The interactive effects of predator stress, predation, and the herbicide Roundup

As the number of studies examining the effects of contaminants grows, ecologists are becoming increasingly aware that contaminants can interact with natural stressors (e.g., competition and predator cues) in their effects on nontarget animals. In amphibians, predator cues can make contaminants more lethal under laboratory conditions, but the opposite outcome has been observed under more natural conditions with stratified water columns; stratification causes more pesticide to be present near the surface while predator cues scare spring‐breeding amphibians down to the benthos. I examined whether this phenomenon also occurs in three species of summer‐breeding amphibians ( Hyla versicolor, Rana clamitans, and Rana catesbeiana ) that were raised in outdoor mesocosms. Specifically, I asked how amphibian survival was affected by multiple concentrations of a common herbicide (glyphosate; commercial name: Roundup), the herbicide combined with chemical cues from predators (caged larval dragonflies; Anax junius ), and the herbicide combined with lethal predators. Environmentally relevant concentrations of the herbicide caused high rates of tadpole mortality, but this outcome was substantially reversed by the addition of predator cues. With lethal predators, the tadpoles experienced such high mortality that the herbicide caused no additional effect. Roundup also induced morphological changes in Hyla versicolor, and the induced traits were different from those induced by predators. Collectively, these results suggest that while predator cues can make pesticides less lethal when thermal stratification occurs, highly lethal predators can overwhelm these effects. Thus, the impacts of such contaminants can be dramatically different in environments that do or do not contain high‐risk predators.