In seeking data sources, we placed the highest priority on data that are available on a national basis, with a sufficient historical record to provide meaningful trends. Unfortunately, data at these preferred levels of coverage were available for only about 60 percent of the selected measures.

Established national monitoring programs often provide such coverage, but these programs do not address all the measures selected for this Report. In some cases, monitoring may have begun only recently, as a result of new or modified programs or improvements in technology (such as remote sensing). This type of information is valuable because, as monitoring is repeated over time, trend data will become available. Data from programs that cover only a portion of the United States, but that are expected to cover additional geographical areas in the future, are also valuable. Despite their limitations, all of these are considered "good data" for purposes of the figure below.

For the remainder of our measures, we included either limited data or no data at all. Limited data are those that are available from only one or a few places, or for only one or a few time periods; in most cases, there are no plans for expanding or continuing the data collection efforts. These are considered "data with significant limitations" in the figure below. The Coasts & Oceans section includes more one-time data sets and data for limited geographical areas than do the other sections.

The following sections define and explain the rationale for selecting the 12 major properties included in the Report, and summarize the findings of this prototype with respect to each one

These measures describe the physical dimensions, basic composition, and intensity of use of each ecosystem. These are fundamental aspects that can determine the amounts and types of goods and services provided by an ecosystem. In addition, these measures provide an important context for descriptions of flows of goods or services or other aspects of ecosystem condition.

Extent: the overall quantity of a system, such as how much forest there is in the United States; or of specific components, such as certain crops, forest types, or coastal habitats.

Landscape Pattern: the degree to which parts of an ecosystem are intermingled with other land uses, or separated into isolated patches. Such areas function differently from intact areas.

Management and Stewardship: public or private restrictions on the use of lands and waters. Measures address the intensity of use, degree of government restriction, and voluntary stewardship actions.

Croplands cover about 20 percent of the land area of the United States; about 15 percent of U.S. lands are considered "prime farmland." The area used for production fluctuates, with the amount set aside in government programs exhibiting the greatest variation. The cropland landscape includes wetlands, buffer areas, windbreaks, field borders, and other areas, although there is little consistent information on these areas.

Forests cover about one-third of the United States, and their area declined slightly over the past two decades. Most western forests are public; most eastern ones are private. Most public forests are managed for uses that include timber harvest, recreation, and other activities. Harvest is prohibited in 15 percent of public forests. Fragmentation of forests into smaller and more isolated patches remains a concern, but there is little consensus on how to measure this condition.

Under international law, the Nation's coastal and ocean waters extend to 200 miles from shore. Although this zone is unchanging in total area, there is almost no consistent national information on the area of important habitat types found in these waters and on the coastal fringe, or on land use protection or management along the coast.

Ecosystems provide food, fiber, and other products; recreational opportunities; and other ecosystem services such as building soil, providing clean water, and controlling erosion or flooding. This Report focuses primarily on food and fiber production and recreation. Because many ecosystem services are provided by intact natural systems, measures of extent are at present the only feasible surrogates for nationwide measures of many ecological services. Ecosystem services are generally difficult to measure directly, and the Report usually does not quantify them.

Over the past 50 years, the amount of crops grown in the United States has more than doubled, the amount of fish landed has doubled, and the amount of timber cut has nearly doubled. Much of the increase in fish catch has occurred in Alaska since the late 1980s. Increases in timber harvest have been more gradual, with increases seen largely on private lands and in the South. Crop harvest has also grown steadily.

Recreation is a significant use of forests, coasts & oceans, and, to a lesser degree, croplands. However, it is difficult to report on recreation taking place in specific ecosystems, because information on recreation is generally reported by activity only, not by where the activity occurs. Consideration is being given to including a broad national overview of recreation in the 2001 Report. There is information on the number of beach closures—more than 7,000 in 1998, indicating significant restrictions on recreational use due to water quality conditions.

Key aspects of ecosystem condition include chemical, physical, and biological properties.

Plant Growth and Primary Productivity: Productivity, or plant growth, is considered a key indicator of overall ecosystem condition because it reflects a fundamental driving force in an ecosystem—the conversion of sunlight into plant matter. Productivity is generally inferred from other data rather than measured directly. Measures include the yield of harvested crops, tree growth, satellite measurements of plant growth, and amount of carbon stored in ecosystems.

Physical Conditions: Soil and water provide the basic foundation for ecosystems; both serve as habitat for many species and provide nutrients that support plant life. Measures include the condition of soils (e.g., erosion, compaction) and water (e.g. salinity, temperature).

Nutrients: Nitrogen and phosphorous are key plant nutrients. Excesses can contribute to ecosystem degradation, whereas deficiencies can hinder plant growth.

