High-frequency monitoring equipment deployed on Peter Lake. Photo: Michael Pace

Early Warning of Ecosystem Tipping Points

Summary

This project developed and tested early warning indicators for ecosystem tipping points. The project provided the first proof-of-concept that tipping points can be anticipated in advance at scales relevant to ecosystem management and policy making. Results were communicated to researchers through journal articles and conference presentations, and the general public through news media. Two statistical packages for applying early warning indicators were created and made freely available.

Project Status

Complete

Project Description

In the field of ecology, regime shifts are massive changes in function and character that occur when an ecosystem passes a tipping point. Regime shifts sometimes have severe consequences for human well-being through losses of ecosystem services, including desertification in arid regions and marine fisheries collapses. These changes are difficult to predict and sometimes impossible to reverse. For these reasons, understanding how to anticipate and prevent regime shifts is one of the most important challenges faced by environmental scientists.

This project developed and tested early warning indicators for ecosystem regime shifts. This research was centered around a whole-lake experiment conducted with collaborators on two small lakes in the Upper Peninsula of Michigan. We manipulated the fish community of one lake to cause a trophic cascade—a type of regime shift—and made measurements throughout the food web to determine whether early warning indicators were detectable before the tipping point. An adjacent reference lake was monitored for comparison.

A whole-lake experiment pushed a food web past a tipping point from prey dominated regime to predator dominated regime.

Peter and Paul Lakes in northern Michigan were the experimental and references ecosystems, respectively. Photo: NSF

Early warning indicators based on autocorrelation and variance responded strongly. Early warning signals were recorded up to a year and a half before the tipping point and were present throughout the food web including in time series of prey fish abundance, zooplankton biomass, and phytoplankton biomass. There were no early warning signals in the reference system. This was the first ecosystem-scale proof-of-concept that early warning indicators can be detected before tipping points. These results suggest that ecosystem managers may one day be able to use adaptive management to avert unwanted regime shifts.

Top: Average number of fish caught in minnow traps distributed around the edge of the experimental (black line) and reference (red line) lakes. This is an index of prey abundance. The whole-lake experiment occurred during four consecutive summers, but here the data are concatenated into a continuous time series for aesthetic reasons. Bottom: Early warning of the regime shift based on moving-window conditional heteroskedasticity tests applied to chlorophyll-a concentrations from four summers concatenated into a single time series. Chlorophyll-a concentration is an index of phytoplankton biomass that strongly reflects the variability generated by the regime shift in the fish community. There was early warning for the entire summer the year before the regime shift and until the tipping point was passed during the year of the regime shift.

Outside of the lake experiments, this project included contributions to an international synthesis project that culminated with the development of freely available statistical software that can be used to apply the early warning indicators to time series or spatially explicit data.

The early warning indicators we developed and tested are generated by phenomenon that are general to complex systems and are not restricted to ecosystems. After our experiment, other research groups identified early warning indicators before regime shifts in a variety of complex systems, including in economic records before the subprime housing loan crisis, before the self-termination of epileptic seizures, and before shifts in social networking activity on Twitter. Hence, this project is exemplary of how fundamental ecological research can generate tools and concepts that provide diverse benefits to society.

Broader Impacts

Education

I mentored two undergraduate students in research as part of this project. Our work together resulted in a peer-reviewed journal article co-authored by the students.

Tools

I contributed to the development of a website that features non-technical descriptions of theories and application of early warning indicators for ecosystem tipping points. Specifically, I wrote a case-study of application of early warning indicators during the whole-lake experiment. Related to this effort, I was part of an international synthesis group that developed two statistical packages for application of early warning indicators. These are freely available online and are run using the R statistical software.

Selected Media Reports

Press Releases

R Packages

Publications

. Early warnings of regime shifts: evaluation of spatial indicators from a whole-ecosystem experiment. Ecosphere, 2014.

PDF Dataset Project

. Early Warning Signals of Ecological Transitions: Methods for Spatial Patterns. PLoS ONE, 2014.

PDF Code Project

. Asymmetric response of early warning indicators of phytoplankton transition to and from cycles. Theoretical Ecology, 2013.

PDF Project

. Methods for Detecting Early Warnings of Critical Transitions in Time Series Illustrated Using Simulated Ecological Data. PLoS ONE, 2012.

PDF Code Project

. Conditional Heteroskedasticity Forecasts Regime Shift in a Whole-Ecosystem Experiment. Ecosystems, 2012.

PDF Project

. Conditional Heteroscedasticity as a Leading Indicator of Ecological Regime Shifts. American Naturalist, 2011.

PDF Project F1000 Recommendation

. Early Warnings of Regime Shifts: A Whole-Ecosystem Experiment. Science, 2011.

PDF Dataset Project F1000 Recommendation