Regime Shifts and Leading Indicators Michael Pace University of Virginia

Outline Human accelerated environmental change Regime shifts Theory of leading indicators An experiment to test indicator theory Microbial communities and regime shifts

Humans Global climate Loss of biodiversity change Land-use changes Stratospheric ozone depletion Invasions of Exotic Species Toxification of the Biosphere Environmental change caused by multiple and interacting human activities. From Likens 1998

Regime Shifts Massive reorganizations of ecosystems that occur abruptly and led to an alternate state May be driven external causes or internal processes Shifts may be difficult or impossible to reverse Feedbacks that control ecosystem processes are different after a regime shifts Example: switch during eutrophication from limitation by external nutrient loading to internal nutrient recycling

Regime Shift Second Regime Resilience First Regime

Anderson et al. TREE in press

ENSO – Example of Regime Shift

El Nino Forecast

Shallow Lakes Shift from clear water, vegetation-dominated condition To turbid, algal-dominated state These shift are triggered by several switches

Algae + + grazers Turbidity Nutrients fish - - Vegetation waves resuspended sediments - - Vegetation waves

Zebra Mussels – Dreissena polymorpha

Zebra Mussel Dynamics 1991-2002

Hudson River Zebra Mussel Invasion

Effects on Hudson Ecosystem Direct consumption – plankton Change in light transmission - plants Fouling – native bivalves Competition – other filter feeders Propagation through food web – fish Creation of structure – benthic animals

Problem Regime shifts are difficult to predict Ideally manage to avoid “going over the cliff” Are there leading indicators that would warn of shifts prior to initiation?

Possible Leading Indicators of Regime Shifts From Theory Increase in variance of time series Variance shifts to lower frequencies (“red shift”) Recovery rate from small perturbations increases (“critical slowing down”) Known from physics, models of ocean thermohaline circulation, economics, climate studies, and ecological models

Carpenter et al. 2008 Ecology Letters

Conclusions from Model Signals of regime can be detected well in advance if the driver of the regime shift (e.g. fish harvest) changes more slowly than key ecological variables (e.g. phytoplankton production, zooplankton biomass) However regime shifts can occur after the driver has already passed the critical point (i.e. ecosystem is already committed to going over the cliff when indicators go off)

Paul Peter

Continuous Observation Methods Daytime: Nighttime:

Can Microbial Communities Drive Regime Shifts? Perhaps rapid response and diversity of microbes results simply in community shifts to groups adapted to the new conditions However competitive interactions, metabolic capacity, genetic composition or other phenomena might drive regime shifts that are determined by relative abilities with profound consequences for ecosystem structure and function

Initial Model Adapted modeling framework of Fellows et al. Start with simple two group model of phytoplankton based on relative ability to compete for N and P (e.g. cyanobacteria vs. greens)

Status Elaborating model to build an approach to an experimental Idea would be to fertilize a lake varying N:P ratio through time Continuously monitor changes in phytoplankton (and others) Measure leading indicators

The hypoxic zone of the Gulf of Mexico has approximately doubled in size per unit nitrogen input. This partially reflects the accumulation of organic carbon in sediments that supports increased oxygen demand Turner et al. 2008 EST

Perspective Not all environmental change is abrupt (many transitions are gradual) Not all regime shifts will have leading indicators Continuous measurement systems provide opportunity to detect systems in transition (whether slow or fast) How do we manage to avoid transitions Defining indicators Ability to detect “signals” of change Proactive means to stay away from “the edge” and/or avoid “slow progressive” changes What microbial capacities and processes will drive regime shifts particularly in relation to climate change?