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Executive Summary Indicators are designed to inform us quickly and easily about some- thing of interest. They communicate information about conditions, and, over time, about changes and trends. Like economic indicators, environmental indicators are needed because it is not possible to measure everything. Developing indicators and monitoring them over time can help to determine whether problems are developing, whether any action is desir- able or necessary, what action might yield the best results, and how successful past actions have been. To develop and implement sound environmental policies, data are needed that capture the essence of the dynamics of environmental systems and changes in their functioning. These kinds of data then need to be incorporated into indicators. Although no current indicators of environmental conditions or trends have the stature of the most influential economic indicators, some envi- ronmental indicators, such as global mean temperature, sea surface tem- peratures, and atmospheric carbon dioxide concentrations, are attracting considerable attention. Developing indicators of comparable power for ecological processes will help focus appropriate attention on ecological conditions, providing clues that could help guide significant and informed policy choices. Ecological indicators are also needed as yardsticks to measure the need for and performance of public policies and programs. During recent decades, a variety of efforts have laid important groundwork for the development of national-level indicators to inform major policy decisions. Our work builds on these efforts.

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2 ECOLOGICAL INDICATORS FOR THE NATION This report has several goals: (1) to suggest criteria for selecting useful ecological indicators, (2) to provide methods for integrating com- plex ecological information into indicators that summarize in simple but powerful ways conditions and changes in important ecological processes and products, (3) to propose indicators that meet the suggested criteria, (4) to identify sources of data that can be used to design and compute the numerical value of indicators, and (5) to offer guidance for gathering, storing, interpreting, and communicating information from ecological monitoring. This report concentrates on ecological indicators; this was the charge to the committee and an area in which better indicators are urgently needed. Our report does not cover other important types of environ- mental indicators, such as physical and chemical indicators of climate change, ozone depletion, acid precipitation, or air and water quality, although those indicators are no less important than the ones to which we give our primary attention here. SCALES AND APPLICABILITY OF INDICATORS Indicators can be useful at many levelscommunity, state, ecoregional, watershed, national, and international and better indicators are needed at all such scales. In addition, better ways are needed of matching the scales at which indicators are useful to the scales of ecological processes. In this report, we concentrate on indicators that can support national decision making, but we also show how the methods we recommend can be used to develop indicators whose primary use would be at local and regional scales. National ecological indicators are difficult to develop for a country as large and diverse as the United States. Determining appropriate and useful ways to aggregate information collected at small scales into indica- tors covering the entire country is challenging. Indeed, this problem has not yet been fully solved for all of the indicators we propose. Some will require further development and pilot studies before they are fully imple- mented to better understand how they respond to temporal and spatial variability. Yet national-level indicators are needed because many envi- ronmental policies are made or implemented nationally, and many inter- national agreements need national-level information to help establish international standards. For all these reasons, the committee has focused most of its efforts on indicators that are potentially useful at a national level. Ecological indicators that describe the state of the nation's ecosystems and command credibility and attention from the public and decision makers have been elusive. In part, this results from the complexity of

