Ecological thresholds and regime shifts: approaches to identification

Abstract

There is an apparent gap between the prominence of present theoretical frameworks involving ecological thresholds and regime shifts, and the paucity of efforts to conduct simple tests and quantitative inferences on the actual appearance of such phenomena in ecological data. There is a wide range of statistical methods and analytical techniques now available that render these questions tractable, some of them even dating half a century back. Yet, their application has been sparse and confined within a narrow subset of cases of ecological regime shifts. Our objective is to raise awareness on the range of techniques available, and to their principles and limitations, in order to promote a more operational approach to the analysis of ecological thresholds and regime shifts. The observation that managed ecosystems often fail to respond smoothly to changing pressures has generated perplexity and eventually lead researchers to draw parallels between the behaviour of ecological systems and other complex systems with non-linear dynamics, such as the global climate, the human immune system, and the world economy (cf.[1] for a popular account). Initial reports of kelp forest disturbance and recovery [2], freshwater ecosystem shifts engineered by beavers [3], and vegetation shifts affected by fire [4] have lead on to an ever-growing research effort on ECOLOGICAL THRESHOLDS and REGIME SHIFTS (see Glossary), whose underlying theoretical framework [5,6] (Box 1) has been shown to be applicable to a broad range of ecosystems from coral reefs to forests and lakes [7,8]. These concepts are now also making their way into the minds and discussions of policy makers and might soon be translated into legislative frameworks [9].Ecological regime shifts can be defined as abrupt changes on several trophic levels [10], leading to rapid ecosystem reconfiguration between alternative states. These shifts are generally thought to be driven by external perturbations (e.g. climatic fluctuations, overexploitation, eutrophication, and invasive species), but the exact mechanism is often unclear. The subject has become a fast growing scientific discipline, manifested by a 12-fold increase in publications between 1991 and 2006, twice as fast as the growth rate of research effort in ecology as a whole (7.7 % year -1 , ISI Web of Science). Most of the reported cases of ecological regime shifts are inferred from time series of monitoring data, while direct evidence by controlled experiments of the existence of alternative states is difficult to find [11]. Surprisingly, the general techniques available to test for regime shifts and thresholds have only to a limited extent been applied to these data sets. As formal tests of regime shifts have a long history in the context of climate change research (e.g. that formal statistical tests for ecological regime shifts have mostly been restricted to the effects of climate change on marine communities [13]. These observations suggest that there is a need to increase the awareness of ecologists on the availability...

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