Early Warnings of Regime Shifts: A Whole-Ecosystem Experiment

Ecological regimes whether terrestrial or aquatic, can shift in ways that are detrimental to themselves and to other ecosystems, local or global. Through understanding warning signs of an ecosystem that is on the verge of a regime shift, measures may be taken to slow or potentially reverse these often damaging shifts; however, little research has been done to explore the overall or long term effectiveness of efforts to reverse ecological regimes changes. In a study by S. R. Carpenter et al. (2011), warning signs are sought out in advance of an aquatic regime shift which was planned out and executed in the confines of a single lake, using a nearby lake for reference against the results of their manipulated ecosystem.  This study concludes that statistical indicators can prove useful in recognizing early warning signs that announce catastrophic ecological regime changes in nonlinear systems. –Lindon Pronto
Carpenter, S.R, Cole, J. J., Pace, M. L., Batt, R., Brock, W. A., Cline, T., Coloso, J., Hodgson, J. R., Kitchell, J. F., Seekell, D. A., Smith, L., and Weidel, B. 2011. Early Warnings of Regime Shifts: A Whole-Ecosystem Experiment. Science 332, 1079-1082.

This experiment spanned the course of three years and used two separate lakes, one as reference and the other as the manipulated environment. The objective was to use top predators (largemouth bass in this case) to destabilize the food web in the manipulated lake, while using data from a nearby lake with naturally present largemouth bass as a reference. Destabilizing a food web through addition or removal of predators causes trophic alterations affecting primary production, ecosystem respiration, and nutrient cycling. These statistical indicators were discerned through the observation of nonlinear dynamics of fish, zooplankton and phytoplankton populations as they were affected by the artificially introduced predators. The three indicators were monitored daily by sensors during three years of summer stratification in both lakes between 2008 and 2010. Carpenter et al. hypothesized that through a chain reaction initiated by the introduction of largemouth bass into an ecosystem, the food web would undergo an ecological regime shift toward the piscivore-dominated food web of the non-manipulated lake. In fact, the study was able to produce significant warning signs indicating a regime change shortly after additional bass were added.  
This experiment purposefully created ideal conditions for early warning signal recognition, and monitored potential indicators including rising autocorrelation, steep increases in variance, extreme changes in skewness, and a shift in variance spectra. The results were consistent with modeling predictions that warning signs would become evident soon after the addition of the bass in 2008 and would match the decline in zooplankton biomass which would continue to decline through 2009, becoming highly oscillatory in 2010. Finally, the authors suggest that subsequent studies should address the reversibility of ecosystem regime shifts. 

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