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EXECUTIVE SUMMARY 3 ecological systems, but more attention should be given to the criteria for developing and using successful national ecological indicators. In addi- tion, many current ecological indicators are applicable to only limited areas, to one type of ecosystem, or to the populations of one or a few species. They are useful for their intended purposes, but they cannot serve as nationwide indicators. Some indicators have been less useful than hoped because the mea- sures used are not clearly linked to underlying ecological processes. As a result, it has been difficult for scientists to interpret changes in those indicators. In other cases, data requirements are so complex and exten- sive that the indicators would be too expensive to use. Another difficulty is the frequent need to combine very different kinds of variables into a single indicator. These types of problems in indicator development and interpretation have Plagued scientists and managers for many years. CRITERIA FOR EVALUATING INDICATORS To avoid the above pitfalls and to provide a common framework for indicators, the committee developed a general checklist of criteria for evaluating them. The checklist can be used to assess the potential impor- tance of a proposed indicator, its properties, its domain of applicability, and its limitations, and thus how the indicator might be used. The items in this checklist follow. General Importance. Does the indicator provide information about changes in important ecological and biogeochemical processes? Does the indicator tell us something about major environmental changes that affect wide areas? Conceptual Basis. Is the indicator based on a well-understood and generally accepted conceptual model of the system to which it is applied? Is it based on well-established scientific principles? The conceptual model provides the rationale for the indicator, suggests how it should be com- puted, and enables us to understand the features of the indicator and how they change. Reliability. What experience or other evidence demonstrates the indicator's reliability? The best evidence for the reliability of an indicator is, of course, successful previous use. Nevertheless, all existing indicators should be analyzed retrospectively before assuming that their use should be continued. An indicator that is newly proposed inevitably lacks a historical record of reliability. Nonetheless, if it is based on a well- established scientific theory, and if a retrospective analysis has indicated

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4 ECOLOGICAL INDICATORS FOR THE NATION that it probably would have informed us about important changes in an environmental process or product of concern, its reliability is provision- ally established. Some of the indicators we have proposed are new, and like other new indicators, development and experience will be needed to make them operational. Temporal and Spatial Scales. Does the indicator inform us about national, regional, or local ecological conditions, processes, and products? Are the changes measured by the indicator likely to be short-term or long- term? Can the indicator detect changes at appropriate temporal and spatial scales without being overwhelmed by variability? To determine what an indicator indicates, the kinds of data needed to compute it, and how changes in it should be interpreted, the temporal and spatial scales of the processes measured by the indicator need to be clear. Statistical Properties. In the areas of accuracy, sensitivity, precision, and robustness, has the indicator been shown to serve its intended pur- pose? Is the indicator sensitive enough to detect important changes but not so sensitive that signals are masked by natural variability? Are its statistical properties understood well enough that changes in its values will have clear and unambiguous meaning? Data Requirements. How much and what kinds of information are necessary to permit reliable estimates of the indicator to be calculated? How many and what kinds of data are required for the indicator to detect a trend? Most ecological indicators depend on data gathered by means of long-term monitoring. The challenge is deciding which rates of change to watch, and to determine which of the changes observed represent signifi- cant departures from expected natural variability. Once an indicator is selected, monitoring must be used to gain experience with the likely mean- ing of changes in the indicator's values. Experimental studies them- selves requiring monitoring should be used to determine whether the stress/response relationships suggested by the monitoring program are indeed causal. The use of the indicator may change as additional insights are gained into its behavior and the underlying processes that cause it to change. Skills Required. What technical and conceptual skills must the col- lectors of data for an indicator possess? Does the collection of input data require highly technical, specialized knowledge if the data are to be accu- rate, or is data collection a relatively straightforward process? An indica- tor capable of commanding broad attention must be based on data that are accurate and, equally important, perceived by all to be accurate.

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EXECUTIVE SUMMARY 5 Because the collection of data for ecological indicators (i.e., monitoring) is sometimes perceived by scientists as boring or less interesting and presti- gious than "scientific research" (i.e., hypothesis-driven investigation), it is important to provide incentives for consistent and accurate data collection. One way to do that is to design monitoring programs so that the informa- tion also has scientific value (i.e., can be used to help to answer research questions). The indicators we have proposed embody hypotheses about the functioning of ecosystems. To the degree that such hypotheses can be made explicit in the design of indicators, their development and the sub- sequent monitoring of them should generate a great deal of valuable scientific information. Data Quality. No indicator of environmental quality is reliable unless the underlying data that are used to construct or calculate it are accurate. Attention to data quality during the archiving and computa- tional phases cannot substitute for the quality of the input data. In this critical sense, the ultimate responsibility for data quality must lie with the investigators who collect them. Clear documentation of sampling and analytical methods is necessary if future investigators are to understand exactly how each indicator was calculated. This requirement is particu- larly important as methods and instrumentation change, so that data from early parts of the time series are quantifiably comparable to data from later parts of the same time series. Data Archiving. A monitoring system to track ecological indicators requires archiving capabilities that provide interested parties access to the data. For indicators that are direct representations of environmental samples, the archive simply needs to save a record of the measurements. In general, the minimum number of physical samples saved should ensure the ability to recalibrate the entire data set, should that become necessary because of changes in sampling or analytical technologies. The costs of preserving physical samples in forms that do not decay or otherwise change must be weighed against the opportunity cost of not being able to recalibrate a data set with improved or modified measurement techniques. The complete description and availability of the models and the data used to calculate indicators are just as important as the availability of the un- derlying data themselves; otherwise, future comparisons might actually not compare the same things. The archive must be robust enough to ensure that the time series of the indicator can be reprocessed as models Improve. Robustness. For our purposes here, we define robustness in a nonstatistical sense, as an indicator's ability to yield reliable and useful

