When can noise induce chaos and why does it matter: a critique

Noise-induced chaos illustrates how small amounts of exogenous noise can have disproportionate qualitative impacts on the long term dynamics of a nonlinear system. This property is particularly clear in chaotic systems but is also important for the majority of ecological systems which are nonchaotic, and has direct implications for analyzing ecological time series and testing models against field data. Dennis et al. point out that a definition of chaos which we advocated allows a noise-dominated system to be classified as chaotic when its Lyapunov exponent lambda is positive, which misses what is really going on. As a solution, they propose to eliminate the concept of noise-induced chaos: chaos should retain its strictly deterministic definition'', hence ecological populations cannot be strictly chaotic. Instead, they suggest that ecologists ask whether ecological systems are strongly influenced by underlying skeletons with chaotic dynamics or whatever other dynamics - the skeleton being the hypothetical system that would result if all external and internal noise sources were eliminated. We agree with Dennis et al. about the problem - noise-dominated systems should not be called chaotic - but not the solution. Even when an estimated skeleton predicts a system's short term dynamics with extremely high accuracy, the skeleton's long term dynamics and attractor may be very different from those of the actual noisy system. Using theoretical models and empirical data on microtine rodent cycles and laboratory populations of Tribolium, we illustrate how data analyses focusing on attributes of the skeleton and its attractor - such as the deterministic Lyapunov exponent lambda(0) that Dennis et al. have used as their primary indicator of chaos - will frequently give misleading results. In contrast, quantitative measures of the actual noisy system, such as lambda, provide useful information for characterizing observed dynamics and for testing proposed mechanistic explanations.