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Expression of parasite virulence at different host population densities under natural conditions

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Abstract

It has recently been suggested that the expression of parasite virulence depends on host population density, such that infected hosts have a higher sensitivity to density, and thus reach their carrying capacity earlier than uninfected hosts. In this scenario, parasite-induced reduction in fitness (i.e., virulence) increases with host density. We tested this hypothesis experimentally, using outdoor mesocosm populations of Daphnia magna infected by the microsporidian Octosporea bayeri. Contrary to the prediction, virulence was independent of host density. In a competition experiment with initial prevalence of 50%, O. bayeri reduced the competitive ability of infected Daphnia within the asexual growth phase independent of initial host population density. In an additional experiment we set up populations with 100% and 0% prevalence and followed their population dynamics over the whole season. Consistent with the competition experiment, we found no difference in population dynamics within the asexual growth phase of the host, suggesting that infected hosts are not more sensitive to density than uninfected hosts. The additional experiment, however, included more than the initial growth phase as did the competition experiment. Eventually, after 100 days, 100% infected populations assumed a reduced carrying capacity compared to uninfected populations. We identify and discuss three reasons for the discrepancy between our experiment and the predictions.

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Acknowledgements

We thank Curt Lively and an anonymous reviewer for their comments improving this manuscript. Thanks to Florian Altermatt and Thomas Zumbrunn for ideas and help during the whole project. We also thank David Duneau and Dita Vizoso for helpful comments on the manuscript and Suzanne Zweizig for improving the English. A. B. was supported by the Werenfels fonds (FAG) and the Anna Caroline Stiftung. The study was supported by the Swiss National Science Foundation. This work is part of project no. 97524006 at Tvärminne Zoological Station. This research was carried out in compliance with the current laws of Finland governing ethical conduct and the use of animals in research.

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Authors and Affiliations

  1. Zoologisches Institut, Universität Basel, Vesalgasse 1, 4051, Basel, Switzerland

    Annette Bieger & Dieter Ebert

  2. Tvärminne Zoological Station, 10900, Hanko, Finland

    Annette Bieger & Dieter Ebert

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  1. Annette Bieger
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  2. Dieter Ebert
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Correspondence to Annette Bieger.

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Communicated by R. Brandl.

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Bieger, A., Ebert, D. Expression of parasite virulence at different host population densities under natural conditions. Oecologia 160, 247–255 (2009). https://doi.org/10.1007/s00442-009-1297-x

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  • DOI: https://doi.org/10.1007/s00442-009-1297-x

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