# U.S. Climate Change Science Program 

# Synthesis and Assessment Product 4.2

January 2009 

# Thresholds of Climate Change in EcosystemsSAP 4.2: Thresholds of Climat e Change in Ecosystems



# Thresholds of Climate Change in Ecosystems

Final Report, Synthesis and Assessment Product 4.2

U.S. Climate Change Science Program And the Subcommittee on Global Change Research 

### Lead Agency U.S. Geological Survey 

### Contributing Agencies National Oceanic and Atmospheric Administration National Science Foundation U.S. Department of Agri culture, Forest Service U.S. Department of Energy U.S. Environmental Protection AgencySAP 4.2: Thresholds of Climat e Change in Ecosystems



## Federal Executive Team Director, Climate Change Science Program William J. Brennan Director, Climate Change Science Program Office Peter A. Schultz Lead Agency Principal Representa tive to CCSP; Senior Advisor for Global Change Programs, U.S. Ge ological Survey Thomas R. Armstrong Product Lead, U.S. Geological Survey Colleen W. Charles Synthesis and Assessment Product Advisory Group Chair; Associate Director, EPA National Center for Environmental Assessment Michael W. Slimak Synthesis and Assessment Product C oordinator, Climate Change Science Program Office Fabien J.G. Laurier

Editorial and Production Team Layout editor, U.S. Geological Survey Anna Glover Copy editor, U.S. Geological Survey Jeanette Ishee

This document, part of the Synthesis and Assessment Products described in the U.S. Climate Change Science Program (CCSP) Strategic Plan, was prepared in accordance with Section 515 of the Treasury and General Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554) and theinformation quality act guidelines issued by the Department of the Interior and the U.S. Geological Survey pursuant to Section 15). The CCSP Interagency Committee relies on the Department of the Interior and the U.S. Geological Survey certifications regarding compliance with Section 515 and Department guidelines as the basis for determining that this product conforms with Section 515. For purposes of compliance with Section 515, this CCSP Synthesis and Assessment Product is an “interpreted product” as that term is used in U.S. Geological Survey guidelines and is classified as “highly influential”. This document does not express any regulatory policies of th e United States or any of its agen cies, or provide recommendations for regulatory action.

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January 2009, Members of Congress: On behalf of the National Science and Technol ogy Council, the U.S. Climate Change Science Program (CCSP) is pleased to transmit to th e President and the Congress this Synthesis and Assessment Product (SAP) Thresholds of Climate Change in Ecosystems . This is part of a series of 21 SAPs produced by the CCSP aimed at providing current assessments of climate change science to inform public debate, policy, and operational decisions. These reports are also intended to help the CCSP develop future program research priorities. The CCSP’s guiding vision is to provide the Nation and the global community with the sciencebased knowledge needed to manage the risks and capture the opportunities associated with climate and related environmental changes. The SAPs are important steps toward achieving that vision and help to translate the CCSP’s extens ive observational and research database into informational tools that directly address key qu estions being asked of the research community. This SAP addresses and synthesizes the current state of scientific understanding regarding potential abrupt state changes or regime shifts in ecosystems in response to climate change. It was developed in accordance with the Guide lines for Producing CCSP SAPs, the Information Quality Act (Section 515 of the Treasury and General Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554)), and the guidelines issued by the Department of Interior and the U.S. Geological Survey pursuant to Section 515. We commend the report’s authors for both the thoroug h nature of their work and their adherence to an inclusive review process. Sincerely,

Carlos M. Gutierrez Secretary of Commerce

Chair, Committee on Climate Change Science and Technology Integration Samuel W. Bodman Secretary of Energy Vice Chair, Committee on Climate Change Science and Technology Integration John H. Marburger III Director, Office of Science

and Technology Policy Executive Direct or, Committee on Climate Change Science and Technology Integration

iiiSAP 4.2: Thresholds of Climat e Change in Ecosystems 

## Author Team for this Report Lead Authors : Daniel B. Fagre, USGS; Colleen W. Charles, USGS.



### Authors : Craig D. Allen, USGS; Charles Birkeland, USGS -University of Hawaii; F. Stuart Chapin, III, University of Alaska; Peter M. Groffman, Institute of Ecosystem Studies; Glenn R. Guntenspergen, USGS; Alan K. Knapp, Colora do State University; A. David McGuire, USGS-University of Alaska; Patrick J. Mu lholland, Oak Ridge National Laboratory; Debra P.C. Peters, USDA Agricultural Resear ch Service; Daniel D. Roby, USGS-Oregon State University; George Sugihara, Scripps Institute of Oceanography and University of California at San Diego.


### Contributors: Brandon Bestelmeyer, Jornada Basin LTER; Julio L. Betancourt, USGS; Jeffrey E. Herrick, Jornada Basin LTER
Thresholds of Climate Change in Ecos ystems Federal Advisory Committee An eight-member Federal Advisory Committ ee oversaw the scien tific review and development of this Synthesis and Assessmen t Product (SAP) at the request of the U.S. Geological Survey. Linda Joyce, Forest Service Steven McNulty, Forest Service Ronald Neilson, Forest Service Dennis Ojima, John H. Heinz III Center fo r Science, Economics and theEnvironment David Peterson, Forest Service Franklin Schwing, National Oceanic and Atmospheric Administration Fisheries Service John P. Smol, Queen’s University, Kingston, Ontario Leigh Welling, National Park Service

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## Acknowledgments CCSP Synthesis and Assessment Product 4.2 (SAP 4.2) was developed with the benefit of a scientifically rigorous first draft peer review conducted by a committee appointed under the Federal Advisory Committ ee Act—Thresholds of Climate Change in Ecosystems Federal Advisory Committee (FAC). The resultant review was instrumental in shaping the final version of SAP 4.2 and in improving its completeness, sharpening its focus, and communicating its co nclusions and recommendations. We would like to acknowledge Susan Haseltine, Associate Director-Biology USGS, Robert Szaro, Chief Scientist-Biol ogy USGS (retired), Bruce Jones, Chief Scientist- Biology, and Jeffrey Kershner, Northern Rocky Mountain Science CenterUSGS for the support, encouragement, and accommodation provided in order to carry out this complex task.

The authors extend their sincere thanks and appreciation to Fabien Laurier, CCSP, for his patience, guidance, and encouragem ent all through the development of SAP 4.2.

We wish to thank the members of the ReviewFAC: Linda Joyce, USDA Forest Service; Steven McNulty, USDA Forest Service; Rona ld Neilson, USDA Forest Service; Dennis Ojima, The H. John Heinz III Center for Science, Economics and the Environment; David Peterson, USDA Forest Service; Fr anklin Schwing, Na tional Oceanic and Atmospheric Administration Fisheries Serv ice; John P. Smol, Queen’s University, Kingston, Ontario; Leigh Welli ng, National Park Service. We would like to thank Anthony C. Ja netos, Joint Global Change Research Institute, for providing guidance and s upport in developing this document and

viSAP 4.2: Thresholds of Climat e Change in Ecosystems

vii encouragement in getting this information out . We also extend appreciation to Patricia Jellison, USGS, for her support in completing SAP4.2. The authors also express deep gratitu de to Lindsey Bengtson and Kimberly Dueling, Jeannete Ishee, and Anna Glove r, USGS, for their invaluable time and dedication to the production of SAP 4.2. We are also grateful to all the review ers who provided valuable comments during the public comment period. The author team carefully considered all comments and the subsequent revisions resulted in improving the quality and clarity of this report.SAP 4.2: Thresholds of Climat e Change in Ecosystems 

## Synopsis As defined in this Synthesis and Assessment Report, ‘an ecological threshold is the point at which there is an abrupt change in an ecosystem quality, property, or phenomenon, or where small changes in one or more external conditions produce large and persistent respons es in an ecosystem’. Ecological thresholds occur when exte rnal factors, positive feedbacks, or nonlinear instabilities in a system cause ch anges to propagate in a domino-like fashion that is potentially irreversible. This report reviews threshold changes in North American ecosystems that are potentially induced by clim atic change and addr esses the significant challenges these threshold crossings im pose on resource and land managers. Sudden changes to ecosystems and the goods and se rvices they provide are not well understood, but they are extremely important if natural resource managers are to succeed in developing adaptation strate gies in a changing world. The report provides an overview of what is known about ec ological thresholds and where they are likely to occur. It also identifies those areas where research is most needed to improve knowledge and understa nd the uncertainties re garding them. The report suggests a suite of potential actions th at land and resource managers could use to improve the likelihood of success for the resources they manage, even under conditions of incomplete understanding of what drives thresholds of change and when changes will

occur. Key examples of climate-induced threshol d changes are presented. This synthesis effort identified a suite of potential actions that , taken together or se parately, can begin to

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ix improve the understanding of thresholds and increase the likelihood of success in developing management and adaptation strategi es in a changing climate, before, during, and after thresholds are crossed. In general, it is essential to increase the resilience of ecosystems and thus to slow or prevent th e crossing of threshol ds; to identify early warning signals of impending threshold changes; and to employ adaptive management strategies to deal with new conditions, new successional trajectories and new combinations of species.SAP 4.2: Thresholds of Climat e Change in Ecosystems



## Recommended Citation For the Report as a Whole: CCSP, 2009: Thresholds of Climate Change in Ecosystems. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [Fagre, D.B., C.W. Charles, C.D. Allen, C. Birkeland, F.S. Chapin III, P.M. Groffman, G.R. Guntensper gen, A.K. Knapp, A.D. McGuire, P.J. Mulholland, D.P.C. Peters, D.D. Roby, and George Sugihara]. U.S. Geological Survey, Reston, VA, 156 pp.

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## Table of Contents

Federal Executive Team ..................................................................................................... ii Letter to Congress............................................................................................................. . iv Author Team for this Report............................................................................................... v Acknowledgments.............................................................................................................. vi Synopsis ....................................................................................................................... ....viii Recommended Citation....................................................................................................... x Executive Summary............................................................................................................ 1 Introduction................................................................................................................... 1 Examples of Ecosystem Thresholds ............................................................................. 2 Recommendations......................................................................................................... 5 Chapter 1—Introduction and Background........................................................................ 10 1.1 The Problem of Sudden Change in Ecological Systems....................................... 10 1.2 The Response of the Climate Change Community............................................... 13 1.3 The Goal of SAP 4.2............................................................................................. 15 1.4 Standard Terms ..................................................................................................... 17 Chapter 2—Ecological Thresholds................................................................................... 18 2.1 Introduction........................................................................................................... 18 2.2 Early Development ............................................................................................... 20 2.3 Current Discussions of Threshold Phenomena..................................................... 22 2.4 Ecological Thresholds Defined for SAP 4.2......................................................... 25 2.5 Factors That Influence Resilience......................................................................... 27 2.6 The Bottom Line................................................................................................... 29

xiSAP 4.2: Thresholds of Climat e Change in Ecosystems Chapter 3—Case Studies .................................................................................................. 32 Case Study 1: Ecological Thresholds in Alaska ......................................................... 32 Case Study 2. The Mid-Continent Prai rie Pothole Region: Threshold Responses to Climate Change......................................................................... 46 Case Study 3. Broad-Scale Forest Die- Back as a Threshold Response to Climate Change in the Southwestern United States ....................................... 53 Case Study 4. Thresholds in Climate Change for Coral-Reef Ecosystem Functioning ..................................................................................................... 62 Chapter 4—Examples of Threshold Change in Ecosystems ............................................ 74 4.1 Background........................................................................................................... 74 4.2 Example of Thresholds from the Past................................................................... 75 4.3 Temperature Increases Are Pushing Ecosystems Towards Thresholds................ 76 4.3.1 Climate Interactions Drive Ecosystems to Thresholds ................................ 78 4.3.2 Climate Variability Increases Likelihood of Threshold Shifts .................... 79 4.3.3 Other Human Stressors and Climate Change............................................... 80 4.3.4 Ecosystem Vulnerability and Climate Change ............................................ 84 Chapter 5—What Can Be Done?...................................................................................... 87 5.1 Integration of Management and Research ............................................................ 87 5.1.1 Need for Conceptual Models ....................................................................... 87 5.1.2 Scaling.......................................................................................................... 89 5.1.3 Applying Models from Other Disciplines.................................................... 91 5.2 Adaptive Management to Increase Resilience...................................................... 92 5.2.1 Role of Monitoring ...................................................................................... 94

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xiii 5.2.2 Role of Experiments .................................................................................... 96 5.3 Management by Coping........................................................................................ 97 5.3.1 Reducing Multiple Stressors........................................................................ 98 5.3.2 Triage ........................................................................................................... 99 5.3.3 System-Level Planning and Policy ............................................................ 100 5.3.4 Capacity Building and Awareness............................................................. 101 5.4 Summary............................................................................................................. 102 Chapter 6—Summary and Science Recommendations .................................................. 103 Summary................................................................................................................... 103 Science Recommendations ....................................................................................... 104 Before ................................................................................................................. 104 During ................................................................................................................. 107 After ................................................................................................................. 110 Conclusion ................................................................................................................ 110 References Cited ............................................................................................................. 11 2 Appendix A—Glossary................................................................................................... 154 Contact Information........................................................................................................ 157SAP 4.2: Thresholds of Climat e Change in Ecosystems 

## Executive Summary
Introduction As defined in this assessment, an ecological threshold is the point at which there is an abrupt change in an ecosystem quality, property, or phenomenon, or where small changes in one or more external conditions produce large and persistent responses in an ecosystem . Ecological thresholds occur when exte rnal factors, positive feedbacks, or nonlinear instabilities in a system cause ch anges to propagate in a domino-like fashion that is potentially irreversible. Once an ecolo gical threshold is crossed, the ecosystem in question is not likely to return to its previous state. Over the past three decades, climate change has become a recognized driver of ecosystem change. Changes in phenology, ra sh ifts of species, and increases in such disturbances as wildland fire s are all examples of ecosy stem-scale responses to a warming biosphere. Much ecosystems resear ch focuses on enhancing understanding of climate change impacts on ecosystems (and vice versa ) and in developing the capability to predict the potential impacts of future clim ate change. In addition to the gradual types of climate-related change mentioned above, there is increasing r ecognition that small changes in climate can trigger major, abrupt responses in ecosystems when a threshold is crossed The potential for sudden, unanticipated shifts in ecosystem dynamics make resource planning, preparation, and manage ment intensely difficult. These sudden changes to ecosystems and the goods and se rvices they provide are not well understood, but they are extremely important if natural resource managers are to succeed in developing adaptation strategies in a changing world. This re port provides an overview of

1SAP 4.2: Thresholds of Climat e Change in Ecosystems what is known about ecological thresholds and where they are likely to occur. It also identifies those areas where research is most needed to improve knowledge and understand the uncertainties regarding them. The report suggests a suite of potential actions that land and resource managers coul d use to improve the likelihood of success for the resources they manage, even under conditions of incomplete understanding of what drives thresholds of change and when changes will occur. The focus of this report is on North American ecosystem threshold changes and what they mean for human society. 

### Examples of Ecosystem Thresholds There are numerous examples of sudden ecological change that fit the current qualitative definition of an ecological thres hold and that were likely caused by climatic changes such as warming temperatures. A clear example comes from recent observations of the Arctic tundra, where the effects of warmer temperatures have included reduced snow cover duration, which leads to redu ced reflectivity of the surface. Reduced reflectivity causes greater absorption of so lar energy, resulting in local warming, which, in turn, further accelerates the loss of s now cover. This amplified, positive feedback effect quickly leads to warmer conditions th at foster the invasion of shrubs into the tundra. The new shrubs themselves then further reduce albedo and add to the local warming. The net result is a relatively sudden, do mino-like chain of even ts that result in conversion of the arctic tundra to shrubland, triggered by a relatively slight increase in temperature. Examples like this illustrate the im portance of positive feedbacks. Positive feedbacks are those that tend to increase al teration of the nature of the system, while negative feedbacks tend to minimize these chan ges. Ecosystems include both positive and

2SAP 4.2: Thresholds of Climat e Change in Ecosystems negative feedbacks. Changes in external or internal factors that favor and strengthen positive feedbacks can lead to a change in conditions that may overwhelm other components of the system, leading to thresh old changes. For example, the invasion and spread of a highly flammable grass in dese rts will change the susceptibility of that landscape to fire. As another example, pe rsistent drought will push an ecosystem’s vegetation toward the limits of its physiol ogical tolerance to wa ter stress, creating conditions that favor drought-tolerant species at the expense of thirstie r plants; this leads to system change, until a new state (with different, more drought-t olerant species) is achieved. Ecosystems are not simple, and complex interactions between multiple factors and feedbacks can lead to even greater nonlin ear changes in their dynamics. For example, the interaction of drought t ogether with overgrazing can trigger desertification. Disturbance mechanisms, such as fire and insect outbreaks, shape many landscapes and may predispose many of them to threshold change when the additional stress of climate change is added. Furthermore, climate change will alter not only the landscape, but it will also affect the disturbance mechanisms th emselves; in the example above, a warmer climate may not only lead to vegetation changes, but may also favor increased dryness, which will increase the likelihood of fire. On a global scale, such alte red disturbance regimes may influence rates of climate change. For example, as mentioned above, warm, dry conditions favor fire, and more fires release more carbon di oxide from burning vegetation, which in turn favors more warming. Adding additional complexity to alre ady-complex systems, human actions also interact with natural driver s of change, producing multifaceted ecosystem changes that

3SAP 4.2: Thresholds of Climat e Change in Ecosystems have important implications for the services provided by those ecosystems. For instance, the introduction of exotic, invasive plants may change the way in which an ecosystem responds to drought, and the conversion of woodl and to farmed fields or urban areas will change the manner in which that la ndscape responds to intense storms. The stories of several im portant ecosystems provide concrete examples of ecological thresholds, and illustrate the kinds of complex change that natural resource managers are facing, and that they must manage in the future. As mentioned briefly above, a key exampl e of observed climate-related threshold change is the warming of Alaska. Warming has caused a number of effects, including earlier snowmelt in the spring, reductions in sea-ice coverage, warming of permafrost, and resultant impacts to ecosystems includi ng dramatic changes to wetlands, tundra, fisheries, and forests, including increases in the frequency and spatial extent of insect outbreaks and wildfire. During the 1990s, south-central Alaska experienced the largest outbreak of spruce bark beetle s in the world. Milder winter s and warmer temperatures increased the over-winter survival of the spruce bark beetle and allowed the bark beetle to complete its life cycle in 1 year instead of th e normal 2 years. Added to this were 9 years of drought stress, which resulted in spruce trees that were too weak to fight off the beetle infestation. For these forests, multiple climat e-triggered stresses amplified each others’ effects to cause a profound ecosystem change. The Alaskan spruce bark beetle outbreak and consequent forest die-off are an example of an actual climate-induced thresh old crossing. There are additional ecosystems for which conditions suggest an approachi ng climate-related threshold. These include coral reefs, prairie pothole we tlands, and southwestern forests. Climate-related processes

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that affect coral reefs include sea-level rise, ocean acidifi cation, and the increased water temperatures that are responsible for cora l bleaching events. The Prairie Pothole Region of north-central North America is one of the most ecologically valuable freshwater resources of the Nation, with numerous wetla nds that provide cr itical habitat for waterfowl populations. Climate models suggest a warmer, drier future climate for the Prairie Pothole Region, which would result in a reduction in, or elimination of, wetlands that provide waterfowl breeding habitat. Sim ilarly, predicted warmer, drier conditions in the semiarid forests and woodlands of the s outhwestern United States would place those forests under more frequent water stress, resu lting in the potentia l for shifts between vegetation types and distributions , and could trigger rapid, exte nsive, and dramatic forest dieback. In each of these cases, the anticipated cha nges would also be expected to tie to other nonlinear feedback relationships and other ecologica l disturbance processes, potentially leading to additional nonlinear threshold behaviors. Understanding and predicting the outcome of such complex interactions is not a trivial endeavor. Ecological systems are multivariate in nature, but current ecological forecasting model capabilities are comparatively simple and generally do not address the possi bility or consequences of thresholds. Complex situations like those involving ecological thresholds thus tend to be beyond the limits of existing predictive capab ilities. The end result is surprises for
managers. 

