CHAPTER 6 

ECOLOGICAL CONDITION 

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ECOLOGICAL CONDITION CHAPTER CONTENTS 

6.1 INTRODUCTION ........................................................................................................................6-5

6.1.1 The Ecological Condition Paradigm................................................................................6-6

6.1.2 Overview of the Data .......................................................................................................6-7

6.1.3 Organization of This Chapter...........................................................................................6-8

6.2 WHAT ARE THE TRENDS IN THE EXTENT AND DISTRIBUTION OF THE 

NATION’S ECOLOGICAL SYSTEMS? ..................................................................................6-11

6.2.1 Introduction....................................................................................................................6-11

6.2.2 ROE Indicators ..............................................................................................................6-13

INDICATOR: Forest Extent and Type ........................................................................6-14

INDICATOR: Forest Fragmentation ...........................................................................6-17

INDICATOR: Ecological Connectivity in EPA Region 4...........................................6-20

INDICATOR: Relative Ecological Condition of Undeveloped Land in EPA 

Region 5.........................................................................................................................6-22

6.2.3 Discussion......................................................................................................................6-25

What These Indicators Say about Trends in Extent and Distribution of the Nation’s 

Ecological Systems........................................................................................................6-25

Limitations, Gaps, and Challenges ................................................................................6-27

6.3 WHAT ARE THE TRENDS IN THE DIVERSITY AND BIOLOGICAL BALANCE OF 

THE NATION’S ECOLOGICAL SYSTEMS?..........................................................................6-29

6.3.1 Introduction....................................................................................................................6-29

6.3.2 ROE Indicators ..............................................................................................................6-30

INDICATOR: Bird Populations ..................................................................................6-32

INDICATOR: Fish Faunal Intactness..........................................................................6-34

INDICATOR: Non-Indigenous Species in the Estuaries of the Pacific Northwest.....6-37

6.3.3 Discussion......................................................................................................................6-40

What These Indicators Say About Trends in the Diversity and Biological Balance of 

the Nation’s Ecological Systems ...................................................................................6-40

Limitations, Gaps, and Challenges ................................................................................6-41

6.4 WHAT ARE THE TRENDS IN THE ECOLOGICAL PROCESSES THAT SUSTAIN THE 

NATION’S ECOLOGICAL SYSTEMS? ..................................................................................6-43

6.4.1 Introduction....................................................................................................................6-43

6.4.2 ROE Indicators ..............................................................................................................6-44

INDICATOR: Carbon Storage in Forests....................................................................6-45

6.4.3 Discussion......................................................................................................................6-48

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What This Indicator Says About Trends in the Ecological Processes that Sustain the 

Nation’s Ecological Systems .........................................................................................6-48

Limitations, Gaps, and Challenges ................................................................................6-48

6.5 WHAT ARE THE TRENDS IN THE CRITICAL PHYSICAL AND CHEMICAL 

ATTRIBUTES AND PROCESSES OF THE NATION’S ECOLOGICAL SYSTEMS?..........6-50

6.5.1 Introduction....................................................................................................................6-50

6.5.2 ROE Indicators ..............................................................................................................6-51

INDICATOR: U.S. and Global Mean Temperature and Precipitation ........................6-53

INDICATOR: Sea Surface Temperature .....................................................................6-58

INDICATOR: Sea Level..............................................................................................6-61

6.5.3 Discussion......................................................................................................................6-67

What These Indicators Say About Trends in Critical Physical and Chemical 

Attributes of the Nation’s Ecological Systems..............................................................6-67

Limitations, Gaps, and Challenges ................................................................................6-70

6.6 WHAT ARE THE TRENDS IN BIOMARKERS OF EXPOSURE TO COMMON 

ENVIRONMENTAL POLLUTANTS IN PLANTS AND ANIMALS? ...................................6-72

6.6.1 Introduction....................................................................................................................6-72

6.6.2 ROE Indicators ..............................................................................................................6-72

6.6.3 Discussion......................................................................................................................6-73

What These Indicators Say About Trends in Biomarkers of Exposure to Common

Environmental Pollutants in Plants and Animals...........................................................6-73

Limitations, Gaps, and Challenges ................................................................................6-74

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6.1 INTRODUCTION 

Ecological condition refers to the state of the physical, chemical, and biological characteristics of the 

environment, and the processes and interactions that connect them. Understanding ecological condition is 

crucial, because humans depend on healthy ecological systems for food, fiber, flood control, and other 

benefits,1

 and many Americans attribute deep significance and important intangible benefits to ecological 

systems and their diverse flora and fauna.2

 As noted in the introduction to this report, this chapter focuses 

on critical characteristics of ecosystems that are affected simultaneously by stressors in multiple media, 

rather than those whose trends can be definitively shown to be the results of trends in particular air, water, 

or land stressors. The ability to report on ecological condition remains significantly limited by the lack of 

indicators, but this chapter at least provides a framework for examining those conditions. 

EPA’s mission, broadly stated, is “to protect human health and to safeguard the natural environment–air, 

water, and land–upon which life depends.”3

 The translation of the mission into programs, initiatives, and 

research efforts continues to evolve within the Agency and is reflected in program goals, regulatory 

programs, and collaborative and educational efforts. EPA, other federal agencies, and state agencies 

collectively bear responsibility for ensuring the protection of ecological systems, including forests, public 

lands, oceans and estuaries, and particular species or groups of species. Trends in ecological conditions 

provide insight into the degree to which the natural environment is being protected. 

In this chapter, EPA seeks to assess trends in critical attributes of ecological condition on a national scale 

using indicators to address five fundamental questions: 

• What are the trends in the extent and distribution of the nation’s ecological systems? This 

section examines trends in the overall extent (e.g., area and location) of different kinds of 

ecological systems (e.g. forests, undeveloped lands, and watersheds) and of spatial patterns in 

the distribution of ecological systems that affect interactions of nutrients, energy, and 

organisms. 

• What are the trends in the diversity and biological balance of the nation’s ecological 

systems? This section explores trends in the types and numbers of species that live within 

ecological systems. The section also examines biological balance in terms of the proportional 

distributions of species and the influence of interactions among native and invasive species 

on the stability of ecological systems. 

• What are the trends in the ecological processes that sustain the nation’s ecological 

systems? This question focuses on trends in the critical processes that sustain ecological 

systems, such as primary and secondary productivity, nutrient cycling, decomposition, and 

reproduction. 

• What are the trends in the critical physical and chemical attributes and processes of the 

nation’s ecological systems? This question addresses trends in physical attributes such as 

1

 Daily, G.C, ed. 1997. Nature’s services: societal dependence on natural ecosystems. Washington, DC: Island Press. 

2

 Norton, B. 1988. Commodity, amenity, and morality: the limits of quantification in valuing biodiversity. In: 

Wilson, E.O., ed. Biodiversity. Washington, DC: National Academy Press. p. 521. 

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 U.S. EPA. About EPA. <http://www.epa.gov/epahome/aboutepa.htm#mission> 

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climatological patterns, hydrology, and light. This section also examines how chemical 

processes influence conditions such as pH, oxidation-reduction potential, and nutrient cycles. 

• What are the trends in biomarkers of exposure to common environmental pollutants in 

plants and animals? This question examines trends in biomarkers of exposure to pollutants 

that are particularly important to the health of plants and animals as well as to humans who 

consume such organisms. 

These ROE questions are posed without regard to whether indicators are available to answer them. This 

chapter presents the indicators available to answer these questions, and also points out important gaps 

where nationally representative data are lacking. 

While the ecological (and, in the previous chapter, human health) indicators may be more or less directly 

influenced by pollutants, other environmental stressors, and complex interactions among these factors, the 

indicators are not intended to confirm direct causal relationships. 

6.1.1 The Ecological Condition Paradigm 

Because ecological systems are dynamic assemblages of organisms that have more or less continuously 

adapted to a variety of natural stresses over shorter (e.g., fire, windstorms) and longer (climate variations) 

periods of time, measuring ecological condition is a complicated endeavor. It is not as straightforward as 

monitoring water or air for temperature or concentrations of pollutants. The complexity of interactions 

comprising ecological systems makes determination of the condition of a natural system difficult.4

 In 

addition, people have altered natural ecological systems to increase the productivity of food, timber, fish, 

and game and to provide the infrastructure needed to support a modern society. How should the 

ecological condition of these altered ecological systems be measured and against what reference points? 

Ecological systems are not necessarily naturally occurring entities with well-defined, mutually exclusive 

boundaries; rather, they are constructs with boundaries determined for human scientific or management 

purposes. Consequently there are many ways to define ecological systems, including by the predominant 

biota, spatial scales, and physical characteristics. These factors further complicate the definition and 

measurement of ecological condition. Several recent reports by experts in the field have provided 

guidance for current and future efforts, however. 

The National Research Council (NRC) report Ecological Indicators for the Nation5 provides an 

introduction to recent national efforts to measure ecological condition and a thoughtful discussion of the 

rationale for choosing indicators. EPA’s Science Advisory Board (SAB) also proposed a Framework for 

Assessing and Reporting on Ecological Condition.6 The framework identified six essential ecological 

attributes of ecological systems: landscape condition, biotic condition, chemical and physical 

characteristics, ecological processes, hydrology and geomorphology, and natural disturbance regimes. 

4 Ehrenfeld, D.H. 1992. Ecosystem health and ecological theories. In: Costanza, R., et al., eds. Ecosystem health: 

new goals for environmental management. Washington, DC: Island Press. pp. 135-143. 

5 National Research Council. 2000. Ecological indicator for the nation. Washington, DC: National Academies Press. 

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 U.S. EPA. 2002. A framework for assessing and reporting on ecological condition: an SAB report. 

EPA/SAB/EPEC-02/009. <http://www.epa.gov/sab/pdf/epec02009a.pdf> 

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The SAB report is organized around questions 

about trends in each of these attributes, 

consolidating the last three into a single attribute. 

Neither report identifies specific methodologies, 

network designs, or actual datasets. The SAB and 

NRC documents provide the foundation for the 

questions that are addressed within this chapter. 

Exhibit 6-1 is a conceptual depiction of the events 

that link environmental changes and ecological 

outcomes in this paradigm. “Stressors,” indicated 

by arrows, represent factors such as insect 

outbreaks or 

pollutants affecting the system. These act directly 

on one or more of the "essential ecological 

attributes" shown in the circles in the center of the 

diagram. Most of these attributes can, in turn, act 

on and be acted on by others. The web of arrows 

among the indicators illustrates some of the 

possible interactions. Effects on ecological 

attributes can be direct or indirect. The diagram 

illustrates the fact that changes in ecological 

structure and processes provide important 

feedback on the chemical and physical structure 

of the environment in which these changes occur. The overall changes in the attributes result in altered 

structure and function of ecological systems, which in turn lead to outcomes (positive or negative) about 

which society is concerned. 

There have been other notable efforts conducted by EPA and other federal agencies and institutions to 

describe the ecological condition of the nation, either in total or by type of ecological systems. These 

efforts include both indicator-based and integrative approaches. The indicator-based approaches, such as 

this report, use indicators to assess ecological condition. The integrated assessments do not rely on 

indicators; rather, they comprehensively assess a wide range of data in order to arrive at an overall picture 

of the status and trends in ecological systems. Indicator approaches offer the advantage of drawing 

attention to important trends and don’t require an extensive background in ecology, but are not able to 

capture the complex interactions that characterize ecological systems. 

6.1.2 Overview of the Data 

This chapter, like the others in this report, is not intended to be exhaustive. Rather, it provides a snapshot, 

at the national level, of current indicators of U.S. ecological condition and the status based on important 

endpoints and data sources with study design, quality assurance, and maturity. Because ecological 

condition depends critically on the physical and chemical characteristics of land, air, and water, this 

chapter draws on indicators from Chapters 2 through 4 of this report. Those chapters should be consulted 

for the data sources of those indicators. Many of the indicators continue to be drawn from The H. John 

Heinz III Center for Science, Economics, and the Environment (The Heinz Center) report, The State of 

the Nation’s Ecosystems: Measuring Lands, Waters, and Living Resources of the United States (2002; 

Web update 2005). 

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Most of the data relied upon come from surveillance and monitoring surveys. The key data sources for 

this chapter reflect the fact that monitoring ecological condition is a multi-organizational task. 

Organizations in addition to EPA that are responsible for collecting the data to support indicators in this 

chapter include the U.S. Department of Commerce (National Oceanic and Atmospheric Administration), 

National Aeronautics and Space Administration, U.S. Department of Agriculture (Forest Service, 

Agricultural Research Service, National Agricultural Statistics Service, and Natural Resource 

Conservation Service), U.S. Department of Interior (U.S. Geological Survey and U.S. Fish and Wildlife 

Service), and NatureServe (a private foundation). 

Programs such as the U.S. Department of Agriculture Forest Inventory and Analysis (FIA) program and 

the Natural Resources Inventory (NRI) have a long history because they measure aspects of the 

environment that are critical to multi-billion dollar industries (e.g., timber, crops, etc.). Programs with a 

strictly “ecological” focus (e.g., the USDA Forest Service Forest Health Monitoring [FHM] Program, the 

U.S. Geological Survey National Water Quality Assessment Program [NAWQA], the multi-agency 

Multi-Resolution Land Characterization Consortium [MRLC], and EPA’s Environmental Monitoring and 

Assessment Program [EMAP]) are more recent, but equally informative. 

The major challenges involve adequate coverage of the diverse aspects that comprise ecological 

condition. For example, there are numerous groups of animals and plants, but there are ROE indicators 

for only some of these. Major groups known to be undergoing changes, such as the amphibians, are not 

captured by the ROE indicators. These challenges and limitations are described in each of the subsections. 

This chapter presents only data that meet the ROE indicator definition and criteria (see Chapter 1, 

Introduction). Note that non-scientific indicators, such as administrative and economic indicators, are not 

included in this definition. Thorough documentation of the indicator data sources and metadata can be 

found online at [insert url]. All indicators were peer-reviewed during an independent peer review process 

(see [insert url] for more information. Readers should not infer that the indicators included reflect the 

complete state of knowledge on current indicators of U.S. ecological condition. Many other data sources, 

publications, and site-specific research projects have contributed to the current understanding of status 

and trends in indicators of U.S. ecological condition, but are not used in this report because they do not 

meet some aspect of the ROE indicator criteria. 

6.1.3 Organization of This Chapter 

The remainder of this chapter is organized into five sections, corresponding to the five questions EPA is 

seeking to answer regarding trends in ecological condition. Each section introduces the question and its 

importance, presents the National Indicators selected to help answer the question, and discusses what the 

indicators, taken together, say about the question. Some of the National Indicators presented are broken 

down by EPA Regions or other appropriate regions. In addition, several Regional Indicators are presented 

that capture regional trends of particular interest to EPA Regions. These Regional Indicators serve as 

models that could potentially be expanded to other EPA Regions in the future. A map showing the EPA 

regions (and states within each region) is provided in Chapter 1 (Exhibit 1-1).Each section concludes by 

highlighting the major challenges to answering the question and identifying important gaps and emerging 

issues. 

The table below shows the indicators used to answer each question and the location where they are 

presented. 

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1 Table 6.1.1. Ecological Condition—ROE Questions and Indicators 

Question Indicator Name Section Page # 

What are the trends in the extent 

and distribution of the nation’s 

ecological systems? 

Land Cover (N/R) 

Forest Extent and Type (N/R) 

Forest Fragmentation (N/R) 

Wetland Extent, Change and Sources of Change 

(N) 

Land Use (N) 

Urbanization and Population Change (N) 

Land Cover in the Puget Sound/Georgia Basin 

(R) 

Ecological Connectivity in EPA Region 4 (R) 

Relative Ecological Condition of Undeveloped 

Land in EPA Region 5 (R) 

4.2.2 

6.2.2 

6.2.2 

3.4.2 

4.3.2 

4.3.2 

4.2.2 

6.2.2 

6.2.2

4-11 

6-14 

6-17 

3-53 

4-24 

4-31 

4-17 

6-20 

6-22 

What are the trends in the 

diversity and biological balance of 

the nation’s ecological systems? 

Coastal Benthic Communities (N/R) 

Benthic Macroinvertebrates in Wadeable 

Streams (N) 

Bird Populations (N) 

Fish Faunal Intactness (N) 

Submerged Aquatic Vegetation in the 

Chesapeake Bay (R) 

Harmful Algal Bloom Outbreaks Along the 

Western Florida Coastline (R) 

Non-Indigenous Species in the Estuaries of the 

Pacific Northwest (R) 

3.5.2 

3.2.2 

6.3.2 

6.3.2 

3.5.2 

3.5.2 

6.3.2 

3-71 

3-35 

6-32 

6-34 

3-74 

3-81 

6-37 

What are the trends in the 

ecological processes that sustain 

the nation’s ecological systems? 

Carbon Storage in Forests (N) 6.4.2 6-45 

What are the trends in the critical 

physical and chemical attributes 

of the nation’s ecological systems? 

U.S. and Global Mean Temperature and 

Precipitation (N) 

Sea Surface Temperature (N) 

High and Low Stream Flows (N) 

Streambed Stability in Wadeable Streams (N) 

Sea Level (N) 

Nitrogen and Phosphorus Discharge from Large 

Rivers (N) 

Nitrogen and Phosphorus in Wadeable Streams 

(N) 

Nitrogen and Phosphorus in Streams in 

Agricultural Watersheds (N) 

Lake and Stream Acidity (N) 

Extent of Hypoxia in the Northern Gulf of 

Mexico and in Long Island Sound (R) 

6.5.2 

6.5.2 

3.2.2 

3.2.2 

6.5.2 

3.2.2 

3.2.2 

3.2.2 

2.2.2 

3.5.2 

6-53 

6-58 

3-14 

3-19 

6-61 

3-28 

3-22 

3-25 

2-62 

3-77 

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Question Indicator Name Section Page # 

What are the trends in biomarkers 

of exposure to common 

environmental pollutants in plants 

and animals? 

Coastal Fish Tissue Contaminants (N/R) 

Ozone Injury to Forest Plants (N) 

Contaminants in Lake Fish Tissue (N) 

3.8.2 3-103 

2.2.2 2-37 

3.8.2 3-107 

1 N = National Indicator 

2 

3 

4 

R = Regional Indicator

N/R = National Indicator displayed at EPA Regional scale

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6.2 WHAT ARE THE TRENDS IN THE EXTENT AND DISTRIBUTION OF THE 

NATION’S ECOLOGICAL SYSTEMS? 

6.2.1 Introduction 

Ecological systems,7

 ranging from forests and watersheds to wetlands and coral reefs, are the foundation 

of the environment. An ecological system can be defined as a spatially explicit unit of the Earth that 

includes all of the organisms, along with all components of the abiotic environment, within its 

boundaries. Ecological systems are not isolated but blend into and interact with other systems. The spatial 

coverage and arrangement of ecological systems influence the types of animals and plants that are 

present; the physical, chemical, and biological processes in the system; and the resiliency of the systems 

to perturbations.8

 Ecological systems influence water and nutrient cycles, the building of soils, the 

production of oxygen, sequestration of carbon, and many other functions important for the health of the 

planet and the people who depend on them. 

