Distinguishing random environmental fluctuations from ecological catastrophes for the North Pacific Ocean

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Abstract
The prospect of rapid dynamic changes in the environment is a pressing concern that has profound management and public policy implications1,2. Worries over sudden climate change and irreversible changes in ecosystems are rooted in the potential that nonlinear systems have for complex and ‘pathological’ behaviours1,2. Nonlinear behaviours have been shown in model systems3 and in some natural systems1,4,5,6,7,8, but their occurrence in large-scale marine environments remains controversial9,10. Here we show that time series observations of key physical variables11,12,13,14 for the North Pacific Ocean that seem to show these behaviours are not deterministically nonlinear, and are best described as linear stochastic. In contrast, we find that time series for biological variables5,15,16,17 having similar properties exhibit a low-dimensional nonlinear signature. To our knowledge, this is the first direct test for nonlinearity in large-scale physical and biological data for the marine environment. These results address a continuing debate over the origin of rapid shifts in certain key marine observations as coming from essentially stochastic processes or from dominant nonlinear mechanisms1,9,10,18,19,20. Our measurements suggest that large-scale marine ecosystems are dynamically nonlinear, and as such have the capacity for dramatic change in response to stochastic fluctuations in basin-scale physical states.