Acidic deposition as an unlikely cause for amphibian population declines in the Sierra Nevada, California

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The Sierra Nevada of California is one of many regions worldwide that has recently experienced dramatic declines in amphibian populations. During the past two to three decades many populations of at least two species (Rana muscosa and Bufo canorus) have disappeared in national parks and designated wilderness areas at high elevation, whereas populations of a third widespread species (Pseudacris regilla) have remained stable or declined to a lesser extent. Acidification from atmospheric deposition has been suggested as a cause for these disappearances primarily because most surface waters in these areas are exceptionally low in acid neutralizing capacity (ANC), and thus are vulnerable to changes in water chemistry due to acidic deposition. We tested the hypothesis that acidification of habitats has adversely affected amphibian populations by eliminating populations from waters most vulnerable to acidification, i.e. low in pH or ANC, or from waters low in ionic strength, a condition that increases the sensitivity of amphibians to low pH. A survey of 235 potential breeding sites in 30 randomly selected survey areas failed to reveal significant differences in water chemistry parameters between sites with and sites without each of the three species. Moreover, the water chemistry parameters did not differ among sites inhabited by the three species in a manner paralleling their degrees of acid tolerance. These findings contra-indicate acidic deposition as a cause of recent amphibian population declines in the Sierra Nevada at high elevation.