Regime shifts in the ocean: reconciling observations and theory

The discussions in the Villefranche Workshop covered a wide range of issues. The term regime shift was originally confined to spatial or temporal correspondences between climatic indices and population abundance. The body of evidence for physical–biological coupling has certainly generated a much better appreciation of the natural decadal scale variability in marine systems. It is difficult, however, to deduce from these time series, the mechanisms or trophic pathways that produce the correspondence. Ideally, we would need experimental manipulations such as those used in small lakes, to unravel the causal connections. Since this is impossible in the open sea, we must use comparisons between systems subject to different types of perturbation or stress. We focused at the Workshop on the effects of over-fishing in different marine regimes. The consequences of large scale changes in community structure imposed by excessive fishing give valuable case studies. Coral reefs, rocky shores, freshwater and terrestrial ecosystems provide other examples. The possible existence of similar processes across such diverse systems raises corresponding questions about common ecological principles. The adaptive benefits of maximizing resilience (defined as minimizing the largest eigenvalue of the perturbed system) were considered. The corollary of this assumption is that, at the limits of adaptation, there will be switching between communities, providing a potential ground for a broad definition of regime shifts.