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6 ECOLOGICAL INDICATORS FOR THE NATION numbers in the face of external perturbations. In other words, is the indicator relatively insensitive to expected sources of interference? Are technological changes likely to render the indicator irrelevant or of limited value? Can time series of measurements be continued in compatible form when measurement technologies change? To continue to gather data by outdated methods is undesirable. Nevertheless, because long-term data sets are essential for detecting most environmental trends, technological changes must be incorporated into monitoring programs in ways that do not destroy the continuity of the data sets or render consistent interpreta- tion of the changes impossible. As pointed out in Chapter 2, cross- calibration of measurements is especially important for remotely sensed data. International Compatibility. Is the indicator compatible with indica- tors being developed by other nations and international groups? Not all indicators used in the United States, especially those relating to specific regions, ecosystems, or species, need to be compatible with indicators developed and used in other nations. However, national-level indicators signal changes that are likely to transcend national boundaries. Effective responses to these changes may require international action. If the signals that trigger actions are not meaningful to the affected nations, appropri- ate multinational responses are certain to be more difficult to mount. Costs, Benefits, and Cost-Effectiveness. Costs and benefits associated with implementing proposed ecological indicators are important because resources for monitoring are limited and should be used efficiently. The costs of developing and monitoring an indicator, which can continue to accrue as the indicator is used and refined and as new data and technolo- gies develop, can be estimated objectively. The benefits the value of the information obtained are more difficult to estimate. The greater the benefits of an indicator, the higher the costs that can be justified in devel- oping and implementing it. Cost-effectiveness is also an important crite- rion. If one assumes that the information an indicator yields is essential, can it be obtained for less cost in another way? If so, the indicator is not cost-effective. The value of the information was the committee's first consideration in every indicator we recommend. THE COMMITTEE'S CONCEPTUAL MODEL FOR CHOOSING INDICATORS To guide its selection of ecological indicators, the committee used the above criteria and a conceptual model of the factors that most strongly influence ecosystem functioning, described in Chapter 2. The goods and

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EXECUTIVE SUMMARY services that ecosystems provide to humans depend directly or indirectly on ecosystems' productivity, i.e., their ability to capture solar energy and store it as carbon-based molecules. Productivity is strongly influenced by temperature, moisture, soil fertility, and the structure and composition of ecological communities. Measures of the presence of native and exotic species are also important inputs to national ecological indicators. How the committee used this conceptual model is described in Chapter 4, where we recommend a set of indicators of the key factors that influence ecosys- tem functioning. POLICY PERSPECTIVES For use in policy making, indicators need to be understandable, quan- tifiable, and broadly applicable. They should provide information about important ecological processes. Indicators are more likely to be influential if there are relatively few of them and if they convey information in a form that the public and policy makers understand. It is crucially impor- tant for public confidence in an indicator that its numerical values be independent of who does the calculating: the rules for calculating an indicator from measurement data must be objective and clear. The indi- cators need to be credible, and therefore the people and organizations that produce them need to be credible. This recommendation is especially critical if the indicators recommended here are used as input for reporting on the status of the nation's ecosystems, as we hope they will be. THE RECOMMENDED INDICATORS Based on consideration of the desirable characteristics of indicators, the sources of data that underlie them, the models that support them, the criteria summarized above, and the conceptual model we used, the com- mittee recommends the following national ecological indicators in three categories: As indicators of the extent and status of the nation's ecosystems, the committee recommends land cover and land use. As indicators of the nation's ecological capital, the committee rec- ommends total species diversity, native species diversity, nutrient runoff, and soil organic matter. As indicators of ecological functioning or performance, the com- mittee recommends carbon storage, production capacity, net primary produc- tion, lake trophic status, stream oxygen, and for agricultural ecosystems, nutrient-use efficiency and nutrient balance.