### Recommendations If climate change is pushing more ecosystems toward thresholds, what can be done by land and resource managers and othe rs to better cope w ith the threat of

5SAP 4.2: Thresholds of Climat e Change in Ecosystems transformative change? Although the science of ecological th resholds is still in its infancy, one outcome of this synthesis effort was the identification of a suite of potential actions that, taken together or separately, can improve th e understanding of thresholds and increase the likelihood of success indeveloping management and adaptation strategies in a changing climate, before, during, and after thres holds are crossed: Support Research to Identify Thresholds.— While conceptually robust and widely acknowledged as already occu rring, thresholds and thre shold crossings have had relatively few empirical studies addressing them. Reliable id entification of thresholds across different systems should be a nationa l priority because of the potential for substantive surprises in the manage ment of our natural resources. Enhance Adaptive Capacity .—Given that threshold changes are increasingly likely to occur, it is important to prepare fo r them by increasing societal and ecological resilience. Managers that unde rstand ecological diversity and the other factors that influence the resilience of the systems they manage are in a better position to implement changes that reduce the likelihood that thresholds will be crossed. Monitor and Adjust Multip le Factors and Drivers.— Once the key factors controlling adaptive capacity and resilience are known, monitoring strategies should include those factors. Cons ideration should be given to monitoring indicators of ecosystem stress rather than the resources and ecological services of management interest. Develop Scenarios of the Consequences of Alternative Management Options for Dealing with Potential Changes.— In some cases, the kinds of external factors that can precipitate threshold changes are well known, and furthe rmore are known in advance (for

6SAP 4.2: Thresholds of Climat e Change in Ecosystems example, hurricanes, wildfire, or invasive spec ies). In these cases, scenario analysis is a powerful tool for predicting and understandi ng the potential consequences of specific management actions. Collate and Integrate Informati on Better at Different Scales.— Because agencies and institutions have different management mandates, there can be a focus on those resources and at their scales of interest to the exclusion of others. Better information sharing and integration have great pote ntial for improving the understanding of thresholds and identifying when they might occur. Reduce Other Stressors.— Many trigger points for abr upt change in ecosystems that are responding to climate change are not recognized, because human civilizations have not previously witnessed climate change of this magnitude. However, other stressors for which reliable information ex ists can be reduced to make ecosystems healthier and more resili ent as climate changes. Manage Threshold Shifts.— There may be constraints to reducing or reversing climate-change-induced stresses to component s of an ecosystem. If a threshold seems likely to occur but the uncertainties remain high as to when it will occur, contingency plans should be created. These can be implem ented when the threshold shift begins to occur or can be carried out in advance if the approaching th reshold is clear. Project Impacts to Natural Resources.— There are many efforts to project climate change (for example, general circulation models) and ecosystem responses to climate change (for example, mapped atmosphere-plant-soil systems) using simulation modeling and other tools. These models generally proj ect ecosystem trends and shifts, but do not explicitly consider the possibility of thresh olds. A concerted effort must be made to

7SAP 4.2: Thresholds of Climat e Change in Ecosystems understand, model, and project ecosystem res ponses to climate change with explicit acknowledgment of thresholds. Recognize Need for Decisionm aking at Multiple Scales.— Much of the recent information on climate change impacts suggest s that changes are o ccurring more quickly than forecast only a few years ago. It is al so apparent that many changes are causing secondary, or cascading, domino-like, changes inother parts of ecosystems. Management policies that were developed during relati vely stable climate conditions may be inadequate for a variable world with more surprises. A shift toward multiple scales of information integration and subsequent decisionmaking can enhance and leverage existing management resources. Instigate Institutional Change to Increase Adaptive Capacity.— In many cases, current institutional structures are geared to wards disciplinary and jurisdictional isolation by agencies and, therefore, they do not facil itate synthesis across re sources, regions or issues. The capacity for such synthesis will be critical for identifying potential thresholds in ecosystem processes on multiple scales. Identify Research Needs and Priorities to Address Thresholds.— At this point in time, very little is understood about thresholds in ecosystems. The major research needs and priorities that will enhance the ability inthe future to forecast and detect abrupt changes in ecosystems caused by climate cha nge must be articulated. The ubiquity of threshold problems across so many fields s uggests the possibili ty of finding common principles at work. The cross- cutting nature of the problem of large-scale system change suggests an unusual opportunity to leverage effort from other fields and apply it to investigating systemic risk of crossing thresholds.

8SAP 4.2: Thresholds of Climat e Change in Ecosystems In summary, the science of understanding and predicting ecological thresholds is still in its infancy, and our existing understa nding of many aspects and potential impacts of these thresholds is qualitative at best. Th e challenge is to improve the science needed to support decisionmaking, while recognizing that managing lands and resources is a continual process and that stra tegies are needed to inform management decisions that must be made under conditions of high uncerta inties regarding potential thresholds. To better understand and prepare for ecological th reshold crossings and their consequences, it is essential to increase the resilience of ecosystems and thus to slow or prevent the crossing of thresholds; to identify early wa rning signals of impendi ng threshold changes; and to employ adaptive management strate gies to deal with new conditions, new successional trajectories and new combinations of species. Better integration of existing monitoring information across a range of spatia l scales will be needed to detect potential thresholds, and research will need to focu s on ecosystems undergoing a threshold shift to better understand the underlying processes. In a world bei ng altered by climate change, natural resource managers may also have to be increasingly nimble, and adjust their goals for desired states of resources away from static, historic benchmarks and focus on increased resilience, biodiversity, and adaptive capacity as measures of success.

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## Chapter 1—Introduction and Background
1.1 The Problem of Sudden Chan ge in Ecological Systems The carbon dioxide (CO 2 ) concentration in the earth’s atmosphere has reached 385 parts per million (ppm), a level that is unprecedented over the past one-half million years (based on ice core data) to 24 million years (based on soil data) (Hoegh-Guldberget et al. 2007). CO 2 levels have been increasing during th e past 150 years, with most of the change occurring in just the past few decades. Global mean temperature has risen in response to increased CO 2 concentration and is now higher than at any time in the past 1,000 years (based on tree rings) to 160,000 years (based on oxygen 18 ( 18 O) and deuterium (D) isotopes in ice). The relativelysudden increase in the energy balance of the planet, due to an increase in greenhouse ga s concentrations, has led to abrupt global climate changes that alter physical processe s and biological systems on many scales and will certainly affect ecosystems that s upport human society (IPCC 2007). One of the ways that a rapidly changing climate may affect ecosystems is by causing sudden, irreversible effects that fundamentally change the function and structure of the ecosystem with potentially huge impacts to human society (Wamelink et al.2003). Even small, gradual change can induce threshold changes. For instance, in 1976– 1977, major shifts occurred in sea surface temp eratures, fisheries landings, zooplankton abundance, and community composition in th e North Pacific (Hare and Mantua, 2000). Later analysis suggested that nonlinear regime sh ifts operate in this ecosystem, such that even small changes in physical conditions canprovoke a regime shift that may not be easily or symmetrically reversed (for example , an increase in temperature from global warming, even as small as 0.5 0 C, has led to responses that have been well documented)

10SAP 4.2: Thresholds of Climat e Change in Ecosystems (IPCC 2007; Hsieh et al. 2006). This tende ncy can be compounded by additional environmental stressors that predispose ecosystems to expe rience threshold changes in response to climate change. For example, in North America in the late 1990s, forests, woodlands, grasslands, and shrublands exhi bited extensive dieback across the arid southwestern United States as overgrazing, fire suppression, and climat e variability led to massive insect outbreaks and an unprecedented br eadth of area consumed by fire (Allen, 2007). Abrupt changes in ecosystems may result in dramatic reductions in ecosystem services, such as water supplies for human use. In the Klamath River basin in the Pacific Northwest, for example, the delicate socio ecological balance of wa ter allocation between needs for irrigated agriculture and habitat fo r endangered species of fish, which had been established in 1902, collapsed in 2002 during a multiyear drought because the system’s resilience to maintain water quality in the face of climatic variability was degraded by long-term nutrient loading (NRC 2002). Thresh olds pose perhaps the greatest challenge currently facing climate change scientists. There is clear evidence that climate change has the potential to increase thres hold changes in a wide range of ecosystems, but the basic and practical science necessary to predic t and manage these changes is not well developed (Groffman et al. 2006) . In addition, climate change interacts with other natural processes to produce threshold changes. Distur bance mechanisms, such as fire and insect outbreaks (Crutzen and Goldammer 1993, L ovett et al. 2002, respectively), shape landscapes and may predispose many of them to threshold change when the stress of climate change is added (Swetnam and Betancourt 1998). To complicate matters further, climate change can alter the disturbance mechanisms themselves and, on a global scale,

11SAP 4.2: Thresholds of Climat e Change in Ecosystems altered disturbance regimes ma y influence rates of climate change. Another challenge is the multiscaled nature of threshold change s. These changes almost always involve coupled socioecological dynamics where human actions interact with natural drivers of change to produce complex changes in ecosyst ems that have important implications for the services provided by the ecosy stems (Wamelink et al. 2003). A sense of urgency regarding thresholds ex ists because of the increasing pace of change, the changing features of the driver s that lead to thresholds, the increasing vulnerabilities of ecosystem services, and the challenges the existence of thresholds poses for natural resource management. These challenges include the potential for major disruption of ecosystem services and the possi bility of social upheav al that might occur as new ways to manage and adapt for climate change and to cope w ith the unanticipated change are required. Research on ecological thres holds is being assessed cr itically. The Heinz Center conducted several workshops that presented cas e studies of likely threshold change and began looking at possible social and policy re sponses. Another effort included numerous case studies focused on nonlinearities in ecolo gical systems (Burkett et al. 2005) and considered how thresholds are nonlinear res ponses to climate cha nge. Recently, specific requests for proposals have been issued fo r research on thresholds (for example , see http:// es.epa.gov/ncer/rfa/2004/2004_aqua_sys.html ; http://cfpub.epa.gov/ncer_abstrac ts/index.cfm/fuseaction/recci pients.display/rfa_id/422/r ecords_per_page/ALL ), and there are active efforts to bridge the gap between research and application in this area (see, for example , www.ecothresholds.org). Assessment of

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the “state of the science” as it relates to ecosystems in the United States and for articulation of critical re search needs is needed. 

### 1.2 The Response of the Climate Change Community Climate change is a very complex issu e, and policymakers need an objective source of information about the causes of c limate change, its potential environmental and socioeconomic consequences, and the adaptation and mitigation strategies to respond to the effects of climate change. In 1979, the first World Climate Conference was organized by the World Meteorological Organization. This conference expr essed concern about man’s activities on Earth and the potential to “cause significant ex tended regional and even global changes of clim ate” and called for “global c ooperation to explore the possible future course of global climate and to take this new unders tanding into account in planning for the future development of human society” (IPCC 2007a). A subsequent conference in 1985 focused on the assessment of the role of CO 2 and other greenhouse gases in climate variations and associated imp acts, concluding that an increase of global mean temperature could occur that would be greater than at any time in humanity’s history. As a follow up to this conference, the Advisory Group on Greenhouse Gases, a precursor to the Intergovernmental Pane l on Climate Change (IPCC), was set up to ensure periodic assessments of the state of scientific knowledge on climate change and the implications of climate change for so ciety. Recognizing the need for objective, balanced, and internationally coordinated sc ientific assessment of the understanding of the effects of increasing con centrations of greenhouse gases on the earth’s climate and on ways in which these changes may potentiall y affect socioeconomic patterns, the World Meteorological Organizati on and the United Nations Environment Programme

13SAP 4.2: Thresholds of Climat e Change in Ecosystems coordinated to establish an ad hoc intergove rnmental mechanism to provide scientific assessments of climate change. Thus, in1988, the IPCC was established to provide decisionmakers and others interested in cl imate change with an objective source of information about climate change. The role of the IPCC is to assess ( on a comprehensive, objective, open, and transparent basis) the scient ific, technical, and socioeconomic information relevant to understanding the scientific ba sis of risk of human-induced climate change, its potential impacts, and options for adaptation and mitigation and to provide reports on a periodic basis that reflect existing viewpoints within the scientific community. Because of the intergovernmental nature of the IPCC, the reports provide decisionmakers with policyrelevant information in a policy neutral way (IPCC, 2007a). The first IPCC report was published in 1990, with subsequent re ports published in 1995, 2003, and 2007. In 1989, the U.S. Global Change ResearchProgram began as a Presidential initiative and was codified by Congress in the Global Change Research Act of 1990 (Pub. L. 101–606), which mandates development of a coordinated interagency research program. The Climate Change Science Program (CCSP, www.climatescience.gov ), a consortium of Federal agencies that perform climate scienc e, integrates the research activities of the U.S. Global Change Resear ch Program with the U.S. Climate Change Research Initiative. The CCSP integrates federally supported research on global change and climate change as conducted by the 13 U.S. Government departments and agencies involved in climate science. To provide an open and tran sparent process for assessing the state of scientific information relevant to underst anding climate change, the CCSP established a

14SAP 4.2: Thresholds of Climat e Change in Ecosystems synthesis and assessment program as part of its strategic plan. A pr imary objective of the CCSP is to provide the best science-ba sed knowledge possible to support public discussion and government and private se ctor decisionmaking on the risks and opportunities associated with ch anges in the climate and related environmental systems. The CCSP has identified an initial set of 21 synthesis and assessment products (SAPs) that address the highe st priority research, observa tion, and decision-support needs to advance decisionmaking on climate change-r elated issues. This assessment, SAP 4.2, focuses on abrupt ecological responses to c limate change, or thresholds of ecological change. It examines the impacts to ecosystems when thresholds are crossed. It does not address those ecological changes that ar e caused by major disturbances, such as hurricanes. These externally driven change s, or exogenous trigge rs, are distinguished from changes caused by shifts in the ecosystem ’s response to a driver, such as a gradual rise in temperature. These internal changes in system response, or endogenous triggers, are the focus of this SAP. This SAP is one of four reports that address the Ecosystems research element and Goal 4 of the CCSP stra tegic plan to understa nd the sensitivity and adaptability of different na tural and managed ecosystems and human systems to climate and related global changes. 

### 1.3 The Goal of SAP 4.2 This SAP summarizes the present state of scientific unders tanding regarding potential abrupt state changes or regime sh ifts in ecosystems in response to climate change. The goal is to identify specific difficu lties or shortcomings in our current ability to identify the likelihood of abrupt state changes in ecosystems as a consequence of climate change.

15SAP 4.2: Thresholds of Climat e Change in Ecosystems Questions addressed by this SAP include:
1. What specifically is meant by abrupt st ate changes or regime shifts in the structure and function of ecosystems in response to climate change?
2. What evidence is available from current ecological theory, ecological modeling studies, or the paleoecological record that abrupt changes in ecosystems are likely to occur in response to climate change?
3. Are some ecosystems more likely to exhi bit abrupt state ch anges or threshold responses to climate change?
4. If abrupt changes are likely to occur in ecosystems in response to climate change, what does this imply about the ability of ecosystems to provide a continuing supply of ecosystem goods a nd services to meet the needs of humans?
5. If there is a high potential for abrupt or threshold-type changes in ecosystems in response to climate change, what changes must be made in existing management models, premises, and practices in order to manage these systems in a sustainable, resilient manner?
6. How can monitoring systems be designed and implemented, at various spatial scales, in order to detect and antici pate abrupt or threshold changes in ecosystems in response to future climate change?
7. What are the major research needs and pr iorities that will enhance the ability in the future to forecast and detect abrupt changes in ecosystems caused by climate change?

16SAP 4.2: Thresholds of Climat e Change in Ecosystems


### 1.4 Standard Terms The 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report ( IPCC, 2007 ) is the most comprehensive and up-to-date report on the scientific assessment of climate change. This assessment (SAP 4.2) uses the standard terms defined in the IPCC’s Fourth Assessment Report with re spect to the treatmen t of uncertainty and the likelihood of an outcome or result base d on expert judgment aboutthe state of that knowledge. The definitions are shown in figure 1.1. This set of definitions is for descriptive purposes only and is not a quantitative appro ach from which probabilities relating to uncertainty can be derived.

Figure 1.1. Degrees of outcome likelihood as defined in the IPCC’s Fourth Assessment Report (AR4) (IPCC, 2007). Likelihood terminology Like lihood of the occurrence/ outcome Virtually certain > 99% probability Extremely likely > 95% probability Very likely > 90% probability Likely > 66% probability More likely than not > 50% probability About as likely as not 33 to 66% probability Unlikely < 33% probability Very unlikely < 10% probability Extremely unlikely < 5% probability Exceptionally unlikely < 1% probability

17SAP 4.2: Thresholds of Climat e Change in Ecosystems 

## Chapter 2—Ecological Thresholds
2.1 Introduction Temperature, precipitation, and relate d climate variables are fundamental regulators of biological processe s and it is reasonable to expect that significant changes in the climate system may alter linkages and feedbacks between ecosystems and regional climate systems. Increasing focus is bei ng placed on the existence and likelihood of abrupt state changes or thres hold responses in the structur e and functioning of ecosystems (Holling 1986; Scheffer et al., 2001; Higgins et al. 2002; Foley et al. 2003; Schneider 2004; Burkett et al. 2005; Hsieh et al. 2005). Various interrelated te rms are employed in the scientific literature to characterize these types of discontinuous and rapid changes in ecosystems, including ecosystem tipping point s, regime shifts, threshold responses, alternative or multiple stable states, and ab rupt state changes. Our current understanding of thresholds and ecosystem responses makes it unlikely that we can predict such discontinuities in ecosystems, and these discontinuities are likely to result in profound changes to natural resources that are sensitiv e to climate changes, as well as to human societies that depend on ecosystem goods and services. This assessment, based on the literature and the synthesis teams’ expe rtise, indicates that thresholds are likely to represent large-scale risk and uncertainty and will likely be a major challenge to natural
resource managers. Abrupt transitions have occurred in numerous ecosystems where incremental increases in global temperature have produced sudden and dramatic changes in the state of and the dynamics governing these systems (Anderson et al. 2008). These thresholds of magnified ecological change are a conseque nce of the underlying nonlinear nature of

18SAP 4.2: Thresholds of Climat e Change in Ecosystems ecosystems and are very likely critical to adaptation stra tegies for managing natural resources in a rapidly changing world. S udden, unanticipated shifts in ecosystem dynamics are a major source of uncertaintyfor managers and make planning and preparation difficult. One of the primary objectiv es of this report (SAP 4.2) is to enhance the understanding and ability of managers to forecast the effects of climate change on ecosystems. As discussed elsewhere in this chapter, the occurrence of threshold, or abrupt changes in ecosystems, is suggested by curre nt ecological theory and models, and is documented with laboratory and field examples and even in the paleoecological record. However, on a predictive level, thresholds remain poorly understood, particularly in terms of the underlying causal mechanisms and the general factors that predispose systems to threshold effects. For example, it is unclear under what circumstances climate change (both in its mean state and in its va riance in space and time, including occurrence of extreme weather events) might cause ecosy stem threshold shifts, instead of more gradual, continuous changes in ecosystems and species. Furthe r, it is not known what the resulting effects of very abr upt climate change (that is, cr ossing climate thresholds) on ecosystems will be. However, it will likely increase the likelihood of an ecosystem threshold shift. Thus, while rapid transitions in ecosystems are clear, reaching a level of understanding that enable s one to anticipate or actually predict threshold effects is the main bottleneck to producing results that are useful to managers (Muradian 2001; Bestelmeyer 2006; Groffman et al. 2006; Kinzig et al. 2006).

19SAP 4.2: Thresholds of Climat e Change in Ecosystems 

### 2.2 Early Development The concepts of ecological thresholds, multiple stable states, and regime shifts originated in early theoretical work on the stability or persistence of ecosystems (Margalef, 1963; Lewontin, 1969; Odum, 1969; Holling, 1973; May 1973, 1977). The two key components of stability were considered to be the system’s “resilience,” or the speed at which it would return to its current “stable equilibrium,” and its “resistance,” or ability to maintain its current “stable” st ate in the face of di sturbance of a given magnitude. According to this early thinking, given enough disturbance, systems could be pushed into alternative stable states. This theoretical work was complemented (however sparsely) with early empirical demonstrati ons of multiple stable states in marine experimental systems (Sutherland, 1974) a nd with field data combined with model analysis for terrestrial ecos ystems (Ludwig et al. 1978). “Stability” as a well-defined mathematical concept was central to these early theoretical discussions of thresholds. Lewon tin (1969) reviewed mathematical models of stability and discussed the forces required to move an ecosystem out of a basin of attraction or stable state. May (1973) presented a precise definition of stability and a crater-and-ball analogy to illustrate the concep ts. Later, May (1977) focused attention on the existence of alternative stable states and multiple equilibrium points with an emphasis on the thresholds between them. Holling ( 1973) drew attention to the ability of ecosystems to absorb and respond to disturban ce and introduced the c oncept of resilience. Again, resilience focuses on dynamics far from equilibrium and was used to measure the magnitude of perturbations from which rec overy of a system was no longer possible.

20SAP 4.2: Thresholds of Climat e Change in Ecosystems Although mathematically tractable and well de fined in static e ngineering contexts, in the 1990s “stability” and the implication of “equilibrium” in ecological systems began gradually to give way to grow ing evidence that real ecological systems are neither static nor even well approximated as such. Notions of stable equilibrium, which continue to dominate much of our thinking and research to date are based on models and controlled experiments (for example, on paramecia and fl our beetles) from the middle of the last century where singular static equilibrium was the ideal. Crack s in the equilibrium view began to appear as quantitative evidence mounted from natural systems demonstrating that “change” rather than “constancy” is the rule, and that nonlinear instability, thresholds, and chaos can be ubiquitous in na ture (Dublin et al. 1990; Sugihara and May, 1990; Tilman and Wedin, 1991; Grenfell, 1992; Knowlton, 1992; Hanski et al., 1993; and Sugihara 1994). The possibility that so- called “pathological” nonequilibrium, nonlinear behaviors seen in theoretical treatments coul d be the rule in nature as opposed to a mathematical curiosity, opened the door fo r credible studies of thresholds. Indeed, threshold changes now appear to be everywhere. Recognition and documentation of sudden, not readily reversible changes in ecosy stem structure and function have become a major research focus during the past 10 to 20 years (Scheffer et al. 2001; Scheffer and Carpenter, 2003). One of the important drivers of current interest in nonlinear ecosystem behavior and, in particular, threshold effects has been the recognition of the importance of unanticipated effects of clim ate change (Scholze et al . 2006). Although much climate change research has focused on the direct e ffects of long-term changes in climate on the structure and function of ecosystems, there ha s been increasing recognition that the most

21SAP 4.2: Thresholds of Climat e Change in Ecosystems dramatic consequences of climate change ma y occur as a result of indirect effects, including threshold change s (Vitousek, 1994; Carpenter, 2002; Schneider, 2004; Hobbs et al. 2006). 

