Recent reviews hypothesize that pathogen-associated amphibian declines may be exacerbated by immunosuppression triggered by exposure to contaminants and/or stress hormones released in response to environmental changes. Currently, there are few empirical data in support of these hypotheses. We exposed wood frog tadpoles (Rana sylvatica) to sublethal concentrations of malathion (0, 10, 100 ppb), and cues from caged dragonfly predators (Anax junius). We measured effects of these treatments on life history traits (growth, development and survival) and the susceptibility of wood frog metamorphs to the fungal pathogen Batrachochytrium dendrobatidis (B.d.).
Both treatments had mild negative effects on wood frog development, but not growth. Survival was also slightly lower in the highest pesticide treatment when predators were present, but not when predators were absent, supporting past evidence that these stressors can have synergistic negative effects on survival. Surprisingly, mortality rates in frogs exposed to B.d. were lower in individuals stressed by predator cues, while malathion did not effect survival. Overall these data fail to show that contaminants and predator stress cause higher rates of mortality associated with B.d., although they do show that environmental context can alter life history traits and disease susceptibility. Further tests are needed to show whether such stressors alter immune system function and if these results are robust for other pathogens.