UV-B exposure could have synergistic effects with other possible agents of amphibian decline, such as repressing amphibian immune systems and making individuals more vulnerable to pathogens.