TRANSITIONS FROM STABLE EQUILIBRIA TO CHAOS, AND BACK, IN AN EXPERIMENTAL FOOD WEB.

The question of whether deterministic chaos occurs in natural populations has been discussed since the 1970s following the discovery that simple population models can generate chaotic dynamics. Natural populations undergo a diverse mixture of deterministic and stochastic processes that define population dynamics. In most habitats populations are also exposed to changes in biotic and abiotic parameters. Models predict that shifts in ecological parameters may lead to a transition between deterministic chaos, stable equilibria, and limit cycles, yet clear examples from empirical studies are rare. However, such transitions should be considered when discussing the occurrence of chaos in nature because ecological time series are in general short and have large sampling intervals. Here we document short-term transitions in population dynamics to and from chaos in an experimental system. Manipulation of only one experimental parameter (chemostat dilution rate) in a multi-species food web of two bacteria and a bacterivorous ciliate showed that switching between different dynamic behaviors occured with surprising rapidity in the microbial populations. Thus, short periods of chaotic dynamics may easily be overlooked in field observations.