### 2.3 Current Discussions of Threshold Phenomena As ecologists were exploring the existence of alternative stable states in ecosystems, oceanographers were documenting th e impacts of major cl imatic events on the North Atlantic Ocean (Steele and He nderson, 1984), North Pacific Ocean, and Bering Sea ecosystems. They eventually used the te rm “regime shift” to describe the sudden shifts in biota that are driven by ocean c limate events (Steele, 1996; Hare and Mantua, 2000). More recently, for the California Current Ecosystem (CCE), regime shifts in the biota have been distinguished from random ex cursions in the ocean climate based on the nonlinear signature of the time series (Hsieh et al. 2006). Th e main idea here is that regimes represent different rules governi ng local dynamics (that is, they depend on environmental context), and that inherent positive feedbacks drive the system across thresholds into different dynamic domains. T hus, regime shifts in marine ecosystems are an amplified biological response to ocean climate variation (mainly temperature variation) rather than a simple tracking of environmental variation (Anderson et al. 2008). On the other hand, ocean climate for the CCE in the 20 th century did not have this nonlinear signature because the dynamic rules were the same in both warm and cold periods. Hsieh et al. (2006) and Anderson et al. (2008) suggest nonlinear forecasting methods as a rigorous way to detect thresholds because of the circul arities of statistical methods. Current interest in regime shifts a nd thresholds in marine science has focused

22SAP 4.2: Thresholds of Climat e Change in Ecosystems on understanding the factors that determine thresholds and on ways of extracting dynamics from observational data to make predictions. Muradian (2001) and Walkers and Meyers ( 2004) used a definition of regime shift developed by Scheffer and Carpenter (2003) emphasizing changes in the threshold level of a controlling variable in a system, such thatthe nature and extent of feedbacks change and result in a change in the system itself. Scheffer and Carpenter (2003) built on work in shallow lakes to demonstrate empirically the concept of threshold-like change and used these examples to further reinforce the idea that ecosystems are never stable but are dynamic and that fluctuations (in populations, environmenta l conditions, or ecosystems) are more the rule than not. Given the move in thinking among many ecologists toward nonequilibrium and unstable dynamics, the broader technical concept that may eventually replace “equilibrium” in this context is a more gene ral notion concept that includes equilibrium, stable limit cycles, and nonequilibrium dyna mics or chaos (Sugihara and May, 1990; Hsieh et al. 2006). Depending on wh ether the control variable is thought of as part of the system (an intrinsic variable) or as extern al to the system (an extrinsic variable), threshold behavior may be thought of as a ri dge of instability that separates control variables. From a more descriptive point of view, the idea suggests that there are particular states or characteristic combina tions of species (grasslands, chaparral, oakhickory forests, and so forth) that make up the biological component, and that ecosystem thresholds can be identified in the physical pa rt of the system. Part of the nonlinearity or nonequilibrium nature of ecosystems comes from the fact that the biology (especially the

23SAP 4.2: Thresholds of Climat e Change in Ecosystems dynamics) of the system is contingent on its own particular state (s uite and abundance of species), as well as on the physical context in which it resides. The field of range science has a parallel and largely indepe ndent literature on thresholds, resilience, regime shifts, and alte rnative stable states that has engendered a lively debate over how these terms are used in that field. Bestelme yer (2006) argued that there is a lack of clarity in the use of the te rm “threshold” and its ap plication to state-andtransition models that are used in rang e management. State-an d-transition models describe alternative states and the nature of thresholds between states. Bestelmeyer’s argument reflects a broad lack of consensu s or understanding am ong range scientists about how best to define and use the th reshold concept. Watson and others (1996) criticized a focus on the consequences of thre shold shifts at the e xpense of the processes that precede them. Many definitions of threshold phenomena emphasize relatively rapid, discontinuous phenomena (for example, Wissel, 1984, and Denoël and Ficetola, 2007). Others emphasize the points of instability atwhich systems collapse (Radford et al. 2005) or the point at which even small changes in environmental conditions lead to large changes in state variables (Suding et al. 2004) . Still other definitions emphasize changes in controlling variables. According to Walker and Me yers (2004), “a regime shift involving alternative stable states occurs whena threshold level of a controlling variable in a system is passed.” There is clearly a need fo r these concepts to be tested across a wider set of ecosystems and have these experiments conducte d with greater consistency and rigor to better evaluate the veracity of these concepts developed under rangeland conditions to other ecosystems and environmental conditions . One point of consensus underlying both

24SAP 4.2: Thresholds of Climat e Change in Ecosystems the theoretical and empirical approaches to th e topic of thresholds is that changes from one ecological condition to another take pl ace around specific points or boundaries. But further advancement and agreement is limite d by the small number of empirical studies that address this topic. While some belie ve that further advancement will depend on rigorous statistical testing for reliable identi fication of thresholds across different systems (Huggett, 2005), many in fields outside of range science see the danger of circularity in such arguments and suggest dynamic testing for determining threshold behavior (Hsieh et al. 2005). 

### 2.4 Ecological Thresholds Defined for SAP 4.2 Because of the variety of ways that the concept of thresholds has been developed, this assessment (SAP 4.2) uses the following general definition of ecological thresholds: An ecological threshold is the point at wh ich there is an abrupt change in an ecosystem quality, property, or phenomenon or where small changes in an environmental driver produce large, persistent responses in an ecosystem. Fundamental to this definition is the idea that positive feedbacks or nonlinear inst abilities drive the domino-like propagation of change that is po tentially irreversible. In line with this definition, thresh old phenomena are particular nonlinear behaviors that involve a rapi d shift from one ecosystem state (or dynamic regime) to another that is the result of (or that provokes) instabilityin any ecosystem quality, property, or phenomenon. Such instability al ways involves nonlinear amplification (some form of positive feedback) and is often the result of the particular structure of the interactions or the complex web of interactio ns. This definition dis tinguishes thresholds from other biological changes that are simp le responses to exte rnal environmental

25SAP 4.2: Thresholds of Climat e Change in Ecosystems change. Thus, bifurcation cascades (the point at which events take one of two possible directions with important final consequences, making dynamic systems evolve in a nonlinear way with successive disruptions, dive rgences, or breaks from previous trends), nonlinear amplification (Dixon et al. 1999), and the propagation of positive feedback (increasing instabilities) through complex we bs of interactions are all interrelated attributes that fit our general work ing definition of threshold phenomena. “Systemic” risk, or risk that affects the w hole ecosystem rather than just isolated parts of the system, provides a useful anal ogy. Systemic risk corresponds to widespread change in an ecosystem characterized by a break from previous trends in the overall state of the system. Runaway changes are propagated by positive feedbacks (nonlinear instabilities) that are often hidden in the co mplex web of interconnected parts. Recovery may be much slower to achieve than the coll apse, and the changes may be irreversible, in that the original state may not be fully r ecoverable (Chapin et al. 1995). Our concept of threshold transitions include so-called bifurcation cascades where, for example, small changes in a controlling variable , such that the nature and extent of feedback change, leads to a sudden destabi lization of the system. Several examples of threshold crossings or transitions that illustrate this definition are described in Groffman et al., 2006. Thes e include the interac tions of drought and overgrazing that trigger runawa y desertification, and the exceeding of some critical load, as with the toxicity limit of a contaminan t or elimination of a keystone species by grazing, so that when one component of the system fails, it provokes a domino-like cascade of instability that substantially alters the rest of the system. Other examples are discussed in more detail in the case studies presented in Chapter 3.

26SAP 4.2: Thresholds of Climat e Change in Ecosystems


### 2.5 Factors That Influence Resilience At a general level, systems can be viewed as consisting of mixtures of positive and negative feedbacks, with positive feedbacks tendi ng to alter the natu re of the system, and negative feedbacks tending to minimize these changes (Chapin et al. 1996). Changes that strengthen positive feedbacks (for ex ample, the invasion and spread of highly flammable grass in a desert) can lead to a ch ange in conditions (for example, the fire regime) that may exceed the tolerance of othe r components of the system. This, in turn, leads to destabilization and threshold change s. Threshold crossings occur when positive feedbacks amplify changes in system charact eristics in ways that exceed the buffering capacity of negative feedbacks that tend to main tain the system in its current state or the current limits of the control variables. Viewed from an adaptive management perspective, threshold crossings occur when changes in the system exceed the adaptive capacity of the system to adjust to change (Groffman et al . 2006). Because systems have adapted to the natural variability experienced in the past, anyt hing that disrupts that variability can make them vulnerable to further change and amplif ied instability (Walker et al. 2006; Folke, 2006). The following is a partial list of factors that are believed to come into play in determining a system’s resilience, and sensitiv ity to threshold behavi or (see also May and McLean, 2007):
1. A higher diversity of very weakly conn ected and substitutable components are thought to enhance resilience. Such ar guments were made in the classic stability complexity debate [see reviews by Pimm (1984) and McCann (2000)].

27SAP 4.2: Thresholds of Climat e Change in Ecosystems

2. Compartmentalization of interactions into guilds is a way to make model ecosystems more resilient to syst emic events (May et al. 2008). Compartmentalization acts as a fire-br eak that prevents the spread of a system’s collapse.
3. A predominance of weak linkages in a system with a few strong linkages leads to relatively low connectance (McCann, 2000; May et al. 2008) and is thought to increase resilience. Real ec ological systems are thought to have a lognormal distribution of interaction streng ths, which has been associated with increased resilience (Sala and Graham, 2002).
4. Ecosystems are resilient by virtue of th eir existence. They are the selected survivors of billions of years of upheaval and perturbation (continental drift, meteor extinctions, and so forth), and show some remarkable constancy in structure that persists for hundreds of millions of years (for example, the constancy of predator-to-prey ratio s). As such, enumerating the common attributes of these diverse naturally selected surviving systems, including those that change without experiencing thresholds, could be of interest to understanding thresholds.
5. Higher measured nonlinearity (greater instability) in the dynamics that provoke an increase in boom and bust population variability (Anderson et al. 2008) is directly associated with regime shifts. This is true in exploited marine fish populations, which show greater swings in abundance than their unexploited counterparts from the same environment. Exploited species show an amplified response to regime shifts , with greater extremes in abundance.

28SAP 4.2: Thresholds of Climat e Change in Ecosystems

6. In line with the so-called “para dox of enrichment” (Rosensweig, 1971), fertilizing a system to increase gr owth rates and carrying capacity can differentially advantage some species a nd provoke a rapid loss of species to a much simpler state.
7. Increasing time lags involved in population regulatoryresponses can destabilize systems (May 1977), and th is effect becomes more pronounced with higher growth rates. This is an alogous to a large furnace (rapid growth) with a poor thermostat (regulator y delay), which tends to produce undershooting and overshooting of temperature in a way that predisposes the system to large-scale failure.
8. Reductions in variance, as might occur when managing systems for a stable flow of one particular good or service, tends to favor those species and components that are typical of this set of conditions at the expense of species that function more effectively under other conditions. Consequently the system as a whole remains stable under a narrower range of conditions. 

### 2.6 The Bottom Line To manage risks associated with ecological thresholds, it is essential to be able to forecast such events and to plan for and study alternative management scenarios. Because of the multiscale nature of thresholds, better integration of existing monitoring information from the local to the largest pos sible spatial scales will be required to monitor and identify ecosystems that are appr oaching and undergoing critical transitions. Field research that focuses on ecosystems unde rgoing a threshold shif t can help clarify the underlying processes at work. The rapid fo rest dieback in the southwestern United

29SAP 4.2: Thresholds of Climat e Change in Ecosystems States, described in detail in the next chapter, is an example of a threshold shift for which field research identified the trigger (sudden tree mortality) that caused multiple other ecosystems changes. And natural resource managers will very likely have to adjust their goals for the desired states of resources aw ay from historic benchmarks that may no longer be achievable in a nonequilibrium wo rld that is continually changing and now being altered by climate change (Julius et al. 2008). Such changes in methods and outlook as the following may be required:

• Abandon classic management models th at assume a constant world in equilibrium (for example, maximum sustained yield models).

• Acknowledge in our management strategies and in our models that ecosystems are nonlinear, interd ependent, and nonequilibrium systems.

• Use near-term forecasting tools, statistical and otherwise, that are appropriate to this class of system (for example, nonlinear time series prediction coupled with scenario models).

• Continue to identify the characteri stics of systems that make them more or less vulnerable.

• Continue to identify early warn ing signals of impending threshold changes (and to monitor for those signals).

• Survey the major biomes to iden tify which systems might be most vulnerable to current climatic trends.

• Employ adaptive management strategies, such as skillful short-term forecasting methods coupled with scen ario exploration models that are

30SAP 4.2: Thresholds of Climat e Change in Ecosystems capable of dealing with new successional scenarios and novel combinations of species.

31SAP 4.2: Thresholds of Climat e Change in Ecosystems 

## Chapter 3—Case Studies Thresholds of ecological change can occu r at many spatio-temporal scales and in a diversity of ecosystems. The following exam ples were chosen to illustrate that thresholds probably have already been cro ssed in ecosystems in response to climate change and that the crossing of these thre sholds will likely have implications at continental and global scales. Because these changes will likely impact American society significantly, these examples make clear the us efulness of consideri ng thresholds in the monitoring and management of natural resources. Four case studies are presented below in detail. They cover distinctly different types of ecosystems, all of which are poten tially undergoing threshold-type changes. These studies are arranged in order of latitude, beginning with the highest. The first study is at a latitude in the far north where climat e change has resulted in large temperature changes. The next study is of the mid-latitu de Prairie Pothole Region where continental drying is expected because th e subtropical high-pressure zone is broadening. The third case study is of forests of the West and Southw est, which are at slightly lower latitude, are generally already water-limited, and w ill be sensitive to the decreased water availability that will profoundly impact the we stern half of the United States. Finally, in the lowest latitude example, the effects of c limate change in forcing threshold changes in coral reef ecosystems are examined. 

### Case Study 1: Ecological Thresholds in Alaska In recent decades, Alaska has warmed at more than twice the rate of the rest of the United States. The statewide annual average te mperature has increased by 3.4°F since the mid-20 th century, and the increase is much greater in winter ( 6.3°F). A substantial portion

32SAP 4.2: Thresholds of Climat e Change in Ecosystems of the increase occurred during the regime sh ift of the Pacific Decadal Oscillation in 1976–1977. The higher temperatures of recent de cades have been associated with changes in the physical environment, such as earlier snowmelt in the spring (Dye, 2002; Stone et al., 2002; Dye and Tucker, 2003; Eu skirchen et al., 2006, 2007), a reduction of sea-ice coverage (Stroeve et al., 2005), a re treat of many glaciers (Hinzman et al., 2005), and a warming of permafrost (Osterkamp, 2007) . In parallel with th ese changes in the physical environment, substantial changes in ecological systems have been observed, including major increases in the frequency of large-fire years in interior Alaska (Kasischke et al., 2002), drama tic changes in the wetlands of interior Alaska (Yoshikawa and Hinzman, 2003), vegetation changes in the tundra of northern Alaska (Goetz et al., 2005), and ecological changes that are affecti ng fisheries in the Be ring Sea (Overland and Stabeno, 2004; Mueter and Litzow, 2008). Becau se Alaska is experiencing substantial changes in ecological systems, we divide th e Alaska case study into four themes that focus on (1) changes in insect and wildfire regimes, (2) changes in wetlands, (3) vegetation change in northern Alaska, and (4 ) changes in Bering S ea Fisheries. For each of these themes we evaluate the occurrenc e and implications of threshold responses. Ecological Thresholds and Changes in Inse ct and Wildfire Regimes of Interior Alaska .—Analyses of historical insect and fire disturbance in Alaska indicate that the extent and severity of these disturbances ar e intimately associated with longer and drier summers (Juday et al. 2005; Balshi et al. 2008). Between 1970 and 2000, the snow-free season increased by approximately 10 days acr oss Alaska, primarily because of earlier snowmelt in the spring (Euskirchen et al. 2006, 2007). Longer summers have the potential to be beneficial to the growth of plan ts; however, th e satellite record suggests

33SAP 4.2: Thresholds of Climat e Change in Ecosystems that the response of plan t growth to warming differs in diffe rent regions of the State, with aboveground vegetation growth increasing in the tundra of northern Alaska and decreasing in the boreal forest of interior Alaska (Jia et al

. 2003; Goetz et al. 2005). Analysis of forest growth data indicates that the growth of white spruce forests in interior Alaska is declining because of drought stress (Barber et al. 2002), and there is the potential that continued warming could lead to forest dieback ininterior Alaska (Juday et al. 2005). The drought stress that has been expe rienced by trees in Alaska during recent decades makes them particularly vulnerable to attack by insects. During the 1990s, south-central Alaska experienced the largest outbreak of spruce bark beetles in the world (Juday et al. 2005). This outbreak was associated with a threshold response to milder winters and warmer temperatures that increased the overwinter survival of the spruce bark beetle and allowed the bark beetle to complete its life cycle in 1 year instead of the normal 2 years. This was superimposed on 9 years of drought stress between 1989 and 1997, which re sulted in spruce trees that were too stressed to resist the infestat ion. The forests of interior Al aska are now threatened by an outbreak of spruce budworms, which generally erupt after hot, dry summers (Fleming and Volney 1995). The spruce budworm has been a major insect pest in Canadian forests, where it has erupted approximately every 30 years (Kurz and Apps 1999), but was not able to reproduce in interior Alaska before 1990 (Juday et al. 2005). Areas that experience the death of trees over large areas of forest are vulnerable to wildfire as the dead trees are highly flammable. This is of particular concern in interior Alaska where the frequency of large-fire years has been increasing in recent decades.

34SAP 4.2: Thresholds of Climat e Change in Ecosystems The area burned in the North American boreal region has tripled from the 1960s to the 1990s owing to the increased freque ncy of large-fire ye ars (Kasischke and Turetsky 2006). For example, two of the three most extensive wildfire seasons in Alaska’s 56-year record occurred in 2004 and 2005, and half of the y ears with the largest fires during this 50-year time period have been since 1990 (Kasischke et al. 2002, 2006; Kasischke and Turetsky 2006). The increase in fi re frequency in Alaska appears to be primarily associated with the shift in the Paci fic Decadal Oscillation that occurred in the late 1970s, as large-fire years occurred once every 6 years before the shift and increased to once every 3 years after the shift (Kasischke et al. 2002). Analyses of fire probability in interior Alaska indicate th at fire probability increases as a step function when the mean temperature in June increases above 14°C or when the August mean precipitation decreases below 40 millimeters (mm). Because the mean June temperature has been increasing in interior Alaska during the la st several decades, th e crossing of these thresholds will likely lead to substantial increases in area burned in interior Alaska, and there is the potential that the large-fire years of 2004 and 2005 in Alaska may occur several times a decade instead of once or twice every 50 years. Analyses of the response of fire to scenar ios of future climate change indicate that the average area burned per year in Al aska will double by the middle of the 21 st century for scenarios of both moderate and high rates of fossil fuel burning (Balshi et al. 2008). By the end of the 21st century, fire is projec ted to triple in Alaska for a scenario of moderate rates of increase in fossil fuel burning and to quadruple for scenarios of high rates of increase in fossil fuel burning. Such increases have the potential to release large stocks of carbon stored in Alaska soils to the atmosphere, which would be a positive

35SAP 4.2: Thresholds of Climat e Change in Ecosystems feedback to climate warming (Balshi et al . 2008). The projected increase in the burned area also increases the fire risk to rura l indigenous communities, reduces subsistence opportunities, and has implications for fire policy (Chapin et al. 2008). Ecological Thresholds and Changes in Wetlands of Interior Alaska .—There has been a documented decrease in the ar ea of closed-basin lakes (that is , lakes without stream inputs and outputs) duri ng the latter half of the 20 th century in the southern twothirds of Alaska (Klein et al

. 2005; Riordan et al. 2006). Th e decrease in lake area appears to be caused by greater evaporation associated with longer and drier summers and by sudden drainage associated with th awing of permafrost in areas where the temperature of permafrost is close to melting. A decrease in the area of closed-basin lakes has also been documented in Siberia in areas of “warm” permafrost (Smith et al. 2005). Discontinuous permafrost in Alaska is warming and thawing, and extensive areas of thermokarst terrain (marked subsidence ofthe surface resulting from thawing of icerich permafrost) are now developing as a re sult of climatic change. Estimates of the magnitude of the warming at the discontinuous permafrost surface are 0.5° to 1.5°C (Osterkamp and Romanovsky 1999). Thermokarst is developing in the boreal forests of Alaska where ice-rich discon tinuous permafrost is thawing. Thaw subsidence at the thermokarst sites is typically 1 to 2 meters (m) with some sites experiencing subsidence of up to 6 m (Osterkamp et al. 1997). Much of the discontinuous permafrost in Alaska is warm and is highly susceptible to thermal degradation if regional warming continues. Warming of permafrost may be causing a sign ificant loss of open water across Alaska as thawing of permafrost connects closed wate rsheds with groundwater (Yoshikawa and Hinzman 2003).