This section examines trends in the extent and distribution of ecological systems. Extent refers to the 

physical coverage of an ecological system. This may be reflected as area or percent compared to a 

baseline or total area. Distribution includes the pattern or arrangement of the components of an ecological 

system and is dependent on the scale of analysis. For example, the national distribution of forests can be 

estimated by a percent coverage, but within a stand of trees the distribution may involve patterns of gaps, 

species, and edge/interior ratios. As noted in Section 6.1.1, ecological systems can be defined by 

predominant biota, spatial scales, and physical characteristics. Extent indicators typically are based on 

physical and biological characteristics that are observable by remote sensing, with indistinct boundaries 

operationally defined according to some scientific or resource management construct.9 

As noted in Chapter 1 (Introduction), safeguarding the natural environment is an integral part of the 

EPA’s mission. EPA traditionally has been most concerned with maintaining the quality of air, water, and 

land necessary to support balanced biological communities and the processes that support them; however, 

the success of these efforts requires that ecological systems not be altogether lost or fragmented. The 

potential influences of pollutants on the extent and distribution of ecological systems are a prime concern, 

and, in turn, the extent and distribution of ecological systems has far-reaching influences on air and water 

quality. 

Apparent trends in extent and distribution of ecological systems depend on the temporal and spatial scale 

of assessment. For this reason, National and Regional Indicators are particularly valuable. Temporal 

changes occur naturally over long time scales, such as those associated with geological and climatological 

forces (e.g., glaciation). Change can also occur more quickly as a result of direct shifts in land use (e.g., 

forest to development and historical filling of wetlands), alterations of nutrient and hydrological cycles 

7

 Likens, G. 1992. An ecosystem approach: its use and abuse. Excellence in ecology, book 3. Oldendorf/Luhe, 

Hermany: Ecology Institute. 

8 Wilson, E.O. 1992. The diversity of life. Cambridge, MA: Belknap Press, 1992. 

9

 The H. John Heinz III Center for Science, Economics, and the Environment. 2005. The state of the nation’s 

ecosystems: measuring the lands, waters, and living resources of the United States. New York, NY: Cambridge 

University Press, September 2002. Web update 2005. <http://www.heinzctr.org/ecosystems/forest/frgmnt.shtml> 

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(e.g., dam removal), introduction of invasive species (e.g., Asian carp), hazardous waste exposure (e.g., 

acid rain), or extreme weather events, which all act over comparatively short time periods. Thus, trends 

can be the result of natural forces or may be accelerated by human activity. 

The spatial scale of alterations also represents a significant factor in tracking ecological condition. 

Alterations that are short in duration and local in nature (e.g., seasonal droughts or a windfall in a closed 

forest canopy) may not have large-scale or lasting effects on ecological systems. Alterations that are 

chronic in nature and occur over large areas may affect entire ecosystems over long periods of time, 

especially if they affect soil formation, microclimate, refugia for recolonizing species, etc. Particularly 

relevant discussions of the importance of scale in ecological processes, monitoring, and management can 

be found in a number of relatively recent publications.10,11,12 

Different regions and different ecological systems respond to stressors in different ways, resulting in 

unique regional distributions of species and habitats. The result is that across any slice of landscape the 

extent and distribution of ecological systems may shift.13 In the case of habitat loss, large impacts may 

occur and the extent of coverage may be reduced or eliminated altogether. More subtle changes in 

ecological systems can occur which are not captured in simple metrics of extent and distribution. These 

are discussed in later sections of this chapter. 

Fragmentation, the division of previously uninterrupted habitat, can have either negative or positive 

impacts on communities.14 Examples of fragmentation include building highways through a forest, 

damming a river in a manner that limits migration of fish, or developing waterfronts in a manner that 

splits apart bordering marshlands. Fragmentation and the increasing area of edge habitat may force 

migrating species to find new transport corridors, may allow new species (e.g., competitors, pathogens, 

weeds) to enter areas previously blocked from immigration, and in some cases may actually increase 

biodiversity.

15 Regardless of the impact, fragmentation likely will result in shifting distributions of 

species. 

10 Peterson, D.L., and V. Thomas Parker. 1998. Ecological scale: theory and applications. New York: Columbia 

University Press. 615 pp. 

11 Niemi, G., and M. McDonald. 2004. Application of ecological indicators. Annu. Rev. Ecol. Evol. Syst. 35:89­

111. 

12 Findlay, C.S., and L. Zheng. 1997. Determining characteristic stressor scales for ecosystem monitoring and 

assessment. J. Environ. Manage. 50(3):265-281. 

13 The H. John Heinz III Center for Science, Economics, and the Environment. 2005. Forest pattern and 

fragmentation. In: The state of the nation’s ecosystems: measuring the lands, waters, and living resources of the 

United States. New York, NY: Cambridge University Press, September 2002. Web update 2005. 

<http://www.heinzctr.org/ecosystems/forest/frgmnt.shtml> 

14 Fahrig, L. 1997. Relative effects of habitat loss and fragmentation on population extinction. J. Wildl. Manage. 

61(3):603-610. 

15 Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity. Annu. Rev. Ecol. Syst. 34:487-515. 

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Trends in ecological system extent and distribution are highly dependent on the evaluation scale. At one 

scale, coastal wetlands may appear to be uninterrupted and uniform. However, at a more refined scale, 

edges, patches, corridors associated with tidal creeks, and discontinuous distributions of species become 

evident. Defining systems in terms of local organization or predominant species facilitates discussion and 

analysis, but may also obscure the important linkages among systems across landscapes. Therefore, while 

it is helpful to discuss trends in the extent and distribution of systems such as wetlands or forests, each 

system is tied into global water, nutrient, carbon, and energy cycles. 

The indicators discussed in this section fall into three broad categories: indicators of the extent and 

distribution of forests, indicators of the extent and distribution of wetlands, and indicators of land use. 

6.2.2 ROE Indicators 

In this question, trends in the extent and distribution of ecological systems are evaluated for a subset of 

systems including forests, wetlands, undeveloped lands, and developed lands. 

To answer the question on extent and distribution of ecological systems, this report relies primarily on six 

National Indicators and three Regional Indicators (Table 6.2.1). Data on trends in extent and distribution 

of ecological systems come from a variety of sources, including satellite remote sensing, geographic 

information systems, and independent field studies. Information for the indicators discussed in this 

section is drawn from several national assessments including the USDA Forest Service Forest Inventory 

Analysis System, the National Wetlands Inventory Status and Trends Survey, the National Level Cover 

Data Set (NLDC) for 1992, and the USDA National Resources Inventory. 

Table 6.2.1. ROE Indicators of Trends in Extent and Distribution of the Nation’s Ecological 

Systems 

NATIONAL INDICATORS LOCATION 

Land Cover (N/R) 4.2.2 – p. 4-11 

Forest Extent and Type (N/R) 6.2.2 – p. 6-14 

Forest Fragmentation (N/R) 6.2.2 – p. 6-17 

Wetland Extent, Change and Sources of Change 3.4.2 – p. 3-53 

Land Use 4.3.2 – p. 4-24 

Urbanization and Population Change 4.3.2 – p. 4-31 

REGIONAL INDICATORS 

Land Cover in the Puget Sound/Georgia Basin 4.2.2 – p. 4-17 

Ecological Connectivity in EPA Region 4 6.2.2 – p. 6-20 

Relative Ecological Condition of Undeveloped Land in EPA Region 5 6.2.2 – p. 6-22 

N/R = National Indicator displayed at EPA Regional scale 

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INDICATOR: Forest Extent and Type 

The forests of the U.S. cover extensive lands in both the eastern and western thirds of the country. While 

the amount of forest land has remained nearly unchanged since the beginning of the 20th century, regional 

changes both in amount and types of forest cover have occurred as a result of changing patterns of 

agriculture and development. The distribution of various forest cover types is a critical determinant of the 

condition of forest ecosystems. 

This indicator is based on data from the USDA Forest Service Forest Inventory and Analysis (FIA) 

system. The FIA program, using a statistical survey design and comparable methods across the U.S., 

collects various data that help assess the extent, type, age, and health of forest land in the United States. 

Because the surveys are repeated over time, the FIA data provide an indication of trends in both the extent 

and composition of forest land. The extent data are collected for all forest lands across the nation, but 

species composition data over time are only available for timberland as defined by FIA data collection 

procedures (that is, forests capable of producing at least 20 cubic feet per acre per year of industrial wood 

and not withdrawn from timber utilization by statute or regulation). Timberland makes up 94 percent of 

the forest land area in the eastern U.S. and 39 percent of forest land in the western U.S. as of 2002 (Smith 

et al., 2004). Extent data are collected for individual states, but have been summarized by EPA Region for 

this indicator.

What the Data Show

After a slight increase in forest land nationwide between 1907 and 1938, forest acreage decreased by 

more than 16 million acres between 1938 and 1977, before increasing by 5.3 million acres over the past 

three decades (Exhibit 6-2). There are variations in trends in forest cover among the different EPA 

Regions. For example, between 1907 and 2002, forest land declined by roughly 22 million acres in 

Region 6 and more than 12 million acres in Region 9. Over the same period, forest land increased by 13 

million acres in Region 3 and by 10 million acres in Region 5. 

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In addition to changes in the extent of forest, there have been changes in the types of forests over time 

(Exhibits 6-3 and 6-4). The largest changes in the eastern U.S. over the period 1953-2002 occurred in the 

maple-beech-birch forest type and the oak-hickory forest type, gaining 27.5 million acres and 23 million 

acres, respectively, since 1953. In the West, the fir-spruce type and Western hardwood type also have 

increased (about 11.5 million acres each) since 1953, while the hemlock-Sitka spruce, pinyon-juniper, and 

ponderosa-Jeffrey pine forest types have decreased by about 13.6 million, 8.8 million, and 8.7 million 

acres respectively. The Western white pine forest type has decreased by 5.3 million acres, or about 96 

percent of its 1953 acreage. 

Indicator Limitations 

• Data on extent of forest land have an uncertainty of 3 to 10 percent per million acres for data 

reported since 1953. In 1998 Congress mandated that the FIA move to annual inventories. 

While data now are collected more often, fewer data are collected in any given year. Because 

area estimates now are based on a smaller sample size, the precision of the national estimates 

may be reduced relative to pre-1998 dates. 

• Most of the specific data related to species and age classes are only collected on lands 

classified as timberland and not forest land in general. 

• In addition to extent and species class, age class also influences the use of forest land as 

habitat by different species. Younger and older stands of forest have increased over the past 

half-decade, while middle-aged stands of more merchantable timber have decreased (Smith et 

al., 2001, 2004). 

Data Sources 

This indicator is based on data from two USDA Forest Service reports (Smith et al., 2001, 2004), which 

provide current and historical data on forest extent and type by state. Most data were obtained from the 

2004 report; the 2001 report was consulted only for 1963 data, which were excluded from the more recent 

report. Data were originally collected by the Forest Service’s Forest Inventory and Analysis (FIA) 

Program; original survey data are available from the FIA database (USDA Forest Service, 2005) 

(http://www.fia.fs.fed.us/tools-data/data/). 

References 

Smith, W.B., P.D. Miles, J.S. Vissage, and S.A. Pugh. 2004. Forest resources of the United States, 2002. 

General Technical Report NC-241. St. Paul, MN: USDA Forest Service, North Central Research Station. 

<http://ncrs.fs.fed.us/pubs/gtr/gtr_nc241.pdf> 

Smith, W.B., J.S. Vissage, D.R. Darr, and R.M. Sheffield. 2001. Forest resources of the United States, 

1997. General Technical Report NC-219. St. Paul, MN: USDA Forest Service, North Central Research 

Station. <http://www.ncrs.fs.fed.us/pubs/gtr/gtr_nc219.pdf> 

USDA Forest Service. 2005. Forest Inventory and Analysis, national FIA data base systems. Accessed 

2005. <http://www.fia.fs.fed.us/tools-data/data/> 

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1 INDICATOR: Forest Fragmentation

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The amount of forest land in the United States monitored by the USDA Forest Service has remained 

nearly constant over the past century, but the patterns of human land-use have affected its distribution 

from one region of the U.S. to another. Forest fragmentation involves both the extent of forest and its 

spatial pattern, and is the degree to which forested areas are being broken into smaller patches and pierced 

or interspersed with non-forest cover. 

Forest fragmentation is a critical aspect of the extent and distribution of ecological systems. Many forest 

species are adapted to either edge or interior habitats. When the degree or patterns of fragmentation 

change, it can affect habitat quality for the majority of mammal, reptile, bird, and amphibian species 

found in forest habitats (Fahrig, 2003). As forest fragmentation increases beyond the fragmentation 

caused by natural disturbances, edge effects become more dominant, interior-adapted species are more 

likely to disappear, and edge- and open-field species are likely to increase. 

This indicator of forest fragmentation was developed by the USDA Forest Service and has appeared in 

other recent reports (USDA Forest Service, 2004; Heinz Center, 2005). The indicator is based on the 1992 

National Land Cover Dataset (NLCD), which was constructed from satellite imagery (Landsat) showing 

the land area of the contiguous U.S. during different seasons (i.e., leaves-on and leaves-off) during the 

early 1990s. In many locations, the best available Landsat images were collected between 1991 and 1993, 

with data in a few locations ranging from 1986 to 1995. The USDA Forest Service’s Southern Research 

Station performed a re-analysis of NLCD, aggregating the four NLCD forest cover classes (coniferous, 

deciduous, mixed, and wetland forest) into one forest class and the remaining land cover classes into two 

classes: a non-forest class and a “missing” class consisting of water, ice/snow, and bare ground (Riitters et 

al., 2002). Land cover in the “missing” class is not considered to fragment forest in this indicator because 

no forest would be expected in those locations. A model that classifies forest fragmentation based on the 

degree of forest land surrounding each forest pixel (a square approximately 30 meters on each edge) for 

various landscape sizes (known as “windows”) provides a synoptic assessment of forest fragmentation for 

the contiguous United States by assessing each pixel’s “forest neighborhood” within various distances. 

Results are based on four degrees of forest cover: “core” if a subject pixel is surrounded by a completely 

forested landscape (no fragmentation), “interior” if a subject pixel is surrounded by a landscape that is 90 

to 100 percent forested, “connected” if a subject pixel is surrounded by a landscape that is 60 to 90 

percent forest, and “patchy” if the subject pixel is surrounded by less than 60 percent forest. The window 

(landscape) size used for this analysis was 9 by 9 pixels, 270 meters on each edge (percent forest was 

resampled from 30 meter pixel data and aggregated by State to develop the EPA Region-specific 

breakouts), or about 5900 hectares (14,500 acres). The window is shifted one pixel at a time over the map, 

so the target population for the indicator is all forested pixels in the contiguous United States. 

What the Data Show

Slightly more than 21 percent of the forested pixels in the U.S. represent “core” forest, which appear as 

landscapes dominated by forest (Exhibit 6-5). However, the data for “interior” and “core” forests suggest 

that fragmentation is extensive, with few large areas of complete, un-perforated forest cover. About 47 

percent of forest pixels in the U.S. occur in a landscape where less than 60 percent of the “neighborhood” 

is forest (i.e., forest cover is “patchy”). 

There is considerable regional variation in forest fragmentation (Exhibit 6-5). Regions 1, 2, 3, and 10 

have more than 30 percent “core” forest pixels, while less than 10 percent of the forest pixels in Regions 7 
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and 9 are “core” forest. From the opposite perspective, 

fewer than 17 percent of forest pixels are surrounded by 

less than 60 percent forest in Region 1, while almost 80 

percent of the forest pixels in Region 7 are surrounded 

by less than 60 percent forest. 

Indicator Limitations 

• Trend information is not available for this 

indicator. The 1992 NLCD data on which 

this indicator is based derive from satellite 

imagery that is more than a decade old. 

Trends in this indicator will depend on the 

availability of NLCD data in the future. 

• The degree of connectivity is dependent on 

the size of the window: Riitters (2003) 

determined that the percentages for all 

categories (especially “core” and 

“connected” forest pixels) decrease rapidly 

as the size of the window is increased 

progressively from 18 to 162, 1459, and 

13132 acres. 

• Because the non-forest land cover classes 

were aggregated, this indicator does not 

distinguish between natural and 

anthropogenic fragmentation (although 

such a distinction has been made for global 

fragmentation by Wade et al., 2003). 

• Fragmentation by roads is only partly 

captured using NLCD maps, which show 

some roads, but not all. Excluding such 

roads probably underestimates 

fragmentation on most public forest lands 

(Riitters et al., 2004). 

• The 1992 NLCD data do not include 

Hawaii or Alaska, which account for about 

one out of every six acres of forest land in 

the United States. 

Data Sources 

This analysis was previously published in Riitters (2003) and Heinz Center (2005), with results presented 

for the nation as a whole or broken down by Forest Service RPA region. EPA calculated results by EPA 

Region based on the national map provided by Riitters (2003). The original analysis is based on land 

cover data from the National Land Cover Dataset (USGS, 2005). 

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References 

Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity. Annu. Rev. Ecol. Evol. Syst. 34:487­

515. 

The H. John Heinz III Center for Science, Economics, and the Environment. 2005. Forest pattern and 

fragmentation. In: The state of the nation’s ecosystems: measuring the lands, waters, and living resources 

of the United States. New York, NY: Cambridge University Press, September 2002. Web update 2005. 

<http://www.heinzctr.org/ecosystems/forest/frgmnt.shtml> 

Riitters, K.H. 2003. Report of the United States on the criteria and indicators for the sustainable 

management of temperate and boreal forests, criterion 1: conservation of biological diversity, indicator 5: 

fragmentation of forest types. Final report. FS-766A. In: Darr, D., ed. Data report: a supplement to the 

National Report on Sustainable Forests. Washington, DC: USDA Forest Service. 

<http://www.fs.fed.us/research/sustain/contents.htm> 

Riitters, K., J. Wickham, and J. Coulston, J. 2004. Use of road maps in national assessments of forest 

fragmentation in the United States. Ecol. Soc. 9(2):13. 

<http://www.ecologyandsociety.org/vol9/iss2/art13/> 

Riitters, K.H., J.D. Wickham, R.V. O’Neill, K.B. Jones, E.R. Smith, J.W. Coulston, T.G. Wade, and J.H. 

Smith. 2002. Fragmentation of continental United States forests. Ecosystems 5:815-822. 

<http://www.srs.fs.usda.gov/pubs/ja/ja_riitters002.pdf> 

USDA Forest Service. 2004. National report on sustainable forests—2003. FS-766. 

<http://www.fs.fed.us/research/sustain/> 

USGS. 2005. National Land Cover Dataset 1992 (NLCD 1992). Accessed 2005. 

<http://landcover.usgs.gov/natllandcover.php> 

Wade, T.G., K.H. Riitters, J.D. Wickham, and K.B. Jones. 2003. Distribution and causes of global forest 

fragmentation. Conserv. Ecol. 7(2):7. <http://www.consecol.org/vol7/iss2/art7/> 

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INDICATOR: Ecological Connectivity in EPA Region 4 

As part of their natural functioning, ecological systems remove particulate matter and carbon dioxide 

from the air, purify surface and ground water, reduce flooding, and maintain biological diversity. These 

functions depend on a connected ecological “framework” of high-quality land consisting of central hubs 

interconnected by corridors that provide for the movement of energy, matter, and species across the 

landscape. This framework of connectivity is threatened by agricultural and silvicultural practices, road 

development, and “urban sprawl” that fragment the landscape and threaten the ecological framework. 

Maintaining ecological connectivity protects the entire system. 

The Ecological Connectivity Indicator (ECI) developed by EPA Region 4 (Durbrow et al., 2001) consists 

of a framework that captures the connectivity of important natural areas and ecological systems across the 

landscape of the Region (i.e., in Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South 

Carolina, and Tennessee). Four ecological aspects contribute to the functionality of the ECI infrastructure 

(see Carr et al., 2002, for additional details). The most important of the four, Hub and Corridor 

Connectivity, forms the basis for this indicator. Hub and Corridor Connectivity shows the connections

among critical ecological systems in the Region. Hubs are large areas of important natural ecosystems 

such as the Okefenokee National Wildlife Refuge in Georgia and the Osceola National Forest in Florida. 