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8 ECOLOGICAL INDICATORS FOR THE NATION For each indicator recommended in this report, information is pro- vided on the following points insofar as possible: . Why the indicator is useful. The ecological model that underlies the indicator. The range of values the indicator can take and what the values mean. The temporal and spatial scales over which the indicator is likely to change. Whether the needed input data are already being gathered, and, if so,by whom. If the needed data are not being gathered, what new data are needed and who should collect them. The probable effects of new technologies on our ability to make the required measurements and how soon significant technological changes are likely. In some cases, noted for each indicator, some experience will need to be gained on details of its behavior, but all the indicators are based on soundly established scientific principles and experience. The proposed indicators are in general applicable to both managed (e.g., agricultural and silvicultural) and unmanaged ecosystems; the indicators of nutrient- use efficiency and overall nutrient balance are specific to agricultural eco- systems. Indicators of Ecosystem Extent and Status The largest ecological changes caused by humans result from land use. The changes include replacing native biological communities with agricultural systems, changing hydrological and biogeochemical cycles, changing the Earth's surface by creating buildings and transportation corridors, and so on. These changes affect the ability of ecosystems to provide the goods and services that society depends on. For this reason, it is necessary to know about land cover and land use. In addition, an indicator of land cover provides a rough inventory of the nation's biologi- cal capital, an essential part of any suite of indicators. Information on land cover provides a reference point and is needed to calculate several other proposed indicators. This information provides a standard against which to detect and measure changes. The information and technology to calculate land cover are currently available; land use is in some ways more informative, but considerable synthesis of existing information and some new information will be required to develop that indicator. The committee recommends a land cover indicator that includes

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EXECUTIVE SUMMARY 9 aquatic and dryland ecosystems. It records the percentage of land in each of many land cover categories. Each time land cover is computed, the proportions in each category should be compared with those at the previ- ous recording time. Data must be entered and stored separately for many categories of land cover types. Because the proportion of land in each category changes relatively slowly, land cover needs to be reported only every five years, but its values should be computed annually so it can be used as inputs to other indicators. The land cover indicator measures the proportion of the landscape occupied by each member of a set of land cover types that comprise the total area of the nation. The major questions concern how many land cover types to recognize, how to account for their spatial configurations, and how to accommodate changes in the number and kinds of categories recognized. Changes in the proportional representation of various land cover types is the variable of interest. Satellite imagery can identify many categories of land cover. The classification of land cover types must be comprehensive to serve other indicators that are derived from it. The number of classes of land cover types that will be needed will likely be different for each indicator, and therefore the input data should be archived at the most highly resolved and disaggregated levels. Some indicators of land cover are currently used on less than a national scale, such as the U.S. Department of Agriculture's National Resources Inventory. The inventory covers 800,000 sites on private lands and has provided valuable information on ecological extent and condi- tion. A national indicator that could be applied to all U.S. lands and even beyond would be even more useful. When sufficient information is developed on land use, the committee recommends that a similar land use indicator be developed. Indicators of Ecological Capital The capacity of ecosystems to provide goods and services depends on the natural capital, both biotic and abiotic, that constitutes them. The essential capital includes physical components such as soil condition, as well as the species that drive and maintain ecosystem processes. There- fore the committee recommends indicators of species diversity, soil con- dition, and nutrient runoff. The United States has affirmed many times through law, policy, and action its commitment to preserve its biological resources. Because loss of a species is irreversible, species richness is especially important to moni- tor. Ecological capital includes the number of species still present in the country relative to their number at the time of European contact, their