36SAP 4.2: Thresholds of Climat e Change in Ecosystems Examination of satellite imagery indicat es that the loss of water can occur suddenly, which suggests catastrophic drainage associated with thawing of permafrost (Riordan et al. 2006). However, the reducti on of open water bodies may also reflect increased evaporation under a warmer and e ffectively drier climate as the loss of open water has also been observed in permaf rost-free areas (Klein et al. 2005). In wetland complexes underlain by ice-ri ch permafrost in areas of hydrologic upwelling (for example , wetland complexes abutting up against the foothills of large mountain ranges), the thawing of that permafrost may result in wetland expansion as trees die when their roots are regularly flo oded, causing wet sedge meadows, bogs, and thermokarst ponds and lakes to replace forests (Osterkamp et al. 2000). The Tanana flats, which extends nearly 70 miles from the northern foothills of the Alaska Range to Fairbanks, Alaska, is underlain by ice-rich permafrost that is thawing rapidly and causing birch forests to be converted to minerotrophic floating mat fens (Jorgenson et al. 2001). It is estimated that 84 percent of a 260,000-hect are (ha) (642,000-acre) area of the Tanana flats was underlain by permafrost a centuryor more ago. About one-half of this permafrost has partially or totally degrad ed. These new ecosystems favor aquatic birds and mammals, whereas the previous forest ecosystems favored land-based birds and mammals. During the past 50 years, it appears that wa rming has generally resulted in the loss of open water in closed-basin lakes inwetland complexes located in areas of discontinuous permafrost in th e southern two-thirds of Alas ka (Riordan et al. 2006). The Tanana flats near Fairbanks is the only area where an increase in water area has been documented (Jorgenson et al. 2001), and clos ed-basin lakes in the tundra region of

37SAP 4.2: Thresholds of Climat e Change in Ecosystems northern Alaska have shown no changes in ar ea during the past 50 years (Riordan et al. 2006). The loss of area of closed-basin lakes in interior Alaska may be indicative of a lowering of the water table that has the potential to convert wetland ecosystems in interior Alaska into upland vegetation. A substantial loss of wetlands in Alaska has profound consequences for management of na tural resources on nati onal wildlife refuges in Alaska, which cover about 3.1 million hectar es (more than 77 million acres) and make up 81 percent of the National Wildlife Ref uge System. These refuges provide breeding habitat for millions of waterfowl and shorebirds that winter in more southerly regions of North America. Reduction of habitat area w ould present a substa ntial challenge for waterfowl management across the National Wild life Refuge System (Julius et al. 2008). Wetland areas have also been traditionally im portant in the subsis tence lifestyles of native peoples in interior Alaska, as many villages are located adjacent to wetland complexes that support an abunda nce of wildlife subsistence resources. Thus, the loss of wetland area has the potential to affect the su stainability of subsis tence lifestyles of indigenous peoples in interior Alaska. Ecological Thresholds and Vegeta tion Changes in Northern Alaska .—Shrub cover in northern Alaska has increased by about 16 per cent since 1950 (Sturm et al. 2001; Tape et al. 2006), and the tree line in Alaska is expa nding in most places (Lloyd and Fastie 2003; Lloyd, in press). This is consistent with satellite observations, which show an approximately 16 percent increase per decad e in the normalized difference vegetation index (NDVI) (Jia et al. 2003; Goetz et al. 2005). The increased growth of vegetation at or above the tree line appears to be a res ponse to longer and warmer growing seasons. Tundra vegetation in northern Alaska may not be experiencing drought stress to the

38SAP 4.2: Thresholds of Climat e Change in Ecosystems extent experienced by forests in interior Alaska because the surface water in tundra regions is not able to drain away thro ugh the ice-rich continuous permafrost. Experimental studies demonstr ate that arctic summer warm ing of 1°C increases shrub growth within a decade (Arft et al. 1999). Sa tellite analyses of relationships between NDVI and summer warming (Jia et al. 2003) suggest that the response of tundra vegetation is linearly related to summer warmth. Thus, it appears that the response of tundra vegetation to warming is not a threshold response. While growth of shrubs and trees may not be threshold responses to warming, the changing snow cover and vegetation in northern Alaska have the potential to result in sudden changes in the absorpti on of heat from incoming sola r radiation and the transfer of that heat to warm the atmosphere. Fo r example, the advance in snowmelt reduces spring albedo, causing the ecosy stem to absorb more heatand transfer it to the atmosphere. The snowmelt-induced increase in heating in northern Al aska has been about 3.3 watts per square meter (W m -2 ) averaged over the summer, similar in magnitude to the 4.4 W m -2 caused by a doubling of atmospheric CO 2 over several decades (Chapin et al. 2005). Thus, gradual warming has caused a rapid advance in the snowmelt date and a very large increase in loca l heating. Although vegetation ch anges to date have had minimal effects on atmospheric heating, convers ion to shrubland would increase summer heating by 8.9 W m -2 , with even larger changes tr iggered by conversion to forest. Warming experiments that increase shrubs al so reduce the abundance of lichens, an important winter food of cari bou (Cornelissen et al. 2001). Mo st arctic caribou herds are currently declining in po pulation, although the reasons are uncertain. In summary, positive feedback associated with earlier snowmelt and shrub expansion is amplifying

39SAP 4.2: Thresholds of Climat e Change in Ecosystems arctic warming and may be altering food-we b dynamics in ways that have important cultural and nutritional implications for northern indigenous people. Ecological Thresholds and Fi sheries of the Bering Sea .—Alaska leads the United States in the value of its commercial fishi ng catch, and most of th e Nation’s salmon, crab, and herring come from Alaska, and specifica lly from the Bering Sea. The Bering Sea is one of the most productive marine ecosystems in the world, supporting some of the largest oceanic populations of fish, seabirds , and marine mammals anywhere (Loughlin et al. 1999). The Bering Sea provides 47 percent of total U.S. fishery production by mass, including the largest single species fishery in the United States, walleye pollock ( Theragra chalcogramma) (Criddle et al . 1998). It is also animportant source of subsistence resources (such as, fish, marine mammals, and seabirds) for more than 30 Alaska Native communities and supports 95 percent of the worl dwide population of northern fur seals, 80 percent of the total num ber of seabirds that breed in the United States, and major populations of tens of thous ands of Pacific walrus, Steller sea lion, and several species of great wh ales. This production is fueled by nutrients annually replenished from slope and oceanic waters across the very broad (more-than-500- kilometer-wide) continental shelf (Stabeno et al. 2001, 2006). Changes in fisheries of the Bering Sea occurred during and after the transition from cool to warm conditions in 1976–1977, in association with a regime shift in the Pacific Decadal Oscillation, and were followe d by historically high commercial catches of salmon and pollock, as well as a shift away from crab dominance on the ocean floor (Overland and Stabeno 2004). In the pa st decade, geographic displacement of marine mammal populations to the north has been documented in the Bering Sea region (Moore

40SAP 4.2: Thresholds of Climat e Change in Ecosystems et al. 2003). The displacement s of walrus and seal populati ons are already apparent to coastal communities. The northward displacements of fauna in the Bering Sea has coincided with a reduction of benthic (organisms that live on or near the ocean floor) prey populations, an increase and nor thward shift in pelagic (those of the open seas and oceans) fishes, an increase in air and ocean temperatures, and a reduction in sea ice (Stroeve et al. 2005; Grebmeie r et al. 2006). Ultimately, popul ations of fish, seabirds, seals, walruses, and other species depend on water temperatures and plankton blooms that are regulated by the thickness, extent, and lo cation of the ice edge in spring (Hunt and Stabeno 2002). As the sea ice continues to retreat, the location, timing, and species makeup of the blooms is changing, subarctic pe lagic food webs are replacing arctic ones, and the amount of food reaching the living th ings on the ocean floor, the benthos, is declining dramatically. This in turn radically changes th e species makeup and populations of fish and other marine life forms, with significant repercussions for fisheries (Anderson and Piatt 1999; Litzow et al. 2008; Hatfield et al. 2008; Julius et al . 2008). Reductions in sea-ice cover also result in reduced albedo (reflectance of solar ra diation), greater sea surface temperatures, and accelerated sea-ice re treat, a positive feedb ack loop that is at least partly responsible for the unexpected and record-setting extent of open water in the Arctic Ocean in recent years. Thus, changes in sea ice are the major driver of concern with respect to threshold changes in fish eries of the Bering Sea (Mueter and Litzow 2008). Seasonal sea-ice extent currently divide s the Bering Sea eastern shelf into two biogeographic provinces, which differ in production pathways. In the subarctic biogeographic province (south of the average annual maximum extent of the sea ice),

41SAP 4.2: Thresholds of Climat e Change in Ecosystems most primary production remains within th e pelagic ecosystem, and pollock is the dominant tertiary consumer (Macklin and Hunt 2004). In contrast, in the arctic biogeographic province, tight coupling betw een pelagic primary production and the benthos benefits benthic foragers, such as gray whales, walrus, and diving ducks (Lovvorn et al. 2003; Grebmeier et al . 2006). The boundary between the two biogeographic provinces varies in location on longer time scal es (decadal or longer) and is expected to move northward as the regi on becomes warmer. The av erage southern edge of the maximum ice extent currently lies nort h of the Pribilof Islands (Byrd et al. 2008). The Bering Sea ecosystem, however, is in a state of rapid flux due to climate change. Present data and climate projections from atmosphere-ocean models predict major loss of sea ice during the next few decades (Overland and Stabeno 2004; Holland et al. 2006); the Bering Sea is particularly sensitive to global warming because of the seasonal nature of sea-ice c over (Grebmeier et al. 2006). R ecent relative temperature extremes (above 2°C) in Alaska and adjacent waters represent the largest recent change on the planet (Hansen et al. 2006). However, these models and empirical data also demonstrate large natural variability. Ecosystems will likely be affected by how the path of such warming occurs—that is, whether th ere will be a continued slow warming trend with little interannual variability (in which case crossing of ecologica l thresholds is less likely) versus a warming trend that incorporates wide swings in temp erature and extent of sea ice (enhancing the likelihood of threshold crossings). Climatic and oceanographic conditions in the Bering Sea during 2007–2008 were unexpectedly cold, supporting the latter scenario.

42SAP 4.2: Thresholds of Climat e Change in Ecosystems Warming of the Bering Sea is altering the geographic distributions and behaviors of humans, marine mammals, seabirds, and fi sh by restructuring their habitats and food webs (Grebmeier et al. 2006; Mueter and Litzow 2008). As a result of warming, changes in the time and place of food production lead to dominance of top-down control processes in the pelagic marine environment and the decline of benthic production. Under a longterm warming scenario with early ice retreat, bottom-up control mechanisms (temperature, sea-ice extent and duration, ocean currents, and nutrient fluxes) set the stage for the emergence and dominance of t op-down control processes in the pelagic marine environment and the decline of be nthic production (Muete r and Litzow 2008), a threshold change akin to that was docume nted after the 1976–1977 regime shift in the Pacific Decadal Oscillation. Increased heat content would increase the combined populations of the subarctic piscivores —arrowtooth flounder, pollock, and cod—in proportion to expanded breedi ng grounds and increased av ailability of food during critical developmental stages (Hunt and Stabeno 2002). Because arrowtooth flounder is not targeted by fishing, it is likely to become the dominant component of the biomass of the three subarctic piscivores in this system and is predicted to be one of the principal agents of top-down control in the Bering S ea, as predator and competitor of the nowdominant, but commercially exploited, pollo ck and cod. Such a rapid and dramatic restructuring of subarctic marine co mmunities is not unprecedented; the 1976–1977 regime shift in the Pacific Decadal Osci llation resulted in threshold community reorganization in the Gulf of Alaska (Anderson and Piatt 1999). Arrowtooth flounder is also an agent of cha nge as a direct and indirect competitor of fur seals, murres, kittiwakes, and other t op trophic-level piscivor es for their respective

43SAP 4.2: Thresholds of Climat e Change in Ecosystems forage species (juvenile pollock, capelin, sand lance, herring, and myctophids). Populations of fur seals, murres, and kittiw akes could decline or increase in the near term, depending on the locality of rookeries and nesting colonies, but long-term overall trends would be downward under warming. Fu r seals, murres, and kittiwakes would further decline owing to competition from humpback and fin whales, with fur seal declines being further accelerated by increa sing killer whale pred ation. Dislocation of feeding hot spots would likely di sadvantage breeding fur seals, murres, and kittiwakes as central place foragers, but would work to the advantage of humpback and fin whales, further exacerbating direct and indirect comp etition between these two groups of species. Dislocations and declines in fur seals, kitt iwakes, murres, pollock, and cod would stress human communities by increasi ng the costs of maintaining a livelihood and obtaining food and by necessitating changes in the types of food taken and the means of harvest. Both commercial fishers based in Dutch harb or and subsistence fishers based in over 30 Native Alaskan communities on the shores of the Bering Sea are facing greater commuting distances and higher risks to explo it fisheries resources that were formerly close to home. The northern Bering Sea, in particular, is experiencing a rapid shift in the structure and function of the fo rmerly arctic community to conditions typical of marine ecosystems of the subarctic (Hunt et al. 2002; Grebmeier et al. 2006). The earlier sea-ice retreat results in a later, warm-water spring phytoplankton bloom, increased grazing by zooplankton, and greater pelagic secondary pr oductivity (Hunt et al . 2002). Concurrently, benthic productivity is decrea sing (Grebmeier et al. 2006). The formerly ice-dominated, shallow marine ecosystem that favored highly productive benthic communities also

44SAP 4.2: Thresholds of Climat e Change in Ecosystems supported high densities of upper trophic-level bottom-feeders, such as Pacific walruses, gray whales, and seaducks, including th e Endangered Species Act (ESA)-listed spectacled eider. The northward flowing Anadyr Current, wh ich originates in the southern Bering Sea, transports nutrient-rich water far onto the Bering Shelf and the northern Bering Sea. This largely wind-forced transp ort creates highly pr oductive shelf waters in the area north of St. Lawrence Island and south of the Bering Strait, known as the Chirikov Basin (Springer et al. 1989; Piatt and Spri nger 2003). Oceanic copepods, such as Neocalanus
cristatus and N. flemingeri, transported by the Anadyr Current, along with the large euphausiid Thysanoessa raschii provide abundant prey for planktivores foraging near St. Lawrence Island (Piatt et al. 1988). The Anadyr Current is highly variable on a seasonal and annual basis, usually reaching its greates t velocity during July (about 1.3 Sv, or 13 million cubic meters per second) (Roach et al. 1995). Consequently, the primary productivity on the Bering Shelf during summer months varies withthe strength of northward flow associated with the Anadyr Current (Springer et al. 1989; Russell et al. 1999). When the Anadyr Current is weaker, planktivores presumably rely more on zooplankton associated with northern Beri ng Shelf waters, such as the small copepod Calanus marshallae and the large amphipod Themisto libellula (Coyle, Chavtur, and Pinchuk 1996; Russell et al. 1999). Neocalanus copepods are larger and have higher energy content per prey item than the small, neritic copepod C. marshallae, which is characteristic of Bering Shelf water. The lipid content of Neocalanus copepods is also probably higher (Obst et al. 1995), making thes e oceanic species more energy-dense than

45SAP 4.2: Thresholds of Climat e Change in Ecosystems their shelf domain counterparts. When preferred Neocalanus copepods are not available, planktivores must switch to other prey type s. The progressively earlier transition from winter to spring in the Bering Sea, changes in prevailing we ather patterns and associated wind forcing, and the resulting changes in primary and secondary productivity are expected to have large impact s on upper trophic-level consumers that rely on the Anadyr Current (Stabeno and Overland 2001; Grebmeier et al. 2006). Projected warming of the Bering Sea is also expected to profoundly alter the structure of the southeastern Bering Sea eco system by changing pathways and fluxes of energy flow, as well as the abundance, spat ial distribution, and species composition of fish, seabirds, and marine mammals, ther eby affecting commercial and subsistence fisheries that support local, regional, a nd national economies (Hunt and Stabeno 2002; Grebmeier et al. 2006; Mueter and Litzow 2008). Climate-induced changes in physical forcing of the Bering Sea modify the partitioni ng of food resources at all trophic levels on the continental shelf through bottom-up processes. The emergent properties of this formerly seasonal sea-ice-dominated mari ne ecosystem under warming are still the subject of intense scientific inquiry, but th e weight of evidence s uggests that the Bering Sea ecosystem has reached a threshold of major ecosystem change and community restructuring. 

### Case Study 2. The Mid-Continent Pr airie Pothole Region: Threshold Responses to Climate Change The Prairie Pothole Region of north-centr al North America is one of the most ecologically valuable freshwater resources of the Nation (van der Valk, 1989). It contains 5 million to 8 million wetlands, which supply cr itical habitat for continental waterfowl

46SAP 4.2: Thresholds of Climat e Change in Ecosystems populations and provide numerous valuable ecosystem services for the region and Nation. The weather extremes associated with this re gion are particularly important for the longterm productivity of waterfowl dependent on these wetlands. The Prairie Pothole Region (fig. 3.1) exhi bits a variable climate, ranging from severe droughts, exemplified by the 1930s wh en agriculture was de vastated, grassland communities shifted eastward, and trees died by the millions (Albertson and Weaver, 1942, 1945; Woodhouse and Overpeck, 1998; Rosenzweig and Hillel, 1993), to periods of deluge, such as occurred in the late 1900s when closed-basin lakes flooded, causing high mortality of shoreline trees and consid erable economic damage to farmland, roads, and towns (Winter and Rosenberry, 1998; Johnson et al. 2005; Shapley et al. 2005). The
20 th -century climate of the Prairie Pothole Region was punctuated by significant droughts. These conditions have occurred over sm all and large areas and lasted as short as several growing seasons to as long as a decade (Skaggs, 1975; Laird and Cumming, 1998; Nkemdirim and Weber, 1999).

47SAP 4.2: Thresholds of Climat e Change in Ecosystems

Figure 3.1. Location of the Prairie Pot hole Region (Prairie Pothole Region) of North America (red highlighted area). (Boldsethet et al. 2007) Wetlands in the Prairie Pothole Region are likely to be strongly affected by gradual changes in climate (Poiani and Johnson, 1991; Covich et et al. 1997). Climate drives surface processes, such as the hydr ologic cycle, and hydrology is the most important factor that controls key wetla nd processes and services (Winter and Woo, 1990). A warmer and drier climate, as indicated by general circulation models for the northern Great Plains (Ojima and Lackett, 2002), could affect the wetland hydroperiod, the ratio of emergent plant cover to op en water, the species composition, wetland permanence, and primary and secondary pr oductivity, among others (van der Valk, 1989). Winter (2000) predicted that the su rface area of seasonal and semipermanent wetlands in the Prairie Pothole Regi on would be reduced by increases in evapotranspiration and reduced summer soil moisture. With increased temperatures,

48SAP 4.2: Thresholds of Climat e Change in Ecosystems summer evapotranspiration would put increas ing demands on groundwat er, resulting in earlier drying of wetlands. Thus, additiona l climate variability of the magnitude suggested by global climate change mode ls would profoundly affect wetland water budgets and the many processes and attr ibutes linked to these wetlands. Changing climate can have direct effect s on the trajectories of these wetland ecosystems and their sustainability. Shifts in climate in this region over decadal time scales could result in longer or more frequent drought periods and may lead to threshold responses by the wetland systems. The intera ction of extrinsic and intrinsic processes reflected in such hydrologically, geologica lly, and biologically linked systems as wetlands and their surrounding watersheds coul d result in rapid nonlinear changes at broad spatial scales that are triggered by small differences in temperature and precipitation if threshold values are exceeded that may also resu lt in these systems exhibiting hysteresis. The first quantitative assessments of th e possible effects of climate change on Prairie Pothole Region wetlands used the WETSIM (WETland SIMulator), which is a rule-based, spatially explicit simulation model that is composed of hydrology and vegetation submodels (Poiani and Johns on, 1991, 1993a, b; Poiani et al. 1995, 1996). Simulations using this model and general circul ation model climate forc ings indicate that semipermanent wetlands would lose their historic highly dynamic character by drying up more frequently and becoming chronically choked with emergent cover. Shortened hydroperiods and continuous stands of emer gent cover for semipermanent wetlands across the Prairie Pothole Region would have strong negative effect s on the continental population of water birds (particularly ducks).

49SAP 4.2: Thresholds of Climat e Change in Ecosystems Johnson et al. (2005) used a simulation m odel (WETSIM) to contrast historical and future wetland conditions across the Prai rie Pothole Region of North America (fig. 3.1). They assembled 95-year climate data sets for 18 weather stations across the Prairie Pothole Region as input to a revised versi on of WETSIM (version 3.1), which enabled a much broader geographic assessment to be cond ucted of the effects of past and future climate variability on wetland conditions acro ss the Prairie Pothole Region. Their model runs reflected the high level of spatial and temporal heteroge neity in wetland water levels historically across the Prairi e Pothole Region. They were able to use model output to simulate the number of completions of the wetland cover cycle across the Prairie Pothole Region (fig. 3.2; Weller, 1965).

Figure 3.2. Wetland cover cycle (modified from Weller, 1965). The wetland cover cycle was highly sensitiv e to alternative future climates. The geographic pattern of return times shifted markedly with changes in temperature and precipitation. A 3°C increase in temperature a nd no change in precip itation resulted in a

50SAP 4.2: Thresholds of Climat e Change in Ecosystems greatly diminished area and ge ographic shift eastward for th e region of fastest return times. However, reduced preci pitation and warmer air temp eratures resulted in no complete cover cycle return times across th e Prairie Pothole Region except in a small area of north-central Iowa (fig. 3.3), thus representing a threshol d response to climate change. Such dramatic shifts in wetland c onditions emphasize the sensitivity of Prairie Pothole Region wetlands to climate variability. Using this information, Johnson et al. ( 2005) simulated the occurrence of highly favorable water and cover conditions for breeding waterfowl (fig. 3.4). The most productive habitat for breeding water birds w ould shift under an effectively drier climate from the center of the Prairie Pothole Region (the Dakotas and southeastern Saskatchewan) to the wetter eastern and norther n fringes (in sync with the changes in the cover cycle return results). Continental waterfowl populat ion cycles are largely di ctated by regional wetland conditions, with population declines bei ng commonplace during periods of drought and then rebounding during wetter periods. Under a warmer, drier climate, wetlands would be especially vulnerable even if precipitation were to continue at histor ic levels (Johnson et al. 2005). The geographic shifts in the mostfavorable region for waterfowl breeding resulting from the model simulation runs will likely affect the rate at which the threshold for waterfowl population sustainability will be reached.