Connections, referred to as “corridors,” are links to support the functionality of the hubs (e.g., the 

Pinhook Swamp which connects the Okefenokee and Osceola hubs). The ECI framework is based on land 

cover data obtained from the 1992 National Land Cover Dataset (NLCD), which was constructed from 

satellite imagery (Landsat) showing the land area of the contiguous U.S. during different seasons (i.e., 

leaves-on and leaves-off) during the early 1990s. In many locations, the best available Landsat images 

were collected between 1991 and 1993, with data in a few locations ranging from 1986 to 1995. 

What the Data Show

The hub and connection framework covers 43 percent of the total land and water resources in EPA 

Region 4—30 percent classified as hubs and 13 percent as corridors (Exhibit 6-6). Currently, 22 percent 

of this framework area is protected as conservation land, 12 percent is in the public domain as open water, 

and an additional 14 percent is classified as wetlands, for a total of 48 percent of hub and corridor acreage 

being afforded some type of long-term protection. 

Indicator Limitations

• Trend information is not available for this indicator. The most important data layer used in 

the ECI development is the National Land Cover Data (NLCD) from 1992-93. Establishing 

trends in the indicator will be limited by the availability of comparable land cover/land use 

data for the period 2002 and beyond. 

• Due to both the limited availability of data (ecological data not available or not in digital or 

GIS format) and the Southeastern Ecological Framework (SEF) parameter that sets a size 

threshold of 5,000 acres for ecological hubs, the results do not comprehensively include each 

and every ecologically important area in the Southeast. 
Data Sources 

The hub and corridor map was provided 

by U.S. EPA Region 4’s Southeastern 

Ecological Framework (SEF) project, 

and is available as a GIS data layer 

from the SEF website’s data page (U.S. 

EPA, 2002) 

(http://geoplan.ufl.edu/epa/data.html). 

The summary statistics shown in the pie 

charts in Exhibit 6-6 are presented in 

Carr et al. (2002). This analysis was 

based on the 1992 NLCD (USGS, 2005) 

(http://landcover.usgs.gov/natllandcover 

.php) and several additional datasets 

described in Carr et al. (2002); input 

data layers can be obtained on CD by 

following instructions on the SEF 

website (U.S. EPA, 2002). 

References 

Carr, M.H., et al. 2002. Final report: 

Southeastern Ecological Framework. 

Region 4. Atlanta, GA: U.S. 

Environmental Protection Agency. 

<http://geoplan.ufl.edu/epa/download/se 

f_report.pdf> 

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Durbrow, B.R., N.B. Burns, J.R. Richardson, and C.W. Berish. 2001. Southeastern Ecological 

Framework: a planning tool for managing ecosystem integrity. In: Hatcher, K.J., ed. Proceedings of the 

2001 Georgia Water Resources Conference. Athens, GA: University of Georgia. 

U.S. EPA. 2002. The EPA Southeastern U.S. Ecological Framework project. 

<http://geoplan.ufl.edu/epa/index.html> 

USGS. 2005. National Land Cover Dataset 1992 (NLCD 1992). Accessed 2005. 

<http://landcover.usgs.gov/natllandcover.php> 

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INDICATOR: Relative Ecological Condition of Undeveloped Land in EPA Region 

5

Ecological condition in EPA’s Report on the Environment is approached using questions broadly relating to 

landscape, biological diversity, ecological function, and the physical and chemical make-up of the 

environment, but no attempt is made at the national level to capture ecological condition in a small number of 

indices. In this indicator, the ecological condition of undeveloped land in EPA Region 5 (Illinois, Indiana, 

Michigan, Minnesota, Ohio, and Wisconsin) is characterized based on three indices derived from criteria 

representing diversity, self-sustainability, and the rarity of certain types of land cover, species, and higher taxa 

(White and Maurice, 2004). 

Geographic units referred to as cells are used to quantify geographic information. A spatially explicit model 

using ecological theory and geographic information system (GIS) technology was used to create twenty data 

layers of 300m x 300m cells from the 1992 National Land Cover Dataset (NLCD), which was constructed 

from satellite imagery (Landsat) showing the land area of the contiguous U.S. during different seasons (i.e., 

leaves-on and leaves-off) during the early 1990s. In many locations, the best available Landsat images were 

collected between 1991 and 1993, with data in a few locations ranging from 1986 to 1995. For this indicator, 

NLCD data were used to generate three indices, which represent estimates of three criteria: 

• Ecological Diversity - the relative diversities of populations (species), communities, and 

ecological systems in any given location on the landscape. Four data layers were used to 

derive this index. 

• Ecological Self-Sustainability - the potential for an ecological system to persist for years 

without external management; it is negatively impacted by two factors: landscape 

fragmentation and the presence of chemical, physical, and biological stressors. Twelve data 

layers were used to derive this index. 

• Rarity - the rarity of land cover, species, and higher taxa. Four data layers were used to derive 

this index. 

The model produces composite layers that are statistically independent. The scores for each criterion are 

normalized from 1 to 100 and each layer contributes equally to the final index (all of the data layers are 

weighted equally). In all the data layers and the resultant criteria layers, scores are normalized from 0 to 100. 

Zero always indicates the lowest quality, the greatest stress, or the least valuable observation, and 100 

indicates the highest quality, least stress, or most valuable observation. While it has not been done for this 

indicator, the three composite scores can be summed to result in a final “ecological condition” score for each 

cell (White and Maurice, 2004). Cell counts (a measure of geographic coverage) are used to indicate the 

distributions of scores associated with three index scores of ecological condition of undeveloped land: 

diversity, sustainability, and rarity. 

What the Data Show 

The frequency distributions of the 1992 baseline scores are quantified and plotted for each criterion (Exhibit 

6-7), and these provide a baseline against which to track future landscape trends in diversity, sustainability, 

and rarity. Diversity scores generally run from 20 to 80 across the region, signifying that most areas are in the 

moderate diversity range. More than 90 percent of the region has sustainability scores above 50, but rarity 

scores above 50 are seldom encountered. The majority of undeveloped land in Region 5 (which has higher 

index scores) lies in the northern forests of Minnesota, Wisconsin, and Michigan and along the large rivers in 

Ohio, Indiana, and Illinois (Exhibit 6-8). 
Indicator Limitations 

• Trend information is not available for this 

indicator. The ability to track trends will be 

dependent on the comparability of the 2001 

round of the NLCD with the 1992 NLCD 

data used to develop this indicator. 

• Although this indicator is designed to be 

comparable across undeveloped land within 

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Region 5, layers were ranked within ecoregions for some of the indicators in order to account 

for different geophysical, geochemical, or climatic features of each ecoregion. 

• Aquatic systems and connectivity resulting from water flow paths are not adequately covered 

and small, but potentially keystone, systems are not a part of the analysis (U.S. EPA, 2005). 

• Equal weighting of the data layers that contribute to each index may not be representative of 

the relative importance of each layer (U.S. EPA, 2005). 

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• The resolution and uncertainty of the results make comparing the ecosystem condition score 

for one individual cell (300m x 300m) with another inappropriate, but this is not the case for 

comparison between larger landscapes (U.S. EPA, 2005). 

• Field validation of the model to insure that modeled results are reflective of actual ecosystem 

condition will not be completed until the summer of 2006. 

Data Sources 

Maps and frequency distributions for the three indices were provided by EPA Region 5. An EPA report 

available online contains several related maps produced by the Critical Ecosystem Assessment Model 

(CrEAM), along with a list of the various datasets used as inputs for the model (White and Maurice, 2004 

[appendices]). Results from the CrEAM model are no longer available as digital map layers. 

References 

U.S. EPA. 2005. SAB review of the EPA Region 5 Critical Ecosystem Assessment Model. 

EPA/SAB/05/011. Washington, DC. <http://www.epa.gov/sab/pdf/cream_sab-05-011.pdf> 

White, M.L., and C. Maurice. 2004. CrEAM: a method to predict ecological significance at the landscape 

scale. Chicago, IL: U.S. Environmental Protection Agency. 

<http://www.epa.gov/region5/osea/science/CrEAM_text.pdf> (text); 

<http://www.epa.gov/region5/osea/science/CrEAM_appendices.pdf> (appendices) 

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6.2.3 Discussion 

What These Indicators Say about Trends in Extent and Distribution of the 

Nation’s Ecological Systems 

While ecological systems are interconnected and overlapping, it is useful to discuss trends in terms of 

major types of systems. There are many ways to define ecological systems, including by the predominant 

biota, spatial scales, and physical characteristics. Most terrestrial systems are defined by predominant 

vegetation types. The current extent of these types has been assessed (see Land Cover indicator, p. 4-11). 

Forests form the predominant land cover in the eastern and northwestern United States while grasslands, 

shrublands, and agricultural lands are the predominant types of vegetation in the central and western parts 

of the country. Trends in forest and wetland ecological systems are considered below. Trends in land 

development also are discussed, as this influences trends in the extent of ecological systems. 

Trends in Extent and Distribution of Forested Ecological Systems 

At a national scale, the percentage of forest land has varied somewhat over the last century with some 

decreases and some recent increases (see Forest Extent and Type indicator, p. 6-14, and Forest 

Fragmentation indicator, p. 6-17). However, shifts in regional distribution, of tree species and age 

structure have occurred. Forested ecological systems have decreased in extent in EPA Regions 6 and 9 

but increased in extent in Regions 3 and 5. The complex of tree species within a forest can have a strong 

influence on the community structure and functioning of a forested ecological system. These assemblages 

change over time, however this change may not be detectable at a national scale. On the scale of broad 

geographical regions, the Maple-Beech-Birch cover type has increased in acreage since 1992 in the 

Northern U.S., while a number of spruce and pine species cover types have decreased in the West These 

compositional changes can be as important as changes in the overall extent of forested ecological systems. 

Age structure differences are also apparent among regions with younger trees in the East and older trees 

in the West. 

At a finer regional scale, forest cover in the Puget Sound and Georgia Basin in the Pacific Northwest also 

was relatively stable during the 1990s (see Land Cover in Puget Sound/Georgia Basin indicator, p. 4-17). 

However, some of the forested watersheds experienced a conversion of small amounts of forest land to 

some other cover type. As discussed below, urbanization of low-elevation forested watersheds is a change 

that is receiving particular attention (see Land Cover in Puget Sound/Georgia Basin indicator, p. 4-17). 

While extent and species composition are important aspects of forested ecological systems, the spatial 

arrangement and contiguity of the systems also influence the functioning of the systems and the 

distribution of wildlife species that use forests and adjacent areas for habitat. Fragmentation of forested 

systems can reduce or redefine the interconnections within forests, modifying the scale of habitat and 

shifting distributions of wildlife species. For example, increasing fragmentation due to forest clearing, 

development, fires, or other activities creates more edge habitat and limits the acreage of interior habitat. 

Groups of wildlife species may prefer one habitat over another and move to maximize the time spent in 

the preferred habitat type. Nationwide, almost half of forests are highly fragmented or “patchy,” although 

more than 30 percent of the forests in the heavily forested Regions 1, 2, 3, and 10 are virtually 

unfragmented “core” forest (see Forest Fragmentation indicator, p. 6-17). The majority of forest 

fragmentation in the eastern United States is anthropogenic, while most fragmentation in the West is 

caused by natural factors. 

Ecosystem connectivity, the degree to which ecosystem “hubs” are connected to each other by “spokes” 

that serve as corridors for the interaction of biota, was shown to account for about 40 percent of the land 

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cover in EPA Region 4, the southeastern United States (see Ecological Connectivity in Region 4 

indicator, p. 6-20). In this indicator, connectivity includes not only forested land but also wetlands and 

open water. It may be coincidental that the degree of forest fragmentation is broadly similar to the degree 

of ecosystem connectivity, but in both indicators about half of the ecosystems in the Southeast are 

fragmented or unconnected. 

Trends in Extent and Distribution of Wetland Ecosystems 

Wetlands are ecosystems of high biological diversity and support a number of ecological functions from 

nursery and breeding areas to food and protection.16 Whether inland or coastal, freshwater or marine, 

wetlands acreage has been declining over the past 50 years (see Wetlands indicator, p. 3-53). The extent 

of the losses varies by type of wetland, with forested wetlands losing the most acreage and coastal 

wetland loss slowing somewhat. 

Trends in Land Development 

Land use refers to the visible effects of human use (see Land Use indicator, p. 4-24). Changes in land use 

from forested or wetland systems to urban or agricultural environments have a direct impact on the 

ecological systems within which the change occurs as well as on systems that are interconnected with the 

altered areas (e.g., watersheds and coastal areas). Some changes can create edge environments that are 

favored by certain wildlife species. Therefore, trends in land development are important considerations 

with respect to overall trends in the extent and distribution of ecological systems. 

Changes in land use sometimes result in changes in land cover and conversion from one major ecosystem 

type to another, but sometimes they do not. For example, gains in agricultural productivity have caused 

significant changes in the extent and location of crop and pasture land uses. Some land that had been used 

for crops or pasture has reverted to forest. Timber production may convert cropland to forest, or it may do 

little more than substitute one forest type or age-class distribution for another. At the same time, growth 

in population has driven an increase in the extent of developed land, much of which has converted crop or 

pasture land to developed land. 

At a national scale, crop and farm acreages have decreased, timberland (productive forest land) has 

remained constant since 1997, and developed lands have increased between 1982 and 2002 (see Land Use 

indicator, p. 4-24). Within the larger scale trends, many subtle shifts occur at smaller scales. The increase 

in developed lands has received particular attention in National and Regional Indicators. 

Increases in the numbers and changes in the spatial distribution of human populations explain part of the 

increase in developed lands. However, developed land increased by almost two times the increase in 

population from 1982 to 2002 suggesting that during this period people were making a proportionally 

greater use of the landscape (see Urbanization and Population Change indicator, p. 4-31). Geographically, 

the rate of development was four times the population growth rate in the Northeast, one to three times the 

population growth rate in the South and Midwest, and nearly equal to the growth rate in the West.17 The 

16 Dahl, T.E. 2000. Status and trends of wetlands in the conterminous United States 1986 to 1997. Washington, DC: 

U.S. Department of the Interior, U.S. Fish and Wildlife Service. 

17 U.S. Census Bureau. 2004. Table 8: Annual Estimates of the Population for the United States, Regions, and 

Divisions: April 1, 2000 to July 1, 2004. NST-EST2004-08. U.S. Census, Population Division. 

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increases in developed land suggest there were comparable decreases in other types of lands. To the 

extent that these other lands afford habitat to animals and plants, shifts in land use result in shifts in the 

extent and distribution of ecological systems. 

The degree of change in developed lands appears to be associated with types of locations which emerge 

as focal points for increasing stress on ecological systems. For example, in the Puget Sound and Georgia 

Basin area of the Pacific Northwest, forest conversion to other types of land use is occurring along the 

coast while older growth forests are observed at higher elevations (see Land Cover in Puget 

Sound/Georgia Basin indicator, p. 4-17). Further, trends indicate that impervious surface coverage is 

increasing to the point where detrimental impacts to aquatic resources may occur.18 In the Great Lakes 

region, most of the undeveloped lands occur in the northern forests or along the major rivers (see 

Condition of Undeveloped Land in Region 5 indicator, p. 6-22). Proximity to developed areas has an 

obvious effect on the quality of these ecological systems. The highest quality systems comprise about 3 

percent of the total and are located in the most remote and/or protected areas. The potential for future land 

use changes with increasing urbanization is the major determinant for judging potential fragmentation of 

ecological systems in EPA Region 5 (Great Lakes area). 

Limitations, Gaps, and Challenges 

While many of the indicators in this section provide baseline information, trend information is available 

for only a few of the major types of systems–forests and wetlands. There are no ROE indicators for other 

types of terrestrial or aquatic systems including grasslands, shrublands, and marine hard bottom 

communities including coral reefs. Filling these gaps in information would help EPA to better evaluate 

trends in ecological condition. 

One of the challenges in capturing meaningful changes relates to location and scale. The importance of 

location-specific changes is evident in some of the indices. For example, small changes in certain areas, 

such as near-coastal areas of the Pacific Northwest, could have disproportionately large effects on coastal 

waters relative to a similar change in the middle of an expansive prairie. In addition, the appearance of 

fragmentation in ecological systems depends on the area over which data were extracted.19 Thus, 

choosing locations and assessment areas have obvious impacts on trend assessment. Conversely, the 

implications of trends are manifested at scales that are location- and area-specific. Important 

consequences of changes can be captured or missed depending on how the information is aggregated and 

presented. 

Another challenge relates to understanding the factors underlying changes that occur over various time 

scales and their effects on human health and ecological condition. Principal among these is recognizing 

that natural cycles and natural variability bring about changes that may appear as “trends” over one time 

scale but will appear as cycles or variations over longer time scales. Familiar examples include population 

variations among predators and prey or temperature variations associated with the advance and retreat of 

ice ages. Distinguishing these natural cycles and variations from trends caused by human-induced 

perturbations is yet another challenge. In some cases the relationships may be evident, as in the influence 

of urbanization on watersheds or the impact of lost sand dunes on subsequent beach erosion. In other 

18 Klein, R.D. 1979. Urbanization and stream water quality impairment. Water Resour. Bull. 15(4):948-963. 

19 USDA Forest Service. 2004. National report on sustainable forests—2003. FS-766. Washington, DC. p. 19. 

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cases factors influencing changes may be difficult to discern, such as long-term shifts in major plant 

communities.

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6.3 WHAT ARE THE TRENDS IN THE DIVERSITY AND BIOLOGICAL BALANCE 

OF THE NATION’S ECOLOGICAL SYSTEMS? 

6.3.1 Introduction 

Trends in the biological diversity of the nation’s ecological systems can be viewed in terms of both the 

numbers of species present in an ecological system and the extent to which some of the species are 

threatened or endangered. Biological balance refers to the inter-relationships among organisms, including 

the structure of food webs and the ability of ecological systems to maintain themselves over time. Balance 

is a dynamic characteristic rather than a fixed state. 

The biological diversity and balance within ecological systems are often used to judge the health of the 

system, and their reduction often represents a response to pollutants or other stressors. Restoring 

biodiversity and biological balance have constituted a focus of EPA’s attention over the past three 

decades. Reversing declines of species such as the brown pelican caused by pesticides and brook trout 

caused by acid rain, replacing nuisance algal blooms caused by excess nutrients with balanced 

communities of phytoplankton, replacing beds of sludgeworms due to sewage discharges with balanced 

communities of benthic invertebrates, and restoring biological communities previously decimated by 

improper handling of toxic and hazardous wastes are well-known examples. 

 The significance of biological diversity also stems from the fact that, for many people, biological 

diversity contributes to the quality of life.20 Everyone recognizes the importance of species as 

commodities (if those species produce products that can be bought and sold), and some argue that species 

have moral value in and of themselves. 

Diversity and biological balance are also of interest because of how they may influence the functioning 

and stability of ecological systems.21,22 While scientists debate the exact relationship between the 

diversity and the functioning and stability of ecological systems, it is generally agreed that as the number 

of species in any particular type of ecological system declines, there is a potential loss of “resilience” 

within that system.

23. It is also recognized that these relationships are not straightforward and can vary in 

degree depending on the types of species introduced or removed from a system.

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Diversity and balance have important time and space components. Diversity arises over time as 

adaptation results in new species that fill available “niches” in the environment. This is a dynamic process 

involving colonization, evolution of species adapted to new conditions, and extinction of species that are 

less well adapted to a changing environment. This process has occurred over thousands or millions of 

20 Norton, B. 1988. Commodity, amenity, and morality: the limits of quantification in valuing biodiversity. In: 

Wilson, E.O., ed. Biodiversity. Washington, DC: National Academy Press. p. 521. 