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10 ECOLOGICAL INDICATORS FOR THE NATION distribution in today's natural and human-modified environments, and the number of species present today that are nonnative. The first recommended indicator of ecological capital, total species diversity, measures the ecological capital actually present. It combines a measure of diversity with information about land cover. To avoid dis- counting rare species, the diversity measure is species richness, which is not weighted by population abundances. The diversity measure, based on decades' worth of experience with species-area curves for many taxa, is described in detail in Chapter 4. The indicator will be based on the land cover indicator: a diversity score will be assigned to each category of land cover based on its contribution to total species diversity. The average for the whole country the national indicator of total species diversity- is computed by multiplying each diversity score by the total area in its land cover category, summing scores, and dividing the total by the area of the nation. This indicator of species richness should be calculated for a few representative taxa that are reasonably well known and easy to sample. The indicator's chief value is in providing a measure of total species richness. It can reflect human impacts, especially severe ones, and it also reflects many other environmental variations. It thus allows one to compare the species richness in various land cover types as well as the effects on species richness of various natural environmental and human-caused changes. The second indicator, native species diversity, reflects human impact on the land. Land that has been so transformed by people that it cannot support native species that would otherwise be there carries a heavy burden caused by human activities. Thus, the indicator compares the number of native species an area of land supports with the number of native species one would expect such a landscape type to support. Total species diversity includes all the species present, both native and non- native. Native species diversity includes only natives, because its purpose is to reflect human impacts. If humans cause a native species to be replaced by a nonnative one, native species diversity will change, indicat- ing an impact even though total species diversity does not change. Both indicators depend similarly on land cover (or land use) for their calcula- tion. Although lack of adequate information on many taxa will make developing these indicators difficult, the work including developing better information about species diversity of many taxa should be started now. Doing so will provide an incentive to learn about taxa that are not well known at present, and enough is known about some taxa to be useful now. Soil is the source of nutrients and energy for soil biota, and soil struc- ture influences its capacity for water retention, its susceptibility to ero- sion, and the fate of pollutants such as pesticides. The best indicator of

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EXECUTIVE SUMMARY 11 soil condition is soil organic matter (SOM). SOM strongly influences all of these processes. Concentrations of SOM generally range from 1 to 10 percent, and it can recover or be maintained through careful manage- ment, just as it can decline through inappropriate management. Thus it is a useful indicator of agricultural soil condition as well as of the condition of unmanaged soils. The committee recommends an indicator of nutrient runoff, which measures the loss of essential nutrients from the soil and is related to soil erosion, because excess nutrients, especially nitrogen and phosphorus, reduce water clarity, increase nuisance algal blooms, and increase the incidence of hypoxia (low oxygen) in waters. The adverse effects are seen in fresh waters, estuaries, and coastal waters. For these reasons, the com- mittee recommends nutrient runoff, measured by total nitrogen and phos- phorus, as an indicator of water quality. The indicator can take values from 0 (no discharge) to thousands of kilograms per square kilometer per year, with lower values being more desirable for most purposes. Because nitrogen and phosphorus runoff is largely a result of human activities, it can also be an indicator of the need for and effectiveness of environmental management. Indicators of Ecosystem Functioning Changes in ecosystems' productivity are usually accompanied by changes in their ability to provide goods and services important to hu- mans. Usually, declines in productivity are undesirable, but in freshwa- ter ecosystems, increases in productivity associated with eutrophication can be undesirable. The committee recommends three indicators of ter- restrial productivity and two aquatic indicators. Energy in the form of light is captured by chlorophyll and converted to chemical energy in the form of carbon, a process called primary pro- duction, the basis of terrestrial productivity. The committee recommends an indicator of production capacity, measured by total chlorophyll per unit area. It provides a direct measure of the energy-capturing capacity of terrestrial ecosystems. An equivalent measure for lakes would be total chlorophyll per unit volume. Total chlorophyll is an excellent indicator because it is strongly correlated with an ecosystem's actual capacity to capture energy. The chlorophyll per unit area ranges from 2.8 g/m2 in tropical forests to 0.5 g/m2 in tundra and desert ecosystems. Next, the committee recommends an indicator of net primary pro- duction (NPP), which is a direct measure of the amount of energy and carbon that has been brought into an ecosystem; it also is a measure of productivity as understood in forestry and agriculture, i.e., the amount of plant material produced in an area per year. Values of NPP range from