51SAP 4.2: Thresholds of Climat e Change in Ecosystems

A B

D C

Figure 3.3. Geographic patterns of the speed of the wetland co ver cycle, simulated for the Prairie Pothole Region under historic (a) and alternative future (b, c, and d) climatic conditions. (Johnson et al. 2005)

Figure 3.4. Simulated occurrence of highly favorable water and cover conditions for waterfowl breeding (occurrence of at least one return time and hemi-mar sh conditions at more than 30 percent frequency) across the Prairie Pothole Region under historic (a) and alternative (b, c, and d) future climatic conditions. (Johnson et al. 2005)

52SAP 4.2: Thresholds of Climat e Change in Ecosystems 

### Case Study 3. Broad-Scale Forest Die-Ba ck as a Threshold Response to Climate Change in the Southwestern United States The ecological dynamics of semiarid forests and woodlands in the southwestern United States are observed to respond strongl y to climate-driven variation in wateravailability, with major pulses of woody pl ant establishment and mortality commonly corresponding to wet and dry periods (Swe tnam and Betancourt, 1998). Although human management of these forests is also a factorin tree mortality, it is clear that climateinduced water stress can trigger rapid, extensiv e, and dramatic forest dieback (Breshears et al. 2005), exemplifying significant ecosystemthreshold responses to climate. Broadscale tree mortality can shift ecotones be tween vegetation types (Allen and Breshears 1998) and alter regional distribut ions of overstory and understo ry vegetation (Gitlin et al. 2006; Rich et al. 2008). Rapid forest dieback also has nonlinear feedbacks at multiple spatial scales with other ecol ogical disturbance processes, su ch as fire and erosion (Allen, 2007), which, in some cases, leads to additi onal nonlinear threshold behaviors. Massive forest mortality is an example of a thres hold phenomenon with substantial implications for future ecosystem dynamics and manage ment of lands undergoing such changes (Millar et al. 2007). Assessments of potential global change impacts in itially focused on how vegetation types matched given climatic e nvelopes (IPCC, 1996). Subsequent research has considered how vegetation patterns might migrate in response to a changing climate with a focus on rates of plant establishment, has documented that forest turnover rates follow global and regional patterns of produc tivity (significantly driven by climate) (Stephenson and van Mantgem, 2005), a nd has increasingly moved toward dynamic

53SAP 4.2: Thresholds of Climat e Change in Ecosystems global vegetation models that try to incor porate more realistic disturbance dynamics (Scholze et al. 2006; Purves and Pacala, 2008) . Currently, climate-induced dieback of woody plants is being recognized as an importa nt vegetation response to climate variation and change, with examples of forest dieb ack emerging from around the world (Allen and Breshears, 2007). [It should also be noted that other recent studies have documented increased tree growth in dry forests, perhaps because of increased water use efficiency (Soule and Knapp 1999).]. Recent research shows that water stress appears to be driving increases in background tree mortality rates in western North American forests (van Mantgem and Stephenson, 2007). In addition, observations of extensive tree die-off— especially from semiarid ecosystems where woody plants are near their physiological limits of water stress tolerance—are bei ng documented globally, for example, in Australia (Fensham and Holman, 1999), Afri ca (Gonzalez, 2001), west Asia (Fisher, 1997), Europe (Dobbertin et al. 2007), South America (Suarez et al. 2004), and North America (Breshears et al. 2005). Climate-induced water stress over extended time periods can exceed the physiological tolerance thresholds of individual plants and directly cause mortality through either 1) cav itation of water columns in the xylem conduits (“hydraulic failure”) or 2) forcing plants to shut down photosynthesis to conserve water, leading to “carbon starvation” (McDowell et al. 2008). Th ese individual-scale threshold responses to climate stress can trigger tree mortality th at propagates to landscape and even regional spatial scales (Allen, 2007), sometimes amplified by biotic agents (like bark beetles) that can successfully attack and re produce in weakened tree popula tions and generate massive insect population outbreaks with positive fee dbacks that greatly in crease broad-scale forest mortality (Kurz et al. 2008).

54SAP 4.2: Thresholds of Climat e Change in Ecosystems Ecotones are areas where vegetation change s in response to climate are expected to be most rapid and prominent (Beckage et al. 2008), as highlighted by a southwestern case study of drought effects on vegetati on during the 1950s (fig. 3.5; Allen and Breshears, 1998). Severe drought across the so uthwestern United Stat es during the 1950s caused ponderosa pine ( Pinus ponderosa ) trees at lower, drier site s to die, resulting in an upslope shift of the ponderosa pine forest and piñon-juniper woodl and ecotone of as much as 2 kilometers (km) in less than 5 y ears, producing a rapid a nd persistent change in dominant vegetation cover. Similarly, within the distributional range for the piñon pine ( Pinus edulis ), many trees at lower or drier sites also died (Swetnam and Betancourt, 1998).

Allen and Breshears 1998, PNAS

Evidence of 1950s dieback: -remnant dead wood -air photos -documents Figure 3.5. Changes in vegetation cover between 1954 and 1963 at Frijolito Mesa, Jemez Mountains, New Mexico, showing the persistent ponderosa pine forest (365 ha), the persistent piñon-juniper woodland (1527 ha), and the ecotone shift zone (486 ha) where forest changed to woodland (from Allen and Breshears, 1998).

55SAP 4.2: Thresholds of Climat e Change in Ecosystems Although tree mortality almost certainly occurred across much of the southwestern United States in response to the 1950s drought (and probably for previous regional-scale droughts as well), few studies exist that allow scientists to test projections about the rapidity and extent of potentia l vegetation die-off re sponses to drought. A recent drought beginning in the late 1990s a nd peaking in the early 2000s affected most of the western United States. This was the most severe drought in the Southwest since the 1950s. Substantial mortality of multiple tr ee species has been observed throughout the Southwest during this 2000s dr ought (fig. 3.6; Gitlin et al. 1996; U.S. Forest Service, 2006; Allen, 2007). For example, mortality of the piñon pine spanned major portions of the species’ range, with substantial die-o ff occurring across at least 1,000,000 ha from 2002 to 2004 (Breshears et al. 2005; U.S. Forest Service, 2006). For both droughts, much of the forest mortality was associated with bark beetle infestat ions, but the underlying cause of dieback appears to be water stress associated with the drought conditions.

56SAP 4.2: Thresholds of Climat e Change in Ecosystems

0 500 1000 1500 2000

1997 1998 1999 2000 2001 2002 2003 YearAcres (1,000) Ponderosa Pine 0 500 1000 1500 2000 2500 3000 3500

1997 1998 1999 2000 2001 2002 200 3 YearAcres (1,000) Pinyon Pine

17.6 32.4 6.1 13.3 38.4 USFS aerial surveys for forest insect and disease activity, cumulative map of affected areas in Southwest for 2000-2003.

Figure 3.6. Graph of the acreage of piñon pine ( Pinus edulis) and ponderosa pine ( Pinus ponderosa) dieback from 1997–2004 in the Four Corners States of Arizona, New Mexico, Colorado, and Utah; map showing cumulative area from 2000 to 2004. Based upon annual aerial forest insect and disease activity inventories by the U.S. Forest Service. The precipitation deficit that triggered the recent regional-scale die-off of the piñon pine across the Southwest was not as seve re (dry) as the prev ious regional drought of the 1950s, but the recent 2000s drought wa s hotter than the 1950s drought by several metrics, including mean, maximum, minimum, and summer (June–July) mean temperature (Breshears et al. 2005). Although historic data from the 1950s is limited, available data suggest that piñon pine morta lity in response to the recent drought has been more extensive, affected great er proportions of more age classes, and occurred at higher elevation and wetter sites than in the 1950s drought. Hence, the warmer temperatures associated with the 2000s drought may have driven greater plan t water stress through increased evapotranspirational demand and resu lted in more extensive tree die-off. Because global change is projected to result in droughts under warmer conditions

57SAP 4.2: Thresholds of Climat e Change in Ecosystems (referred to as “global-change type drought”) the severe piñon pi ne dieback from the recent drought may be a harbinger of vegetation response to future global-change type droughts (Breshears et al. 2005). In addition to the die-off of dominant overstory tree species, high levels of dieback also were observed in other Southwes tern U.S. species and life forms in response to the warm regional drought in the 2000s (Gitlin et al. 2006; Allen, 2007). These include species where bark beetles are unimportant or nonexistent, including one-seed juniper ( Juniperus monosperma )—a co-dominant with piñon pine for much of its range; shrubs such as wavy-leaf oak ( Quercus undulate ) and mountain mahogany ( Cercocarpus

montanus ); and blue grama ( Bouteloua gracilis ), the dominant herbaceous species in many of these woodland systems. In addition to direct climate-induced mort ality, severe protracted drought also can cause substantial reductions in the productivity and soil surface cover of herbaceous plants, which in turn affects numerous ot her ecological processes. In particular, reductions in herbaceous ground cover can trigger a nonlinear increase in soil erosion once a threshold of decreased herbaceous cover has been crossed, through increased connectivity of bare soil patc hes (fig. 3.7; Davenport et al. 1998; Wilcox et al. 2003; Ludwig et al. 2005). On the other hand, dieb ack of woody canopies tends to cause an immediate successional shift to ward greater cover of understo ry vegetation if moisture conditions are adequate (for example, Rich et al. 2008), which propa gates a different set of effects.

58SAP 4.2: Thresholds of Climat e Change in Ecosystems

CLIMATE (drought, temperature);

OVERGRAZING by livestock

High tree density, death of individual trees Patchy standscale mortality Landscape-scale forest mortality FOREST DIEBACK

FIRE

Fine Intermediate Broad S P A T I A L S C A L E Beetle outbreak overwhelms even healthy trees Beetle population grows rapidly, colonizing more trees Physiological tree stress, beetles attack weak and dying trees Within-patch fuel connectivity

Fire ignition and surface fire spread Torching and between-patch fire spread Long-distance fire spread beyond L fuel connections

Explosive crown fire. Self-generated weather

EROSION r Grass cover,  within-patch bare connectivity

 Runoff and erosion, drier microsite, within IC patch  Hillslope runoff/erosion Percolating network of bare IC patches  L-scale connectivity of sediment flux

 Net loss of water and soil from watershed Treefall

Between-patch fuel connectivity

Figure 3.7. Diagram representing interactions across spatial s cales for three different disturbance processes (forest dieback, fire, and erosion) in northern New Mexico landscapes (from Allen 2007). Dashed black arrows represent pattern-process feedbacks within thre e different spatial-scale domains, with one example of pattern and process shown for each domain for each disturbance. Solid black arrows indicate the overarching direct effects of widespread environmental drivers or disturbances (such as climate and overgrazing) on patterns and processes at all scales . Blue arrows indicate the point at which altered feedbacks at finer spatial scales induce changes in fe edbacks at broader scales (for example, fine-scale changes cascade to broader scales), and also where changes at broader scales overwhelm pattern-process relationships at finer scales. Red dashed arrows illustrate some examples of amplifying (positive feedback) interactions between disturbance processes within and between spatial scales; green dashed arrows illustrate dampening (negative feedback) interactions between disturbance processes. Abbreviations: L = landscape; IC = intercanopy (int erspaces between tree canopies). Overall, the dieback of overstory ve getation affects numerous key ecosystem processes, which are tied to site-specific distributions of incoming energy and water (Zou et al. 2007), and has multiple cascading ecol ogical effects. Widespread tree mortality may propagate additional pervasive changes in various ecosystem patterns and processes. Breshears (2007) summarizes the important ecological role of woody plant mosaics in semiarid ecosystems: A large portion of the terrestrial biosphe re can be viewed as lying within a continuum of increasing coverage by w oody plants (shrubs and trees), ranging

59SAP 4.2: Thresholds of Climat e Change in Ecosystems from grasslands with no woody plants to forests with nearly complete closure and coverage by woody plants (Breshear s & Barnes, 1999; Breshears, 2006). The characteristics of woody plants de termine fundamental descriptors of vegetation types including grassland, shrubland, savanna, woodland, and forest. Because woody plants fundament ally affect many key aspects of energy, water and biogeochemical patter ns and processes, changes in woody plant cover are of particul ar concern (Breshears, 2006). Climate-driven, rapid forest dieback has feedbacks with other ecological disturbance processes, such as fire and eros ion, in some cases leading to further nonlinear ecosystem threshold behaviors (fig. 3.7). Wa rming and drying climate conditions are driving higher-severity fire ac tivity at broader scales in the southwestern United States directly (Swetnam and Betanc ourt, 1998; Westerling et al. 2006), and probably also indirectly where forest dieback changes fuel conditions (fig. 3.7: Bigler et al. 2005). High-severity stand-replacing fires within w oodlands and forests can almost instantly cause large reductions in tree canopies and soil surface covers, thereby also triggering dramatically increased rates of runoff and so il erosion for several years post-fire until vegetation regrowth restores adequate land surface cover (V eenhuis, 2002). Forest dieback, fire, and erosion also have significant effects on ecosystem carbon pools (Breshears and Allen, 2002; Kurz et al. 2008). The combined interactive effects of climate-driven ecological disturbance proces ses (vegetation diebac k, fire, and erosion) are highlighted by the major changes in w oodland and forest ecosystems that have occurred in northern New Mexico duri ng the past 60 years (fig. 3.8; Allen, 2007). Climate-induced forest dieback, fire, and accelerated erosion already may be causing

60SAP 4.2: Thresholds of Climat e Change in Ecosystems permanent type-conversion changes to some southwestern ecosystems. Even without factoring in ongoing or predicted climate changes, it will be at least several decades to centuries before reestablishment of predisturbance tree canopy covers will occur on many semiarid woodland and forest sites in this region (Allen and Breshears, 1998; Savage and Mast, 2005).

Figure 3.8 Increased herbaceous cover has developed since recent piñon pine forest dieback in the Jemez Mountains of New Mexico and may promote surface fire regimes and changes in runoff and erosion patterns. July 2004. Examples of drought-induced tree die-o ff in semiarid forests and woodlands highlight the rapidity and extensiveness with which climate stress can trigger pervasive and persistent ecosystem changes. Climate change has the potential to drive multiple nonlinear or threshold-like processes that can interact in complex ways, including tree mortality, altered fire regimes, energy and water budget changes, and soil erosion thresholds (Allen, 2007), making ecological pr edictions difficult (McKenzie and Allen, 2007). For example, the projections of state- of-the-art dynamic global vegetation models

61SAP 4.2: Thresholds of Climat e Change in Ecosystems “are currently highly uncertain, making vegetatio n dynamics one of the largest sources of uncertainty in Earth system models” (Pur ves and Pacala, 2008). Additional research, including research on threshol d responses, is needed to improve projections of the nonlinear ecological effects of expected climate changes, such as broad-scale forest dieback, associated ecosystem dynamics, and effects on carbon budgets and other ecosystem goods and services (Breshears and Allen, 2002; Millennium Ecosystem Assessment, 2005; Millar et al. 2007). 

### Case Study 4. Thresholds in Climat e Change for Coral-Reef Ecosystem Functioning Corals are perpetually subjected to envi ronmental changes in time and space. As adult colonies, corals are sessile, remaining in one location over time, and therefore, are subjected to changes in environmental factors through a temporal scale. As larvae, corals are motile, and each must select a location from a complex and variable array of available sites. Corals are resilient to changes, both spatially and temporally, through acclimatization, adaptation, local environm ental ameliorations, initial community composition, and the morphological characteristics of the reef. It is reasonable to assume that most corals will not go extinct with global climate change because of their abilities to acclimatize, to adapt, and to broadcast thei r larvae over a large scale landscape (Paulay 1997). Systems consist of mixtures of positive and negative feedbacks, with positive feedbacks tending to alter the nature of the system, and negative feedbacks tending to minimize these changes (Chapin et al. 1996). The threshold, or tipping point, for coralreef ecosystems is the point along the environm ental gradient at which the ecological or biological processes change from negative feedback for net accretion to positive feedback

62SAP 4.2: Thresholds of Climat e Change in Ecosystems

or reef erosion. When net accre tion decreases to a point of net erosion of the reef, the resiliency of the system to return to a functioning coral ecosystem has been greatly reduced, potentially affecting the rate of r eaching a threshold of coral mortality. Natural stressors, which are the results of anthr opogenic stressors (for example, overfishing, pollutants, sedimentation, habitat destruction), that can lead to positive feedbacks, potentially decreasing the threshold level of coral mortality, include the following (Birkeland 2004):

• inverse density dependen ce (or Allee effect);

• algal abundance at levels beyond the cap acity of herbivores to keep in balance;

• predation of corals at a rate higher than the rate of recovery and coral population replenishment;

• bioerosion of corals;

• the prevalence of crustose coralline algae, which weakens binding of the substratum, is decreased and thereby decreases successful coral recruitment; and

• invasive species—the establishment of introduced species, which modify the habitat in ways that favor the survival and dominance of the introduced species and displacement of natural species. Such processes as these stressors and the feedback mechanisms of corals to these stressors have determined the substantial degradation of coral reefs over the past 3 decades in the tropical western Atlantic Ocean (Gardner et al. 2003) and in the IndoPacific Ocean (Bruno an d Selig, 2007). It is likely that the crossing of thresholds in coral

63SAP 4.2: Thresholds of Climat e Change in Ecosystems ecosystems began nearly 3 decades ago with no evidence that the rate of degradation is decreasing (Birkeland 2004). Although anthropogenic modification of local ecological processes has been the dominant force in coral-reef degradation (Birkeland, 2004) and tipping points have been crossed decades ago in many areas (Gardne r et al. 2003; Bruno a nd Selig, 2007), global changes in climate and oceanic characterist ics are now becoming more apparent. Global processes that are affecting co ral reefs are sea-level rise, th e decline in pH of seawater, and the increase in seawater temperature, whic h are related to the increased concentration of atmospheric CO 2 . Rise and Fall of Sea Level. —Coral reef ecosystems have experienced rise and fall of sea levels several times in geological history with associated effects on reef functioning (Hallock 1997) (w ith “reef functioning defined as constructing reefs upwardly). Reef accretion has stopped for pe riods of time in excess of 10 million years (Copper 1994, Hallock 1997), the threshold for the cessation of reef upward growth being the time of decreasing sea level (Hallock 1997, Hubbard 1997). It is hard to determine the effect of climate change alone on whether corals will keep pace with sea level rise, increasing water temperatures, and change in ocean pH. The rate of sea level rise alone does not provide a predictable tipping point fo r reef deposition that can be generalized over a region (Hallock et al. 1993, Kleypas et al. 2001, Garrison et al. 2003). Whether coral reefs keep up with sea level rise depends on a multitude of local environmental factors and the degree to which these factor s stress the corals themselves which will affect the rate at which the threshold for coral mortality will be reached (Hubbard 1997).

64SAP 4.2: Thresholds of Climat e Change in Ecosystems
Decrease in Seawater pH. —The concentration of CO2 in the atmosphere is generally expected to reach two times the pr eindustrial (late 18th century) levels by 2065 (Houghton et al. 1996). As CO2 concentration increases in the atmo sphere, the surface seawaters take up more CO2. The increased uptake of atmospheric CO2 by the surface waters of the ocean leads to a decrease inpH of surface waters, an increase in the proportion of bicarbonate ions ( 3 HCO − ), and a decrease in th e proportion of carbonate ions ( CO )(Feely et al. 2008). The overall effect on the rate of precipitation of coral skeleton is negative. 2 3 −

2 3 3 2 CO HCO H CO H − + −

+⇔ + + 2 2 H O + ⇔ The oceans have already taken up an additi onal one-third to one-half of industrialage emissions of CO 2 , and the concentrations of carbona te ions in the oceans have decreased from 11 percent (preindustrial), to 9 percent (now) and are projected to decrease to 7 percent when carbonate c oncentrations are double the preindustrial concentrations, perhaps in 3 to 5 decades (ISRS, 2007). Kleypas and others (1999) determ ined that doubled atmospheric CO 2 will lead to a 14 percent to 30 percent decrease in reef calci fication rates. This was estimated to be a general threshold from net carbonate accre tion to net carbonate loss by Kleypas and others (2001). Net reef accretion is potent ially reduced to zero when increased CO 2 in the atmosphere reaches about 500 to 600 ppm. On the other hand, CO 2 is less soluble in seawater at higher temperatures. While in creased concentrations of atmospheric CO 2 may be accelerating the uptake of CO 2 by surface seawater, global warming may be slightly damping the uptake. But of more substantial influence in accelerati ng the tipping point of

65SAP 4.2: Thresholds of Climat e Change in Ecosystems net reef accretion are the synerg istic biological effects on corals of reduced growth in the face of natural and anthropogenic stressors. Sabine and others (2004) showed that uptake of anthropogenic CO 2 by subtropical Atlantic waters has been greater than by Paci fic waters. The north At lantic occupies only 15 percent of the world’s total ocean area and stores 23 percent of the total anthropogenic (fossil-fuel and cement-ma nufacturing emissions) CO 2 taken up by the world oceans. Pacific waters are less re ceptive to the uptake of CO 2 and therefore are buffered from a decrease in pH because of higher concentrations of dissolved inorganic carbon. As seawater becomes warmer coral reef net accretion will probably become slightly more restricted in latitude (Kleypa s et al. 1999, 2001) because of the changes in chemistry from
CO 2 uptake in the world’s oceans. Studies have shown that the resilience of corals to lower pH of ocean waters decreases with input of nutrients from c ontinents. Anne Cohen of the Woods Hole Oceanographic Institution has take n core samples from 20 large Diploria labyrinthiformis colonies in Bermuda and found th at the rate of ca lcification has signi ficantly declined since 1959 (Cohen et al. 2008). This is consiste nt with the decrease in pH of the ocean waters of the northern Atlantic (Sabine et al. 2004) and the concomitant lowering of the saturation level of aragonite in coral skelet ons. The corals are, nevertheless, doing well, and the coral-reef ecosystem is intact in Bermuda (Murdoch et al. 2008), which is relatively distant from Continental land masses. In contrast, the coral reefs have been degrading for decades in the Caribbean and western Atlantic (Gardner et al. 2003), which are close to continental land masses and asso ciated land-surface runoff. (The input of fixed nitrogen from excess fertilizer runoff from the Mississippi River into the western

66SAP 4.2: Thresholds of Climat e Change in Ecosystems Atlantic has averaged 1.6 million metric tons per year since the 1980s and the input of phosphate has averaged a hundred thousand metric tons per year).

Ocean Uptake of CO 2

Figure 3.9 . Status of oceanic uptake of CO 2 . Source: Sabine, S.L., R.A. Feel y, Nicolas Gruber, R.M. Key, Kitack Lee, J.L. Bullister, Rick Wanninkhof, C.S. Wong, D.W.R. Wallace, Bront e Tilbrook, F.J. Millero, T.-H. Peng, Alexander Kozyr, Tsueno Ono, and A.F. Rios. 2004. The oceanic sink for anthropogenic CO 2 . Science , 305(5682) , 367–371. Done et al. (in press) report that coral communities on the Great Barrier Reef have been losing their resilience since about 1997. Done et al. found that loss in resilience on the Great Barrier Reef is correlated with nutrient (fixed nitrogen) input. A number of studies presented at the 11 th International Coral Reef Symposium reported that coral-reef system s are still resilient in areas far from continental land masses (for example, the Andaman and Maldive Arch ipelagoes, Chagos and Maldives in the Indian Ocean, Moorea, Fiji and American Samoa in the Pacific, and Bermuda in the Atlantic).

