Effects of Pesticide Mixtures on Host-Pathogen Dynamics of the Amphibian Chytrid Fungus

RESEARCH ARTICLE Effects of Pesticide Mixtures on HostPathogen Dynamics of the Amphibian Chytrid Fungus Julia C. Buck1,2*, Jessica Hua3¤a, William R. Brogan, III3¤b, Trang D. Dang2, Jenny Urbina4, Randall J. Bendis3¤c, Aaron B. Stoler3¤d, Andrew R. Blaustein2,4, Rick A. Relyea3¤c 1 Texas Research Institute for Environmental Studies, Sam Houston State University, Huntsville, Texas, United States of America, 2 Department of Integrative Biology, Oregon State University, Corvallis, Oregon, United States of America, 3 Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America, 4 Environmental Sciences Graduate Program, Oregon State University, Corvallis, Oregon, United States of America OPEN ACCESS Citation: Buck JC, Hua J, Brogan WR, III, Dang TD, Urbina J, Bendis RJ, et al. (2015) Effects of Pesticide Mixtures on Host-Pathogen Dynamics of the Amphibian Chytrid Fungus. PLoS ONE 10(7): e0132832. doi:10.1371/journal.pone.0132832 Editor: Christopher Joseph Salice, Towson University, UNITED STATES Received: January 29, 2015 Accepted: June 19, 2015 Published: July 16, 2015 Copyright: © 2015 Buck et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All files are available from the DRYAD database (accession number doi:10. 5061/dryad.45595). Funding: This material is based on work supported under a National Science Foundation grant to R.A.R. and A.R.B. (DEB 11-19430). URL of funder's website: nsf.gov. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. ¤a Current Address: Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana, United States of America ¤b Current Address: Smithsonian Environmental Research Center, Edgewater, Maryland, United States of America ¤c Current Address: Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, New York, United States of America ¤d Current Address: Department of Biological Sciences, Oakland University, Rochester, Michigan, United States of America * Abstract Anthropogenic and natural stressors often interact to affect organisms. Amphibian populations are undergoing unprecedented declines and extinctions with pesticides and emerging infectious diseases implicated as causal factors. Although these factors often cooccur, their effects on amphibians are usually examined in isolation. We hypothesized that exposure of larval and metamorphic amphibians to ecologically relevant concentrations of pesticide mixtures would increase their post-metamorphic susceptibility to the fungus Batrachochytrium dendrobatidis (Bd), a pathogen that has contributed to amphibian population declines worldwide. We exposed five anuran species (Pacific treefrog, Pseudacris regilla; spring peeper, Pseudacris crucifer; Cascades frog, Rana cascadae; northern leopard frog, Lithobates pipiens; and western toad, Anaxyrus boreas) from three families to mixtures of four common insecticides (chlorpyrifos, carbaryl, permethrin, and endosulfan) or herbicides (glyphosate, acetochlor, atrazine, and 2,4-D) or a control treatment, either as tadpoles or as newly metamorphic individuals (metamorphs). Subsequently, we exposed animals to Bd or a control inoculate after metamorphosis and compared survival and Bd load. Bd exposure significantly increased mortality in Pacific treefrogs, spring peepers, and western toads, but not in Cascades frogs or northern leopard frogs. However, the effects of pesticide exposure on mortality were negligible, regardless of the timing of exposure. Bd load varied considerably across species; Pacific treefrogs, spring peepers, and western toads had the highest loads, whereas Cascades frogs and northern leopard frogs had the lowest loads. The influence of pesticide exposure on Bd load depended on the amphibian species, timing of PLOS ONE | DOI:10.1371/journal.pone.0132832 July 16, 2015 1 / 17 Effects of Multiple Stressors on Amphibians pesticide exposure, and the particular pesticide treatment. Our results suggest that exposure to realistic pesticide concentrations has minimal effects on Bd-induced mortality, but can alter Bd load. This result could have broad implications for risk assessment of amphibians; the outcome of exposure to multiple stressors may be unpredictable and can differ between species and life stages. Introduction As ecosystems are increasingly threatened by anthropogenic factors, ecologists attempt to understand the impacts of these stressors on sensitive organisms. However, environmental stressors rarely occur in isolation. Instead, anthropogenic stressors such as contaminants can interact with natural stressors such as pathogens to produce unpredictable effects [1,2]. When organisms are exposed to contaminants, their ability to launch effective immune responses may be compromised, rendering them susceptible to disease [3–5]. Alternatively, contaminants may affect the pathogen itself, for example, by inhibiting production of the infective stage [3]. Environmental influences on host-pathogen dynamics are complex, context-dependent, and require continued examination [6,7]. Although the loss of biodiversity affects all taxonomic groups, amphibians are declining at especially alarming rates. One estimate suggests that extinction rates of amphibians may be 211 times greater than the background rate of extinction [8] and more than 40% of amphibian species have experienced population declines or extinctions in recent decades [9]. Possible causes of amphibian population declines include anthropogenic threats such as habitat loss, climate and atmospheric changes, and contaminants, and natural stressors such as competition, predation, and disease [9–11]. In this study, we investigated the potential for interactions between two key factors implicated in amphibian population declines and extinctions worldwide: pesticide exposure and a fungal pathogen. Due to their widespread use, pesticides are commonly found in aquatic habitats. In the United States, 30–60% of shallow ground water and 60–95% of streams are contaminated with at least one pesticide [12]. In these habitats, pesticides can have lethal and sublethal effects on amphibians including reduced growth, altered behavior, and immune suppression [13,14]. While most ecotoxicological studies examine effects of individual pesticides on sensitive species, in natural systems, organisms are often exposed to pesticide mixtures [12]. For amphibians, pesticide mixtures can have additive and non-additive effects, depending on environmental context and life stage [15–18]. Pathogens also play a prominent role in amphibian popula (...truncated)