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2 ECOLOGICAL INDICATORS FOR THE NATION 1,400 g/m2/year in tropical rainforests to 50 g/m2/year in American deserts. In some agricultural systems, it can reach 6,000 g/m2/year. The committee also recommends an indicator of carbon storage, a direct measure of the amount of carbon sequestered or released by eco- systems. It is the difference between the sum of all nonplant respiration in an ecosystem all the CO2 carbon produced by detritivores and animals and net primary production. It measures the change in the total amount of carbon in an ecosystem, and hence indicates the ecosystem's carbon balance. This indicator is important in light of concerns about greenhouse gas emissions because the carbon released by a region equals the region's fossil-fuel emissions minus its ecosystem carbon storage. A fourth indicator, stream oxygen, is recommended by the committee as an indicator of the ecological functioning of flowing-water ecosystems. It captures the balance between instream primary production and respi- ration. High stream oxygen indicates much photosynthetic activity and the likelihood of high nutrient concentrations, algal blooms, and rapid growth of leafy aquatic plants. Low stream oxygen indicates higher respiration than photosynthesis and the likelihood of organic enrichment from wastewater or high plant production upstream. Low stream oxygen usually indicates that the water is not suitable for many species of aquatic animals, including fish. Finally, the committee recommends trophic status of lakes as an indicator of aquatic productivity. Such an indicator can be developed from a few key characteristics that determine the functional properties of lakes and their ability to provide the many goods and services valued by society. The key characteristics nutrient status, net biological produc- tion, and water clarity are closely interrelated and they are influenced by management of fertilizers, sewage, and other nutrient sources. Net biological production and water clarity can be measured by satellite imagery as well as by ground-based methods. Together, these character- istics define a lake's trophic state and have been combined into a trophic state indicator (TSI) that can be aggregated nationally by computing a frequency distribution of trophic states across lakes. The frequency distri- bution of trophic states (but not an average of TSI values) should be used as a national-level indicator, because changes in this distribution provide the most useful information. The national indicator should also record the number of lakes that are hyper-eutrophic, in addition to reporting changes in frequency distributions of lakes and trophic states. In addition to the above five indicators that are directly related to productivity, soil condition and land use also are related to ecosystem functioning. Agricultural ecosystems are a large and important fraction of the land