Chemical Contaminants: The accumulation of synthetic organic chemicals or abnormal levels of heavy metals can harm plants and animals and raise human health concerns.

Biological Community Condition: Plant and animal communities are unique natural elements, and they provide the biological "neighborhood" within which individual species exist. Data on their condition provide information about both the communities and their component species.

Native Species: Species have different ecological functions, levels of human interest, and preferences for different habitat. Measure address, for selected groups of native animal species, whether the group is secure or imperiled and whether its population is increasing, decreasing, or stable.

Biological Invasions, Outbreaks and Diseases: Biological disturbances are both a natural part of ecosystems and potentially significant forces that can change and degrade systems. Measures report on nonnative invasive species and outbreaks of parasitic insects, diseases, and other native species.

Productivity and Plant Growth

Crop yield per acre has grown steadily over the past 50 years. For forests, more wood grows than is harvested each year, and U.S. forests grow more per acre today than they did in the 1950s. There are insufficient data to make statements about changes in coastal productivity.

Physical Conditions and Nutrients

The percentage of cropland with highly erosion-prone soil conditions decreased from 30 percent in 1982 to 24 percent in 1992. In most states, undesirably high soil acidity is found in about one-fourth of all soils tested. Little consistent information is available regarding organic matter content, soil compaction or soil salinity. Irrigated acreage has grown by 25 percent since 1969, while the amount of water applied per acre has dropped by 25 percent. Streams and ground water in agricultural areas generally have higher concentrations of nitrogen and phosphorous pollution than do forested areas. About 5 percent of stream samples and 3 percent of ground water samples from agricultural areas exceeded drinking-water standards for nitrate.

Streams in forested areas generally have very low levels of nitrate. Streams with the lowest levels of nitrate typically are found in watersheds with the greatest percentage of forest cover. Little is known about the national status of forest soils because the data are confined to specific research locations.

There is no consistent national information available to track changes in concentrations of nitrogen and phosphorous, salinity, and temperature in coastal waters.

Contaminants

Streams and fish in cropland areas generally have fewer pesticides than do those in urban or mixed land use areas, whereas ground water in agricultural areas generally has more. Damage to forest plants from elevated levels of ozone has been found in some areas, but not all areas have been surveyed. In coastal areas, contaminant levels in shellfish remained fairly constant over the past decade. Although contaminants were detected in most coastal sediments surveyed, concentrations were rarely high enough to suggest potential biological impacts.

Biological Community Condition

About one-fourth of the more than 1,400 plant communities or "associations" found in U.S. forests occur infrequently enough, or on few enough acres, to be considered at risk. About 30 percent are "secure" or "apparently secure." Consistent data are not available on the acreage of old-growth forest or the number of very large trees. Overall forest mortality has remained fairly stable over the past 50 years, never exceeding 1 percent per year. Acreage burned by wildfire in recent decades has been far below the levels of earlier this century but it has increased in the past few years.

We are developing, but have not completed for this prototype Report, a measure or set of measures for the condition of key species groups in each of several coastal habitat types.

Native Species

Most grassland and upland game birds in the central United States (where croplands predominate) are "apparently secure," with some populations declining and others increasing. Most forest birds of prey (e.g., hawks, owls), all birds preferring mature forest, and all birds preferring forest edges are "apparently secure," and more species are increasing in number than are decreasing. The number of seabird populations that are stable or increasing exceeds the number in decline. All 10 sea turtle populations found in U.S. waters are classified as "at risk;" four of these are stable or increasing. One-third of the 300 commercially fished stocks whose status is known are overfished or approaching an overfished condition.

In all three ecosystems, there are important groups of species for which there is insufficient information to evaluate national status and trends. These include most pollinators, most forest carnivores, many seabirds, more than 60 percent of commercially caught fish, and more than 80 percent of marine mammals.

Biological Invasions, Outbreaks, and Disease

No consistent or comprehensive national information is available on crop losses to pests or diseases, or on resistance to pesticides. An indirect indication of pest pressure is the fact that farmers in all regions of the country apply herbicides against weeds, whereas treatment for other pests varies greatly by region. Five species of insects collectively affect about 3 percent of U.S. forests (21 million acres) a year. One disease (fusiform rust) affects about 6 percent of southern forests, and one parasitic plant (dwarf-mistletoe) affects about 12 percent of forests in the West.

Nonnative invasive species are a concern in all three ecosystems, but consistent national information is generally unavailable. Nonnative plants occupy one-fourth of the area of forest understory (smaller trees and shrubs) in California, and 20 percent in the Mid-Atlantic region; the program that collects these data is expanding in coverage. Finally, case studies and expert opinion indicate that harmful algal blooms are increasing in frequency, and there is significant concern over fish lesions, cancer, and other diseases, but there are no consistent or comprehensive national data that can document these concerns.