21 Chapin III, F.S., et al. 1997. Biotic control over the functioning of ecosystems. Science 277(5325): 500-504. 

22 Wilson, E.O. 1992. The diversity of life. Cambridge: Belknap Press, 1992. 

23 McCann, K.S. 2000. The diversity-stability debate. Nature 405(11): 228-233. 

24 Srivastava, D.S. and M. Vellend. 2005. Biodiversity-ecosystem function research: Is it relevant to conservation? 

Annu. Rev. Ecol. Syst. 36: 267-294. 

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years over large geographic areas, punctuated occasionally by large events such as meteor strikes, periods 

of intense volcanism, and ice ages. Ecological systems that are stable in the short term evolve into 

different systems in the long term. Disturbances that reduce biological diversity or disrupt balance on a 

small scale may not have an effect on a larger scale or over longer time periods. 

Changes (decreases and increases) in biological diversity have likely occurred throughout the history of 

the United States in response to regional land use changes (e.g., the reforestation of the Southeast during 

the past century), water management, intentional and unintentional introductions of species, and 

environmental pollution. Other changes in diversity and the composition of the biological community can 

be rapid and dramatic. Introduced plants and plant pathogens can rapidly transform landscapes as some 

species, such as the American chestnut, are lost and others, such as kudzu, thrive. Introduction of the sea 

lamprey to the Great Lakes led to sweeping changes in the entire food chain, from lake trout all the way 

down to the phytoplankton.

25 Declining sea otter populations led to loss of kelp forests, as sea urchins 

formerly preyed upon by otters grazed the kelp down to the sea floor.26 The decimation of grazers such as 

the American Bison or predators such as grizzly bear or wolves has had cascading impacts on upland 

vegetation, wetlands, fish, and other species.27 Toxic chemical pollution can create wastelands where only 

the most resistant species can survive, and nutrients and acid rain have had indirect effects on diversity 

and balance by causing sweeping changes in the chemical habitat. 

Indicators of diversity and biological balance incorporate information about primary producers and 

invertebrate and vertebrate consumers, especially keystone species which play critical roles in structuring 

habitat or serve major roles as primary producers, top predators, or important prey species. Indicators of 

invasive species are also important with respect to assessing trends in the diversity and biological balance 

because these species can alter the nation’s ecological systems by displacing indigenous species, 

potentially changing the structure of biological communities. 

6.3.2 ROE Indicators 

Trends in diversity and balance are evaluated from four National Indicators and three Regional Indicators 

(Table 6.3.1). The focus for this question is on national- or regional-scale trends in biological diversity or 

balance over time spans of one to three decades. The data on biological diversity and balance come from 

a variety of sources, including both systematic monitoring and ad hoc data collection.28 Systematic 

probability surveys are now providing national pictures of the biological diversity of benthic communities 

in estuaries and in rivers and streams. The Breeding Bird Survey is a private sector effort that provides 

valuable national-level data on trends in bird populations. 

25 Eck, G.W., and L. Wells. 1987. Recent changes in Lake Michigan’s fish community and their probable causes, 

with emphasis on the role of the alewife (Alosa pseudoharengus). Can. J. Fish. Aquat. Sci. 44(Suppl. 2):53-60. 

26 Estes, J.A., and J.F. Palmisano. 1974. Sea otters: their role in structuring nearshore communities. Science 

185:1058-1060. 

27 Pritchard, J.A. 1999. Preserving Yellowstone’s natural conditions: science and the perception of nature. Lincoln, 

NE: University of Nebraska Press. p. 370. 

28 There are no systematic national efforts to quantify trends in the diversity of other vertebrate, invertebrate, plant, 

or microbial species, but a private sector organization, NatureServe, working in concert with state Natural Heritage 

Programs, has done much to assimilate and integrate data from ad hoc studies to quantify populations of more than 

200,000 species in the United States. 

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Trends involving longer-term effects associated with climate change are not included. Many issues 

regarding biodiversity at subregional and local scales (e.g., tall grass prairie or the Okefenokee Swamp) 

that cannot be covered here are no less important. 

Table 6.3.1. ROE Indicators of Trends in Diversity and Biological Balance of the Nation’s 

Ecological Systems 

NATIONAL INDICATORS LOCATION 

Coastal Benthic Communities (N/R) 3.5.2 – p. 3-71 

Benthic Macroinvertebrates in Wadeable Streams 3.2.2 – p. 3-35 

Bird Populations 6.2.2 – p. 6-32 

Fish Faunal Intactness 6.2.2 – p. 6-34 

REGIONAL INDICATORS 

Submerged Aquatic Vegetation (SAV) in the Chesapeake Bay 3.5.2 – p. 3-74 

Harmful Algal Bloom Outbreaks Along the Western Florida Coastline 3.5.2 – p. 3-81 

Non-Indigenous Species in the Estuaries of the Pacific Northwest 6.2.2 – p. 6-37 

N/R = National Indicator displayed at EPA Regional scale 

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1 INDICATOR: Bird Populations

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Bird populations are among the most visible and important biological components of ecological systems 

and support a number of important ecological functions including seed dispersal, plant pollination and 

pest control. Some birds migrate over entire continents, while others have more restricted ranges and 

habitats, but in all cases trends in bird populations and in the abundance of species integrate the influences 

of changes in landscape and habitat, the availability and quality of food, toxic chemicals, and climate. The 

North American Breeding Bird Survey (BBS) began in 1966 with approximately 600 surveys conducted 

in the U.S. and Canada east of the Mississippi River. Today there are approximately 3,700 active BBS 

routes across the continental U.S. and southern Canada (Sauer et al., 1997). 

Trends have been computed for observed population sizes of 418 bird species for the period 1966-2003 

(Sauer et al., 2004). The Audubon Society (2004) categorized each species according to its primary 

habitat: grassland, shrubland, woodland, urban, and water and wetlands. This indicator reflects the 

number of species with “significant” increases or decreases in the number of observations (not a change 

in the number of species) for which adequate trend data exist between 1996 and 2003. Significant 

increases or decreases were defined for this study as those in which the observed populations on BBS 

routes increased or decreased by more than two-thirds between 1966 and 2003 and does not necessarily 

imply a statistically significant trend. 

What the Data Show 

The results point to dynamic changes in bird 

populations in all habitat types (Exhibit 6-9), although 

there were no consistent increases or decreases. 

• Of 27 grassland species for which 

adequate data are available, only 2 species 

(7 percent) showed significant population 

increases and 19 species (70 percent) 

showed significant decreases. 

• Of 78 shrubland species for which 

adequate data are available, 11 species (14 

percent) showed significant increases, 

while 28 species (36 percent) showed 

significant declines. 

• Of 164 woodland species for which 

adequate data are available, 48 species (29 

percent) showed significant population 

increases and 42 species (26 percent) 

showed significant decreases. 

• Of 43 primarily urban species for which 

adequate data are available, 17 species (40 

percent) showed significant population 

increases and 10 species (23 percent) had 

significant decreases. 
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• Of 106 water and wetland bird species for which adequate data are available, 40 species (38 

percent) showed significant increases and 14 species (13 percent) showed significant 

decreases. 

Indicator Limitations 

• The BBS produces an index of relative abundance rather than a complete count of breeding 

bird populations. The data analyses assume that fluctuations in these indices of abundance are 

representative of the population as a whole. 

• The BBS data do not provide an explanation for the causes of population trends. To evaluate 

population changes over time, BBS indices from individual routes are combined to obtain 

regional and continental estimates of trends. Although some species have consistent trends 

throughout the history of the BBS, most do not. For example, populations of permanent 

resident and short-distance migrant species (birds wintering primarily in the U.S. and 

Canada) are adversely affected by periodic episodes of unusually harsh winter weather. 

• Few species have consistent population trends across their entire ranges, so increases or 

decreases in this indicator may not reflect the situation across the entire range of the species. 

Data Sources 

Trend data were obtained from the Audubon Society’s 2004 “State of the Birds” report (Audubon 

Society, 2004). Audubon’s analysis used raw data from the National Breeding Bird Survey (USGS, 

2004), which can be downloaded from http://www.pwrc.usgs.gov/bbs/retrieval/menu.cfm. 

References 

Audubon Society. 2004. State of the birds USA 2004. Audubon Magazine September-October. 

<http://www.audubon.org/bird/stateofthebirds/> 

Sauer, J.R., J.E. Hines, and J. Fallon. 2004. The North American Breeding Bird Survey, results and 

analysis 1966-2003. Version 2004.1. Laurel, MD: USGS Patuxent Wildlife Research Center. 

<http://www.mbr-pwrc.usgs.gov/bbs/bbs.html> 

Sauer, J.R., J.E. Hines, G. Gough, I. Thomas, and B.G. Peterjohn. 1997. The North American Breeding 

Bird Survey, results and analysis. Version 96.4. Laurel, MD: USGS Patuxent Wildlife Research Center. 

<http://www.mbr-pwrc.usgs.gov/bbs/genintro.html> 

USGS. 2004. North American breeding bird survey. Laurel, MD: USGS Patuxent Wildlife Research 

Center. Accessed 2004. <http://www.pwrc.usgs.gov/bbs/index.html> 

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INDICATOR: Fish Faunal Intactness

Intactness, the extent to which ecological communities have retained their historical composition, is a 

critical aspect of the biological balance of the Nation’s ecological systems (NRC, 2000). It is of particular 

importance in freshwater systems that are impacted by pollution, habitat alteration, fisheries management, 

and invasive species. 

This indicator tracks the intactness of the native freshwater fish fauna in each of the nation’s major 

watersheds by comparing the current faunal composition of those watersheds with their historical 

composition. In this case, historical data are based on surveys conducted in 1970. The indicator 

specifically measures the reduction in native species diversity in each 6-digit USGS hydrologic cataloging 

unit (HUC) in the 48 contiguous states. Intactness is expressed as a percent based on the formula: 

reduction in diversity = 1 – (# of current native species / # of historic native species). 

The native species diversity indicator proposed by the NRC (NRC, 2000) compared expected native 

species diversity (projected from species-area-curve models) with observed diversity. This “Fish Faunal 

Intactness” indicator makes use of empirical, rather than modeled, data sets and focuses on a well-known 

group of organisms with a fairly strong historical record. 

Reductions in watershed diversity may be due either to the overall extinction of a species (at least 12 U.S. 

freshwater fish species are known to be extinct and another 3 species are known only from historical 

records and may be extinct) or, more commonly, to the extirpation of a species from selected watersheds. 

In the case of regional extirpations, opportunities may exist for restoring the species to watersheds in its 

historic range.

The fish distributional data underlying this indicator were gathered by NatureServe, a non-profit research 

organization, and are derived from a number of sources, including species occurrence data from state 

natural heritage programs, a broad array of relevant scientific literature (e.g., fish faunas), and expert 

review in nearly every state. These data were assembled during the period 1997-2003. The underlying 

data include distributions for 782 native freshwater fish species across small watersheds (8-digit HUC). 

For this indicator, data were pooled and reported by larger 6-digit HUCs to reduce potential errors of 

omission in the smaller watersheds. 

What the Data Show

Watersheds covering about one-fifth (21 percent) of land area in the contiguous United States are fully 

intact, retaining their entire complement of fish species (Exhibit 6-10). Watersheds covering nearly a 

quarter (24 percent) of land area, however, have lost 10 percent or more of their native fish fauna since 

1970. Reductions in diversity are especially severe in the Southwest (e.g., the lower Colorado River 

watershed) and the Great Lakes, with eight major watersheds (representing 2 percent of total land area) 

having lost more than half of their native fish fauna. 

Because the indicator is expressed as a ratio, it does not reflect the magnitude of species losses in a given 

watershed. The southeastern United States, for instance, is far richer in numbers of freshwater fish species 

by HUC than the southwestern United States (Exhibit 6-11). Although some southeastern HUCs have 

experienced losses in the absolute number of species (e.g., in the state of Mississippi), due to the large 
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number of species overall, the fish fauna can still appear relatively intact when viewed on a percentage 

rather than numeric basis. 

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Indicator Limitations 

• The incomplete historical record for freshwater fish distributions and inconsistent inventory 

records for contemporary fish distributions are sources of uncertainty. 

• Although NatureServe has attempted to compile the most complete distributional information 

possible for these species at the 8-digit HUC level, these data are dynamic; new records 

frequently are added and existing records are revised as new information is received and as 

taxonomic changes occur. Consequently, these distributional data could benefit from 

additional quality control, updating, and expert review. 

Data Sources 

This indicator presents a summary of data available from the NatureServe Explorer database 

(NatureServe, 2006) (http://www.natureserve.org/getData/dataSets/watershedHucs/index.jsp). The 

identity and status (current vs. historical) of all native fish species recorded in each 8-digit HUC are 

available from this database, along with species-by-species distribution maps at the 8-digit HUC level. 

Analyses based on these data have previously been reported in Master et al. (1998), Master et al. (2003), 

and Stein et al. (2000). 

References 

Master, L., A. Olivero, P. Hernandez, and M. Anderson. 2003. Using small watershed fish, mussel, and 

crayfish historical and current presence data to describe aquatic biogeography and inform its 

conservation. Abstract #PO67. Society for Conservation Biology Annual Meeting, Duluth, Minnesota. 

Master, L.L, S.R. Flack, and B.A. Stein. 1998. Rivers of life: critical watershed for protecting freshwater 

biodiversity. Arlington, VA: The Nature Conservancy. 

<http://www.natureserve.org/publications/riversOflife.jsp> 

NatureServe. 2006. NatureServe explorer. Accessed 2006. <www.natureserve.org/explorer> 

NRC (National Research Council). 2000. Ecological indicators for the nation. Washington, DC: National 

Academy Press. <http://www.nap.edu/openbook/0309068452/html/> 

Stein, B.A., L.S. Kutner, and J.S. Adams. 2000. Precious heritage: the status of biodiversity in the United 

States. New York, NY: Oxford University Press. 

<http://www.natureserve.org/publications/preciousHeritage.jsp> 

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1 INDICATOR: Non-Indigenous Species in the Estuaries of the Pacific Northwest 

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Non-indigenous species (NIS) are one of the greatest threats to aquatic ecosystems and can impact local 

and regional economies (Lowe et al., 2000). The number of invasive species in estuaries of the Pacific 

Northwest (including Puget Sound, Columbia Estuary, and Coos Bay) is rising and these areas can then 

become sources of invasives to other locales. Coastal waters are particularly vulnerable to NIS 

transported in ballast water and introduced via aquaculture (Puget Sound Action Team, 2002). It is 

becoming apparent that NIS are capable of impacting estuaries along the west coast, even though they are 

rarely addressed in routine monitoring studies. One limitation is the lack of standardized invasion metrics 

and threshold values. 

This indicator focuses on estuarine soft-bottom communities of the Columbian Biogeographic Province 

located along the Pacific coast from Cape Mendocino, CA north to the Strait of Juan de Fuca at the 

entrance to Puget Sound, WA. It is limited to sites with salinities >= 5 parts per thousand. The indicator is 

based on the percent abundance of NIS individuals relative to the combined abundance of native and NIS 

individuals in a benthic grab sample. 

The data for this indicator were collected by the Environmental Monitoring and Assessment Program 

(EMAP) using a probability design over the period 1999-2001 (Nelson et al., 2004; in review) and by a 

special study focusing on estuaries not exposed to ballast water or aquaculture. Probability sampling 

provides unbiased estimates of the percent abundance of natives and NIS in all estuaries in the study area, 

but, because the data for the special study have not yet been statistically expanded, data for this indicator 

is based on stations sampled rather than area. 

Background levels for the indicator are based on observations in estuaries with minimal exposure to invasion 

from ballast water discharges or to aquaculture of exotic oysters. Three threshold levels of invasion were 

assigned to the indicator: “minimally invaded” or “background” when NIS constituted a small proportion 

of the individuals (0-10 percent), “highly invaded” when the NIS were the numerical dominants (>50 

percent of the individuals), and “moderately invaded” when the NIS were relatively abundant but not the 

numerical dominants (10-50 percent of the individuals). Estuaries were further classified into “exposed” 

and “minimally exposed. “Exposed” estuaries are those exposed to ballast water discharges from 

international shipping and/or aquaculture of exotic oysters. “Minimally exposed” estuaries are those 

without international shipping or oyster culture, though there may have been historical exposure as well 

as exposure through regional fishing boats.

What the Data Show

Approximately 15 percent of the stations in the Columbian Province were highly invaded (abundance of 

NIS > abundance of natives) and another 20 percent were moderately invaded (Exhibit 6-12). The study 

showed that non-indigenous species were among the most frequently occurring anthropogenic stressors in 

this biogeographic region when compared to indicators of sediment contamination or eutrophication 

(Nelson et al., 2004). 

The extent of invasion was not uniform, however, among exposed and minimally exposed estuaries. 

Estuaries with greater exposure to these invasion vectors were more invaded; 44 percent of the stations in 

the exposed estuaries were moderately to highly invaded compared to only 21 percent of the stations in 

minimally exposed estuaries (Exhibit 6-12). Nonetheless, the observation that 21 percent of the stations in 

these “pristine” estuaries were at least moderately invaded indicates that non-indigenous species can 
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disperse widely once they are introduced into a region, 

so even estuaries with no direct exposure to ballast 

water or aquaculture are at risk of invasion. 

Indicator Limitations 

• This indicator presents baseline data only; 

trend information is not yet available. 

• Studies in the San Francisco Estuary (Lee 

et al., 2003) and in Willapa Bay, WA 

(Ferraro and Cole, in progress) have 

shown that the percent of NIS can vary 

substantially among communities so that 

regional background values for the 

Columbian Province as a whole may not 

be appropriate for specific community 

types. 

• This indicator represents percent NIS in 

individual benthic grabs of the soft-bottom 

community, but does not characterize the 

total number of NIS in the estuaries. It 

also does not include fish or other taxa not 

collected by benthic grabs. 

• The data for the indicator were only 

collected during the summer index period 

and thus do not capture seasonal 

variations. 

• The threshold values for “minimal,” 

“moderately invaded,” and “highly 

invaded” are preliminary and require 

further research in order to establish their 

ecological significance. Specific values 

may differ in other biogeographic provinces. 

Data Sources 

Data for this indicator were collected by two different studies: EPA’s National Coastal Assessment 

(Coastal EMAP) and a special EPA study of minimally exposed estuaries. The complete results from 

these studies were not publicly available at the time this report went to press, but summary data from the 

1999 Coastal EMAP are available from Nelson et al. (2004; in review), and the underlying sampling data 

from 1999 can be obtained from EPA’s National Coastal Assessment database (U.S. EPA, 2005) 

(http://www.epa.gov/emap/nca/html/data/index.html). Coastal EMAP sampling data and summary 

statistics for 2000 and 2001 are expected to be made available in the near future. Results from the special 

study of minimally exposed estuaries will also be published in the near future. Until then, data for this 

indicator may be obtained from EPA’s Western Ecology Division. 

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References 

Lee II, H., B. Thompson, and S. Lowe. 2003. Estuarine and scalar patterns of invasion in the soft-bottom 

benthic communities of the San Francisco Estuary. Biol. Invasions 5:85-102. 

Lowe, S., M. Browne, S. Boudjelas and M. De Poorter. 2000. 100 of the world’s worst invasive alien 

species—a selection from the Global Invasive Species Database. Auckland, New Zealand: International 

Union for the Conservation of Nature, Invasive Species Specialist Group. 

<http://www.issg.org/booklet.pdf> 

Nelson, W.G., H. Lee II, and J. Lamberson. In review. Condition of estuaries of California for 1999: a 

statistical summary. Washington, DC: U.S. Environmental Protection Agency. 

Nelson, W.G., H. Lee II, J.O. Lamberson, V. Engle, L. Harwell, and L.M. Smith. 2004. Condition of 

estuaries of the western United States for 1999: a statistical summary. EPA/620/R-04/200. Washington, 

DC: Environmental Protection Agency. 

