"People have been looking for a smoking gun to explain the amphibian die-offs, and there isn't just one smoking gun"
SANTA CRUZ, CA --
With many populations of amphibians around the world mysteriously dying, researchers think the cause might be the mix of acidic water and ultraviolet light, according to a laboratory study at the University of California, Santa Cruz.
Researchers exposed the eggs of a common species of frog to various levels of acidity and UV-B radiation, the most harmful form of ultraviolet light. By themselves, even the most acidic water or the most severe UV-B radiation did not kill many embryos. But when the team combined the two environmental assaults, as many as half of the eggs failed to hatch.
Stresses such as habitat loss, a thinning ozone layer, acid rain, pesticides, disease, and global climate change may combine to wreak more havoc on frogs, toads, and salamanders than any one factor could by itself. Indeed, some scientists now suspect that for any given group of amphibians in trouble, two or more such factors may interact synergistically -- that is, the combined effects of the stresses are far greater than the sum of their individual effects.
"People have been looking for a smoking gun to explain the amphibian die-offs, and there isn't just one smoking gun," says Michael Soule, coauthor of the study and professor emeritus of environmental studies at UCSC. "There's quite an armamentarium out there. We're just beginning to look at the interactions among these possible causes."
The UCSC group published its work in the October 1995 issue of the scientific journal Conservation Biology.
"We should look not just for single effects, but also for combinations"
not trying to say that a combination of UV radiation and low pH is the answer for why a particular amphibian population is declining, or even that this is happening now in the field," says biologist Leslie Long, the first author of the study. "The main message is that synergisms can be important and are worth looking for. We may miss a lot if we only look at each factor in isolation."
Another recent study supports Long's view. Ecologists Joseph Kiesecker and Andrew Blaustein at Oregon State University found that a synergism between UV-B radiation and a fungal disease can kill embryos of the Cascades frog and the western toad. The combined effects of ultraviolet light and infection by the fungus magnified the harm done to the eggs beyond the sum of the individual effects of each factor. The work appeared in the November 21 issue of the Proceedings of the National Academy of Sciences.
In 1994, Blaustein's group also showed that UV-B light by itself can kill frog and toad embryos in the field. "Both our new paper and the Long paper show that we should look not just for single effects, but also for combinations," Blaustein says. "We don't think low pH is a factor in Oregon, but it may interact with UV radiation in other parts of the country. UV is important here, but it doesn't play a big role in the tropics. There will not be a single global cause."
For their study, Long and her colleagues used three levels of acidic water (pH readings of 6.0, 5.0, and 4.5) and three levels of ultraviolet light: no UV-B (as a control), a "normal" level for high elevations, and a level forecast if the ozone layer continues to thin. The team exposed several dozen eggs of the leopard frog, Rana pipiens, to one of the nine possible combinations of factors. About one-quarter of the embryos died under the conditions of lowest pH and moderate UV, while half died when subjected to the lowest pH and the highest UV. In all other treatments -- including the lowest pH by itself and the highest UV by itself -- at least 90 percent of the eggs hatched.
Several caveats apply, the authors note. The highest level of UV-B radiation simulated a future earth with an ozone layer 30 percent thinner than it was in 1979. The most acidic water had a pH of 4.5, a reading seen infrequently in nature. Further, the study considered only the lab effects of those factors on the eggs of one species. In the wild, each of the many species of amphibian would react differently at each stage of life to these and other stresses.
Acid rain can send pH levels as low as between 4.6 and 5.4 in lakes in the eastern U.S., Canada, and Europe, research has shown. Other workers have found pH levels of 4.5 in northern California and 5.0 in the Rocky Mountains. In addition, surges of acidified water may occur in mountainous areas when snow melts in the spring. High-elevation frogs breed in that season -- a time of year when UV radiation is near its peak, the UCSC team notes.
Soule hopes to work with a molecular biologist to explore why the pH-UV double whammy harms the eggs. The team observed that embryos died when their egg membranes did not expand, a "curling defect" that trapped the growing tadpoles inside in tight coils.
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