Trophic dynamics in an aquatic community: interactions among primary producers, grazers, and a pathogenic fungus
Free-living stages of parasites are consumed by a variety of predators, which might have important consequences for predators, parasites, and hosts. For example, zooplankton prey on the infectious stage of the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), a pathogen responsible for amphibian population declines and extinctions worldwide. Predation on parasites is predicted to influence community structure and function, and affect disease risk, but relatively few studies have explored its consequences empirically. We investigated interactions among Rana cascadae tadpoles, zooplankton, and Bd in a fully factorial experiment in outdoor mesocosms. We measured growth, development, survival, and infection of amphibians and took weekly measurements of the abundance of zooplankton, phytoplankton (suspended algae), and periphyton (attached algae). We hypothesized that zooplankton might have positive indirect effects on tadpoles by consuming Bd zoospores and by consuming phytoplankton, thus reducing the shading of a major tadpole resource, periphyton. We also hypothesized that zooplankton would have negative effects on tadpoles, mediated by competition for algal resources. Mixed-effects models, repeated-measures ANOVAs, and a structural equation model revealed that zooplankton significantly reduced phytoplankton but had no detectable effects on Bd or periphyton. Hence, the indirect positive effects of zooplankton on tadpoles were negligible when compared to the indirect negative effect mediated by competition for phytoplankton. We conclude that examination of host-pathogen dynamics within a community context may be necessary to elucidate complex community dynamics.
KeywordsBatrachochytrium dendrobatidis Food web Pathogen Structural equation modeling Trophic cascade
We thank M. James, S. Moyers, L. Biga, and P. Buck for field assistance. Members of the Blaustein laboratory, E. Borer, and M. Albins provided advice regarding experimental design and execution, data analysis, and comments on the manuscript. We also thank the R. Tanguay, J. Spatafora, B. Menge, and S. Hacker laboratories, and E. Scheessele for use of equipment and protocol, and S. Robbins and D. Hinds-Cook for assistance at the Horticulture Farm. We thank J. Trexler, R. Alford, and several anonymous reviewers for insightful comments and suggestions that improved the manuscript substantially. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under grant no. 0802268 and a Howard Hughes Medical Institute Summer Undergraduate Research Fellowship to K. I. S. Supplementary funding was provided by the Oregon State University Zoology Research Fund and the Society of Wetland Scientists.
Conflict of interest
The authors declare that they have no conflict of interest.
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