Puget Sound Action Team. 2002. Puget Sound update: the eighth report of the Puget Sound Ambient 

Monitoring Program (PSAMP). Olympia, WA: Puget Sound Water Quality Action Team. 

<http://www.psat.wa.gov/Publications/update_02/ps_update_2002-sec.pdf> 

U.S. EPA. 2005. National Coastal Assessment. Accessed 2005. 

<http://www.epa.gov/emap/nca/index.html> 

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6.3.3 Discussion 

What These Indicators Say About Trends in the Diversity and Biological 

Balance of the Nation’s Ecological Systems 

Few national programs track diversity and biological balance. However, there are ROE indicators 

available for invertebrate communities and select vertebrates (birds and fish) and regionally for invasive 

species (as these can be important disruptors of ecosystem balance) and important communities of 

submerged aquatic vegetation. Some of these indicators show reduced or declining diversity for particular 

groups of animals and plants, but this is not consistent across all the ROE indicators. The particular trends 

of available ROE indicators are discussed below by plant and animal groupings, followed by the 

limitations of the available information and future challenges. 

Primary Producers 

Primary producers range from the microscopic plants of the oceans to the giant redwoods of California. 

The types of plants and the biomass they produce are fundamental to ecological systems. There have also 

been well-documented, modest shifts in forest types and age classes in the past decade, as discussed in 

Section 6.2.3. 

Submerged aquatic vegetation (SAV) is an important biological component of aquatic systems, 

contributing to diversity and balance by providing habitat and food. While there is no National Indicator 

of trends in SAV, the Chesapeake Bay provides a regional example of a potentially wider-spread 

phenomenon (see SAV in Chesapeake Bay indicator, p. 3-74). SAV has increased in the Bay over the past 

25 years, but remains below its historic coverage, and the species composition also has changed 

somewhat. Contributing factors in the Bay include excessive nutrients, sediment loads, diseases, and 

physical disturbance. 

Too much plant growth can shift the balance within aquatic ecosystems, and blooms of algae can sap 

aquatic systems of necessary oxygen and even create toxic conditions. Blooms of toxic algae in the Gulf 

of Mexico occur annually, but there have been fewer blooms in recent years as compared to 1996 (see 

HAB Outbreaks in Western Florida indicator, p. 3-81). Such “red tides” occur along other coastal waters 

and appear to be influenced by a combination of physical, chemical, and biological factors. 

Invertebrates 

Invertebrates such as worms, insects, and crustaceans are among the most diverse group of organisms. 

Collectively they comprise the largest component of animal biomass on the planet and are critical 

components of aquatic and terrestrial food webs. Trends in the composition of invertebrate communities 

can reflect important environmental changes. 

In the nation’s coastal systems, baseline measures of invertebrate biodiversity and species composition 

indicate that about one-fifth exhibit low biological condition (see Coastal Benthic Communities indicator, 

p. 3-71). Because benthic invertebrates live on or in sediments, it is not surprising that most of these areas 

also exhibit low sediment and/or water quality. For streams, the benthic macroinvertebrate Index of 

Biological Integrity (IBI) exhibits a broad and even distribution from low to high values (see Benthic 

Macroinvertebrates in Wadeable Streams indicator, p. 3-35). Exhibit 3-12 provides a basis for evaluating 

future changes. 

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Vertebrates 

The biodiversity of fish, amphibians, reptiles, birds, and mammals is influenced by available food 

resources, the size and arrangement of suitable habitats, influxes of new species, climate and weather, and 

the presence of contaminants. Vertebrates often receive much attention because they are highly visible 

and are often near the top of the food chain. 

Among vertebrates the most reliable indicator of national trends is for birds, which have been tracked 

since 1966 (see Bird Populations indicator, p. 6-32). Bird populations are in dynamic flux. There appears 

to be a net decline of populations most commonly found in the grassland and shrublands, comparable 

increases and decreases in populations in woodlands, and some gains in populations inhabiting urban and 

water/wetlands areas. 

Fish are distributed throughout most of the nation’s aquatic and marine ecological systems. Comparisons 

between current and historic species compositions (see Fish Faunal Intactness indicator, p. 6-34) indicate 

that one-fifth of the watersheds retain their full complement of fish species, while about a quarter have 

experienced of 10 percent or more loss in species. The losses occurred primarily in the Southwest and 

Great Lakes. 

Invasive Species 

The infiltration of new species into areas is a natural phenomenon but can be accelerated through 

intentional and unintentional introductions. Introduction of species such as kudzu, zebra mussels, grass 

carp, starlings, and nutria have had profound effects on ecological systems.29 Many newly introduced 

species may lack predators or parasites that kept these species under control in their native habitats, 

allowing them to out compete resident species and even dominate entire systems. While national data are 

lacking, the Non-Indigenous Estuarine Species in Pacific Northwest indicator (p. 6-37) shows that in the 

Columbian Biogeographic Province (from California to Washington), about one-third of the stations 

sampled were highly or moderately invaded with non-indigenous invertebrates. 

Limitations, Gaps, and Challenges 

A number of additional ROE indicators would help EPA better address the question of trends in diversity 

and biological balance. While there are ROE indicators for the extent and distribution of vegetation types, 

there remain gaps with respect to indicators of plant biodiversity in terrestrial and aquatic ecological 

systems. There is no ROE indicator for threatened and endangered species. Also, there are no ROE 

indicators for freshwater systems that are comparable to the coastal algal bloom indicator (e.g., for the 

extent of nuisance aquatic plants such as the prolific growths of Eurasian milfoil and water chestnut in 

lakes and ponds, which continue to create water management problems

30,31). ROE Indicators of climate29 Lowe, S., M. Browne, S. Boudjelas, and M. De Poorter. 2000. 100 of the world’s worst invasive alien species: a 

selection from the Global Invasive Species Database. Auckland, New Zealand. World Conservation Union, Invasive 

Species Specialist Group. 12 pp. 

30 Madsen, J.D., J.W. Sutherland, J.A. Bloomfield, L.W. Eichler, and C.W. Boylen. 1991. The decline of native 

vegetation under dense Eurasian water-milfoil canopies. J. Aquat. Plant Manage. 29:94-99. 

31 Lake Champlain Basin Program, Federal Agencies Work Group. 2005. Opportunities for federal action: managing 

aquatic non-native nuisance plants and animals. <http://nh.water.usgs.gov/champlain_feds/nonnative.htm> 

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related vegetation changes also are lacking (e.g., fluctuations in the extent of kelp beds on the west coast 

related to El Nino events32). 

There are no ROE indicators for major groups of vertebrate biota including amphibians, reptiles, and 

mammals. Because amphibians live both on land and in the water, their diversity and trends in their 

abundance could be influenced by a wide range of stressors to air, water, and land. Recent reported 

declines in amphibian populations worldwide indicate that losses are attributable in some areas primarily 

to overharvesting, in others to loss of habitat, and in still others to unknown causes,33 but at this time there 

is no National Indicator that meets the criteria for this report. There also are no ROE indicators for trends 

in important insect or freshwater shellfish species. 

Modern transportation and international trade in biota for food have caused invasive species to remain a 

potentially important but poorly quantified source of stress to the diversity and balance of native species. 

While the Non-Indigenous Estuarine Species in Pacific Northwest indicator (p. 6-37) provides some 

insight into the potential importance of invasive species, the full significance of accelerated species 

introductions is not captured by any ROE indicator. 

In addition to indicator gaps and limitations, there are challenges to developing indicators of biological 

diversity and balance even if the data were available. For example, establishing an appropriate time scale 

for assessing trends in diversity and balance poses a major challenge. Biological variation is expected at 

annual, decadal, and even longer time scales. Because of the limited time frames over which observations 

have been made, parsing “normal” fluctuations in diversity and balance from longer-term trends is 

difficult. Also the level of interest and care of observation can also change with time, confounding the 

determination of actual trends. 

Appropriate spatial scales are equally important. Regional Indicators provide helpful insights into 

stressors affecting diversity and biological balance in some kinds of ecological systems for which there 

are no National ROE Indicators. In fact, because many ecological systems vary so much by geographic 

region, compilations of Regional Indicators may provide the only rational approach for identifying 

meaningful trends. Especially important examples for biological diversity are unique ecosystems such as 

the Arctic and Pacific islands. Trends in physical characteristics and processes can have far-reaching 

effects. For example, polar bears represent important keystone species in the nation’s arctic regions where 

they are stressed by warming of coastal waters that limit the duration of ice formation and Pacific island 

biota are stressed by invasive species and a number of other stressors. 

32 Dayton, P.K., M. Tegner. 1984. Catastrophic storms, El Nino, and patch stability in a southern California kelp 

community. Science 224(4646):283-285. 

33 Stuart, S.N., et al. 2004. A global census shows that most of the 5743 known amphibian species are in decline and 

one-third are currently endangered. Science 306(5702):1783-1786. 

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6.4 WHAT ARE THE TRENDS IN THE ECOLOGICAL PROCESSES THAT 

SUSTAIN THE NATION’S ECOLOGICAL SYSTEMS? 

6.4.1 Introduction 

Ecological systems are sustained by a number of biological processes that produce organic matter using 

energy (photosynthesis and chemosynthesis), transfer carbon and nutrients (through food webs and 

through decomposition), and enable the reproduction of organisms. Ecological systems are also shaped 

and sustained by physical and chemical attributes and processes. These physical and chemical 

components of ecological systems are considered in Section 6.5. Biological, physical, and chemical 

processes collectively maintain ecological systems. 

Ecological processes influence the extent, distribution, and biodiversity of systems. If primary production 

declines, energy flow to higher trophic levels is diminished, potentially compromising the sustainability 

of animal populations dependent on the plants for food. Primary production is influenced by the 

availability of nutrients. Decreases and increases in nutrients can affect the amounts of primary 

production as well as the types of plants with subsequent effects on animals. The successful reproduction 

of plants and animals depends on the physical and chemical regimes of their environment. 

Too much primary production can also cause problems, such as those that occur in eutrophic lakes that 

experience an overload of nutrient inputs. Eutrophic conditions can alter the composition of the animal 

and plant life and result in reduced oxygen levels due to decomposition of organic matter. For these 

reasons, management of nutrient inputs is commonly driven by the potential for excessive plant growth. 

Primary production and associated carbon cycling (which form the base of food webs), nitrogen cycling 

(ammonification and nitrification), nutrient cycling (e.g., phosphorous and other essential elements for 

sustainability of carbon-based life), and hydrogen/oxygen cycles (implicating hypoxic/anoxic conditions) 

are fundamental ecological processes within systems. Processes related to the production, transfer, and 

loss of biomass and the reproduction and death rates of individuals within populations are reflected in 

various “end states” in time, snapshots of the outcomes of integrated processes. The standing stock of a 

population or the amounts and types of carbon stored within an ecological system are measures of these 

end states. While not processes themselves, trends in end states provide some insight into the relative 

balance among processes. Carbon storage in forests, discussed in this section, is an example of such an 

end state. 

EPA has long been concerned with the impacts of human activities that can affect the rates, types, and 

timing of ecological processes. In particular, activities that upset the balance between primary production 

and respiration (e.g., biochemical oxygen demand, nutrients from fertilizers and human waste, and the 

effects of UV-radiation) and activities that affect sediment erosion and transport are important factors in 

water quality management. Many pesticides, chemicals used in industry, pollutants, and waste products 

have the potential to interfere with species reproduction (one of the most important of ecological 

processes). At local and regional scales, changes in land use that alter the extent and distribution of 

ecological systems (Section 6.2) directly affect ecological processes within and adjacent to particular 

areas. Concomitant changes often occur in primary production, nutrient cycling, and erosion and sediment 

transport. For example, shifts from forested to urban or agricultural lands influence the amounts and types 

of primary producers, the infiltration of water into soils, and the storage and cycling of carbon and 

nutrients. 

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6.4.2 ROE Indicators 

This section examines trends in the ecological processes that sustain ecological systems for one National 

Indicator (Table 6.4.1). Information for this indicator comes from satellite remote sensing, geographic 

information systems, and independent field studies. Sources include the USDA Forest Service Forest 

Inventory and Analysis (FIA). It is important to note that the data presented for carbon storage in forests 

includes only forests classified as “timberland” which excludes about one-third of the forest land cover. 

Timberland is defined as forests capable of producing at least 20 cubic feet per acre per year and not 

withdrawn from timber utilization by regulation or statute. This is an important distinction between 

previously illustrated trends in forest extent and type with the following discussion of carbon inventories. 

Table 6.4.1. ROE Indicators of Trends in the Ecological Processes that Sustain the Nation’s 

Ecological Systems 

NATIONAL INDICATORS LOCATION 

Carbon Storage in Forests 6.4.2 – p. 6-45 

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After carbon dioxide is converted into organic matter by photosynthesis, carbon is stored in forests for a 

period of time in a variety of forms before it is ultimately returned to the atmosphere through the 

respiration and decomposition of plants, animals, and the paper and wood products that result from tree 

harvest. A substantial pool of carbon is stored in woody biomass (roots, trunks, and branches). Another 

portion eventually ends up as organic matter in the upper soil horizons. Carbon storage in forest biomass 

and forest soils is an essential physical and chemical attribute of stable forest ecosystems. 

This indicator, developed by the USDA Forest Service (USDA Forest Service, 2004), tracks carbon 

storage in the pools of living and dead biomass in forests in the contiguous 48 states. The carbon pools for 

this indicator are estimated using USDA Forest Service Forest Inventory and Analysis (FIA) data from 

five historical periods (circa 1953, 1963, 1977, 1987, and 1997). These data cover 37 states, mostly east 

of the Mississippi, in the Rocky Mountains, or on the Pacific Coast (Smith et al., 2001, 2004). Alaska and 

Hawaii are not included because of limited historical data. Carbon storage is estimated by the FIA 

program using on-the-ground measurements of tree trunk size from many forest sites and statistical 

models that show the relationship between trunk size and the weight of branches, leaves, coarse roots 

(>0.1 inch in diameter), and forest floor litter combined with estimates of forest land area obtained from 

aerial photographs and satellite imagery. These values are converted into carbon storages based on the 

results of previous field studies (Smith and Heath, 2002; Smith et al., 2003; Birdsey, 1996). Forest floor 

litter includes all dead organic matter above the 

mineral soil horizons, including litter, humus, 

small twigs, and coarse woody debris (branches 

and logs greater than 1.0 inches in diameter lying 

on the forest floor). Organic carbon in soil is not 

included.

What the Data Show

The change in carbon inventories from year to 

year represents the net growth of trees minus the 

amount of carbon removed in harvested timber. 

The average rates of net carbon storage in forests 

increased between the 1950s and the 1980s but 

declined somewhat during the 1990s (Exhibit 6­

13). This trend varies among regions of the 

country, but net storage has been positive in all 

regions during the past two decades (Exhibit 6­

14).

The rate of storage for the last period of record 

(1987-1996) decreased to 135 million metric tons 

of carbon per year (MtC/yr), with declining 

sequestration evident in live, dead, and understory

pools (Exhibit 6-13). This decline is thought to be 

due to a combination of increased harvests relative 

to growth, more accurate data, and better 

accounting of emissions from dead wood (USDA 

Forest Service, 2004). The rate of storage over this 
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period is equivalent to approximately

9 to 10 percent of the U.S. carbon 

dioxide emissions over a comparable 

period (U.S. EPA, 2005). 

During the last period of record 

(1987-1996), the North region 

sequestered the greatest amount of 

carbon, followed by the Rocky

Mountain region (Exhibit 6-14). The 

trend of decreasing sequestration 

between 1953 and 1996 in the South is 

due to the increase in harvesting 

relative to growth. Some of the 

harvested carbon is sequestered in 

wood products (USDA Forest Service, 

2004). 

Indicator Limitations 

• The data include only 

forest classified as 

“timberland,” which 

excludes about one-third 

of U.S. forest land cover. 

Historical data from 

Alaska and Hawaii are 

insufficient for inclusion in this indicator. Urban trees, agricultural soils, and yard trimming 

are estimated by EPA to add about 15 percent to annual sequestration (U.S. EPA, 2005), but 

these data are of much lower quality than the timberland data and are not included in the 

indicator. 

• Data are derived from state inventories that do not correspond exactly to the decades 

identified in Exhibits 6-13 and 6-14. 

• Carbon stored in forest soil is not included. 

• Carbon pools are not measured, but are estimated based on inventory-to-carbon relationships 

developed with information from ecological studies. 

These limitations are discussed in detail in Smith and Heath (2000, 2001) and Heath and Smith (2000). 

Data Sources 

Exhibits 6-13 and 6-14 were previously published in the data supplement to USDA Forest Service (2004); 

the numbers depicted in these figures have not been published, but were provided by the USDA Forest 

Service. The raw measurements used as inputs in the carbon storage models can all be obtained from the 

Forest Service’s Forest Inventory and Analysis (FIA) database (USDA Forest Service, 2005) 

(http://fia.fs.fed.us/tools-data/data/). 

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References 

Birdsey, R.A. 1996. Carbon storage for major forest types and regions in the conterminous United States. 

In: Sampson, R.N., and D. Hair, eds. Forests and global change, volume 2: forest management 

opportunities for mitigating carbon emissions. Washington, DC: American Forests. pp. 1-25, 261-308. 

Heath, L.S., and J.E. Smith. 2000. An assessment of uncertainty in forest carbon budget projections. 

Environ. Sci. Policy 3:73-82. 

Smith, J.E., and L.S. Heath. 2000. Considerations for interpreting probabilistic estimates of uncertainty of 

forest carbon. In: Joyce, L.A., and R. Birdsey, eds. The impact of climate change on America’s forests. 

General Technical Report RMRS-59. Fort Collins, CO: USDA Forest Service, Rocky Mountain Research 

Station. pp. 102-111. 

Smith, J.E., and L.S. Heath. 2001. Identifying influences on model uncertainty: an application using a 

forest carbon budget model. Environmental Management 27:253-267. 

Smith, J.E., and L.S. Heath. 2002. Estimators of forest floor carbon for United States forests. Res. Pap. 

NE-722. Newtown Square, PA: USDA Forest Service, Northeastern Research Station. 37 pp. 

Smith, J.E., L.S. Heath, and P.B. Woodbury. 2004. How to estimate forest carbon for large areas from 

inventory data. J. Forest. 102:25-31. 

Smith, J.E., L.S. Heath, and J.C. Jenkins. 2003. Forest volume-to-biomass models and estimates of mass 

for live and standing dead trees of U.S. forests. General Technical Report NE-298. Newtown Square, PA: 

USDA Forest Service, Northeastern Research Station. 57 pp. 

Smith, W.B., J.S. Vissage, D.R. Darr, and R.M. Sheffield. 2001. Forest resources of the United States, 

1997. General Technical Report NC-219. St. Paul, MN: USDA Forest Service, North Central Research 

Station. <http://www.ncrs.fs.fed.us/pubs/gtr/gtr_nc219.pdf> 191 pp. 

USDA Forest Service. 2004. National report on sustainable forests—2003. Washington, DC: USDA 

Forest Service. <http://www.fs.fed.us/research/sustain/> (main site); 

<http://www.fs.fed.us/research/sustain/one_pagers/indicator%2027.pdf> (data supplement: summary); 

<http://www.fs.fed.us/research/sustain/documents/Indicator%2027/c5i27.pdf> (data supplement: graphics 

and metadata) 

USDA Forest Service. 2005. Forest Inventory and Analysis (FIA) database. Accessed 2005. 

<http://fia.fs.fed.us/tools-data/data/> 

U.S. EPA 2005. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2003. EPA/430/R-05/003. 

