Parasite abundance decreases with host density: evidence of the encounter-dilution effect for a parasite with a complex life cycle
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The relationship between host density and parasitism depends on a parasite’s life history. The abundance of a directly transmitted contagious parasite should increase with host density, whereas the abundance of a directly transmitted parasite that seeks its host might decrease due to the encounter-dilution effect. For parasites with complex life cycles, previous studies have found no association between parasite abundance and host density. We tested the relationship between host density and metacercarial abundance of a trematode parasite (Posthodiplostomum minimum) in two species of centrarchid fishes (Lepomis macrochirus and L. auritus) from eight small creeks. We found that host density was negatively associated with parasite abundance. Thus, our study represents the first evidence of the encounter-dilution effect for a parasite with complex life cycle in a natural system. We also report a positive association between total P. minimum population abundance and Lepomis spp. density, indicating that at low host density, cercarial mortality could moderate the encounter-dilution effect.
KeywordsEncounter-dilution Lepomis Posthodiplostomum minimum Sunfish Trematode
We thank S.M. Anderson, R.A. Fiorillo L. Dent, D.S. Millican, and D.K. Wasco for assistance related to this research. We thank K.D. Lafferty, A.M. Kuris, A. Smith-Herron, T.J. Cook, and two anonymous reviewers for useful discussions and comments on the manuscript. This work was supported by a grant from the Engineer Research and Development Center (ERDC) and the Civil Engineering Research Lab (CERL) of the US Army Corps of Engineers (ERDC-CERL Contract #DACA 42-00-C-0047) to W.I.L. We acknowledge the assistance provided by Gordon A. Plishker and all personnel of the Texas Research Institute for Environmental Studies (TRIES) at Sam Houston State University. We also thank Hugh Westbury of the Strategic Environmental Research and Development Program (SERDP), Ecosystem Management Project (SEMP), and Harold (Hal) Balbach of the US Army ERDC and CERL for their sincere interest in and support of this project.
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