Living on the edge of chaos: Population dynamics of Fennoscandian voles

Although ecologists were among the first scientists to study the implicatio ns of chaotic dynamics, there is still no widely accepted example of chaos in a field population. If current developments in nonlinear dynamics are to be of relevance to population ecology, however, it is necessary to determi ne whether chaos occurs in nature. Here we bring together two lines of evid ence suggesting that the dynamics of vole populations in northern Fennoscan dia may be chaotic: nonlinear analysis of time-series data, and a model bas ed on biological mechanisms. We review how a mechanistic population model c an be used to produce short-term population forecasts, even when one of the species is not observed, and then use this benchmark of model accuracy to support our choice of a mechanistic population model for vole dynamics.By a djusting model parameters to optimise the forecasting accuracy on the vole time series, we can obtain parameter estimates for the model. In tests on s imulated data, this new approach to parameter estimation for stochastic dyn amic models yields parameter estimates such that the fitted model produces dynamics qualitatively and quantitatively similar to those observed. Howeve r, some parameters are poorly identified (i.e., many different parameter co mbinations yield nearly identical model dynamics). For the best-fit paramet ers, the model dynamics are characterized by quasi-chaotic behavior; the gl obal Lyapunov exponent is very close to (and statistically undistinguishabl e from) zero, but the median local Lyapunov exponent is significantly posit ive. Our findings of recurrent short-term chaos suggest that vole dynamics in northern Fennoscandia are characterized by a blend of order and irregula rity. The irregularity is the same type as would occur in a noisy chaotic s ystem (i.e., exogenous noise amplification by sensitive dependence on initi al conditions) but only occurs during specific portions of the oscillation.