Washington, DC. 

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6.4.3 Discussion 

What This Indicator Says About Trends in the Ecological Processes that 

Sustain the Nation’s Ecological Systems 

The ROE indicator provides data on trends in primary production and carbon cycles for terrestrial 

systems. Primary producers capture, store, and supply solar derived energy to other species in the 

system.

34 In the forest, the energy currency is organic matter. Primary producers convert carbon dioxide 

into organic matter, which is then available to species throughout the ecological system as an energy 

resource and ultimately returns to the atmosphere (see Carbon Storage in Forests indicator, p. 6-45). For 

forests, the stability of the system may depend on the balance between carbon stored in standing stock and 

carbon lost from the system due to logging. Net carbon storage increased between 1950 and 1980, 

followed by declines through the 1990s. The decline is thought to be due to external “technologies”, i.e. 

increased harvest, more accurate data, and improved dead wood assessment. During the 1987-1996 time 

period, the greatest carbon storage occurred in the North and Rocky Mountain regions where there is 

more tree growth (compared to harvesting), while the greatest decline in sequestration occurred in the 

South where harvesting has been increasing relative to growth. The distribution of carbon has received 

much attention not only from a biological point of view but also with respect to global cycles of carbon. 

Increases and decreases in carbon storage suggest that other pools of carbon (e.g., within the aquatic and 

atmospheric environments) are also changing. The distribution of carbon among all these pools reflects a 

combination of processes and can also influence chemical, physical, and biological processes. 

Limitations, Gaps, and Challenges 

A major limitation of the indicator presented here is that it provides very little insight into ecological 

processes across the nation. Indicators are lacking for primary production in aquatic systems, nutrient 

cycling, secondary production, and reproduction and growth rates of populations. EPA recognizes this as 

a gap in understanding trends in ecological processes. To some degree, information presented in Sections 

6.2 and 6.3 gives insight into the net result of ecological processes. Trends in the extent and distribution 

of ecological systems and in the biodiversity and balance of those systems reflect underlying processes 

that produce food, cycle nutrients, and sustain populations of plants and animals. Sections 6.2 and 6.3 can 

be thought of as addressing “end states” that indicate the results of underlying ecological processes. 

Trends in these end states may or may not pick up important trends in the underlying processes because 

systems are dynamic and internal relationships are rarely linear. 

While the indicators of trends in ecological processes offer insights into potential threats to ecological 

systems, they are also subject to limitations, including natural variability, application to narrow selection 

of land uses (e.g., U.S. Forest Service timberland versus all forest lands protected and active, 

grassland/shrubland waterways compared to forested or urban waterways), human-induced impacts, and 

uncertainty about findings. Trends may be identified, but should be viewed within the context of the 

indicator design. 

34 Whitmarsh, J., and Govindjee. 1999. The photosynthetic process. In: Singhal, G.S., G. Renger, S.K. Sopory, K.-D. 

Irrgang, and Govindjee, eds. Concepts in photobiology: photosynthesis and photomorphogenesis. New Delhi, India: 

Narosa Publishers; Dordrecht, The Netherlands: Kluwer Academic Publishers. pp. 11-51. 

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A limitation in the assessment of trends in carbon storage in forests is that the National Indicator captures 

above-ground carbon and does not include a very important carbon storage medium (soil). In addition, 

statistical models are employed to estimate carbon storage relationships between different tree 

components. Carbon storage trends are important for assessing the future viability of ecological systems 

and have increasing utility in evaluating global carbon cycles and potential climate change. 

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6.5 WHAT ARE THE TRENDS IN THE CRITICAL PHYSICAL AND CHEMICAL 

ATTRIBUTES AND PROCESSES OF THE NATION’S ECOLOGICAL 

SYSTEMS? 

6.5.1 Introduction 

Numerous physical and chemical attributes and processes influence and sustain ecological systems. 

Physical processes shape the physical conditions of ecological systems. Examples include soil generation, 

erosion, sediment transport and deposition, changes in sea level, and physical disturbance regimes 

involving periodic flooding and fires. Critical attributes are those that have shaped the evolutionary 

history of species, govern the very nature of systems, and drive other processes. Critical physical 

attributes include temperature, light, and hydrologic regimes (rainfall, soil moisture, flow rates). Critical 

chemical attributes include oxygen, nutrients, pH, and salinity.

35 

Species have evolved within particular physical and chemical environments. These are characterized by 

mean (i.e., long-term average) conditions as well as by fluctuations on time scales of a day (e.g., tidal and 

light/dark cycles), seasons (e.g., temperature and hydrological cycles), years (e.g., periodic climatic and 

fire events), and longer time scales. The occurrence of ice ages every 40,000 to 100,000 years reflects one 

of the longer time scales. Because critical attributes and processes influence so many aspects of ecological 

systems, small changes in average conditions or changes in temporal variations can potentially have large 

effects on the extent and distribution of ecological systems and on the biodiversity of these systems. 

Average conditions and the degree and periodicity of fluctuations vary over the surface of the globe, and 

species have evolved with specific niche requirements that reflect the physical and chemical states of the 

ecological systems in which they live. For this reason, a species that has evolved in tropical waters would 

have temperature requirements that are higher and narrower (the species is less able to tolerate 

fluctuations) than a species that has evolved in temperate waters where temperatures are lower and more 

variable. Reproduction and other activity patterns of species are often related to physical and chemical 

cues such as temperature, light, and salinity. Because species have evolved coincident with the presence 

(or absence) of physical disturbances, reproductive strategies may be linked with the occurrence of events 

which otherwise appear destructive. Thus, disturbances such as periodic fires or flooding may be essential 

for sustaining certain species and ecological systems where these disturbances have been present over 

evolutionary time scales. 

Critical physical attributes and processes reflect, in part, the influence of solar radiation. Solar radiation 

warms land and water masses and drives hydrologic cycles. The amount of light reaching the surface of 

the earth and penetrating into its waters determines levels of photosynthesis which is essential to the 

support of biological systems. Other examples of physical, chemical, and biological processes that are 

influenced by the amount and periodicity of light include temperature and weather conditions, 

photoactivation of chemicals, mutations, and the timing of reproductive cycles. Solar radiation can also 

have potential harmful effects on some species. Light regimes can be influenced by changes in solar 

energy reaching the earth, changes in the transparency of water, and changes in sea level and these change 

35 Information on nutrients and potentially toxic chemicals is presented in Chapters 2, 3, and 4 of this report. 

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the degree of light penetration reaching the sea floor, coral reefs, and kelp forests. The implication of 

climate change for changes in many aspects of ecological condition has received broad attention.36

EPA has been actively involved over its three decades in assessing and managing factors that alter the 

critical chemical and physical characteristics of ecological systems (e.g., temperature, pH, 

electrochemical (redox) potential, and the transparency of air and water). For example, the use of water 

for cooling purposes can result in temperature increases in receiving waters of a river, acid rain can lower 

the pH levels of lakes in sensitive regions, and wastewater and fertilizer can lead to low redox potentials 

which affect biological communities and the cycling of both toxic and non-toxic materials. Although EPA 

is not directly involved in the control of hydrology, an important physical factor in the environment, 

hydrology greatly influences the fate and transport of pollutants in aquatic ecosystems. Changes in the 

amount of runoff can affect ground water levels as well as flows into streams and rivers. Flood control 

efforts can alter flooding and sedimentation processes that sustain particular types of systems. Because 

ground water is a primary source to surface water bodies in many parts of the nation, changes in the 

quantity (water level) and quality of ground water influence ecological conditions not only in the 

hyporheic zone (subsurface in sediments and soils) but also in surface waters. The potential impacts of 

climate change (whether natural or human-induced) have important consequences for virtually every 

aspect of ecological structure and function. 

6.5.2 ROE Indicators 

The evaluation of trends in the critical physical and chemical attributes of the nation’s ecological systems 

relies primarily on nine National Indicators and one Regional Indicator (Table 6.5.1). Information comes 

from a variety of sources, including satellite remote sensing, geographic information systems, monitoring 

programs, visual surveys, and independent field studies. Indicator data in this section are drawn from the 

Heinz Center,37 National programs such as the USDA Forest Service Forest Inventory and Analysis (FIA) 

program, EPA’s Wadeable Streams Assessment (WSA), National Research Council,38 National 

Atmospheric Deposition Program, National Aeronautics and Space Administration (NASA), National 

Oceanic and Atmospheric Administration's (NOAA’s) National Climate Data Center, U.S. Geological 

Survey’s National Water Quality Assessment Program, U.S. Geological Survey stream flow and water 

quality monitors, and National Emissions Inventory. 

36 Millennium Ecosystem Assessment Board. 2005. Living beyond our means: natural assets and human well being. 

<http://www.maweb.org/en/products.aspx> 

37 The H. John Heinz III Center for Science, Economics, and the Environment. 2005. Forest pattern and 

fragmentation. In: The state of the nation’s ecosystems: measuring the lands, waters, and living resources of the 

United States. New York, NY: Cambridge University Press, September 2002. Web update 2005. 

<http://www.heinzctr.org/ecosystems/forest/frgmnt.shtml> 

38 National Research Council. 2001. Climate change science: an analysis of some key questions. Committee on the 

Science of Climate Change. Washington, DC: National Academy Press. 

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Table 6.5.1. ROE Indicators of Trends in the Critical Physical and Chemical Attributes of the 

Nation’s Ecological Systems. 

NATIONAL INDICATORS LOCATION 

U.S. and Global Mean Temperature and Precipitation 6.5.2 – p. 6-53 

Sea Surface Temperature 6.5.2 – p. 6-58 

High and Low Stream Flows 3.2.2 – p. 3-14 

Streambed Stability in Wadeable Streams 3.2.2 – p. 3-19 

Sea Level 6.5.2 – p. 6-61 

Nitrogen and Phosphorus Discharge from Large Rivers 3.2.2 – p. 3-28 

Nitrogen and Phosphorus in Wadeable Streams 3.2.2 – p. 3-22 

Nitrogen and Phosphorus in Streams in Agricultural Watersheds 3.2.2 – p. 3-25 

Lake and Stream Acidity 2.2.2 – p. 2-62 

REGIONAL INDICATORS 

Extent of Hypoxia in the Gulf of Mexico and Long Island Sound 3.5.2 – p. 3-77 

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1 INDICATOR: U.S. and Global Mean Temperature and Precipitation 

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Air temperature and precipitation are two important properties of climate and are the most widely 

measured variables. Changes in these indicators may have wide-ranging direct or indirect effects on 

ecological condition and human health. These impacts may be positive or negative, depending on the 

effect, the magnitude of change, and the location. For example, changes in temperature can affect heat 

and cold-related mortality and illness due to altered frequency and magnitude of heat waves and cold 

spells. Changes in temperature may also change the range and distribution of animal and plant species. 

Precipitation changes affect water availability and quality which can have important effects on 

agricultural, forest, animal, and fisheries productivity, as well as human nutrition. Indirect effects of 

temperature and precipitation changes include changes in the potential transmission of vector-borne 

infectious diseases. These may result from alterations in the ranges and seasons of animals that carry 

disease or from accelerated maturation of certain 

infectious parasites.

This indicator shows trends in temperature and 

precipitation based on instrumental records from 

1901 to 2005 (except for Alaska and Hawaii, where 

records begin in 1918 and 1905, respectively). Air 

temperature and precipitation trends are summarized 

for the contiguous U.S., as well as for eleven climate 

regions of the U.S., including Alaska and Hawaii 

(these climate regions are different from the ten EPA 

Regions). For context, this indicator also shows 

trends in global temperature (over land and sea) and 

global precipitation (over land) from 1901 to 2005. 

Temperature and precipitation data are presented as 

trends in anomalies. An anomaly represents the 

difference between an observed value and the 

corresponding value from a baseline period. This 

indicator uses a 30-year baseline period of 1961 to 

1990. To generate the temperature time series, 

measurements were converted into monthly 

anomalies, in degrees Fahrenheit. The monthly 

anomalies then were averaged to get an annual 

temperature anomaly for each year. Precipitation 

trends were calculated in similar fashion, starting 

with anomalies for total monthly precipitation, in 

millimeters. Monthly anomalies were added to get an 

annual anomaly for each year, which was then 

converted to a percent anomaly—i.e., the percent 

departure from the average annual precipitation 

during the baseline period. Trends in temperature 

and precipitation were calculated from the annual 

time series by ordinary least-squares regression. For 

each of the eleven climate regions, this indicator also 
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shows a smoothed time series, which was created from the annual series using a nine-point binomial filter 

(four years on each side, averaged with decreasing weights further from the center year). 

What the Data Show 

Over the past century, temperatures rose across the 

contiguous United States at an average rate of 0.11 

°F per decade (1.1 °F per century) (Exhibit 6-15, 

panel A). Average temperatures rose at an 

increased rate of 0.56 °F per decade from 1979 to 

the present. For reference, the most recent eightyear (1998–2005), nine-year (1997–2005), and tenyear (1996–2005) periods have been the warmest 

on record for the United States (NOAA, in 

progress). Warming occurred throughout the U.S., 

with all but three of the eleven climate regions 

showing an increase of more than 1 oF since 1901 

(Exhibit 6-16). The greatest temperature increase 

occurred in Alaska (3.3 °F per century). 

Trends in global temperature and precipitation 

provide a context for interpreting trends in 

temperature and precipitation in the U.S. 

Instrumental records from land stations and ships 

indicate that global mean surface temperature 

warmed by about 1.1 oF during the 20th century 

(Exhibit 6-15, panel B), similar to the rate of 

warming within the contiguous U.S. From 1979 to 

present, however, the U.S. warmed at nearly twice 

the global rate. 

As global mean temperatures have risen, global 

mean precipitation also has increased (Exhibit 6­

17, panel B). This is expected because evaporation 

increases with increasing temperature, and there 

must be an increase in precipitation to balance the 

enhanced evaporation (IPCC, 2001). Precipitation 

over land increased by 1.6 percent globally since 1901, but the trends vary spatially and temporally. Over 

the contiguous U.S., total annual precipitation increased at an average rate of 6.1 percent per century since 

1901 (Exhibit 6-17, panel A), although there was considerable regional variability (Exhibit 6-18). The 

greatest increases came in the East North Central climate region (11.6 percent per century) and the South 

(11.1 percent). Hawaii was the only region to show a decrease (-9.25 percent). 

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1 Indicator Limitations 

2 • Biases may have occurred as a result of changes over time in instrumentation, measuring 

3 procedures (e.g., time of day), and the exposure and location of the instruments. Where 

4 possible, data have been adjusted to account for changes in these variables. 

5 • Uncertainties in both the temperature and precipitation data increase as one goes back in time, 

6 as there are fewer stations early in the record. However, these uncertainties are not sufficient 

7 to mislead the user about fundamental trends in the data. 

8 Data Sources 

9 Data were provided by NOAA’s National Climatic Data Center (NCDC), which calculated global, U.S., 

10 and regional temperature and precipitation time series based on monthly values from a network of long11 term monitoring stations. Data from individual stations were obtained from the U.S. Historical Climate 

12 Network (USHCN) and the Global Historical Climate Network (GHCN), which are NCDC’s online 

13 databases (NOAA, 2006). 

14 References 

15 IPCC (Intergovernmental Panel on Climate Change). 2001. Climate change 2001: the scientific basis. 

16 Contribution of Working Group I to: Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der 

17 Linden, X. Dai, K. Maskell, and C.A. Johnson, eds. Third assessment report of the Intergovernmental 

18 Panel on Climate Change. Cambridge, United Kingdom, and New York, NY: Cambridge University 

19 Press. 881 pp. 

20 NOAA (National Oceanic and Atmospheric Administration). 2006. National Climatic Data Center. 

21 Accessed October 2006. <http://www.ncdc.noaa.gov/oa/ncdc.html> (NCDC home page); 

22 <http://www.ncdc.noaa.gov/oa/climate/research/ushcn/ushcn.html> (U.S. Historical Climate Network); 

23 <http://www.ncdc.noaa.gov/oa/climate/research/ghcn/ghcn.html> (Global Historical Climate Network) 

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INDICATOR: Sea Surface Temperature

Sea surface temperature (SST) is a critical physical attribute of the oceans and coastal ecological systems. 

Water temperature directly affects biological and physical process rates, water column stability, and the 

presence and functioning of species of plants (e.g., algae, sea grasses, marsh plants, and mangroves) and 

animals (e.g., microscopic animals, larger invertebrates, fish, and mammals). Increases in temperature 

have been associated with the timing of breeding in sea turtles (Weishampel et al., 2004), stress and 

bleaching of coral reefs (Brown, 1997; Woodbridge and Done, 2004), alteration of species migration 

patterns, changes in ecological system extent and composition (Helmuth et al., 2002), and changes in the 

frequency or extent of blooms of harmful algae (Ostrander et al., 2000). On longer time scales (decades to 

centuries), rising SST may result in decreases in the supply of nutrients to surface waters from the deep 

sea which could trigger a cascade of effects leading to decreases in primary production and declines in 

fish production (Pratchett et al., 2004), wetland loss, reductions in coastal storm buffering, and losses of 

local tourism. SST is both an indicator of, and a profound influence on, the climate system. Changes in 

SST may result from long-term cycles in ocean circulation, climate variability, or secular trends in 

climate (Committee on the Bering Sea Ecosystem et al., 1996). 

This SST indicator, developed by the National Climate Data Center (NCDC) of the National Oceanic and 

Atmospheric Administration (NOAA) and the National Center for Atmospheric Research (NCAR), 

describes the long-term variability and change in global mean sea surface temperature for the period 1880 

to 2005. This reconstruction provides consistent spatial and temporal data with their associated 95 percent 

confidence intervals. The data are compiled from in-situ measurements from the International 

Comprehensive Ocean-Atmosphere Data Set (ICOADS) release 2 (Slutz et al., 2002) and—in recent 

years—from satellite imagery. Data are available from multiple sources (e.g., ship reports, buoy monitors, 

oceanographic profiles) from as early as 1854 (Woodruff et al., 1998). By filtering and blending data sets 

that use alternative measurement methods and include redundancies in space and time, this reconstruction 

is able to fill spatial and temporal data gaps and correct for biases in the different measurement techniques 

(e.g., uninsulated buckets, intakes near warm engines, uneven spatial coverage, etc.). The extended 

reconstructed data are shown as anomalies, or differences, from the “normal” (i.e., average) SST from 

1961 to 1990 (Smith and Reynolds, 1998). The long-term average change obtained by this method is very 

similar to those of the “unanalyzed” measurements and reconstructions developed by other researchers 

(e.g., Rayner et al., 2003). 

What the Data Show

The reconstruction of SST

anomalies over all latitudes

indicates that sea-surface 

temperatures during the past 

three decades are at their highest 

levels over the period of record 

(Exhibit 6-19). Warming has 

occurred through most of the 

twentieth century and appears to 

be independent of measured 

inter-decadal and short-term 

variability (Smith and Reynolds, 

2005). The SST warming occurs 

in two parts, the first between 
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1910 and 1940 and the second after 1970, with a roughly stationary period between 1940 and 1970. SST 

appears to have cooled between 1880 and 1910, but confidence intervals are wide for the indicator over 

the early period of record. Despite that uncertainty, warming for the entire period of the indicator and for 

the period from 1900 forward is statistically significant. 

Indicator Limitations 

• The 95 percent confidence interval is wider than other methods for long-term reconstructions 

and in mean SSTs this interval tends to dampen anomalies. 

• The geographic resolution is coarse for ecosystem analyses but reflects long-term and global 

changes as well as variability. 

• The reconstruction methods used to create this indicator remove almost all random “noise” in 

the data. However, the anomalies are also dampened when and where data are too sparse for a 

reliable reconstruction. The 95 percent confidence interval reflects this “damping” effect as 

well as uncertainty caused by possible biases in the observations. 

