ABSTRACT Chemical contamination of aquatic environments is widespread, but we have a limited understanding of how contaminants alter critical host–parasite interactions that can influence disease dynamics. We manipulated Northern Leopard Frog ( Lithobates pipiens ) exposure to pesticides (no pesticides, the insecticide Bacillus thuringiensis israelensis [ Bti ], or the herbicide atrazine) and trematode-infected ( Ribeiroia ondatrae and Echinostoma spp.) snails in outdoor mesocosms. Bti exposure extended host larval period, and atrazine exposure had a nonsignificant trend toward reducing host survival; however, neither pesticide influenced parasite success nor magnified the effects of parasites on their hosts. Parasites negatively influenced tadpole development and, by metamorphosis, parasitized frogs had severe limb deformities and greater mass than unparasitized frogs. The greater mass in parasitized frogs may have resulted from reduced competition between tadpoles and snails for algal resources because parasites decreased snail abundance in mesocosms. Reduced competition between tadpoles and snails may offset the direct negative effects of trematodes on tadpoles, enabling them to survive with high infection intensities. Trematodes may further facilitate their own success by inducing limb deformities that likely increase anuran consumption by definitive hosts. Our results demonstrate how common pesticides and parasites impact amphibians and suggest that, at environmentally relevant concentrations, these pesticides may not dramatically alter host–parasite dynamics.

ABSTRACT Infectious pathogens threaten wildlife populations through effects on host growth, reproduction, and survival. The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd), the cause of the disease chytridiomycosis, has been implicated in worldwide declines of amphibian populations. Documenting conditions under which amphibians are threatened by Bd will allow us to pinpoint at-risk populations, especially in the midwestern United States, an understudied region. We investigated how distributions of Bd differ across host life stages, land cover types, and spatial extents in a susceptible temperate host, the American Toad ( Anaxyrus americanus ). We surveyed a total of 232 adults at 14 sites and 430 metamorphs at 15 sites in southwestern Ohio, USA, and analyzed associations among land use type, Bd prevalence, and Bd load by using model selection. We found Bd infection prevalence in metamorphs was dramatically lower than in adults; only 1.6% of metamorphs were infected compared with 28.0% of adults. These results suggest that Bd transmission occurs after metamorphosis in this species or that infections vary with season, given that we surveyed adults in the spring during breeding events and metamorphs in the summer as they emerged from ponds. In adults, infection prevalence was reduced with increasing open-canopy habitats across spatial scales (from 100 to 1,000 m), whereas infection load increased with the proportion of forested habitats at small spatial scales (100 m). Our study shows that Bd infection risk in a temperate system can be influenced by host life stage and land cover types across local spatial scales.

Pathogens can have a range of effects on organisms across environmental gradients; although lethal effects may receive greater attention, sublethal impacts can have important population- and community-level impacts. In this laboratory study, we examined the effects of exposure to the amphibian chytrid fungus ( Batrachochytrium dendrobatidis [Bd]) on Northern Leopard Frog ( Lithobates pipiens ) metamorphs that had been reared in the presence or absence of crayfish ( Orconectes rusticus ) predators in aquatic environments to determine whether prior predator exposure influenced the effect of Bd on terrestrial growth and survival of frogs. Although Bd exposure following metamorphosis did not impact Northern Leopard Frog survival, exposure did significantly reduce terrestrial growth, indicating that Bd exposure can have consequences that can indirectly impact populations, because size of amphibians is positively correlated with overwinter survival and fecundity. Exposure to predators during larval development, however, did not alter the impact of Bd exposure at metamorphosis on Northern Leopard Frogs. Our study suggests the Bd pathogen associated with global amphibian declines can have important sublethal impacts and play a role in population dynamics even when disease outbreaks do not occur.