Effects of pesticide mixtures on host-pathogen dynamics of the amphibian chytrid fungus

Background/Question/Methods

Anthropogenic and natural stressors often interact additively or synergistically to affect sensitive organisms. Such effects may be seen at the population level through population declines and extinctions. For example, amphibian populations are undergoing unprecedented declines and extinctions with pesticides and emerging infectious diseases implicated as causal factors. Yet, despite being associated with population declines globally, the effects of pesticides and pathogens on amphibians are usually examined in isolation. We tested the hypothesis that exposure of larval and metamorphic amphibians to ecologically relevant concentrations of pesticide mixtures increases their post-metamorphic susceptibility to Batrachochytrium dendrobatidis (Bd), the fungus that causes chytridiomycosis. We exposed five anuran species (Pseudacris regilla, Pseudacris crucifer, Rana cascadae, Rana pipiens, and Anaxyrus boreas) to pesticide mixtures as tadpoles and after metamorphosis. Pesticide treatments included control, and high and low concentrations of an herbicide mixture and an insecticide mixture. After metamorphosis, animals were housed individually for two weeks, and exposed to Bd twice. We compared mortality rates between treatments, and conducted quantitative PCR on Bd-exposed and control animals to quantify and compare Bd loads between pesticide treatments.

Results/Conclusions

The responses of amphibians exposed to pesticides and Bd grouped along phylogenetic lines. Cox proportional hazards models indicated that Bd exposure significantly increased mortality in P. regilla, P. crucifer, and A. boreas, but not in R. cascadae or R. pipiens. However, the effects of pesticide treatment on Bd-induced mortality were negligible, regardless of whether the pesticide exposure occurred during the larval stage or after metamorphosis. qPCR analysis revealed that Bd load varied considerably among species, with P. regilla, P. crucifer, and A. boreas exhibiting the highest loads, and R. cascadae and R. pipiens bearing lower loads. Depending on species and pesticide treatment, exposure to pesticide mixtures could increase or decrease Bd load. P. regilla and P. crucifer exposed to pesticides as tadpoles carried lower Bd loads than individuals exposed after metamorphosis, but the opposite pattern was found for A. boreas. We conclude that exposure to pesticides may alter host-pathogen dynamics of metamorphic amphibians in subtle and complex ways.