• Data screening results in loss of many observations at latitudes higher than 60o N and 60o

 S. 

Although the effects of screening at high latitudes on the indicator are extremely small on the 

global average, its main effect is to lessen anomalies and widen the confidence intervals. 

Data Sources 

This extended reconstruction of SST, called ERSST.v2, was recently published in Smith and Reynolds 

(2004). Data are available from NOAA’s National Climate Data Center (NOAA, 2006b), which provides 

access to monthly SST and error data from this reconstruction (ftp://ftp.ncdc.noaa.gov/pub/data/ersst-v2/) 

as well as a mapping utility that allows the user to calculate average anomalies over time and space 

(http://nomads.ncdc.noaa.gov/#climatencdc). Confidence intervals for the global average dataset were 

provided by NOAA. The ERSST.v2 reconstruction is based on in-situ measurements and satellite data, 

both of which are available from online databases. In-situ measurements are available from NOAA 

(2006a) (http://icoads.noaa.gov/products.html), and satellite data from NASA (2006) 

(http://podaac.jpl.nasa.gov/sst/). 

References 

Brown, B. 1997. Coral bleaching: causes and consequences. Coral Reefs 16:S129-S138. 

Committee on the Bering Sea Ecosystem, Polar Research Board, Commission on Geosciences, 

Environment and Resources, and National Research Council. 1996. The Bering Sea ecosystem. 

Washington, DC: National Academy Press. pp. 196-237. 

Helmuth, B., C.D.G. Harley, P.M. Halpin, M. O’Donnell, G.E. Hofmann, and C.A. Blanchette. 2002. 

Climate change and latitudinal patterns of intertidal thermal stress. Science 298:1015-1017. 

NASA. 2006. Physical Oceanography Distributed Active Archive Center (PO.DAAC), sea surface 

temperature overview. NASA, Jet Propulsion Laboratory. Accessed 2006. 

<http://podaac.jpl.nasa.gov/sst/> 

NOAA. 2006a. International Comprehensive Ocean-Atmosphere Data Sets (ICOADS). Accessed 2006. 

<http://icoads.noaa.gov/> 

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NOAA. 2006b. Sea surface temperature (SST) datasets. NOAA, National Climatic Data Center. Accessed 

2006. <http://www.ncdc.noaa.gov/oa/climate/research/sst/sst.html#grid> 

Ostrander, G.K., K.M. Armstrong, E.T. Knobbe, D. Gerace, E.P. Scully. 2000. Rapid transition in the 

structure of a coral reef community: the effects of coral bleaching and physical disturbance. Proc. Natl. 

Acad. Sci. USA 97(10):5297-5302. 

Pratchett, M.S., S.K. Wilson, M.L. Berumen, and M.I. McCormick. 2004. Sublethal effects of coral 

bleaching on an obligate coral feeding butterflyfish. Coral Reefs 23(3): 352–356. 

Rayner, N.A., D.E. Parker, E.B. Horton, C.K. Folland, L.V. Alexander, D.P. Rowell, E.C. Kent, and A. 

Kaplan. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since 

the late nineteenth century. J. Geophys. Res. 108:4407. doi:10.1029/2002JD002670. 

Slutz, R.J., S.J. Lubker, J.D. Hiscox, S.D. Woodruff, R.L. Jenne, D.H. Joseph, P.M. Steurer, and J.D. 

Elms. 2002. Comprehensive ocean-atmosphere data set; release 1. NTIS PB86-105723. Boulder, CO: 

NOAA Environmental Research Laboratories, Climate Research Program. 268 pp. 

<http://icoads.noaa.gov/Release_1/coads.html#abstract> 

Smith, T.M., and R.W. Reynolds. 1998. A high resolution global sea surface temperature climatology for 

the 1961-90 base period. J. Climate 11:3320-3323. 

<http://lwf.ncdc.noaa.gov/img/climate/research/sst/aoiclim_6190.pdf> 

Smith, T.M., and R.W. Reynolds. 2005. A global merged land air and sea surface temperature 

reconstruction based on historical observations (1880-1997). J. Climate 18(12):2021-2036. 

<http://www.ncdc.noaa.gov/gcag/merge.pdf> 

Smith, T.M., and R.W. Reynolds. 2004. Improved extended reconstruction of SST (1854-1997). J. 

Climate 17:2466-2477. <http://www.ncdc.noaa.gov/img/climate/research/sst/ersst-v2.pdf> 

Weishampel, J.F., D.A. Bagley, and L.M. Erhart. 2004. Earlier nesting by loggerhead sea turtles 

following sea surface warming. Glob. Change Biol. 10:1.4. doi: 10.1111/j.1365-2486.2004.00817.x 

Woodbridge, S., and T. Done. 2004. Learning to predict large-scale coral bleaching from past events: a 

Bayesian approach using remotely sensed data, in-situ data, and environmental proxies. Coral Reefs 

23(1):96-108. 

Woodruff, S.D., H.F. Diaz, J.D. Elms, and S.J. Worley. 1998. COADS release 2 data and metadata 

enhancements for improvements of marine surface flux fields. Phys. Chem. Earth 23(5-6):517-526. 

<http://icoads.noaa.gov/egs_paper.html> 

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1 INDICATOR: Sea Level

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Sea level is an indicator of global and local change and a factor that affects human welfare and coastal 

ecosystem conditions. Coastal areas host a rich set of natural and economic resources and include some of 

the most developed and rapidly growing population centers in the nation. More than 100 million people 

globally live within 1 meter of the mean sea level and more than 40 percent of the U.S. population lives in 

watersheds along U.S. ocean coasts (NOAA, 2005). Changing sea levels can inundate low lying wetlands 

and dry lands (Burkett et al., 2005), erode beaches (U.S. Geological Survey, 1998), change rates of 

sedimentation (Olff et al., 1997), and increase the salinity of marshes, estuaries, and aquifers (Condrey et 

al., 1995; Williams et al., 1999). Documented consequences of sea level rise include loss of buffering 

against storms and floods (Burkett et al., 2005), changes in bird populations (Erwin, 2005) and land cover 

(Williams et al., 1999), property losses (Burkett et al., 2005), and infrastructure damage (Theiler and 

Hammar-Klose, 1999; U.S. Department of Transportation, 2003). 

Approximately 58,000 km2 of land in the contiguous U.S. lie less than 1.5 m above sea level, 80 percent 

of which is in Louisiana, Florida, Texas, and North Carolina (Titus and Richman, 2001). Almost half the 

shoreline studied along the U.S. Atlantic Coast was determined to be highly to very highly vulnerable to 

effects of sea level rise (Theiler and Hammar-Klose, 1999). The areas of highest vulnerability are highenergy coastlines where the coastal slope is low and the major landform type is a barrier island. The risks 

may be minimal if wetlands accretion can match or outpace sea level rises, but accretion rates vary widely 

(Hartig et al., 2000, Table 3). 

A number of factors affect sea level, including, but not limited to, changes in sea temperature, salinity, 

and total water volume and mass (e.g. from melting glaciers or changes in the amount of water stored on 

land). Sea level moves up with warming sea temperatures and down with cooling. Changes in the total 

volume and mass of ocean water also result from the melting or accumulation of Antarctic and Greenland 

ice sheets and non-polar glaciers and changes in the amount of water stored in lakes, rivers, and 

groundwater. As such, global average sea level change is an indicator of the physical and climatic 

stability of the global environment. 

Temporal scale is an important factor in interpreting sea level trends. Sea level changes may reflect 

factors such as seasonality, inter-annual to decadal scale variability such as “El Nino,” and/or long-term 

climate change (decades to centuries). Spatial scale also is important because absolute sea height does not 

change uniformly around the globe. 

This indicator presents trends in absolute and relative sea level. Absolute sea level represents only the sea 

height, whereas relative sea level change is defined as sea height change plus land height changes (due to 

subsidence or uplift and changes in natural land accretion). Relative sea level data are from the tidal 

gauge measurements of the National Water Level Observation Network (NWLON) composed of 

approximately 175 long-term, continuously operating stations located along the United States coast, 

including the Great Lakes and along islands in the Atlantic and Pacific Oceans (Smith, 1980; Gill and 

Schultz, 2001). Tidal gauge data are presented from 1950 to 1999, although a few locations have been 

monitoring since the mid-1800s (NOAA, 2001). Absolute sea level data are from satellite measurements 

from NASA’s TOPEX/Poseidon spacecraft, which uses radar to map the precise features of the ocean 

surface, and the “Jason” satellite which monitors ocean circulation (Leuliette et al., 2006). The two 

satellites use radar altimetry to collect sea level data globally. These data have been available since 1993. 
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What the Data Show 

Relative sea levels (combined land and sea movement) in many locations rose from 1950 to 1999, 

typically at rates of 0-3 millimeters per year (mm/yr) (up to one foot per century) (Exhibit 6-20). Relative 

sea level has risen more rapidly (3-6 mm/yr) along the mid-Atlantic coast from North Carolina to New 

Jersey and at rates as high as 9-12 mm/yr at two stations in Louisiana. Other locations, such as the 

southern coast of Alaska, show relative sea level drop, with a maximum decrease of 16 mm/yr. Average 

relative sea level rise for all U.S. coasts was not calculated because the distribution of tidal gauge stations 

is not spatially representative of aggregate trends, but for reference, an analysis of tidal gauge data 

worldwide estimated that on average, relative sea level rose between 1.5 and 2.0 mm/yr during the 20th 

century (Miller and Douglas, 2004). 

The satellite record shows that global mean absolute sea level (i.e., independent of land movements) has 

increased at a rate of 3 mm (0.12 inches) per year since 1993 (Exhibit 6-21). Absolute sea levels do not 

change uniformly around the Earth, however. Around the U.S., areas with increasing absolute sea level 

include the Gulf coast and portions of the Atlantic coast (Exhibit 6-22). Areas showing a decrease include 

the southern part of the Pacific coast and the western Gulf of Alaska. 

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Indicator Limitations 

• An estimated 50 to 60 years of data are required to obtain linear mean sea level trends having 

a 1 mm/yr precision with a 95 percent statistical confidence interval. 

• Tidal gauge measurements do not represent more generalized (i.e. average) relative sea level 

change along US coasts (or globally). 

• Most local tidal gauge measurements cannot indicate whether changes in relative sea level are 

due to changing water level or land level. 

• Satellite data are not available for a multi-decadal time series needed to separate out mediumterm variability from long-term change. 

• Satellite data are not horizontally precise enough to resolve sea level trends for small water 

bodies (such as many estuaries) or for localized interests (such as a particular harbor or 

beach). 

Data Sources 

Exhibit 6-20 is based on a map and corresponding trend data published by NOAA’s National Oceans 

Service (NOAA, 2006) (http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml). These data also can be 

found in NOAA (2001), along with a list of station coordinates (Appendix 1). Individual station 

measurements are accessible through NOAA (2006). 

Exhibits 6-21 and 6-22 were produced using data provided by Leuliette et al. (2006) (time series at 

http://sealevel.colorado.edu/results.php; map at http://sealevel.colorado.edu/maps.php). Leuliette et al.’s 

analysis was based on measurements from NASA’s Ocean TOPography Experiment (TOPEX) and Jason 

satellite altimeters; results were calibrated using a model documented in Leuliette et al. (2004). Satellite 

measurements can be obtained from NASA’s online database (NASA, 2006) (http://topexwww.jpl.nasa.gov/science/data.html). 

References 

Burkett, V.R., D.B. Zilkoski, and D.A. Hart. 2005. Sea-level rise and subsidence: implications for 

flooding in New Orleans, Louisiana. In: Subsidence observations based on traditional geodetic 

techniques, and numerical models. U.S. Geological Survey, National Wetlands Research Center. 

<http://www.nwrc.usgs.gov/hurricane/Sea-Level-Rise.pdf> 

Condrey, R., P. Kemp, J. Visser, J. Gosselink, D. Lindstedt, E. Melancon, G. Peterson, and B. Thompson. 

1995. Status, trends, and probable causes of change in living resources in the Barataria and Terrebonne 

estuarine systems. Thibodaux, LA: Barataria-Terrebonne National Estuary Program. 

Erwin, R.M. 2005. Atlantic sea level rise, lagoonal marsh loss, and wildlife habitat implications. U.S. 

Geological Survey. Accessed December 29, 2005. 

<http://www.pwrc.usgs.gov/resshow/erwin1rs/erwin1rs.htm 

Gill, S.K., and J.R. Schultz. 2001. Tidal datums and their applications. NOAA Special Publication NOS 

CO-OPS 1. 111 p. 

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Hartig, E.K., F. Mushacke, D. Fallon, and A. Kolker. 2000. A wetlands climate change impact assessment 

for the metropolitan East Coast region. Draft for public review. 

<http://metroeast_climate.ciesin.columbia.edu/reports/wetlands.pdf> 

Leuliette, E.W., R.S. Nerem, G.T. Mitchum, and D.P. Chambers. 2006. Sea level change. Updated 

October 2006 (2006 release #2). <http://sealevel.colorado.edu/> 

Leuliette, E.W., R.S. Nerem, and G.T. Mitchum. 2004. Calibration of TOPEX/Poseidon and Jason 

altimeter data to construct a continuous record of mean sea level change. Mar. Geod. 27(1-2):79-94. 

<http://sealevel.colorado.edu/MG_Leuliette2004.pdf> 

Levermann, A., A. Griesel, M. Hoffmann, M. Montoya, and S. Ramstorf. 2005. Dynamic sea level 

changes following changes in the thermohaline circulation. Clim. Dynam. 24:347-354. 

Miller, L., and B.C. Douglas. 2004. Mass and volume contributions to twentieth-century global sea level 

rise. Nature 428:406-409. <http://www.grdl.noaa.gov/SAT/pubs/papers/2004nature.pdf> 

NASA (National Aeronautics and Space Administration). 2006. Ocean surface topography from space. 

Updated January 2006. <http://topex-www.jpl.nasa.gov/science/data.html> 

NOAA (National Oceanic and Atmospheric Administration). 2006. Sea levels online. Accessed October 

6, 2006. <http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml> (home page); 

<http://tidesandcurrents.noaa.gov/sltrends/slrmap.html> (map); 

<http://tidesandcurrents.noaa.gov/sltrends/msltrendstable.htm> (data table) 

NOAA (National Oceanic and Atmospheric Administration). 2005. Population trends along the coastal 

United States: 1980-2008. <http://www.oceanservice.noaa.gov/programs/mb/supp_cstl_population.html> 

NOAA (National Oceanic and Atmospheric Administration). 2001. Sea level variations of the United 

States 1854-1999. NOAA Technical Report NOS CO-OPS 36. 

<http://tidesandcurrents.noaa.gov/publications/techrpt36.pdf> 

Olff, H., J. De Leeuw, J.P. Bakker, R.J. Platerink, H.J. Van Wijnen, and W. De Munck. 1997. Vegetation 

succession and herbivory in a salt marsh: changes induced by sea level rise and silt deposition along an 

elevational gradient. J. Ecol. 85:799-814. 

Smith, R.A. 1980. Golden Gate tidal measurements. J. Waterw. Port C. Div. 106(WW3):407-410. 

Thieler, E.R., and E.S. Hammar-Klose. 1999. National assessment of coastal vulnerability to sea-level 

rise: preliminary results for the U.S. Atlantic coast. U.S. Geological Survey Open-File Report 99-593. 

<http://pubs.usgs.gov/of/of99-593/index.html> 

Titus, J., and C. Richman. 2001. Maps of lands vulnerable to sea level rise: modeled elevations along the 

U.S. Atlantic and Gulf coasts. Climate Res. 18:205-228. 

<http://yosemite.epa.gov/oar/globalwarming.nsf/UniqueKeyLookup/SHSU5C3J4E/$File/maps.pdf> 

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U.S. Department of Transportation. 2003. Does sea level rise matter to transportation along the Atlantic 

coast? In: U.S. Department of Transportation, Center for Climate Change and Environmental Forecasting. 

The potential impacts of climate change on transportation. 

<http://yosemite.epa.gov/oar/globalwarming.nsf/UniqueKeyLookup/JSAW672M6T/$File/Transportation 

_Paper.pdf> 

U.S. Geological Survey. 1998. The Chesapeake Bay: geologic product of rising sea level. Fact Sheet 102­

98. <http://pubs.usgs.gov/fs/fs102-98/> 

Williams, K., K.C. Ewel, R.P. Stumpf, F.E. Putz, and T.W. Workman. 1999. Sea-level rise and coastal 

forest retreat on the west coast of Florida, USA. Ecology 80(6):2045-2063. 

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6.5.3 Discussion 

What These Indicators Say About Trends in Critical Physical and Chemical 

Attributes of the Nation’s Ecological Systems 

Critical Physical Attributes 

Information is available on trends in temperature and precipitation (see Temperature and Precipitation 

indicator, p. 6-53). Across the contiguous U.S., mean temperature increased over the past century. The 

rate of increase in the past 30 years was higher than in the previous part of the century, amounting to 

more than 0.5 o

F per decade. Some regional trends in temperature are evident, with the southeastern 

United States exhibiting some cooling and the western part of the country and Alaska exhibiting a greater 

warming trend than the rest of the country. 

These general warming trends have occurred concurrently with rising atmospheric concentrations of 

greenhouse gases (see Greenhouse Gas Concentrations indicator, p. 2-100). The Intergovernmental Panel 

on Climate Change concluded that “[t]here is new and stronger evidence that most of the warming 

observed over the last 50 years is attributable to human activities.”39 The National Research Council 

agreed that “[t]he changes observed over the last several decades are likely mostly due to human 

activities,” but cautioned that “we cannot rule out that some significant part of these changes is also a 

reflection of natural variability.”

40 

Virtually every ecological system in the United States is potentially vulnerable to changes in temperature 

regimes that might affect either biological or physical conditions. These include coastal and marine 

areas,41 inland freshwater and wetland systems,42 and terrestrial systems.43 The potential ecological 

implications of a gradual warming trend have received much attention.44,45 

39 IPCC (Intergovernmental Panel on Climate Change). 2001. Climate change 2001: the scientific basis. 

Contribution of Working Group I to: Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, 

K. Maskell, and C.A. Johnson, eds. Third assessment report of the Intergovernmental Panel on Climate Change. 

Cambridge, United Kingdom, and New York, NY: Cambridge University Press. 

40 National Research Council. 2001. Climate change science: an analysis of some key questions. Committee on the 

Science of Climate Change. Washington, DC: National Academy Press. 

41 Kennedy, V.S., et al. 2002. Coastal and marine ecosystems and global climate change: potential effects on U.S. 

resources. Arlington, VA: Pew Center on Global Climate Change. 

42 Poff, N.L., M.M. Brinson and J.W. Day, Jr. 2002. Aquatic ecosystems and global climate change: potential 

impacts on inland freshwater and coastal wetland ecosystems in the United States. Arlington, VA: Pew Center on 

Global Climate Change. 

43 Malcom, J., and L. Pitelka, 2000. Ecosystems and global climate change: a review of potential impacts on U.S. 

terrestrial ecosystems and biodiversity. Washington, DC: Pew Center on Global Climate Change. 41 pp. 

44 National Research Council. 2001. Climate change science: an analysis of some key questions. Committee on the 

Science of Climate Change. Washington, DC: National Academy Press. 

