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CORVALLIS, Ore. – A dramatic increase in deformed frogs and other amphibians is being caused by a range of environmental factors, all of which can ultimately be linked to human impacts on habitat, but the primary cause of many of the deformities is an epidemic of a key parasite.
These findings are the results of eight years of research by scientists around the world, and are presented in the February issue of Scientific American by researchers from Oregon State University and the University of Wisconsin.
Increases in ultraviolet radiation, contaminated water and a parasitic trematode are the leading culprits in the wave of deformed legs, eye damage and other ailments that have now been found in more than 60 species of frogs, toads and salamanders in 46 states and across four continents. Of these three leading causes, the parasite appears to be the major cause of many of the deformities, the scientists say.
"We've finally synthesized from a wide body of research the range of causes that are linked to amphibian deformities," said Andrew Blaustein, a professor of zoology at OSU and co-author of the report with Pieter T.J. Johnson, a doctoral candidate at the University of Wisconsin. "As is often the case in nature, it's now clear that there are multiple causes to this problem, some of which may act in concert," Blaustein said. "But the common thread that runs through the issue is that each cause can eventually be traced to human alteration of our climate or amphibian habitat. And one of the most common deformities, extra or deformed legs, is most often linked to a particular parasite."
The deformity problem first received widespread media attention when deformed frogs were spotted by school children in Minnesota in 1995, but it quickly became apparent that its scope was extraordinarily broad and the impacts severe – in some frog populations, including one near Corvallis, Ore., 75-80 percent of the frogs are deformed.
There have always been some level of deformities in amphibians, scientists say, but nothing of this magnitude. And the sudden increase in deformities may also be one factor in overall population declines.
"Deformities undoubtedly impair amphibian survival and most likely contribute to the dramatic declines in populations that have been recognized as a global concern since 1989," the researchers said in their report. "Both trends are disturbing in their own right and are also a warning for the planet. Chances are good that factors affecting these animals harshly today are also beginning to take a toll on other species."
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Since the issue first gained national attention, a range of differing causes for amphibian deformities has been suggested and studied. Years of research by dozens of investigators have now narrowed the causes down to three primary areas:
- UVB Radiation: Rising levels of ultraviolet radiation, a side effect of the erosion of Earth's protective ozone layer, can be implicated in at least some deformities. Research at OSU and elsewhere has shown UVB radiation can kill amphibian embryos and larvae, cause serious eye damage in adult frogs, and induce various types of bodily deformities in frogs and salamanders. It is less relevant to the range of leg deformities seen in nature, and does not lead to the growth of extra legs.
- Water Pollution: Toxicologists studied whether widespread water pollution, especially that caused by pesticide runoff, might cause embryo deformities or other problems. Laboratory tests suggest there may be some linkage, but field tests make it clear pesticide exposure can't be the sole cause of amphibian deformities.
- Parasites: A tiny trematode appears to cause a significant amount of the leg deformities in amphibians, as it works through a complex ecological cycle that at various times includes aquatic snails, amphibians and birds. The parasite can form a cyst in frogs and disrupt normal limb development.
According to the researchers, the story of the parasitic trematode reveals just how complicated natural ecological processes can be, and how difficult it is to trace problems to their underlying cause. In its life cycle, the parasite at times depends on snails for survival and birds for reproduction and transportation. The amphibians, themselves, are actually just an intermediary host.
"We need to understand the complex relationships among human activity, the parasite and its hosts, and the environment in which they interact," Johnson said.
There are many interrelationships among various factors. Snails, for instance, are necessary to the life cycle of the trematode that can cause frog deformities. But snail populations may be surging at some sites due to fertilizer runoff and cattle manure which cause algal blooms and more food for the snails. A survey of the western U.S. in 2000 found that 44 of the 59 wetlands in which amphibians were infected by this parasitic trematode were reservoirs, farm ponds or other artificial bodies of water.
And water pollutants or UVB radiation, while not directly causing the majority of deformities, may set the stage by weakening an amphibian's immune system and making it more vulnerable to a parasitic infection.
"The challenge to scientists becomes teasing apart these agents to understand their interactions," the researchers said in this article. "Humans and other animals may be affected by the same environmental insults harming amphibians. We should heed their warning."
By David Stauth, 541-737-0787
SOURCE: Andrew Blaustein, 541-737-5356