Landscape forest cover is associated with reduced agrochemical exposure and improved amphibian tadpole health in agricultural drainage ditches

Amphibians in agricultural drainage ditches may be exposed to contaminants through runoff and subsurface drainage, which can result in toxicity. Understanding the agroecosystem management activities that can sustain healthy amphibian populations will support biodiversity and ecosystem services within intensive agricultural areas. This study examined how woody vegetation within agricultural areas is associated with water quality and amphibian health metrics. Cages containing northern leopard frog (Lithobates [Rana] pipiens) tadpoles were placed within nine agricultural ditches differing in their riparian vegetation height and percentage of surrounding forest cover for eight weeks (May-July) in an agriculturally dominated watershed in eastern Canada. Physicochemical water quality measurements and pesticide concentrations indicated that sites higher in percent forest cover (within 1 km radius) had lower specific conductance, atrazine, nitrate, and potassium concentrations. Percent forest cover was positively associated with tadpole growth (snout-to-vent length, tail length, mass) and riparian vegetation height positively related to tadpole development. Furthermore, glucose levels increased with forest cover, while corticosterone and hepatosomatic index remained unchanged, suggesting that elevated glucose was not strongly associated with chronic stress in this study. Finally, tested tadpoles were negative for Batrachochytrium dendrobatidis, B. salamandrivorans and Frog Virus 3, suggesting these pathogens are not currently a threat to these organisms at the sites tested. Collectively, our findings suggest forested habitats within intensive agroecosystems are critical landscape elements for reducing agrochemical exposure and improving tadpole health in drainage ditches, whereas woody riparian buffers provide limited additional benefits for aquatic stages, highlighting the importance of considering life-stage-specific responses.