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Temperature changes can influence the physical aspects of ecological systems, including regional and 

global weather and oceanographic patterns. Impacts associated with warming include the global retreat of 

mountain glaciers, reduction in snow-cover extent, earlier spring melting of ice on rivers and lakes, and 

increases in sea surface temperatures and ocean heat content.46 For example, sea surface temperature 

increased throughout the past century, with the greatest increases occurring in the past three decades (see 

Sea Surface Temperature indicator, p. 6-58). Changes in temperature also have shown variations 

throughout the world’s oceans.47,48 

Temperature changes also can influence the biological aspects of ecological systems. All species have 

preferred ranges of temperature for survival, growth, and reproduction as well as lower and upper thermal 

tolerance limits. Mean temperature, seasonal changes, and other temporal fluctuations constitute species 

temperature regimes. As these regimes change, several types of stresses are placed on species. The first is 

that the species may not be as well adapted to the new regime and may not be able to sustain its 

populations. The second is that other species may be better adapted and able to extend their ranges into 

new areas. Finally, because temperature can affect other biological and physical attributes of systems, the 

ecological system itself may change in a way that is not favorable for the species. 

Temperature patterns are interlinked with air and water circulation patterns, which are critical to the 

dispersal of organisms, the movement of nutrients, and many other processes important to sustaining 

ecological systems. The replenishment of water over land surfaces is particularly critical, as it is a major 

determinant of the sustainability of the varied ecological systems that exist along a gradient of moisture 

from wetlands to deserts. For example, in areas where precipitation is reduced, droughts can have a 

pronounced and rapid influence on vegetation.49 

Overall, precipitation increased in the United States over the past century (see Temperature and 

Precipitation indicator, p. 6-53). Regional differences are apparent, however, with the greatest increases in 

the East North Central climate region and the South, very small increases in other regions, and a decrease 

in Hawaii. It is difficult to assign causes to such local and regional changes in precipitation because of 

natural climate variability (e.g., oscillations such as El Niño and others), complex interactions between 

aerosols (from natural and industrial processes) and clouds, and the effects of urban and rural land use on 

evaporation and transpiration. 

45 Millennium Ecosystem Assessment Board. 2005. Living beyond our means: natural assets and human well being. 

<http://www.maweb.org/en/products.aspx> 

46 National Research Council. 2001. Climate change science: an analysis of some key questions. Committee on the 

Science of Climate Change. Washington, DC: National Academy Press. <http://books.nap.edu/html/climatechange/> 

47 Barry, J.P., C.H. Baxter, R.D. Sagarin, and S.E. Gilman. 1995. Climate-related, long-term faunal changes in a 

California rocky intertidal community. Science 267:672–675. 

48 Levitus, S., J.I. Antonov, T.P. Boyer, and C. Stephens. 2000. Warming of the world ocean. Science 287:2225– 

2229. 

49 Allen, C., and D. Breshears. 1998. Drought-induced shift of a forest-woodland ecotone: rapid landscape response 

to climate variation. PNAS 95(25):14839-14842 

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Physical Processes 

Physical processes shape and sustain ecological systems. Stream flows carve and form aquatic habitats. 

Whether moving sediment under high flow regimes or fostering sedimentation in lower flow regimes, 

stream flow impacts ecological communities. The timing of seasonal flows coincides with the 

reproductive cycles of some species. Trends revealing shifts in high and low flows and changes in no flow 

periods forewarn of instability in ecological systems. Water cycles define habitat boundaries throughout 

each year and across years. In some ecological systems streambeds may require an annual high flow event 

to restore habitat that had been filled with debris and sediment during lower flow periods. The seasonal 

trends are important for species depending on specific conditions (see Stream Flows indicator, p. 3-14). 

Flow magnitude and timing and no-flow periods provide insights into trends in flow volumes that may 

impact ecological processes, particularly the life cycles of species that depend on specific habitat 

conditions.50 In one-half of the streams and rivers there have been shifts in magnitude of high or low 

flows of up to 75 percent and timing shifts of up to 60 days. The trend has been an increasing change in 

magnitude from the 1970s to the present.51 The number and duration of no-flow periods are decreasing. 

Information on the stability of streambeds (see Streambed Stability indicator, p. 3-19) can be used as a 

baseline against which future can be measured. 

In many locations along the U.S. coast, sea level has steadily risen, reflecting changes in water levels as 

well as subsidence in land in some areas (see Sea Level indicator, p. 6-61). These changes can alter the 

ecological conditions in coastal areas, especially where land elevations are low. The rise of sea levels 

results in increased flooding that can be exacerbated during storm events. Rising sea level also can result 

in increased salinity levels in coastal inland waters and soils thereby changing the chemical environment 

of habitats. Freshwater ecological systems are progressively lost as they are transformed into more saline 

inland waters or into open coastal waters. 

Critical Chemical Attributes 

Dissolved oxygen is critical to the support of aerobic animals and plants. In aquatic systems, dissolved 

oxygen levels reflect a balance between that produced by plants, consumption by all biota, and physical 

mixing processes. The spatial extent and timing of reduced oxygen conditions (hypoxia) and no oxygen 

conditions (anoxia) affects the distribution and sustainability of populations of aerobic organisms. As 

hypoxic and anoxic areas increase in size and persistence, animals such as mollusks (snails and clams), 

arthropods (e.g., crabs and shrimp), and fish have proportionally less habitat within which they can thrive. 

For these reasons, trends in oxygen affects the sustainability of populations as well as the overall 

biodiversity of aquatic and marine systems. 

50 The H. John Heinz III Center for Science, Economics, and the Environment. 2005. Forest pattern and 

fragmentation. In: The state of the nation’s ecosystems: measuring the lands, waters, and living resources of the 

United States. New York, NY: Cambridge University Press, September 2002. Web update 2005. 

<http://www.heinzctr.org/ecosystems/forest/frgmnt.shtml> 

51 The H. John Heinz III Center for Science, Economics, and the Environment. 2005. Forest pattern and 

fragmentation. In: The state of the nation’s ecosystems: measuring the lands, waters, and living resources of the 

United States. New York, NY: Cambridge University Press, September 2002. Web update 2005. 

<http://www.heinzctr.org/ecosystems/forest/frgmnt.shtml> 

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Regional information is available on baseline and trend hypoxic conditions in the Gulf of Mexico and 

Long Island Sound (see Hypoxia in Gulf of Mexico and Long Island Sound indicator, p. 3-77). The size 

of the hypoxic zones in both the Gulf of Mexico and Long Island Sound has been highly variable since 

the mid-1980s, with no discernable trend in either area. In both cases, there remain substantial areas in the 

latest year of record (2005) where low dissolved oxygen concentrations make the waters unsuitable to 

support most fish and shellfish species. 

Nutrient cycles are tightly interwoven into ecological processes. In stable, healthy ecological systems 

these cycles supply the nutrients required to maintain and even expand populations. Aquatic systems, 

receiving inputs throughout a watershed, are particularly susceptible to disrupted nutrient cycles. Nutrient 

inputs within a watershed may impact ecological processes at a location far from the origin of the input 

(e.g., input occurs upstream, but impact occurs at the mouth of a river). Indicators focusing on the most 

active nutrients in aquatic systems- phosphorus and nitrogen- provide insights into trends in nutrient 

loads, cycles, and transport. 

Nutrient loads have been examined for the Mississippi, Columbia, St. Lawrence, and Susquehanna Rivers 

(see N and P Discharge from Large Rivers indicator, p. 3-28). The largest of the monitored rivers, the 

Mississippi River, contained more than 15 times the nitrate than the other rivers. The nutrient loads in this 

river more than doubled from the 1950-1960s to the 1980-1990s. In contrast to the consistently upward 

trend of nitrate discharge in the Mississippi River, the nitrate discharge in the Columbia River nearly 

doubled in the 1990s compared to historic discharges, but returned to historic levels by 2002. Nitrate 

levels increased in the St. Lawrence but did not exhibit a particular trend in the Susquehanna. Phosphorus 

discharge trends are unclear in the Mississippi and Columbia Rivers and show a decrease in the St. 

Lawrence and Susquehanna Rivers, likely due to phosphorus controls. 

Baseline information for phosphorus and nitrogen is available for wadeable streams for the 1999 to 2004 

period (see N and P in Wadeable Streams indicator, p. 3-22). The cumulative distributions exhibited 

pronounced tails at the higher concentrations. The information indicates that for phosphorus 80 percent of 

stream miles had concentration of 100 μg/L or less; the remaining 20 percent exhibited a broader range of 

100 to about 900 μg/L. For nitrogen, 70 percent of the stream miles had nitrogen values of 1 mg/L or less; 

the remaining 30 percent ranged from 1 to over 40 mg/L. 

Agriculture-dominated watersheds are often characterized by higher loads of applied nitrogen and 

phosphorus fertilizers to optimize crop development. Streams located within these areas provide an 

indication of the extent of nutrient inputs. Baseline studies confirm that levels of nitrogen and phosphorus 

are elevated in many of these water bodies (see N and P in Agricultural Streams indicator, p. 3-25). 

The pH of air masses and waters is critical to biological functions, can directly affect the viability of 

species, and can affect the bioavailability of chemicals (both nutrients and potential toxics). There has 

been a decrease in wet deposition of sulfur and nitrogen compounds over the past 15 years, as discussed 

in Chapter 2. Associated with the decrease in deposition has been an increase in the acid neutralizing 

capability of water bodies (see Lake and Stream Acidity indicator, p. 2-62). Some sensitive regions— 

New England and Blue Ridge—have not shown improvement from 1990 to 2000. 

Limitations, Gaps, and Challenges 

There are ROE indicators for only a few of the critical physical and chemical attributes and processes. 

EPA would like to have ROE indicators for solar radiation over land and water as well as penetration into 

the nation’s waters. In addition, there are no ROE indicators of disturbance regimes associated with 

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flooding and fire. Still, information is available for a few of the most critical attributes. Trends in 

temperature provide insight into other trends that have important biological and physical ramifications. 

The indicators of trends in chemical and physical life-sustaining parameters are influenced by uncertainty. 

As technology changes, biases develop for data collected over long periods of time. Data collection tools 

may improve, creating new uncertainties when comparing recent data to historic trend data. In historic 

trend analyses, gaps in the record may emerge. Bridging the gaps between data series may require use of 

estimation or interpolation methods, or those time periods may be excluded altogether. All indicators of 

long-term trends are susceptible to changes in monitoring technology and historic data gaps. However, the 

increase in temperature and precipitation is occurring and with the collection of additional data sets 

longer-term trends can be confirmed or refuted. 

Measuring trends in physical processes is subject to limitations. For the assessment of the indicator for 

stream flow, establishing a baseline data set is challenging. In this case the baseline data do not represent 

“natural” conditions because of the presence of dams and other human impacts on flow during the 

baseline data years of 1930-1949. Also, the USGS gauging stations that generate the data for this 

parameter are placed on the larger tributaries and may miss trends in the smaller waterways. However, 

this indicator does provide valuable trend information regarding the general increase in high and low 

flows for larger waterways. For the assessment of acidification, the focus is largely on areas where 

previous studies revealed an impact. This may exclude areas that are impacted to a lesser extent by acid 

rain. 

While the large river surveys provide trend data for a watershed, it is not possible to identify the relative 

contributions of different land uses in the river basin. More detailed studies focus on the most common 

land use contributing to nutrient runoff. Each provides useful information regarding trends in the specific 

system. 

Information contained in the indicators represents baseline, decadal, and even century-level trends. 

However, for hydrologic and temperature patterns, these time periods may be too short to assess longterm changes. The field of paleoclimatology offers some promise for extending information to larger time 

frames.52 In addition, the predictive capability of forecasting the extent of dissolved oxygen deficits in 

regional and coastal water bodies is increasing.53 Information is also available on the distribution of solar 

energy over the surface of the United States. Over time, such information could be used to evaluate trends 

in this physical attribute. 

52 National Oceanic and Atmospheric Administration. 2003. North American drought: a paleo perspective. April 22, 

2003.<http://www.ngdc.noaa.gov/paleo/drought/drght_home.html> 

53 Longstaff, B.J., D. Jasinski, and P. Tango. 2005. Ecological forecast—summer 2005. Monitoring and Analysis 

Subcommittee. Chesapeake Update. 

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6.6 WHAT ARE THE TRENDS IN BIOMARKERS OF EXPOSURE TO COMMON 

ENVIRONMENTAL POLLUTANTS IN PLANTS AND ANIMALS? 

6.6.1 Introduction 

Chemicals can be introduced to the environment intentionally (e.g., fertilizers, pesticides, and herbicides), 

unintentionally, and through accidental spillage or leaks of chemicals used in home and commercial 

applications (e.g., in wastes from municipal and industrial operations). The extent to which the presence 

of mixtures of chemicals influences human health and the environment has long been a focus of EPA 

assessments. 

Biomarkers of exposure can include measures of chemical concentrations in plant and animal tissue. Such 

measures provide insight into the magnitude of chemical exposure that organisms receive from their 

environment. Measures of biological response such as biochemical concentrations (e.g., enzymes and 

ligands) that respond to chemical exposures can also serve as biomarkers of exposure. Examples include 

histopathological anomalies such as plant tissue damage from ozone or tumors in fish exposed to PAHcontaminated sediment. This evaluation examines the trends in biomarkers of exposures to common 

environmental pollutants in plants and animals based on the ROE indicators. It also discusses challenges 

in assessing trends in these biomarkers. 

Chemical stressors can have a detrimental effect on plant and animal communities. Chemical stressor 

exposure to plant and animals can lead to increases in tissue concentrations of the chemical stressor in the 

plants and animals. Once stressor concentrations are above threshold levels, they can affect physiological 

systems within the plants and animals and can begin to have toxic effects on individuals within the 

population. These individual effects can lead to changes in plant and animal community structure when 

chemical stressor concentrations in the environment reach levels that can affect one or more species or 

when the population numbers of a key species are detrimentally affected. Biomarkers of exposure, 

including concentrations of chemical stressors or key biomarkers collected over time within plants and 

animals tissues, can help to gauge the health of a plant and animal communities over time. These 

biomarkers of chemical exposure, when coupled with other information (e.g., toxicity testing results), can 

provide a basis for estimating what levels of a chemical stress can and cannot be tolerated in the 

environment by plant and animal communities. These biomarkers also help explain the recovery of 

certain animal populations (e.g., Brown Pelican) that were once nearly driven to extinction by specific 

chemical stressors. Tissue levels of pesticides, PCBs, and mercury have been used for many years to 

evaluate exposures to such species as the Brown Pelican, Bald Eagle, and Lake Trout and a host of other 

fish and wildlife. The Mussel Watch program relies on sampling lower trophic level organisms (mussels 

and clams) for a broad range of chemicals to evaluate exposures in coastal areas. Therefore, measures of 

bioaccumulative compounds in animal tissues provide an indication of exposure levels throughout food 

webs. 

6.6.2 ROE Indicators 

Although trends in specific contaminants of concern in environmental media (e.g., sediments or air) have 

been available for specific locations, the indicators to evaluate trends in biomarkers of exposure to 

common environmental pollutants in plants and animals are mainly focused on national or regional 

programs that have been measuring chemical stressor concentrations in fish tissue in lakes and coastal 

regions of the United States over less than a decade. An example of such biomonitoring efforts is 

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summarized in the National Coastal Condition Report II,54 which was completed as a collaborative effort 

between EPA, NOAA, USFWS, and USGS.55 

Trends in biomarkers of exposure to common environmental pollutants in plants and animals are 

evaluated using three National Indicators (Table 6.6.1). The focus of this question is on national- or 

regional-scale trends in biomarkers of exposure over the period that measurements have occurred (i.e., the 

last one to three decades, depending upon the biomarkers of exposure). While other subregional or localscale efforts concerning monitoring of biomarkers of exposure cannot be covered here, they are no less 

important. 

Table 6.6.1. ROE Indicators of Trends in Biomarkers of Exposure to Common Environmental 

Pollutants in Plants and Animals 

NATIONAL INDICATORS LOCATION 

Coastal Fish Tissue Contaminants (N/R) 3.8.2 – p. 3-103 

Contaminants in Lake Fish Tissue 3.8.2 – p. 3-107 

Ozone Injury to Forest Plants 2.2.2 – p. 2-37 

N/R = National Indicator displayed at EPA Regional scale 

6.6.3 Discussion 

What These Indicators Say About Trends in Biomarkers of Exposure to 

Common Environmental Pollutants in Plants and Animals 

The ROE indicators provide a baseline of recent conditions against which future trends can be assessed. 

Lipophilic chemicals such as PCBs, DDT, and methyl mercury are present in fish tissues throughout most 

of the nation’s freshwater lakes and coastal systems (Coastal Fish Tissue indicator, p. 3-103; Lake Fish 

Tissue indicator, p. 3-107). This shows widespread exposure to these bioaccumulative compounds. Some 

judgment concerning these levels can be made by reference to benchmarks that relate to tissue residues. 

For example, contaminant levels in estuarine/marine fish were judged to be moderate with respect to 

commonly used benchmarks. There are differences in exposure across EPA Regions. Six EPA Regions 

exhibited what is considered to be poor conditions. The Southeast (Region 4) was the only Region with a 

high (i.e., comparatively good) condition with respect to exposure levels of contaminants in fish. The 

contaminants that were most responsible for varied conditions include polychlorinated biphenyls (PCBs), 

mercury, and polycyclic aromatic hydrocarbons (PAHs). 

54 U.S. EPA. 2002. EMAP research strategy. EPA/620/R-02/002. Washington, DC. 

55 Within USGS, the Biomonitoring of Environmental Status and Trends (BEST) Program under the National 

Biological Service (NBS) is another example of a national program mandated to collect biomarkers of common 

contaminant exposure. Although monitoring of fish contaminant concentrations is a focus of this program, this 

program also monitors common pollutants in many other aquatic and terrestrial receptors, such as upper trophic 

level receptors (fish-eating birds like the bald eagle), and catalogs biomarkers data collected from many sources into 

an online database. [0] 

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Foliar injury from ozone pollution disrupts plant/tree physiology. Baseline data indicate that exposure of 

forests to ozone levels varies geographically (Ozone Injury to Forest Plants indicator, p. 2-37). Severe 

injury from ozone was observed in 9 percent of southern sites, 8 percent of northern sites, 1 percent of 

Pacific Coast sites, and no Rocky Mountain sites. Rocky Mountain and Pacific Coast sites had the least 

overall foliar injury (with 0 percent in the Rocky Mountain sites). 

Limitations, Gaps, and Challenges 

Few national programs involve unbiased assessment that can support indicators of trends in national 

conditions in biomarkers of exposure. While there are tissue-level ROE indicators for fish, there are no 

similar indicators for plants (either aquatic or terrestrial) or wildlife species. This represents a gap in 

EPA’s ability to identify trends in biomarkers of exposure to common environmental pollutants in plants 

and animals. 

Some of the primary challenges that exist relating to monitoring biomarkers of exposure include the 

following: 

• To monitor a single biomarker of exposure on a national or regional scale requires a great 

deal of planning, coordination, and resources. Biomarkers are more costly and timeconsuming to measure than chemical concentrations in other media (e.g., water, sediment, 

air), because the living things that require measurement are more difficult to collect and/or 

analyze for the chemical stressors. 

• The biomarkers of exposure need to be clearly linked to biomarkers of effects to be useful for 

predicting whether the function of plant or animal communities are being affected by the 

concentrations of chemical in the environment. In many cases, capabilities are currently 

lacking to link biomarkers of exposure with biomarkers of effects. In addition, most 

monitoring focuses on the media within which plants and animal live (i.e., air and water), and 

does not address the body burden of the chemical in the plant or animal or biomarkers of 

effects. 

• With a myriad of environmental contaminants in the environment, it is difficult to prioritize 

which contaminants should be monitored in biological tissues. Classically, the organochlorine 

pesticides (e.g., DDT), PCBs, and mercury have been monitored in fish tissues in the aquatic 

environment. However, in the future, new chemicals may emerge as equally or more 

important (see Afterword, Section 7.2). 

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