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EXECUTIVE SUMMARY 13 surface of the United States (and other countries). As concern about the environmental impacts of agricultural nutrient losses has grown, so has research on how to manage nutrients to avoid or reduce those losses while maintaining productivity and profitability. Accordingly, it is use- ful to have indicators of both the overall efficiency of nutrient use in the production of crops and animal products and the overall nutrient balance. The committee recommends indicators of nutrient-use efficiency for both nitrogen (N) and phosphorus (P). For croplands, the indicators com- pare the amount of N and P removed in crop biomass per year with the amount of chemical and animal (manure) fertilizer applied plus the amount of N fixed by legumes. Because livestock production is generally less efficient in nutrient use than crop production, the committee also recommends indicators of nutrient-use efficiency for agriculture overall, which compares the N (or P) content of crops for human consumption plus the N (or P) content of animal products produced with the chemical N (or P) fertilizer applied to croplands. A high value of the indicators- near 1 indicates high efficiency, while a low number near O indicates low efficiency. The committee recommends that the same information be used to compute indicators of overall nutrient balance. The approach is a mass- balance one. For N. the indicator would be N fertilizer applied plus N in animal waste (manure) applied plus N fixed by legumes minus N re- moved in harvested crops. The national-level indicators would integrate nutrient-use efficiency and the contributions from all natural processes. Thus, the indicator of nutrient-use efficiency would allow monitoring of agricultural practices and would recognize improvements in those prac- tices; the overall nutrient balance would measure the overall environmen- tal loadings of nutrients. TIMING AND COST OF IMPLEMENTING THE COMMITTEE'S RECOMMENDATIONS Much of the information that is required as input for the above indi- cators is already being collected at regional scales, and in some cases even at national scales. Nonetheless, full development and implementation of the indicators will be expensive and will take some time, especially for the recommended indicators of species diversity. For this reason, the com- mittee recommends a sequential approach to the development and imple- mentation of the indicators. Because the national land-cover indicator has general importance and because it is essential input for some of the other indicators, the land-cover indicator should be implemented first.

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4 ECOLOGICAL INDICATORS FOR THE NATION LOCAL AND REGIONAL INDICATORS Indicators are needed to inform us about ecological status and trends at all spatial and temporal scales. Indeed, many indicators are useful at several scales. In addition, most policy and management decisions are made at scales defined by laws and regulations established by political entities, such as local municipalities, counties, states, and the federal gov- ernment. Although the committee focused its attention on the national- level ecological indicators recommended in Chapter 4, the methods used to select and formulate those indicators are equally applicable to indica- tors designed for use at smaller spatial scales. Further, many national- level indicators can be reported at various levels of disaggregation to serve as regional ecological indicators. In Chapter 5, we examine a number of local and regional indicators that we judge to be especially important, and show how they can be computed and interpreted. Productivity Indicators In addition to a national-level indicator of ecosystem productivity, it is also useful to have indicators specifically designed to capture the per- formance of particular ecosystem types. In Chapter 5, we give examples of indicators for forested ecosystems; these are described below. Similar indicators can and should be developed for other ecosystem types, such as grasslands, savannas, deserts, and wetlands. For regional forest indicators, we recommend indicators of produc- tivity and species diversity, structural diversity, and sustainability. These attributes support the continued provision of the following goods and services from forests: wood and wood products, opportunities for recreation, tourism, and aesthetic enjoyment, maintenance of wildlife resources, control of erosion and nutrient losses to surface waters, and mitigation of greenhouse-gas emissions. The most valuable indicators for forests are those that can provide early warning of adverse trends in productivity, species diversity, and structural diversity. We recommend that the following forest indicators be given high priority: (1) productivity and tree species diversity, (2) soils, (3) light pen- etration, (4) foliage-height profiles, (5) crown condition, and (6) physical damage to trees. These indicators can be assessed using data that can be collected easily in the field. In addition, the data can be used to calculate other synthetic indices (such as various diversity indices) later in the laboratory or office. They can easily be incorporated into existing inven- tory programs.

