Graduation Year

2013

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Jason R. Rohr

Keywords

agrochemical, amphibian decline, anthropogenic change, emerging disease, pesticide

Abstract

Amphibians are declining at alarming rates globally. Multiple factors contribute to these declines, including chemical contaminants and emergent diseases. In recent years, agrochemical use, especially fungicide applications, has increased considerably. Previous studies have demonstrated that these agrochemicals leave application sites and enter wetlands via runoff and have detrimental effects on non-target organisms. For example, exposure to contaminants can have multifarious effects on amphibians, such as reducing their ability to deal with a secondary stressor, such as disease.

A pathogen that is found concomitant with chemical contaminants in aquatic systems is Batrachochytrium dendrobatidis (Bd). Bd has decimated amphibian populations worldwide. Susceptibility to this pathogen varies across amphibian life stages, and is greater in adults than larvae. Consequently, it is important to examine the effects of simultaneous and serial Bd and agrochemical exposure throughout amphibian development.

I assessed the combined effects of 3 different fungicides and Bd on two amphibian species: Cuban tree frogs (Osteopilus septentrionalis) and grey tree frogs (Hyla versicolor), both simultaneously and across life stages. To elucidate the complexities of these interactions, I conducted two experiments, one in the laboratory and another in outdoor mesocosms. Frogs were exposed to most of the possible combinations of fungicides and Bd as tadpoles and metamorphs.

The presence of fungicides during the tadpole stage caused no difference in timing to metamorphosis and therefore no extension of time animals were exposed to the pathogen. Fungicides did not reduce fungal growth; in fact, tadpoles exposed simultaneously to a fungicide and Bd, regardless of the specific fungicide, had increased fungal loads compared to acetone controls. Additionally, animals exposed to both stressors simultaneously had higher mortality compared to controls or any of the stressors singly. Lastly, the fungicide had persistent effects on amphibian health by affecting susceptibility to Bd later in ontogeny. Frogs exposed to any of the three fungicides as a tadpole had higher Bd prevalence, Bd abundance, and Bd-induced mortality when challenged with Bd after metamorphosis, an average of 71 days after their last fungicide exposure.

In conclusion, I found no benefits of fungicides for amphibians. In fact, results indicate both immediate and delayed negative effects of exposure to fungicides and Bd. These findings highlight the importance of studying multiple potential contributors to amphibian declines, simultaneously and sequentially, to understand net effects of stressors on amphibian performance.

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