Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

For purposes of this report, wetlands are defined using the dominant vegetation (including all rooted aquatic species) and hydrologic properties of the National Wetlands Inventory (NWI; for information about the NWI program, see http://wetlands.fws.gov/; for information on the wetlands classification system, see http://wetlands.fws.gov/Pubs_Reports/Class_Manual/class_titlepg.htm). Wetland plant communities are defined according to the association concept, which is a plant community type of a specific floristic composition resulting from certain environmental conditions and displaying relatively uniform physiognomy. These communities form part of the U.S. National Vegetation Classification System (NVCS), which was adopted as the federal standard for vegetation information by the Federal Geographic Data Committee in 1997. The classification covers uplands as well as wetlands (see http://www.fgdc.gov/standards/status/sub2_1.html for information about this classification system). The conservation status assessment for each association is called a global rank and is based on the relative rarity and degree of imperilment of the association across its entire geographic range. Tracking wetland plant communities at the association level is a way of measuring wetland diversity and provides a tool to assess conditions affecting specific types of wetlands across the entire country.

Riparian areas are the margins of streams, rivers, or lakes. Riparian areas include a range of plant communities, including both upland vegetation communities (often thriving on the increased moisture available near the stream or river) and wetland plant communities on the floodplain. Because riparian vegetation is a mixture of upland and wetland habitats, classification is difficult. In 1997, the U.S. Fish and Wildlife Service developed a classification scheme for the western United States (http://wetlands.fws.gov/Pubs_Reports/Riparian/Riparian.htm), but this system has only begun to be used for collecting data on riparian habitats in that region of the country. As the Service uses this classification to expand its natural resource mapping to riparian habitats, it should be possible to use the resultant inventory to document the status of riparian habitats and their trends in the future. Meanwhile, NatureServe (a nonprofit organization; see www.natureserve.org) and the Natural Heritage Network, which provides status information on wetlands (see below), are developing an approach for reporting on riparian area condition (see “Data Quality/Caveats” below).

The Data

Data Source: NatureServe and its Natural Heritage member programs develop and maintain information on each association in the NVCS. The regions were defined by The Heinz Center and collaborators, using vegetation-based and climate-based ecological regions, the regional boundaries developed by federal land and resource management agencies, vegetation data from remote sensing, and a common-sense approach to regional differences and similarities. Data Collection Methodology: NatureServe ecologists gather, review, and integrate available information about vegetation pattern from Natural Heritage program databases, published and unpublished literature, and ecology experts in each state. They then assess conservation status using standardized Heritage ranking criteria (see http://www.natureserve.org/explorer/ranking.htm). Heritage ranks range from 1 to 5, with 1 meaning critically imperiled; 2, imperiled; 3, vulnerable to extirpation or extinction; 4, apparently secure; and 5, demonstrably widespread, abundant, and secure.

Data Manipulation: The global ranks are summarized into “rounded ranks.” For example, an actual rank may express substantial uncertainty about whether the community is “critically imperiled” or “imperiled.” In all such cases, the rank has been rounded to the more imperiled one.

Data Quality/Caveats: Conservation status ranks are continually reviewed and revised by Natural Heritage program biologists. In addition, as development of the system of classifying plant communities evolves (http://www.fgdc.gov/standards/status/sub2_1.html), more communities will be recognized in geographic areas that are currently “underclassified.” Such revisions could affect the proportion of communities considered at-risk. Some variability exists across the country in how the wetland plant community types were defined and in the amount of survey work done, and the definitions of community types are still under review by ecologists with the NatureServe and Natural Heritage programs.

Data Access: Detailed, periodically updated information on each wetland plant community type, including its status, is available at http://www.natureserve.org/servlet/NatureServe?init=Ecol.

The Data Gap

In the near future NatureServe hopes to augment the associations used in this analysis with an “ecological systems” approach. Ecological systems are biological communities found within a geographic region that share similar ecological processes and gradients (e.g., fire regime, elevation, climate, hydrologic regime), biological dynamics (e.g., succession), and other driving environmental features (e.g., soils, geology). Wetland areas defined by such an approach will bear a more direct relationship to major ecological settings (e.g., riparian types, peatlands, marshes) and thus may be a better basis for this kind of analysis.

This ecological systems approach may help in dealing with the fact that riparian areas are not specifically described in the NVCS and are not assessed by NatureServe. A holistic approach could include the entire moist upland–wet lowland zone as part of the riparian area, facilitating mapping and documentation of these systems across a region.