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EXECUTIVE SUMMARY 15 Indicators of Species Diversity In addition to the national indicators of the status of species diversity recommended in Chapter 4, the nation needs indicators to evaluate the diversity status of a local area, such as a national park or an area exploited for human use. For evaluating the diversity status of such areas, we recommend three indicators: independence of the area, species density, and deficiency of natural diversity. Although we tried to reduce the number of these indicators of diver- sity, and have grounded them in a single well-researched power law, all three are needed because they each inform us about different aspects of diversity. An Indicator of Independence This indicator assesses the degree to which the species richness of an area depends on immigration of individuals from surrounding areas. Two types of species contribute to local diversity. The first consists of source species, whose births exceed their deaths in the area and thus they can provide individuals to populate surrounding areas. The other type, sink species, are present only because immigrants compensate for their excess of deaths over births in the area. Isolating an area reduces immigration and therefore sink species will eventually disappear from the isolate. The indicator of the independence of a local area is computed using the expected values of species diversity for a large area of the same vegeta- tion type as the area under consideration. It provides an estimate of the number of sink species in the sample but it does not specify them. These species need to be identified with traditional demographic techniques. An Indicator of Species Density This indicator assesses whether an area supports more or fewer spe- cies than a reasonably defined reference area does. Managers typically wish to optimize the value of their reserves. It might appear that the more species housed in a reserve, the better its condition, but this is not neces- sarily true. The reason is that changing patterns of land use can squeeze more species into a smaller area that cannot support so many species. As result, species will be lost from the area. The indicator signals whether diversity in the area is likely to increase, decrease, or remain the same, and it estimates the probable final diversity of the area.

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16 ECOLOGICAL INDICATORS FOR THE NATION Indicators of Deficiency in Natural Diversity This indicator assesses the degree to which a site preserves exotic species of little or no conservation value rather than valued native spe- cies. When human uses dominate a landscape, natural assemblages of species disappear, but they are in part replaced by exotic species. In Chapter 4, we recommended a national indicator of native species diver- sity, to indicate the degree to which exotics have replaced native species. A local indicator that quantifies this tendency is also needed. Three factors contribute to the extraordinary abundance of a few species in anthropogenic environments: Exotics may have had more time to adjust to us. Exotics may have escaped many of their natural predators. Only a subset of native species (the tolerants) are preadapted to "degraded" environments. To evaluate the deficiency of diversity in an area, the raw value of species density in the area is decreased by subtracting exotic species that follow human settlement and tolerant natives that would thrive any- where. The remaining native species density provides an estimate of the value of a site in supporting biological diversity. The Index of Biotic Integrity: An Indicator of Species Diversity of Aquatic Ecosystems Additive multimetric indicators have been developed and used to compare the species diversity of aquatic systems with what would be in those systems in the absence of human-caused perturbations (sometimes called appropriate diversity). The most widely used multimetric indicator is the Index of Biotic Integrity (IBI). The use of IBI requires general agree- ment about which organisms indicate poor or good ecological and water characteristics by their abundance or absence. The IBI provides a method for quantifying those qualitative assessments. The IBI is primarily a com- munity-level rather than an ecosystem indicator because it is based on taxonomic assemblages within specific phylogenetic groups and specific biogeographic regions. The original IBI was developed for freshwater fish communities in streams in the Midwest. Recently, similar indicators have been applied to freshwater benthic macroinvertebrate communities in several regions (and even to some terrestrial communities).

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EXECUTIVE SUMMARY CARE AND HANDLING OF ENVIRONMENTAL DATA 17 To ensure the accuracy and credibility of environmental indicators, procedures need to be established and maintained to monitor input data; to standardize measurements; to cross-calibrate instruments, especially when measurement technology is changing; and to document methods so that people not associated with the original data collection can reproduce the methods. Data used as input for national-level indicators should be archived in a highly disaggregated form so that these data are available for computing a variety of regional and local indicators. RESEARCH Although the indicators recommended by the committee are well grounded in theory and supported by extensive data, further research and development are needed to enhance the precision and interpretation of these indicators, as well as the identification and development of new indicators. Research might also suggest new indicators that are better than or that can be added to the set of indicators then in use. Research is especially needed on unusually sensitive species and processes, microbial communities, keystone species, and the temporal and spatial behavior of indicators and how their variability is influenced by underlying ecologi- cal interactions. For some recommended indicators, especially the ones that measure land cover, land use, and species diversity, further work is needed on how best to operationalize the indicators and to help identify future research plans. This work, which should include one or more workshops, should include academic scientists, practitioners, agency scientists, and other interested parties.