67SAP 4.2: Thresholds of Climat e Change in Ecosystems
Seawater Warming.— The thresholds in tolerance of corals to an increase in water temperature and its duration before “bleaching” (expelling the symbiotic zooxanthellae ) is predicted by the degree heating week (DHW) record (a NOAA satellite-derived experimental product), 12-week accumulati ons measured as °C weeks. The DHW product is an accumulation of hotspot values over the bleaching threshold (1°C over the maximum monthly mean. The threshold values of DHW vary from site to site because the maximum monthly mean varies from site to site; thus, corals are likely adapted to their own threshold temperatures at each site. Furthermore, the past history of events in the physical environment and local characterist ics of the physical environment can modify the actual locatio n of the threshold or tipping poi nt (Smith and Birkeland 2007). Based on our knowledge of tolerances and th e gaps in the litera ture on thresholds identified in developing this SAP, corals are likely to reach a threshold with an increase in sea water temperatures. Mechanisms of Reef Resilie nce That Alter Thresholds .—The resilience of corals to environmental changes is largely determined by their cap acity to acclim atize (adjust physiologically and behaviorally). The thresholds of resilience of corals to environmental factors, such as water temper ature and ultraviolet (UV) radi ation, are altered by changes in symbiotic interactions. Reef-building cora ls are dependent on symbiotic dinoflagellate algae ( zooxanthellae ) in their endodermal cells for th eir nutrition and proficiency in deposition of skeleton. There are a number of clades or types of zooxanthellae , and the physiological and ecological attr ibutes of zooxanthellae vary among clades (Abrego et al. 2008; Berkelmans and van Oppen 2006; McClanah an et al.2005; Baker et al. 2004; Baker 2004; Buddemeier et al. 2004; Rowan 2004; Baker 2003; Rowan and Knowlton 1995).

68SAP 4.2: Thresholds of Climat e Change in Ecosystems The symbiotic relationship breaks down unde r stressful conditions of extra warm seawater or strong UV radiation. Under these conditions, corals sometimes expel much of the zooxanthellae of clade C and allow the buildup of clade D, w ith which the coral growth rate is slower but survival under st ressful conditions may be greater. As with morphological adjustments, the symbiotic adju stments of corals may be determined by a balance between the stresses imposed by th e physical environment and by ecological interactions with other speci es (Bruno and Selig 2007). In addition to adjustments in morphology and symbiotic relationships, acclim atization can occur through biochemical conditioning where increased water temperat ure triggers a substantial increase in biochemical activity in corals. Intense biochemical activities (s uch as the increase in the amounts of heat shock proteins and ubiquiti n produced) resulting from changes in water temperature, may indicate processes of bioc hemical conditioning and acclimatization that might increase the resilience of the coral fr om increased seawater temperature (that is, increase the threshold level of coral mortality) (Smith and Birkeland 2007). Whether changes in morphology, symb iotic relationships, physiological conditioning, or production of biochemicals are the mechanisms to shift the threshold for survival from climate change, acclimatizati on costs the coral in terms of energy and materials that would otherwise be available for growth and successful competition. Acclimatization in corals can occur either as an accumulation of a simultaneous array of biochemical mechanisms to resist stress (robustness) or as an array of altern ative paths of development or symbiotic associates (p lasticity). The mound- shaped species of Porites (such as P. lobata ) are robust and live in a wide range of habitats. They are the last to drop out of the coral community near a river mouth or in bays with increasing turbidity.

69SAP 4.2: Thresholds of Climat e Change in Ecosystems Species of Acropora dominated the reef front at the municipal sewer outfall for Koror, Palau, until predation on corals by the cr own-of-thorns starfish and bleaching by the large-scale seawater warming of 1997–98 killed the Acropora spp. but not the Porites spp. (Richmond et al. 2002). Porites can maintain itself rather constantly despite fluctuations in the external physical environment, but at a metabolic cost (fig. 3.11). The relatively rapidly growing Pocillopora eydouxi display plasticity and can differ substantially among habitats in rates of growth, colony morphology, and types of zooxanthellae hosted. Pocillopora are generally more vulne rable to the physical environment and so their growth rates vary am ong habitats and they are more likely to bleach (expel zooxanthellae and/or photosyntheti c pigments) with higher than usual water temperatures and with more intense UV radiation.

Figure 3.10. Branching corals overgrowing mound-shaped corals.

70SAP 4.2: Thresholds of Climat e Change in Ecosystems
Factors that Shift the Thresholds.— Corals are most vulnera ble to infrequent or very frequent environmental changes. Corals can acclimatize (physiological or behavioral response) or adapt (genetic response) to environmental change s of intermediate frequency. (Adaptation is gene tic change in a population in response to natural selection). If the phenomena, such as ex traordinarily warm seawater, are infrequent enough to be unpredictable, corals will not be able to acclimatize or adapt, and if t oo frequent, will not have time to recover between events, thus decreasing the threshold level of coral mortality (Smith and Birkeland 2007). The factor of duration relates to the different effects of acute and chronic disturbances on the resilience of coral communities. The threshold seawater temperature associated with global climate change is dete rmined in part by the duration of the warm water event. In 1997–98, an increased average surface seawater temperature of 1.0° to 1.5°C (to about 30° or 31°C) over a period of several weeks caused ex tensive mortality of corals in the Indian Ocean, the southweste rn Pacific Ocean, and the western Atlantic Ocean (Bruno and Selig 2007). In contrast, dail y fluctuations of 6°C to 6.5°C (to about 34°C or 35.5°C) in reef flat pools in American Samoa are endured in good health by about 80 species of corals. The threshold seawater temperature that se verely affects a coral will be higher in areas of constant or even intermittent strong water motion and the threshold of temperature tolerance will be lower in areas of weak water motion (Smith and Birkeland, 2007). Thresholds in levels of tolerable input of nutrients or sediment will be low in backwaters and relatively much higher in ar eas of strong current (Smith and Birkeland 2007; Garrison et al. 2003). In contrast, it will take substan tially longer for the ecosystem

71SAP 4.2: Thresholds of Climat e Change in Ecosystems to solidify rubble into a stable substratum for reef r ecovery in areas of strong water motion than in areas of low water motion. The threshold of tolerance of corals to infection by disease is sometimes lowered by stress from other environmental factors and by abrasion of surface tissue by predators or other objects (Garrison et al. 2003). The physical and biological environments are a comp lex system of factors that potentially act synergistically to shift the thre shold of the specific factor as sociated with climate change.
Thresholds.— Thresholds should be considered at two stages: the first at which the population is killed or the ecosystem become s dysfunctional, and th e second at which the population or the ecosystem is prevented from becoming reestablished. An acute disturbance to a coral reef is a distinct event wh ereas a chronic disturbance is an ongoing process. Coral-reef communities in the Pacific (American Samoa) have been severely affected by large-scale acute disturbances, su ch as outbreaks of the coral-eating crownof-thorns starfish Acanthaster planci (1938, 1978), hurricanes (1981, 1987, 1990, 1991, 2004, 2005), and bleaching in response to seawater warming (1994, 2002, 2003). This is in contrast to the western Atlantic where th ere has been chronic disturbance resulting in degradation of coral reef systems for a half a century(Gardner et al. 2003). When allowed a 15-year interval between acute disturbances, the Pacific coral communities have recovered (Birkeland et al. 2008). Wher eas in the relatively small area of the tropical western Atlantic, external stressors such as nutrients (Hallock et al. 1993), pollutants (Garrison et al. 2003) , and diseases (Lessios etal. 1984) from wide-scale events on continents (Hallock et al. 1993; Garrison et al . 2003) can disperse across the entire region. This chronic disturbance decreases the threshold of coral mortality. A recent paper by Bruno and Selig (2007) reported that 3,168 square kilometers of reef has

72SAP 4.2: Thresholds of Climat e Change in Ecosystems been dying each year rather uniformly throughout the Indo-Pacific Ocean. Reefs are appearing to be losing th eir resilience globally. Coral reefs in the Pacific (America Samoa) have managed to maintain resilience because disturbances have been acute events. External stressors from overfishing, however, have been chronic, and the fish commu nities have not been as resilient as the corals (Zeller et al. 2006 a, b). Thresholds of coral reef system s need to take into account the whole system and not just the corals to ensure a resilient and adaptive system in the face of climate change.

73SAP 4.2: Thresholds of Climat e Change in Ecosystems 

## Chapter 4—Examples of Thresho ld Change in Ecosystems 4.1 Background The existence of ecological thresholds has long been apparent to people who depend on natural resources. Fisheries collap ses, for instance, have been noted for centuries. However, ongoing climate change has given this issue greater urgency because more ecosystems may be getting pushed toward response thresholds simultaneously, and based on gaps in the literature identified through the development process for this assessment (SAP 4.2), little is known re garding where the tipping points are. Ecosystems are very likely to differ significantly in their potential for climate change to impact them to the point that thre sholds are crossed and substantial alterations occur. Given the magnitude and pervasivenes s of climate change, it is surprising how little is known regarding the sensitivity of different ecosystems to any single aspect of climate change (such as increased temper atures), and even less is known about the impacts of multiple climate change factors. Th is lack of basic understanding represents a critical knowledge gap and rese arch challenge, one that is further complicated by the fact that climate change is only one component of global change and that multiple alterations to climate, biogeochemical cycles, and biodiversity are occurring in tandem. Summarized below are examples of wh ere ecological thresh olds have been crossed; they are less detailed than the case studies of Chapter 3 but represent different geographic areas, ecosystem types, and driver s of change. These examples include the new stressor of climate change and reflect how it leads to new eco system responses. For example, the temperature increases documented for many areas can likely cause an ecosystem changeover when normal droughts are experienced because the additional

74SAP 4.2: Thresholds of Climat e Change in Ecosystems evapotranspiration demand of higher temperat ures exceeds the tolerance capacity of trees, leading to the massive forest dieback described in Case Study 3. 

### 4.2 Example of Thresholds from the Past Thresholds appear to have been crossed in the past, leading to ecosystem changes that persist today. A recent example of thre shold behavior is the encroachment of woody plants into perennial grassla nds that has occurred throughout arid and semiarid regions of the world for at least the pa st several centuries. This br oad-scale land-cover conversion and associated soil degradation (that is , desertification) has local to global consequences for ecosystem services, such as reduced air and water quality (Schle singer et al. 1990; Reynolds and Stafford Smith, 2002). Multiple interacting processes and threshold behavior are involved in these dynamics (Rietkerk and van de Koppel, 1997). Cross-scale linkages among local soil and grass degradation, landscape connectivity of erosion processes, and land-cover and weather feedbacks have been invoked to explain threshold behavior in spac e and time that occur during desertification (Peters et al. 2006). Four stages and three thresholds have b een identified as the spatial extent of desertified land increases through time (Peters et al. 2004). Following introduction of woody plant seeds into a grass- dominated system (Stage 1), local spread often occurs as a result of feedback mech anisms between plants and soil properties interacting with wind and water erosion to produce fertile plant islands surrounded by bare areas that move the system across a thre shold into Stage 2 (Sch lesinger et al. 1990). This rate of spread may be slower than othe r stages as a result of interactions between plant life history characteristics that occur in frequently, such as recruitment, and the low precipitation and high temperatures that char acterize dry regions. As the size and density

75SAP 4.2: Thresholds of Climat e Change in Ecosystems of woody plants increase th rough time, contagious processes among patches, primarily wind and water erosion that c onnect bare soil patches, become the dominant factors governing the rate of desertif ication. As a result, a nonlinear increase in woody plant cover occurs and a second threshold is cro ssed as the system enters Stage 3. Through time, sufficient land area can be converted fr om grassland (low bare area, low albedo) to woodland (high bare area, high albedo) so that regional atmospheric conditions, in particular wind speed, temperatur e, and precipitation, are aff ected. At this point, a third threshold is crossed where land-atmosphere interactions with feedbacks to vegetation control system dynamics (Stage 4) (Pielk e et al. 1997). Feedbacks to broad-scale vegetation patterns have been documented in th e Sahara region of Af rica (Claussen et al. 1999). 

### 4.3 Temperature Increases Are Pushing Ecosystems Towards
Thresholds The impacts of increasing temperatures resulting from climate change are not independent of the effects of other important environmental stressors, and thus, need to be assessed in the context of multiple, interacting stressors. The IPCC (2007) reports with very high confidence that the increased warm ing effect of climate change is strongly affecting natural biological systems in both marine and freshwater systems. The chemical and physical characteristics of lakes expe rience major effects owing to changes in temperature, especially changes in nutrientdynamics. Increased temperatures in lake systems will affect the distri butions, growth, and survival of fish and many other aquatic organisms. Tied with increased temperatures is a change in precipitation, which can cause substantial physical and chemical changes in lakes, streams, and wetlands (as discussed

76SAP 4.2: Thresholds of Climat e Change in Ecosystems in Chapter 3) with large consequences for a quatic biota. In marine systems, increased temperature from climate change is affecti ng coastal resources and habitats because of sea-level rise that is caused by thermal e xpansion of the oceans and the melting of ice cover. It also is affecting the strongly coupl ed atmospheric and oceanic circulation that underpins ecosystem dynamics in wind-driven upwelling shelves and ecosystem susceptibility to modulations of upwelli ng wind stress causing present day global distribution of shelf anoxia (Cha n et al. 2008). This has the pote ntial to affect the rate at which the threshold for mortality will be reached for demersal fish and benthic invertebrate communities in these shallow waters. The rate of sea-level rise is expected to accelerate because of global warming. Salt mars hes, which must increase their vertical elevation at rates that keep pace with sea-level rise or ri sk transformation to a lower position along the marsh gradient, may experi ence a change of marsh type. Transition from one type of marsh to another (for ex ample, high marsh to low marsh) at a given point has been described as ecosystem state change (Miller et al. 2001). The effects of temperature increases on te rrestrial systems are further emphasized in the IPCC Assessment Report for Working Group II (IPCC 2007), wher e it is stated with very high confidence that the overwhelmi ng majority of studies of regional climate effects on terrestrial species reveal consiste nt responses to warming trends, including poleward and elevational range sh ifts of flora and fauna. Responses of terrestrial species to warming across the Northern Hemisphe re are well documente d by changes in the timing of growth stages (tha t is, phenological changes), espe cially the earlier onset of spring events, migration, and lengthening of the growing season. Changes in abundance of certain species, including limited evidence of a few local disappearances and changes

77SAP 4.2: Thresholds of Climat e Change in Ecosystems in community composition over the last few d ecades have been attributed to climate change. A further indication of effects of increased temperatur es is revealed in earlier snowmelt and stream runoff, which affects both aquatic and terrestrial ecosystems and species. Diminished snowpacks that melt earlier in the spring have affected the timing and extent of seasonal wetlands where amphi bians breed. A threshold may occur wherein the reduced amphibian population cannot accomm odate the necessary shift in the timing of breeding or cannot survive multiple dry years, causing local extinction (Corn 2003). There is a need to bette r understand the complexities of ecosystems and the drivers of change within them and to be able to identify the thresholds of these changes in a changing climate. 4.3.1 Climate Interactions Drive Ecosystems to Thresholds As important as the increases in temperatur es and changes in moisture availability are for causing ecosystems to go through thresholds , it is the interactions that are key to driving the change. In general, plants in undisturbed ecosystems are at their moisturelimited capacity for net primary productivity . Therefore, increased temperatures and droughtiness will combine to produce severe stress on plant growth, whereas increased temperatures and increased moisture availa bility will lessen the stress or may promote plant productivity, leading to an ecosyst em with increased resilience. Because evapotranspiration demands on ve getation increase with temperature, thresholds are more likely to occur whenever moisture availabil ity does not simultaneously increase with warming temperatures. The exception is ecosystems that are primarily limited by temperature, such as arctic and alpine ecosy stems. In these latter cases, ample moisture means that vegetation can respond without ev apotranspiration limits but that threshold

78SAP 4.2: Thresholds of Climat e Change in Ecosystems changes can still occur as competitive relati onships are altered between plant species (Hansell et al. 1998). 4.3.2 Climate Variability Increases Likelihood of Threshold Shifts The climate drivers that produce threshold ecosystem responses may be complex and involve the interaction of variability in phenology and weather episodes. The “2007 spring freeze” in the Eastern United States is an example. A very warm late winter/early spring period in much of the Southeastern United States in 2007 led to bud-break and development of forest canopy 2 to 3 weeks earlie r than usual. A very cold Arctic air mass spread across much of the Eastern United St ates in early April (an event not unusual for that time of year), dropping the low daily temperatures well below freezing for several days. The freeze killed newly formed leaves, shoots, and developing flowers and fruits and resulted in a sharp drop in vegetation greenness (NDVI) across a large swath of the southeast. The severity of impact was species specific; but at one site affected by this episode, there was a significan t reduction in forest photosynthetic activity for at least several weeks after this ev ent, and the leaf-area inde x was depressed throughout the summer (Gu et al., 2008). Smaller forest leaf ar ea resulted in increased light availability to the stream draining the forest at this site throughout the la te spring and summer, leading to increased primary and secondary productivity and highe r rates of nutrient uptake and retention in the stream (Mulholland et al., 2009). Long-term climate records at this site showed that average late-winter te mperatures are increasing but the date of the last hard freeze remains variable and show s no trend with time, suggesting that this heretofore unusual weather event may become more common in the future, which could lead to significant effects on forests and stre ams. While our unders tanding of the long79SAP 4.2: Thresholds of Climat e Change in Ecosystems term effects of this episode are unclear, they may likely include significant changes in forest composition due to mortal ity and/or increased susceptibility to pests of the more susceptible species if similar episod es occur in the future (IPCC, 2007). 4.3.3 Other Human Stressors and Climate Change The interaction of human stresses on eco systems (for example, land-use change) and climate change may be most evident for lotic ecosystems (those of rivers, streams, and springs) and may produce threshold res ponses that each stress alone would not produce. Flow variability over time and space is a fundamental characteristic of lotic ecosystems. It is this temporal and spatial flow variability that defines and regulates biotic composition and key ecosystem processe s in streams and rivers (Poff et al., 1997; Palmer et al., 2007). Climate change will a lter flow regimes and generate changes to biotic communities in many of these ecosystem s, although it is not cl ear that these flow alterations will produce threshol d-type responses in these systems that have evolved in response to high flow variability. However, growing water demands combined with climate-change-induced increase s in the severity and duration of droughts in the western United States will likely lead to hydrologi c regime shifts in many drainage basins (Barnett et al., 2008). Recent empirical evidence suggests th at severe droughts can produce more dramatic and long lastin g effects (for example , loss of biodiversity) on the biological communities of streams and river ecosystems than do other changes in the flow regime, such as floods (Boulton et al., 1992; Lake, 2004). Studies of drought effects on macroinvertebrates in Australian streams where drought is a co mmon and widespread phenomenon suggest that there may be a signif icant lag effect that prevents recruitment

80SAP 4.2: Thresholds of Climat e Change in Ecosystems after drought conditions end (Bou lton, 2003). Historical evidence exists of large shifts in river fish communities in response to decades -to-century-scale droughts in the Colorado River basin at the end of the Pleistocene (Douglas et al. 2003), but recent findings indicate large uncertainties in long-term effects of dr ought on fish (Matthews and MarshMatthews, 2003). Many of the expected changes to flow regimes from climate change are similar to those that result from urbanization and othe r human alterations of drainages. Among these are increased flashiness of hydrographs and longer periods of low or intermittent flow, higher water temperatures, and simplif ied biotic assemblages (Paul and Meyer, 2001; Roy et al., 2003; Allan, 2004; Nels on and Palmer, 2007). The increases in urbanization that have occurred and are likely to continue in many regions of the United States will very likely exacerbate the effects of climate change. The strongest evidence for potential threshold effects in rivers and streams appears to be the result of combined impacts of high or increasing human water withdrawals and the likelihood of more fr equent or longer droughts under a warming climate. Defining a water stress index equiva lent to total human water use divided by river discharge, Vorosmarty et al. (2000) showed that the combination of projected population and climate change results in subs tantial increases in water stress over large areas of the eastern and southw estern United States. In an analysis of sustainable water use in the United States, the Electric Power Research Institute (EPRI, 2003) reported that total freshwater withdrawal exceeded 30 per cent of available precipitation over much of the semiarid and arid regions of the United States and over large areas of Florida and other metropolitan areas in the east. High rates of human water use reduce flow and

81SAP 4.2: Thresholds of Climat e Change in Ecosystems extend low flow periods, restricting and degr ading habitat for rive r and stream biota. Using two scenarios from the 2001 IPCC report, Xenopoulos et al. (2005) reported that the combination of climate change and increa sed water withdrawal may result in loss of up to 75 percent of the local fish bi odiversity in global river basins. There are several examples of potential large-scale threshold responses to the combined effects of human water manageme nt and climate-induced drought. In the Columbia River basin of the Pacific north west, multiple stressors (including population growth; conflicts between hydropower, agricu lture, and recreation interests; and ineffective water management institutions and structures) have increas ed the vulnerability of water resources (Payne et al., 2004; Miles et al., 2007) that were already vulnerable as a result of reduced winter snowpack (Barne tt et al., 2005), which generates much of the summer flow, and sustained or repetitive dr oughts projected by climate change models that would drive water supplies to extrem e low levels. Because salmon populations are under considerable stress due to dams, wa ter withdrawals, and other human actions, reduced summer flow under a warmer clim ate may exceed population sustainability thresholds (Neitzel et al., 1991). The Colorado River supplies much of the water needs of a large area of the western United States and nor thern Mexico. The lower portions of the river have become highly vulnerable to drought due to increased demand from population increases. A longterm drought, beginning in about 2000, has lo wered water levels considerably in Lakes Powell and Mead, and many climate models project future conditions that will eventually lead to the drying up of Lake Powell and reduced flow in the Colorado River by more than 20 percent. Water allocations for maintaining the ecological integrity of natural

