Amphibian Decline: Air Pollutants
by Mika Pringle Tolson
UC TSR&TP Trainee Jeff Angermann
Are air pollutants more detrimental to amphibians than UVB exposure? This seems to be the case with the Mountain Yellow-legged Frog, Rana Muscosa. Populations of this species have been rapidly declining in sites on the western slope of the Sierra Nevada, but continue to thrive at high elevations on the eastern slope where UVB exposure is much higher. Due to the rainshadow effect of the Sierras, the eastern slope receives less rainfall and more solar radiation than the western slope. A 1970 study analyzing DDT residues in R. muscosa was the first to document the transport of pesticides from the Central Valley to the Sierra Nevada. It concluded that DDT residues were a result of air pollution and that contamination on the western slope was heavier than on the eastern slope. Jeff Angermann, a graduate student in Environmental Toxicology at UC Davis, hypothesizes that similar air pollutants from the Central Valley are the current cause of the frog's disappearance. The use of toxaphene and PCBs in the last thirty years in California correlates with the widespread disappearance of R. muscosa from western slopes. With his graduate student fellowship from the TSR&TP, Angermann is sampling tadpoles to determine if they have any detectable residues of these organochlorine pesticides. He is currently trying to determine if the air pollution pattern for toxaphene and PCBs matches the pattern of DDT/DDE residues in R. muscosa as identified in the 1970 study. R. muscosa are particularly vulnerable to pollutants because of their fairly unique overwintering behavior. They freeze in lake sediments in the winter as tadpoles because it takes them 3-4 years to metamorphose into adults. Angermann hypothesizes that this behavior puts them at greater risk because "they are exposed to sediment for a longer period of time. Since sediment in lakes and slow-moving rivers are the main repositories for organochlorines, we reasoned that they [R. muscosa] may have higher exposures than other species as a result." Organochlorines are thought to compromise cryoprotection, and Angermann believes that they are the main cause for the decline of R. muscosa. Jeff Angermann hopes that his research will help to change the use of pesticides in the Central Valley. "There hasn't been very much concern with these remote effects of pesticides. Of course there has been concern with the target crop, the target pest, and immediate secondary effects on wildlife in the Central Valley. But when you factor in the possibility of a serious effect on a species that's far removed from the site of application, it opens a whole new dimension of discussion."
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