| In the past decade, declines in amphibian populations have captured scientific and popular interest. The causes of the declines are likely to be complex and involve interactions between several environmental stressors. Using multifactorial experiments, I investigated the combined effects of several anthropogenic stressors on developing amphibians in Oregon, USA. In laboratory experiments, I found that low levels of pH and exposure to high levels of nitrate killed larval Rana cascadae. Moreover, exposure to UV-B radiation and exposure to high levels of nitrate reduced larval activity level. Results suggest that in some cases, the effect of increasing nitrate level on larval activity depended on the pH level. In outdoor experiments, I investigated the combined effect of UV-B and nitrate fertilizer on two species of amphibians at both low and high elevation sites in Oregon. In Hyla regilla, I found that UV-B and nitrate together had a negative effect on larval mass in the lower elevation site but adversely affected survival in the high elevation site. Nitrate increased larval mass in Ambystoma macrodactylum. However, in the higher elevation experiment, this effect occurred only when UV-B was blocked. Finally, using both laboratory and outdoor experiments, I investigated the combined effects of UV-B with the two commonly used pesticides, carbaryl and chlorpyrifos. I studied these effects in the larvae of three species of amphibians native to the highly agricultural Willamette Valley in Oregon: Rana aurora, A. macrodactylum, and H. regilla. Laboratory results for all three species revealed that a formulated pesticide product of chlorpyrifos killed larvae although the active ingredient alone did not, suggesting that some components of the pesticide formulations could be toxic to larval amphibians. In the laboratory study, there were no adverse effects caused by UV-B. However, outdoor studies indicated that ambient levels of UV-B enhance pesticide toxicity in amphibians. Both pesticides were toxic to A. macrodactylum in the presence of UV-B. Chlorpyrifos caused mortality in R. aurora in the presence of UV-B. Overall, results demonstrate the importance of considering multiple environmental stressors together in assessing amphibian population declines. |
