Experimental proof of chaos in food webs

Using a long-term laboratory experiment, the study demonstrates that, even under constant conditions, all species in a food web continued to fluctuate in a chaotic fashion. Chaos makes long-term prediction of species abundances impossible.

Theoretical ecologists already argued in the 1970s that populations of plants and animals might fluctuate in an unpredictable manner, even without external influences. These predictions, derived from chaos theory, attracted a lot of debate. However, only few scientists believed that species in real ecosystems would truly fluctuate in a chaotic fashion. The common perception was that species fluctuations result from changes in external conditions, driven by climate change or other disturbances of the balance of nature.

This classic perspective has been radically changed by new findings of graduate student Elisa Benincà and Professor Jef Huisman of the Institute for Biodiversity and Ecosystem Dynamics of the University of Amsterdam, The Netherlands, in collaboration with colleagues from Wageningen University (The Netherlands), the University of Rostock (Germany), and Cornell University (USA).

The core of their work consists of a laboratory experiment in which a plankton community isolated from the Baltic Sea was studied for more than eight years. The experiment was maintained under constant light and temperature conditions by the German biologist Reinhard Heerkloss, who reported the development of the different plankton species twice a week. To his major surprise, the food web never settled at equilibrium and the species abundances continued to vary wildly. He sent his data to Amsterdam for statistical analysis. This revealed that the fluctuations were caused by the species themselves; competition and predation generated a dynamic food web in which none of the species succeeded in getting the upper hand. Advanced mathematical techniques proved the indisputable presence of chaos in this food web.

According to the authors, these findings have far-reaching implications: “Our results demonstrate that species abundances are essentially unpredictable in the long term. For many years, we thought that a better understanding of all relevant processes would enable sound prediction of changes in species abundances in response to external factors (e.g., climate change). Now we know that things are not as simple as that.” Professors Jef Huisman and Marten Scheffer, both from The Netherlands, had already foreseen the possibility of chaos in plankton communities by means of mathematical models. However, the experimental demonstration of chaos in this study provides the real breakthrough. The limited predictability of species in food webs is comparable to the weather forecast. Benincà: “Short-term prediction is possible, but long-term prediction is not. We can at best indicate within which boundaries species will fluctuate”.

The research was financed by the Earth and Life Sciences Foundation, which is subsidized by the Netherlands Organization for Scientific Research (NWO).