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Experimental demonstration of chaos in a microbial food web


Discovering why natural population densities change over time and vary with location is a central goal of ecological and evolutional disciplines. The recognition that even simple ecological systems can undergo chaotic behaviour has made chaos a topic of considerable interest among theoretical ecologists1,2,3,4. However, there is still a lack of experimental evidence that chaotic behaviour occurs in the real world of coexisting populations in multi-species systems. Here we study the dynamics of a defined predator–prey system consisting of a bacterivorous ciliate and two bacterial prey species. The bacterial species preferred by the ciliate was the superior competitor. Experimental conditions were kept constant with continuous cultivation in a one-stage chemostat. We show that the dynamic behaviour of such a two-prey, one-predator system includes chaotic behaviour, as well as stable limit cycles and coexistence at equilibrium. Changes in the population dynamics were triggered by changes in the dilution rates of the chemostat. The observed dynamics were verified by estimating the corresponding Lyapunov exponents. Such a defined microbial food web offers a new possibility for the experimental study of deterministic chaos in real biological systems.

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Figure 1: Experimental results showing the population dynamics of bacteria–ciliate chemostat systems.
Figure 2: Relationship between trajectory stability of data sets from Fig. 1 and the corresponding dilution rates.


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We thank J. Huisman, M. Milinski, A. Scherwass, D. Stauffer, D. Tautz and M. Weitere for their constructive comments on the manuscript; R. Heerkloss for introducing us to the topic; W. Lampert for stimulating atmosphere during sabbatical in Plön; F. Bartlett for correction of the English manuscript; and C. Barth, R. Bieg and B. Gräfe for technical assistance. The study was supported by a grant from the German Research Foundation (Deutsche Forschungsgemeinschaft) to H.A.

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Correspondence to Hartmut Arndt.

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Becks, L., Hilker, F., Malchow, H. et al. Experimental demonstration of chaos in a microbial food web. Nature 435, 1226–1229 (2005).

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