82SAP 4.2: Thresholds of Climat e Change in Ecosystems communities could drop below thresholds that ensure their viability as scarce water is prioritized for human commun ities (Pulwarty et al., 2008). Even in the humid southeastern United St ates, the combined effects of increased water withdrawals and climate change may ex ceed thresholds in ecosystem response. The Chattahoochee-Apalachicola River basin in Al abama, Florida, and Georgia is both an important water source for agricultural, industr ial, and municipal uses and an important fishery. More than 75 percent of the fish species inhabiting this river system depend on access to floodplain and tributary areas to forage and spawn, and there are flow thresholds below which fish cannot move into these critical areas (Light et al. 1998). Analysis of projected future water wi thdrawals and climate change for the Chattahoochee-Apalachicola River basin in dicates that by 2050, minimum flows will drop below these minimum flow thresholds fo r at least 3 months in summer in some areas (Gibson et al. 2005).This situation will be exacerbated by the increased percentage of flow that is wastewater effluent combin ed with lower minimum flows in this rapidly urbanizing basin. This will increase biological oxygen demand and reduce dissolved oxygen concentrations potentially below threshol d levels required by some species of fish (Gibson et al. 2005). The drying up of streams and wetlands re presents thresholds that involve contraction or elimination of entire aquati c ecosystems. Prairie rivers, streams, and wetlands of the Great Plains may be particular ly vulnerable to these types of thresholds because of the combined effects of water w ithdrawals for agricultural and municipal uses and projected climate changes that will resu lt in longer periods of drought (Johnson et al., 2005). For example, since the late 1970s, the Arkansas River and many of its tributaries

83SAP 4.2: Thresholds of Climat e Change in Ecosystems in Kansas have had long periods of dry ch annels because of extensive surface and groundwater use in its drainage basin (Dodds et al. 2004). The drying up of headwater streams and even some larger streams a nd rivers for extended periods may become common in wetter areas of the United States as well, particularly as a result of the combined effects of increased wate r withdrawal and climate change. Riparian ecosystems are also vulnerable to drought-related thres holds, particularly in the more arid regions of the United Stat es. Riparian forests dominated by cottonwood are being replaced by drought-tolerant shrubs al ong some rivers in the western United States. Increased surface and groundwater w ithdrawals combined with drought have resulted in the replacement of riparian forests of native cottonwood ( Populus fremontii ) and willow ( Salix gooddingii ) by an invasive shrub ( Tamarix ramosissima ), resulting in reduced animal species richne ss, diversity, and abundance ov er extensive areas along the San Pedro River in Arizona (Lite and Stro mberg, 2005). Surface flow and the depth to groundwater appear to be the primary controls on riparian vegetation, with loss of native riparian communities when rivers and streams drop below flow permanence thresholds of 50 percent to 75 percent (Stromberg et al. 2005, 2007). 4.3.4 Ecosystem Vulnerability and Climate Change Some ecosystem attributes may be pa rticularly important in generating differential ecosystem vulnerability to cl imate change, including the likelihood that important thresholds of response are crosse d. For example, most ecosystems have a single or just a few dominant species thatmediate ecological processes, control the majority of the resources (including space) , and/or have disproportionate impacts on species interactions. Thus, if climate change favors a new dominant species, the

84SAP 4.2: Thresholds of Climat e Change in Ecosystems prediction is that it will likely be the rate at which the extant speci es can be replaced and the traits of these new species that will de termine the likelihood that the ecosystem will be altered significantly to resultin threshold behavior in st ate or function. For example, ecosystems dominated by long- lived species (for example , trees) with slow population turnover would be expected to be relatively slow to respond to climate change, whereas those ecosystems dominated by short-lived species (for example , annual plants) are expected to be more vulnerable to experien cing substantial change if the new dominant species replacing the old have ve ry different species traits. Ecosystems can differ dramatically in the sizes of key carbon and nutrient pools, as well as rates of biogeoche mical transformations and turnover. These attributes may also determine the rate and magnitude of ecosy stem response to climate change if climate forcings influence these biogeochemical attr ibutes. For example, ecosystems with large nutrient pools and/or slow turnover rates ar e expected to respond minimally to climatechange-induced alterations in nutrients. In contrast, ecosystems with limited nutrient pools and rapid biogeochemical cycling are expe cted to be more vulnerable to climate change that results in critic al thresholds being crossed. The general hydrologic balance of ecosystems would similarly impact ecosystem sensitivity to any climate change that affects water availability. In general, those ecosystems with a ratio of precipitation-topotential evapotranspiration that is near or below 1:1 will be predicted to be more vulnerable to change than ecosystems where this ratio is greater than 1:1. Levels of biodiversity (func tional traits and species) within an ecosystem may also be important in influenci ng sensitivity to climate change (Grebmeier et al 2006). The number and traits of species may buffer ecosy stems from change and influence the extent

85SAP 4.2: Thresholds of Climat e Change in Ecosystems to which immigration of new species will occur. For example, depending on how well species in an ecosystem functionally compleme nt each other and the ability of species to compensate for the change resulting from the loss of the dominant species, the replacement of a dominant species by anothe r species could result in no change or large changes in ecosystem state. Similarly, inva ding species may result in the rapid crossing of thresholds or may have little or no im pact depending on the traits of these species relative to the traits of native species. Finally, interactions with the natural disturbance regime inherent in an ecosystem, other climate change factors, and other globa l changes, such as habitat fragmentation and species invasions, will more than likely influence whether or not ecosystems cross response thresholds and experience substantia l amounts of change in their structure and function. For example, ecosystems that are hist orically prone to fire may experience more frequent fires with climate change, making th em more susceptible to invasions by exotic species as resources become available postfire.

86SAP 4.2: Thresholds of Climat e Change in Ecosystems 

## Chapter 5—What Can Be Done? Because there is significant potential for abrupt or threshold-type changes in ecosystems in response to climate change, what changes must be made in existing management models, premises, and practices to manage these systems in a sustainable, resilient manner? What can be managed and at what scales, given that climate change is global in nature but manifests itself at local and regional scales of ecosystems? This section reviews the management models th at predict how ecosystems will respond to climate change and examines their ade quacy for addressing threshold behavior. 

### 5.1 Integration of Management and Research With ongoing climate change and the thre at that ecosystems will experience threshold changes, managers and decisionm akers are facing more new challenges than ever. Strong partnerships between research and management can help in identifying and providing adaptive management responses to threshold crossings. Because decisionmakers are dealing with whole ne w ecosystem dynamics, the old ways of managing change do not apply. A new paradi gm in which research and management work closely together is needed. The followi ng sections highlight some of the needs of
managers. 5.1.1 Need for Conceptual Models Most frameworks for nonlinear ecosystem behavior are hierarchical so a small number of structuring processes control eco system dynamics; each process operates at its own temporal and spatial scale (O’Neill et al., 1986). Finer scales provide the mechanistic understanding for behavior at a pa rticular scale, and br oader scales provide the constraints or boundaries on that behavior . Functional relationships between pattern

87SAP 4.2: Thresholds of Climat e Change in Ecosystems and process are consistent with in each domain of scale so th at linear extrapolation is possible within a domain (Wiens, 1989). Thre sholds occur when pattern-and-process relationships change rapidly with a small or large change in a pattern or environmental driver (Bestelmeyer, 2006; Groffman et al ., 2006), although both external stochastic events and internal dynamics can drive system s across thresholds (S cheffer et al., 2001). Crossing a threshold can result in a regime sh ift where there is a change in the direction of the system and the creation of an alternative stable stat e (Allen and Breshears, 1998; Davenport et al., 1998; Walker and Meyers, 2004). Under some conditions, thresholds may be recognized when changes in the rate of fine-scale processes within a defined area propagate to produce broad-scale responses (Gunderson and Holling, 2002; Redman and Kinzig, 2003). In these cases, fine-scale pro cesses interact with processes at broader scales to determine system dynamics. A series of cascading thresholds can be recognized where crossing one pattern-and-process thre shold induces the crossing of additional thresholds as processes interact (Kinzig et al., 2006). Conceptual models are particularly useful in linking hierarchical models across scales, because the existence of cross-scale interactions are often clearly recognized and can be incorporat ed as rules, even if they cannot be precisely parameterized. Field ex periments that iden tify cause-and-effect relationships can then be implemented to test these cross-scale interactions. For example, manipulation of CO 2 or water table depth (global-to-regional driver s of change) can be used to assess impacts on plot-scale patte rns of biogeochemistry of community composition.

88SAP 4.2: Thresholds of Climat e Change in Ecosystems
5.1.2 Scaling Recent theories and ideas about system beha vior have used hierarchy theory as a basis for describing interactions among pro cesses at different scales. Such theories include complex systems (Milne, 1998; A llen and Holling, 2002), self-organization (Rietkerk et al., 2004), panarchy (Gunderson a nd Holling, 2002), and resilience (Holling, 1992; Walker et al., 2006). Cro ss-scale interactions (CSIs, processes at one spatial or temporal scale interacting with processes at another scale that often result in nonlinear dynamics with thresholds) are an integral part of all of these ideas (Carpenter and Turner, 2000; Gunderson and Holling, 2002; Peters et al., 2004). These interactions generate emergent behavior that cannot be predicte d based on observations at single or multiple, independent scales (Michene r et al., 2001). CSIs can be important for extrapolating information about fine-scale processes to broa d-scales or for downscaling the effects of broad-scale drivers on fine-sca le patterns (Ludwig et al., 2000; Diffenbaugh et al., 2005). The relative importance of fine- or broad-scal e pattern-and-process relationships can vary through time and compete as the dominant f actors controlling system dynamics (for example, Rodó et al.2002; King et al , 2004; Yao et al. 2006). Because CSI-driven dynamics are believed to occur in a variety of systems, including lotic invertebrate communities in freshwater streams (Palmer et al.1996) and lakes (Stoffels et al. 2005), mouse populati ons in forests (Tallmon et al. 2003), soil microbial communities (Smithwick et al. 2005), coral reef fish recruitment in the ocean (Cowen et al. 2006), human diseases (Rodó et al. 2002), and grass-sh rub interactions in deserts (Peters et al. 2006)—it is critical that ecologists find ways to measure CSI. It is important to identify the key processes i nvolved in these changing pattern-and-process

89SAP 4.2: Thresholds of Climat e Change in Ecosystems relationships so that thres holds can, at a minimum, be understood and predicted if not averted through proactive measures. Recently, a framework was developed to explain how patterns and processes at different scales interact to create nonlinear dynamics (Peters et al. 2007). This framework focuses on intermediate-scale properties of tran sfer processes and spatial heterogeneity to determine how pattern-and-process relationships interact from fine to broad scales (fig. 5.1). In this framework, within a domain of scale (that is, fine, intermediate, or broad), patterns and processes can reinforce one another and be relatively stable. Changes in external drivers or disturbances can alter pa ttern-and-process relati onships in two ways. First, altered patterns at fine scales can result in positive feedbacks that change patterns to the point that new processes and feedbacks are induced. This shift is manifested in a nonlinear threshold change in pattern and process rates. For example, in arid systems, disturbance to grass patche s via heavy livestock grazing can reduce the competitive ability of grasses and allow shrub colonization. After a certain density of shrubs is reached in an area and vectors of propagule transport (for example, livestock or small animals) are available to spread shrubs to nearby grasslands, shrub colonization and grass loss can become controlled by dispersa l processes rather than by competition. Shrub expansion rates can increase dram atically (Peters et al. 2006). As shrub colonization and grazing diminish grass cover over large areas, br oad-scale wind erosion may govern subsequent losses of grasses and increases in shrub dominance. These broadscale feedbacks downscale to overwhelm fi ne-scale processes in remnant grasslands. Once erosion becomes a pervasive landscape-scale process, neither competition nor dispersal effects have signif icant effects on grass cover.

90SAP 4.2: Thresholds of Climat e Change in Ecosystems
Second, direct environmenta l effects on pattern -and-process relationships at broad scales can similarly overwhelm fine-scale pr ocesses. For example, regional, long-term drought can produce widespread erosion and minimize the importance of local grass cover or shrub dispersal to pa tterns in grasses and shrubs.



### Pattern (patch type)

Process (competition)
fine intermediate broad 

## Spatial Scale Pattern (connectivity) Pattern (land cover)

Process (erosion) Process (dispersal) Environmental drivers/disturbances

Pattern (patch type)

Process (competition)
fine intermediate broad 

## Spatial Scale Pattern (connectivity) Pattern (land cover)

Process (erosion) Process (dispersal) Environmental drivers/disturbances

Figure 5.1. Diagram representing cross-scale interactions . Solid arrows represent pattern-and-process feedbacks within three different scale domains with one example of pattern and process shown for each domain. Green arrows indicate the dir ect effects of environmental drivers or disturbances on patterns or processes at different scales (for example, patch disturbance versus climate). Blue arrows indicate the pointat which altered feedbacks at finer scales induce changes in feedbacks at broader scales (for example, finescale changes cascade to broader scales). Red arrows indicate when changes at broader scales overwhelm pattern-and-process relationships at finer scales. 5.1.3 Applying Models from Other Disciplines Climate requires interdisci plinary approaches. Recent and global environmental changes, including climatic change, changes in atmospheric composition, land-use change, habitat fragmentation, pollution, and the spread of invasive species, have the potential to affect the struct ure and functions of some ecosy stems, and the services they provide. Many ecological effects of global envi ronmental change have the potential for

91SAP 4.2: Thresholds of Climat e Change in Ecosystems feedbacks (either positive or negative) to climatic and other environmental changes. Furthermore, because many global environmenta l changes are expected to increase in magnitude in the coming decades, the potential exists for more significant effects on ecosystems and their services. As climate change manifests itself at local and regional scales of ecosystems, it is necessary not only to downscale forecasting mode ls but also to ensure that models used for predictions take into account not just the physical parame ters that support ecosystems but also the biotic aspects of the ecosyst ems. Biomes and ecosystems do not shift as entities in response to climate change, but they change through the responses of individual species (Scott and Lemieux, 2005). The biogeochemical, temperature, and precipitation requirements of individual spec ies need to be taken into account when predicting these shifts, thus the need for the use of interdisciplinary models that address these variables and their dynamic feedback . Our current understanding suggests that using interdisciplinary models will very lik ely reduce scientific uncertainties about the potential effects of global change on ecosy stems and provide new information on the effects of feedbacks from ecosystems on globa l change processes. The challenge is to create a framework in which interdisciplinary models can work interactively to consider all the feedbacks involved. This integrat ive approach will provide a framework to organize observations and assessments of changes in the system in response to management actions. 

### 5.2 Adaptive Management to Increase Resilience The process of selecting, implementing, monitoring, assessing, and adjusting management actions is called adaptive manage ment or, in the context of this report,

92SAP 4.2: Thresholds of Climat e Change in Ecosystems adaptive ecosystem manage ment (AEM) (Holling 1978; Walters 1986; Prato 2004, 2007a). AEM can be done passively or actively. If passive AEM is used, the decision to adjust management actions or not depends on whether the indicators or multiple attributes of the outcomes of management actions sugge st that the ecosystem is becoming more resilient or more variable and might cross a threshold. If active AEM is used, the decision of whether or not to adjust management actions is determined by testing hypotheses about how the ecosystem state is responding to management actions. Active AEM treats management actions as experiments. Unlik e passive adaptive management, active AEM yields statistically reliable information about ecosystem responses to management actions, although it is more expensive and difficult to apply than passive AEM and requires sufficient monitori ng (Lee 1993, Wilhere 2002). To increase ecosystem resilience, a number of approaches have been put forth for use in adaptive management (Julius et al. 2008). These include avoiding landscape fragmentation and its converse, restoring connectivity; ensuring that refugia are protected so that recolonization of species is possi ble; focusing protection on keystone species where applicable; reducing other stressors such as pollution; removing introduced invasive species; and reducing extraction of ecosystem services for humans (for example, ensuring water flows for aquatic ecosyst ems under stress) (Scott and Lemieux 2005, Groffman et al. 2006). For each ecosystem, AEM potentially provides quantitative documentation as to the relative efficacy of the different approaches to improving resilience (Keeley 2006; Millar et al. 2007: Newmaster et al. 2007).

93SAP 4.2: Thresholds of Climat e Change in Ecosystems
5.2.1 Role of Monitoring Because climate change effects are likely to interact with patterns and processes across spatial and temporal scales, it is clear the monitoring strategies must be integrated across scales. First and foremost, the earth’s su rface must be hierarchically stratified (for example, using the Major Land Resource Area an d Ecological Site Description System of the U.S. Department of Agriculture and Na tional Resources Conser vation Service and U.S. Forest Service ecoregions), and conceptu al or simulation models of possible impacts and feedbacks must be specified for each st ratum (Herrick et al., 2006). The models are used to develop scenarios and to identify key properties and processes that are likely to be associated with abrupt changes. S econd, simultaneous multiple-scale monitoring should be implemented at up to three spatia l scales based on thes e scenarios and the recognition of pattern-and-pro cess coupling developed in the models (Bestelmeyer, 2006), which may feature cross-scale interactions (Pet ers et al., 2004). Remote-sensing platforms can be used to monitor some broad-scale spatial patterns, including significant shifts in plant community composition; vegetation production; changes in plant mortality; bare-g round, soil, and water-surface temperatures; and water clarity. These platforms may also be used to detect rates of change in some contagious processes, such as the spread of readily observable invasive species. Changes in variance across space and time derived from such measures may be a primary indicator of incipient nonlinear change (Carpenter and Brock, 2004). These measures should be coupled with ground-based measures at mesos cale to patch scales. Mesoscale monitoring often requires widely distributed observat ions across a landscape (or ocean) acquired with rapid methodologies including sensor ne tworks. Such widely distributed monitoring

94SAP 4.2: Thresholds of Climat e Change in Ecosystems is necessary in some situations because in cipient changes may materialize in locations that are difficult to predict in advance (such as with tsunami warning systems). In other cases, however, more targeted monitoring is n ecessary to detect mesoscale discontinuities in smaller areas that are likely to first regi ster broad-scale change, such as at ecotone boundaries (Neilson, 1993). Fina lly, patch-scale monitori ng can feature methodologies that focus on pattern-and-process linkages th at scale up to produce systemwide threshold changes, such as when vegetation patches degr ade and bare patches coalesce to result in desertification (Rietkerk et al., 2004; Ludwig et al., 2005). The involvement of land users is particularly important at this scale b ecause recognition of pr ocesses that degrade resilience may be used to mitigate climate-driv en thresholds by way of local management decisions. Consequently, technically sophistic ated approaches should be balanced with techniques suitable for the public at large (for example, Carpenter et al. , 1999; Pyke et al., 2002). Monitoring data across scales must then be integrated, and interpretations generated for key strata. Ground-based monitori ng, for example, may reveal key changes not detected through remote sensing, or conversely, remote sensing may explain apparently idiosyncratic patterns in ground-ba sed data to reveal key vulnerabilities. Multiagency institutions and a “network of networks” could be organized with such efforts in mind and could periodically revi ew data gathered across scales and from different partners (Parr et al., 2003; Betancourt et al., 2007; Peters et al., 2008). Monitoring key ecosystem indicators whic h integrate across ecosystem processes and scales are essential in developing observations for threshold changes. For example, nutrient export via streamflow is a sensitive metric for identifying changes in ecosystem

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structure and function at the watershed scale that may be difficult to detect on complex and spatially heterogeneous systems. For exam ple, nitrate concentration in streams has been used as a sensitive indicator of fore st nitrogen saturation (Stoddard, 1994; Swank and Vose, 1997; Lovett et al. 2000; Aber et al., 2003), effects of in sect pest outbreaks (Eshleman et al. 1998), and eff ects of short-term climate pe rturbations (Mitchell et al. 1996; Aber et al. 2002). Stream chemistry monitoring, partic ularly at gauged sites where discharge is also monitored, can provide sensitive signals of changes in ecosystem biogeochemical cycles. 5.2.2 Role of Experiments A key component of adaptive manageme nt strategies is the role of experimentation. A critical component of desi gning appropriate experi ments is to identify the conditions or systems that are susceptible to threshol d behavior and interactions across scales that include transport processe s at intermediate scales. Observations and experiments to evaluate the sensitivity of these processes and interconnections to anticipated perturbations provide insight to management stra tegies to enhance resilience or to mitigate threshold changes. One approachis to measure responses at multiple scales simultaneously and then test for significant effects of variables at each scale (for example, Smithwick et al. 2005; Stoffels et al. 2005). Experiment al manipulations can also be used to examine processes at fine and intermediate scales and to isolate and measure impacts of broad-scale drivers under c ontrolled conditions (for example, Palmer et al. 1996; King et al. 2004). Stratified-cluster experiment al designs are methods for considering multiple scales in spatial variables and for accounting for distance as related to transport processes in th e design (Fortin et al., 1989; King et al., 2004). Regression

96SAP 4.2: Thresholds of Climat e Change in Ecosystems (gradient)-based experimental designs ma y be superior to analysis of variance (ANOVA)-type designs for predicting threshol ds in ecological re sponse to linear or gradual changes in climate or other drivers. Quantitative approaches also show promise in identifying key processes related to threshold behavior. Statistical analyses based on nonstationarity (Rodó et al., 2002) and nonlinear time series analysis (Pascual et al. 2000) are useful for identifying key processes at different scales. Spatial analyses that combine traditional data layers for fineand broad-scale patterns with da ta layers that use surrogate s for transfer processes at intermediate scales (for example , seed dispersal) can isol ate individual processes and combinations of processes that influence dynamics in both space and time (for example, Yao et al. 2006). Simulation models that use fine-scale models to inform a broad-scale model can be used to examine the relative importance of processes and drivers at different scales to system dynamics as well as interactions of processes and drivers (Moorcroft et al. 2001; Urban, 2005). Coupled biological and physical models that include population processes and connectivity among populations as well as broad-scale drivers have been used to show the conditi ons when connectivity is important, and to identify the locations that are more suscep tible or resilient to management decisions (Cowen et al. 2006). 

