UC Berkeley

Amphibian populations are dramatically reduced from historical numbers on a global scale. Amphibians in agricultural regions experience a diverse set of environmental stressors that may disrupt immune function and increase susceptibility to infection. The draining of wetlands for water and land usage leads to desiccation, crowding, and ultimately temperature extremes. Fertilizers and pesticides further degrade the quality of the available water. American bullfrogs (Lithobates catesbeiana) were collected throughout a gradient of agricultural land use in the Sacramento, San Joaquin, and Salinas River watersheds of California, USA. Animals collected downstream of intense agricultural land use had increased stress hormone (corticosterone) concentrations and altered white blood cell differentials and activity relative to those in upstream locations. An individual stressor of global importance, an agrichemical mix, as well as the amphibian immune response to chronic stress were furthered examined in the laboratory. Exposure to a mixture of pesticides and nutrients commonly applied in California did not affect plasma corticosterone or immune function of adult L. catesbeiana, suggesting that additional factors or exposure scenarios influence amphibians in the wild. The effects of chronic stress on blood cell differentials and function were characterized in lab-bred African clawed frogs (Xenopus laevis) and wild-caught L. catesbeiana. In both species, increased plasma corticosterone was associated with increased whole blood oxidative burst activity and blood neutrophil concentrations, while blood lymphocyte and eosinophil concentrations were decreased. Taken together, our results suggest that wild amphibians experience stressful conditions in agricultural habitats and that stressors may alter immunity and result in disease.