The health of northern leopard frogs (Rana pipiens) and green frogs (Rana clamitans) was evaluated at eight breeding sites (permanent ponds or ditches), four of which were situated in apple orchards. Mature males were weighed and measured and aged. Tissues were taken to determine genetic variation, gonadosomatic, hepatosomatic and fat body indices, detoxification enzyme (EROD) activity, and levels of circulating steroid hormones. Embryos and larvae were subjected to in situ (caging) and ambient pond-water (laboratory) assays, and to toxicity tests of pesticides used in orchards. Significant differences in adult male size, age and condition, and in tadpole growth were detected between study site populations, but differences did not always distinguish reference site individuals from orchard site individuals. Male green frogs at one orchard site exhibited hepatic EROD induction, indicating that they had been exposed to pollutants. Caged embryos and larvae suffered high mortality at some orchard sites during in situ assays. The embryos and larvae caged in reference sites exhibited high hatching success (over 90%) and high survival rates (over 85%) during the two week assays, indicating that mortality in orchard ponds was probably due to stressful environmental conditions. Toxicity tests revealed that the pesticide diazinon, and the formulations Dithane DG, Guthion 50WP, and Thiodan 50WP cause mortality, deformities, and/or growth inhibition in embryos and tadpoles. Residues of three of these compounds were detected in some orchard ponds, and, therefore, could have detrimentally affected wild egg and tadpole stages. High average heterozygosity values in sampled adult populations suggested that extensive immigration and emigration were occurring. This supports suspicions that breeding populations were subpopulations of larger metapopulations.
Megan Harris, C.A.Bishop, J. Struger, J.P. Bogart
Contact: Megan Harris, Dept Zoology, University of Guelph, Guelph, Ontario N1G2W1
519-825-4210 x 6260