### 5.3 Management by Coping If there is a high potential for abrupt or threshold-type changes in ecosystems in response to climate change, existing manageme nt models, premises, and practices must be modified in order to manage these systems in a sustainable, resi lient manner (Millar et al. 2007). Existing management paradigms may have some limited value because of the

97SAP 4.2: Thresholds of Climat e Change in Ecosystems assumption that the future will be similar to the past. This assumption, however, fails to take into account the und erlying uncertainty of the trajectories of ecological succession in the face of climate change. Managers can in stead take a dynamic approach to natural resource management, emphasizing processes ra ther than composition, to best maintain, restore, and enhance ecological functions (W alker et al., 2002). The following sections address some of the mechanisms that can be used to plan for future ecosystem resilience and achieve a balance of positive and negative feedbacks (Millar et al. 2007). 5.3.1 Reducing Multiple Stressors The key to reducing stressors is to identify the factors that infl uence resilience. In many cases, management practices that increas e resilience can be designed from existing knowledge; in other cases, however, it is not clear what manageme nt practices will enhance resilience (Millar et al. 2007). For example, c onnectivity in a fragmented landscape can be restored by creating corrido rs for species movement between suitable habitat patches (Gustafson, 1998) . Alternatively, inadvertent connectivity that has been established and utilized by i nvasive species can be remove d to reduce stress on the native populations remaining. To potentially mitigate for threshold crossing, it is likely that a variety of approaches, including both long-term and short-term strategies based on new information for natural resource management, will need to focus on increasing ecosystem resilience and resistance as well as assisting ecosystem s to adapt to the inevitable changes as climates and environments continue to shif t (Millar et al. 2007; Parker et al. 2000). Increasing management adaptive capacity is the operative action taken to increase resilience in ecosystems. For instance, incr easing water storage cap acity can provide a

98SAP 4.2: Thresholds of Climat e Change in Ecosystems buffer against reaching the trigger point fo r a drought-induced thre shold crossing that would permanently change an arid land ecosystem. The concept of critical loads for organisms is well established but can be productively applied to ecosystems. Based on gaps in the literatu re identified through the development process for this assessment (SAP 4.2) and the synthesis team’s expertise, tools to analyze and detect nonlinearity and thresholds from monitoring data will need to be developed. Increases in the variance of an important ecosystem metric have been suggested as an early sign of system instability. As negative feedbacks w eaken and positive feedbacks strengthen, the likelihood that a threshold will be reached and crossed increases. As identified by the synthesis team in producing this assessment, there is a need for more nonlinear modeling and statistics to be applied to the threshold issue to identify the point at which positive feedbacks dominate. 5.3.2 Triage Scientific evidence shows that climate change in the 21 st century will most likely result in new vegetation successions, wate r regimes, wildlife habitat and survival conditions, permafrost and su rface-ice conditions, coastal er osion and sea-level change, and human responses (Welch 2005). Triage is a process in which things are ranked in terms of importance or priority. The term environmental or ecological triage has been used to describe the priori tization process used by policymakers and decisionmakers to determine targets and approaches to dealing wi th resource allocation (for example, health of ecosystems) that are in high demand and rapidly changing. In the planning process, resource managers can address ecological tr iage under three differe nt priorities: 1) status
quo or do nothing; 2) reaction after disturbance; or 3) proa ctive intervention (Holt and

99SAP 4.2: Thresholds of Climat e Change in Ecosystems Viney 2001). Triage is a useful tool to prioritize actions, espe cially in cases where highly valued resources are at stake, conditions ar e changing rapidly, and decisions are urgent. The approaches to apply after triage ar e adaptive management, and mitigation and adaptation strategies. Enabling ecosystems to respond to climate chan ge will help to ease the transition from current to future stable and resilient states and to minimize threshold changes (Fitzgerald 2000; Holt and Viney 2001; Millar et al. 2007; Millar in press). 5.3.3 System-Level Planning and Policy Expanding management to regional levels is also key, because climate change may be pushing ecosystems to regional synchron y. In order to better understand and to manage thresholds, developing a regional pers pective may prove to be more effective than using a local perspective. This regional approach would take in to account large scale changes in climate regimes while still inco rporating local scale resource issues. An example is that wildland fire is synchr onously increasing throughout the western United States and could lead to majo r recruitment events for species such as lodgepole pine or trigger beetle outbreaks at unpr ecedented scales. These recrui tment events could lead to supercohorts that develop with succession fo llowing subcontinental-scale disturbance. There is little management precedent for th ese types of outcomes that are threshold events on a continental scale, even if they are common on local scales. Adaptive management and structured deci sionmaking will almost certainly be required to deal with increased temperature effects on threshol d crossings and the different trajectories of su ccession that follow in the western United States. Natural systems are out of sync with climate, leadi ng to the greatest potential for new species combinations in many centuries. Therefore, new actions may be considered, such as

100SAP 4.2: Thresholds of Climat e Change in Ecosystems planting different tree genot ypes after large-scale fires, with appropriate followup monitoring to learn from the results. 5.3.4 Capacity Building and Awareness There is, and will be, an urgent need to adapt where climate-change-induced thresholds are crossed and a new ecosystem state will be a reality for the foreseeable future. Capacity building basically increases the resilience of the socioeconomic system to tolerate different states of natural resources and ecosystem functioning (Scott and Lemieux, 2005). If ecosystems become more variable in providing essential ecosystem services, greater flexibility is needed on the human side. An example is the need to add storage capacity for capturing mountain ecosy stem water if a threshold in snow persistence is crossed, leading to smalle r and more variable snowpacks. Building stakeholder tolerance for change is part of the adaptation that will be necessary (Scott and Lemieux, 2005). Adaptation can take many forms. Scenar io planning provides descriptions of plausible future conditions. Scenario planning, done at the local level, makes stakeholders aware of the scope of uncertainty, facilita tes tolerance for change, and motivates the desire to build capacity to better handle threshold changes. Multiscenario approaches used with ecosystem modeling can also be used to develop a range of possible postthreshold conditions to better inform strate gic decisionmaking and planning for natural resource managers (Lemieux and Scott 2005). Impact assessments on specific resources (for example, population viability of indivi dual species) can be expanded to examine the underlying viability of protected areas desi gned to maintain ecosystems (Scott and

101SAP 4.2: Thresholds of Climat e Change in Ecosystems Suffling 2000). These assessments can prep are managers by broadening the scope of planning and ensuring that institutio nal action plans remain flexible. 

### 5.4 Summary As this synthesis makes clear, climat e change increases the likelihood that ecosystems will undergo threshold changes. Th e underlying mix of interacting feedback mechanisms that drive these thresholds is poorly understood. Monitoring of ecosystems to detect early indicators, such as increasing variability in system behavior, is generally inadequate even when it is known what aspect of the system to monitor. Based on gaps in the literature identified by the synthesis team, there is little scientific or natural resource management experience in dealing with eco systems undergoing threshold changes. The degree to which we can reverse a threshol d change is largely unknown. These knowledge gaps present scientists and re source managers with severe challenges in anticipating and coping with threshold change s to the natural systems. The gaps identified include the need to increase the resilience of ecosystems and reduce multiple stressors to avoid threshold crossing. Bo th of these challenges are difficult to plan for but also are consistent with managing ecosystems under conditions of uncertainty such as climate ch ange. After a threshold crossing occurs, viable options are to increase coping mechanisms, adaptive capacity, and stakeholder tolerance. The publication of this assessment (SAP 4.2) will bring the state of scientific understanding to the forefront of the natural resource manageme nt paradigm, identifying a need for greater scientific research on thres holds and ecosystem response to adequately manage natural resources for the future.

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## Chapter 6—Summary and Sc ience Recommendations

This document reviews and summarizes much of what is understood about thresholds of ecological change. This is a na scent field of inquiry and even the definition of thresholds remains somewhat fluid. The di scussion in chapter 2 clarifies what is meant by “threshold” and is intended to help fo cus future research on this topic. 

### Summary Because of the enormous role they are believed to play in the tolerance of ecosystems to climate change, the existence of thresholds should be a key concern of scientists, Federal land managers, and other na tural resource professionals responsible for the state of natural re sources and the ecological services these resources provide. Sudden large-scale changes in ecosystems may present new challenges to resource managers because the capacity to predict, manage, and adapt to threshold crossings is currently limited. One goal of resource management is to minimize the risk of declines and uncertainty in the delivery of ecological goods an d services but, as discussed in chapter 3, thresholds can precipitate such sudden declines and greatly increase management risks. Indeed, efforts by resource managers to reduc e variance in the produc tion of particular goods and services lead to a reduction inecosystem resilience and increase the probability of threshold change. Current re gulatory and legal frameworks do not account for threshold behavior of ecosystems. Fo r this reason and because the social and economic costs of these precipitous collapses are potentially high (for example, the collapse of Atlantic cod population), we recommend the following possible actions be considered as a national priority.

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### Science Recommendations Given the knowledge that eco logical thresholds exist a nd the lack of tools to predict them, scientists need to develop better predictive capabilities, and managers must make adjustments to increase their capacity to cope with surprises. If climate change is pushing more ecosystems toward thresholds, what can be done at the national level? In the development of SAP 4.2 the following poten tial actions were identified. The actions (or approaches) are organized according to th ose that can be take n before, during, and after thresholds of ecological change are crossed. Before Support Research To Identify Thresholds .—Although the existence of thresholds of ecological change is widely acknowledge d, further advancement and agreement on the nature and effects of thresholds is limited by the small number of empirical studies that address this topic. Further advancement will depend on the development and use of rigorous tests to identify thresholds reliably across different systems. Enhance Adaptive Capacity .—Given that threshold changes are increasingly likely to occur, a “no-regrets” policy to pr epare for them would enhance the capacity of the socioecological system to cope with chan ge—that is, it would in crease its resilience. To implement management changes that could reduce the likelihood of threshold changes, resource managers must first determine the factors that influence the resilience of the systems they manage. These determinations should consider the importance both of ecological diversity at patch and landscape scales a nd of economic diversity and innovation. The key components of diversity and adaptive capacity and resilience would need to be determined on a system-by-system basis and should include consideration of

104SAP 4.2: Thresholds of Climat e Change in Ecosystems soil, plant, and animal disturbance, socioeco logical factors, and cr oss-scale interactions. A key assumption is that management plan s that minimize diversity to maximize the provision of one particular ecosystem good or service are likely to increase the susceptibility of the system to threshold changes. Monitor and Adjust Mult iple Factors and Drivers. —Once the key factors that control the adaptive capacity and resilience of a system have been identified, monitoring programs may be altered to include these fact ors, as well as the resources and ecological services of management interest. For exam ple, monitoring the effects of increased salinity and (or) inundation from sea level ri se on vegetation in coastal wetlands may make it possible to predict what degree of stress vegetation can endure before it goes beyond the ability to recover (Burkett et al. , 2005). Monitoring soil conditions in areas that are susceptible to nonnative species inva sions may make it possi ble to predict when invasive species may appear in a stressed ecosystem and push it beyond its threshold. It might also be useful to monito r the variability rather than mean values of an ecological service, because an increase in the amplitude of variability is sometimes an indication of system instability before a threshold is cros sed. Another potential in dicator is a slowing in response time (recovery time) to local pert urbations; in certain th eoretical scenarios, perturbations may grow larger in amplitude with an ever-increasing period of recovery as a threshold is approached (Van Nes and Scheffer, 2007). Current understanding suggests that thresholds are likely to be triggered when resource use pressures interact with gradual changes in climate that are associated with extreme climatic events, such as extended drought periods or hurricanes. Adjusting resource use provides one of the few near-term means available to mitigate thresholds. To

105SAP 4.2: Thresholds of Climat e Change in Ecosystems enable rapid adjustments in resource use in at-risk places and time periods, it would be useful to put in place finer-grained clim ate and ecosystem monitoring systems coupled with administrative mechanisms to expedite policy modifications. Develop Scenarios To Explore Alternative Management Options for Dealing With Potential Changes .—The types of changes that caus e threshold changes often are well known in advance (for example, hurricanes, wi ldfire, or invasive species). Scenario analysis with well-characterized dynamics can explore the potential consequences of taking actions either to redu ce the likelihood of threshold ch ange or minimize the impact of changes that occur. In this way, scenar ios can provide managers with tools for action before the crisis occurs. Collate and Integrate Informati on Better at Different Scales. —Cross-scale interaction, where change in a large-scale vari able, such as climate, alters a local-scale driver of threshold change, such as fire, is a great challenge in assessing and preventing threshold change. Greater efficiency and us e of information is likely to result from coordinating and pooling information from adjo ining jurisdictions and different agencies. For example, trends that are not significant or noticeable at small scales may be clear at larger scales. These and other observations argue for much better integration and coordination of monitoring information, not necessarily more monitoring. Although considerable investment would be needed to make monitoring “smarter” initially, the payoff would be the ability to detect early indicators of ecosystem change that could result in a threshold crossing. Reduce Other Stressors. —The points that may trigger an abrupt change in an ecosystem that is responding to climate change are rarely known because human

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civilizations have not witnessed climate change of this magnitude. However, the likelihood of crossing a threshold is most likely lessened by reducing other stressors on the ecosystem (Scott and Lemieux, 2005; Juliu s et al., 2008). These other stressors might include air and water pollution, regional landscape fragmentati on, and control of invasive plants. To help reduce stressors, decisions could be made to allow larger or more extensive buffers when considering carrying capacity of habitats, minimum habitat sizes for species of interest, or use of ecological services, such as water. During Manage Threshold Shifts. —There may be constraints to reducing or reversing climate-change-induced stresses to component s of an ecosystem. If a threshold seems likely to occur but the uncertainties remain high as to when it will occur, contingency plans can be created (Julius et al., 2008). These plans can be implemented when the threshold shift begins to occur or they can be carried out in advan ce if the onset of the threshold crossing is imminent. Take, for exam ple, an Alpine area in which trees have begun to grow at higher elevations than the current tree line because reduced snowpack has lengthened the growing season. If this tree invasion of formerly open areas reduces animal movement between adjoining mountai n areas, movement corridors can be kept open by mechanical clearing of trees. Project Impacts to Natural Resources. —Many efforts are underway to project climate change ( for example , Global Climate Models) and ecosystem responses to climate change (for example, mapped atmosphere-plant-soil systems) using simulation models and other tools. These models genera lly project ecosystem trends and shifts, but they do not explicitly consider the possibili ty of thresholds as part of the system

107SAP 4.2: Thresholds of Climat e Change in Ecosystems dynamics. To project impacts to natural resources accurately, it is necessary to understand, model, and project ecosystem re sponses to climate change with explicit acknowledgment of thresholds. An example of how the inclusion of thresholds in modeling would be beneficial is the bark beetle outbr eak now occurring in Western forests where one threshold was passed when warmer winters allowed two lifecycles of beetle reproduction per year rather than one and where a second threshold may be passed by the expansion of the forests northward to connect with boreal forests that provide a corridor eastward. Such a scenario could lead to continental-scale beetle infestation (Logan et al., 1998). Recognize Need for Decisionm aking at Multiple Scales. —Climate change often expresses itself across regi onal boundaries which transc end local jurisdiction and management boundaries. The scale of some thres hold crossings, such as the bark beetle example above, is likely to requ ire coordinated decisi ons on larger scales than in the past. Because of different agency management manda tes, levels of resources, or geographic scope, the potential exists for agencies to work at cross-purposes when coping with threshold effects at large scales. Also, the effectiveness of response can be enhanced through economies of scale if several agencies work on the problem simultaneously. Instigate Institutional Change To Increase Adaptive Capacity. —The capacity for synthesis is a critical component of identifying potentia l thresholds in ecosystem processes on multiple scales. Institutional ch anges that promote greater interdisciplinary and interagency scientific and informati on exchange are likely to increase adaptive capacity in general. Such institutional ch anges would be especially helpful when

108SAP 4.2: Thresholds of Climat e Change in Ecosystems implementing comprehensive monitoring to detect and document responses to thresholds in ecosystems. Identify Research Needs and Priorities To Address Thresholds. —Identifying research needs in general can help when ev aluating calls for specific threshold research. The ubiquity of threshold problems across so many fields suggests the possibility offinding common principles at work. The cross-cutting nature of the problem of largescale system change suggests an unusual opportu nity to leverage effort from other fields and apply it to investigating the systemic risk of crossing thres holds. Ecological and economic systems share common elements as complex adaptive systems. To the extent that the analogy holds, these two disciplines have potential for mutual leverage. Beyond the specific analogy between ecology and economics, certain dynamic behaviors and structural (topological network) constraints are common to broad classes of systems. Leverage can also occur by sharing methods acr oss disciplines. Such diverse fields as engineering risk analysis, epidemiology, and ecology employ similar methods and research styles. The aim is not to repl ace conventional approaches but to explore complementary approaches. Exploiting commo nalities is one way that leverage is achieved. As a further reality check on investme nts in research and development, management agencies can expand on efforts to examine their bottom-line performance as a normal part of the feedback and evaluati on process. For many agencies, this will involve evaluating actual forecast skill as a m easure of merit, rather than post-hoc fitting and correlation (the products of which ma y fit an existing paradigm but lack any predictive skill). Obtaining ground truth on th is level can validate whether classical

109SAP 4.2: Thresholds of Climat e Change in Ecosystems management concepts, such as maximum sustained yield in fisheries and other equilibrium concepts and models, are suffici ently useful to be predictive. A periodic evaluation process based on actual (real time ) predictive power should indicate whether the model paradigm currently in use is an ad equate representation of real systems, and whether the current direction of investment s in research and development are on track. This level of verification is essential for e ffective management of threshold transitions. After Although many of the management responses to thresholds should be continued after thresholds have been crossed (for example, monitoring and building ecosystem resilience), human society will largely be faced with adjust ing to different ecosystems. These adaptations may be expensive, requiring significant new physical and administrative infrastructures. Capacity building, scenario planning, and adaptive management must all be applied to quickly improve the ability of management to cope with a different ecosystem and for stakeholders to adjust their expectations of ecosystem services. 

### Conclusion There is a need to develop a deeper unde rstanding of thresh olds of ecological change, especially given our current relative inability to predict wh en and where they will occur. There have, however, been enough o ccurrences with significant economic and social costs to warrant cons ideration of thresholds in natural resource planning and management. Threshold threats to many ecosystems are threats to long-term sustainability of human users as well as biodiversity and biological adaptive capacity. This document has summarized much of what is known about thresholds and has

110SAP 4.2: Thresholds of Climat e Change in Ecosystems suggested approaches to improve understandi ng of thresholds, to reduce the chances of threshold crossing, and to enhance the ability to cope with thresholds that have occurred. Given the magnitude of climate change effect s on ecosystems, the added factor of sudden threshold changes complicates societal re sponses and underscores the importance of continued integration of resear ch and management to devel op appropriate strategies for coping with thresholds.

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## Appendix A—Glossary
adaptive capacity the capacity of organisms, both individuals and groups, to respond to and change in the state of the system (Folke et al., 2003; Wa lker et al., 2004; Adger et al., 2005); depends on initial diversity and the capacity of co mponent organisms to adjust and change bioerosion describes the erosion of hard ocean s ubstrates by living organisms by physical mechanisms such as boring, drilling, raspi ng, and scraping or by chemical mechanisms for dissolution. degradation deterioration of a system to a less desirable st ate as a result of failure to actively adapt or transform degree heating week (DHW) the NOAA satellite-derived Degree Heating W eek (DHW) is an experimental product designed to indicate the accumulated thermal stress that coral reef s experience. A DHW is equivalent to one week of sea surface temperature 1 deg C above the expected summertime maximum. For example, 2 DHWs indicate one week of 2 deg C above the expected summertime maximum ecosystem all the organisms, including people, in an area and the nonbiological materials, such as water and soil minerals, with which they interact ecosystem services

154SAP 4.2: Thresholds of Climat e Change in Ecosystems benefits that people derive from ecosystems, including supporting, provisioning, regulating, and cultural services
exogenous factor factor external to the system being managed and which therefore is not incorporated into the management framework
exposure nature and degree to which the system experi ences environmental or sociopolitical stress mitigation

r eduction in the exposure of a sy stem to a stress or hazard negative feedbacks
i nteraction in which the eff ects of two interacting components on one another have opposite signs; generally buffer against change s in the system; an important mechanism enhancing resilience positive feedback
i nteraction in which the effect s of two interacting components on one another have the same sign (both positive or both negative); tend to amplify changes in the system, leading to threshold changes in the system regime shift sudden shifts in biota that are driven by ocean climate events resilience capacity of a socioecological system to absorb a spectrum of shocks or perturbations and continue to develop with sim ilar fundamental function, structure, identity, and feedbacks, that is, to remain within a given stability domain (Holling, 1973; Gunderson and Holling.

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156 2002; Walker et al., 2004; Folk e, 2006a); includes adaptive capacity but also depends on legacies (for example, seed banks) and st rong negative feedbacks that might balance positive feedbacks that might destabilize the system socioecological system system in which human activities depe nd on resources and services provided by ecosystems and ecosystem organization is influenced, to varying degrees, by human activities steady state

c ondition of a system in which there is no net change in system structure or functioning over the time scale of study sustainability use of the environment and resources to meet the needs of the present without compromising the ability of future ge nerations to meet their own needs
threshold As defined in this assessment, an ecological threshold is the point at which there is an abrupt change in an ecosyst em quality, property, or phenomenon, or where small changes in one or more external conditions produ ce large and persistent responses in an ecosystem. vulnerability the degree to which a system is likely to e xperience harm due to exposure to a specified hazard or stress (Turner et al., 2003; Adger, 2006)SAP 4.2: Thresholds of Climat e Change in Ecosystems



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## Climate Change Science Program and the Subcommittee on Global Change Research
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