It is becoming increasingly apparent that amphibian populations are in decline in a wide variety of geographic regions and habitats.10 However, most reports about this have been anecdotal and few experiments have been based on quantitative field observations. Scientists suspect the declines may be due, but are not limited to, factors such as:1,8,10
Ultraviolet radiation may affect this toad at the Baltimore Zoo. Photographer: Brent Moore Creative Commons
- Ultraviolet radiation
- Habitat destruction
- Water pollution
- Introduction of game fish
- Global warming
- Viral infections
- Grazing by cattle
- Alien predators
Why are some populations declining while others are stable?
Most scientists agree that amphibian species, including salamanders, newts, and toads are showing signs of decline in various locations around the globe.1,2 However, it has also been noted that not all species are declining. Some are even on the rise. For example:
In recent years the Costa Rica golden toad is thought to have become extinct.7
The gastric brooding frog of North America has not been seen since the early eighties.7
The Wyoming toad, with numbers in the fifties, and the Houston toad, at populations under ten thousand, are on the “most endangered” list.
Species such as the Colorado River toad, Yosemite toad, foothill yellow-legged frog, mountain yellow-legged frog, cascades frog, spotted frog, leopard frog, tailed frog, and Western spade-foot toad are all on California’s protected species list.
- The gray treefrog of the Southeastern United States seems undaunted by the declines.11,1
The problem now is to find out what is causing the declines, and what makes some species immune. While it is likely that more than one variable is behind the declines, many single variables are being indicated. Ultraviolet radiation is a prominent factor in some decline situations, so more attention to date has been focused on this agent.9
How is UV radiation a factor?
Recent evidence shows that ultraviolet radiation, perhaps linked to ozone depletion, causes the formation of cyclobutane pyrimidine dimers, leading to a higher mortality rate and slower development time in amphibian embryos. If such is the case, increasing ultraviolet radiation could be a major cause of population declines in the amphibian world.
A zoologist at the University of Oregon, Andrew Blaustein, has been working with ultraviolet radiation and amphibians for a number of years. In a 1997 study he and colleagues determined that ultraviolet radiation causes amphibian embryonic deformities.3
- Multiple studies have shown that ultraviolet radiation causes the formation of cyclobutane pyrimidine dimers (CBPDs), which are cytotoxic, and mutagenic photoproducts of DNA.
- These ultraviolet photoproducts impede gene expression by blocking transcription.4
- The photoreactivating enzyme, photolyase, is the most important means for some organisms to repair CBPDs. Certain species, such as the pacific treefrog, had mortality rates unrelated to ultraviolet radiation. This fact is believed to be linked to relatively high levels of photolyase activity. Species in which ultraviolet radiation negatively affects their mortality rate, such as the boreal toad and cascade frog, were found to have low levels of photolyase activity.4,5
The apparent sensitivity of some amphibians to current levels of ultraviolet radiation has lead to a hypothesis that increasing ultraviolet radiation has contributed to global amphibian declines.3,4 This hypothesis could explain the puzzling recent extensive declines of species in relatively undisturbed habitats. Declines are occurring in such diverse habitats that finding a single common cause is not likely. Declines attributable solely to ultraviolet radiation are therefore suspect.
Why is it important to understand the causes for the declines?
Herpetologists have paralleled amphibians to the proverbial canary in the coalmine. Amphibians are natural indicators of our planet’s well-being. They alert us of its condition.
- Because frogs and toads lead a double life, meaning both terrestrial and aquatic, they are twice as likely as other vertebrates to be effected by their environment.
- What makes frogs such important bioindicators is their permeable skin, which makes them highly vulnerable to toxins and threats present in the water, on land, and in the atmosphere.
- Amphibians are among the first organisms to suffer from the effects of global pollution and climate change — providing an early warning of environmental degradation.
With many populations on the decline in diverse and even pristine habitats, frogs are the medium for a vital message: the earth is in trouble. More research needs to be done and more factors investigated. Perhaps some of the more recent research will reveal some answers. For example, the Department of Biology at Arizona State University has been awarded a grant to fund the study “Host-Pathogen Biology and the Global Decline of Amphibians.” Groups like DAPTF, Declining Amphibian Populations Task Force, are constantly working to produce a reliable “frog census.”6,7
The time for anecdotal evidence is passed; the time has now come for quantifiable field research.
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