Ecological Indicators for the Nation (2000) / Chapter Skim
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2 The Empirical and Conceptual Foundations of Indicators
2 The Empirical and Conceptual Foundations of Indicators
Pages 27-50
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From page 27... ...
2 The Empirical and Conceptual Foundations of Indicators ' ndicators describe and summarize: they can be used for diagnosis and warning, and they can be used to monitor change. No indicator needs ~ to do all these things, but if one wants to know whether an indicator is of value, its intended use must be clear.
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From page 28... ...
Prospective monitoring is common and often required by laws. For example, monitoring concentrations of various gases in the atmosphere and contaminants in water is required by the Clean Air Act and the Clean Water Act.
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From page 29... ...
Useful ecological indicators are based on clear conceptual models of the structure and functioning of the ecosystems to which they apply. The models can be empirical or theoretical, quantitative or qualitative, but, as the following discussion emphasizes, some type of model is essential to ground and rationalize all indicators.
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From page 30... ...
Analytical Predictions Mathematical models of environmental processes generate predictions that can be tested observationally and experimentally in the field. They also may generate predictions of future events that cannot be tested directly, but these predictions may suggest which events should be monitored to determine whether the predictions are borne out.
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From page 31... ...
The calibration of methods during periods of technological change is an essential component of integrating indicators across technological boundaries. HISTORICAL AND PALEOECOLOGICAL DATA AS AIDS TO INDICATOR DEVELOPMENT Paleoecology, taken here to encompass also paleontology, paleolimnology, paleogeochemistry, and paleoclimatology, is concerned with describing past ecological communities and their environments.
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From page 32... ...
showed that fire frequency in this area was greater during the dry medieval warm period than during the subsequent Little Ice Age. · Were communities and ecosystems relatively stable, were they following trajectories of gradual change, or did they exhibit sudden fluctuations or transitions to another state?
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From page 33... ...
Paleoecological data have often revealed major unsuspected environmental changes, over time scales ranging from the mass extinctions observed in the fossil record to the sudden catastrophic loss of submerged macrophytes in Chesapeake Bay in the early 1970s (Brush and Davis 1984, Davis 1985~. Paleoecological data have been especially important in altering views of the operation of the global climate system.
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From page 34... ...
For many of the indicators that the committee recommends, especially for information on terrestrial ecosystem processes and some aspects of ecosystem status, remote sensing offers rapid and relatively accurate sampling. For a few remotely sensed measurements, there already is an approximately 25-year time series of reasonably well calibrated data of nearly global coverage.
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From page 35... ...
Since 1982, the primary instrument has been the Thematic Mapper, with six spectral bands in the visible and short-wave near-infrared, all of which have 30 m spatial resolution, and one thermal band with 120 m spatial resolution. A Landsat scene is approximately 185 km on a side; the combination of swath width and orbital characteristics of the satellite means that any spot on the Earth is revisited at about 16- to 18-day intervals.
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From page 36... ...
Collaborating ground receiving stations around the world have recently developed a more precise measure of absorbed photosynthetically active radiation than is possible with NDVI by using multiple scenes with the sun and sensor in different positions and inverting a radiative transfer model. Once one knows the amount of leaf area in a location, it is a relatively easy matter to predict the region's net primary production (NPP)
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From page 37... ...
Classifying and mapping land cover and vegetation, the most common use of aerial photography, is served well by satellite imagery. The number of vegetation categories used is limited by the number of spectral bands, the resolution of the most commonly used imagery, and the experience of the interpreter.
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From page 38... ...
The USGS has nearly completed a major effort to map existing land cover for the United States at approximately 100 m resolution, also using Landsat TM data. Several national- and continental-scale data sets recently have been acquired by agencies with the explicit intent of promoting studies of land cover and land-cover change in the United States, the humid tropics, and North American boreal forests.
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From page 39... ...
data have been available for several years from European and Japanese satellites and from an experimental SAR flown by NASA on the Space Shuttle. Radar backscatter is sensitive to surface wetness and the dielectric properties of the surface; some results suggest that indicators of soil moisture and vegetation biomass can be derived from SAR data for particular ecosystems.
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From page 40... ...
Low water clarity in reservoirs often results from nonalgal turbidity (clay-like suspended solids) caused by soil erosion, and so different relationships must be developed between reflectance and trophic status in such systems (e.g., Gallie and Murtha 1993~.
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From page 41... ...
Recently declassified data provide an opportunity to analyze time series of land cover in some locations back into the 1960s. Remote Sensing from Aircraft Aerial photographs are available for most developed countries for most of the current century.
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From page 42... ...
Although each site has a different investigator-driven mission, several measurements are made at each site every year. The results, which include estimates of primary production, nitrogen mineralization rates, standing crop, abundances of most soil cations, detritus production, and censuses of dominant plant species, are available in standard form.
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From page 43... ...
trophicdynamic model of energy flow through the food web of Cedar Lake introduced to ecology such important topics as energy transfer efficiency, relationships between production and decomposition, and physical and chemical constraints on biological production. Conceptual models help identify key links between ecosystem components and serve as the basis for developing quantitative models.
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From page 44... ...
. Other conceptual models have been developed to assist the design of biotic indicators and to evaluate the effects of oxygen-demanding wastes on stream ecosystems (Metcalfe 1989, Cairns and Pratt 1993~.
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From page 45... ...
Examples of useful empirically determined relationships are the positive correlation between chlorophyll a levels (a measure of algal biomass) and total phosphorus concentration (the nutrient assumed to be limiting algal growth)
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From page 46... ...
Developed in the mid-1920s, long before the advent of computers, the model expressed changes in oxygen concentration in a river as the difference between a loss term representing biochemical oxygen demand, caused by microbial degradation of organic matter, and a source term representing atmospheric re-aeration. Other source and sink terms, such as planktonic primary production and sediment oxygen demand, later were added to the model; it was computerized in the 1960s.
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From page 47... ...
Although some early simulation models included fishes, these models' outputs failed to accurately portray fish population dynamics. Also, current simulation models of aquatic ecosystems typically exclude benthic invertebrate and macrophyte populations.
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Understanding these connections is difficult because time lags between landscape changes and instream responses are highly variable. THE COMMITTEE'S CONCEPTUAL MODEL FOR CHOOSING INDICATORS To guide its selection of national ecological indicators, the committee assessed the current status of empirical and conceptual knowledge of the factors that most strongly influence ecosystem functioning.
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From page 49... ...
The species composition of biological communities influences primary production in part because species differ in their abilities to photosynthesize under different weather conditions. Although relationships between ecosystem productivity and the number of species in the system are as yet poorly understood, it is clear that without some minimal number of species, ecosystems would function poorly (Grime 1997, Tilman 1996~.
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From page 50... ...
For example, the Bureau of the Census has no policy-making responsibility; so, despite recent political arguments about the validity of sampling as opposed to counting everyone, the population estimates produced by the Bureau are usually trusted. Similarly, the National Weather Service has no responsibility for environmental policies, and so, beyond some scientific questions about the nature and placement of its instruments, its statistics are generally widely respected and trusted.
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