Mathematical models predict that plankton communities may generate chaos. However, these mathematical predictions have never been demonstrated empirically. Here, we present the first demonstration of chaos in a complex planktonic food web (Benincà et al. 2008. Nature 451: 822-825). Our food web was isolated from a coastal ecosystem in the Baltic Sea, and consisted of phytoplankton, zooplankton, heterotrophic bacteria, and detritivores.  The food web was cultured in a laboratory mesocosm experiment for more than 2300 days, and sampled twice a week.

Results/Conclusions

Despite constant external conditions, the species abundances showed striking fluctuations over several orders of magnitude.  The population dynamics were characterized by significantly positive Lyapunov exponents. Predictability was limited to a time horizon of 15–30 days, only slightly longer than the local weather forecast. Hence, our results provide the first experimental demonstration of chaos in a complex food web. This implies that stability is not required for the persistence of complex food webs, and that the long-term prediction of species abundances can be fundamentally impossible.