Classification of Standard Ecological Units

NatureServe Explorer reports information on ecological communities as defined in the International Classification of Ecological Communities: Terrestrial Vegetation of the United States (Grossman et al. 1998). In addition, NatureServe Explorer reports information on ecological systems as defined in Ecological Systems of the United States: A Working Classification of U.S. Terrestrial Systems (Comer et al. 2003) and Ecological Systems of Latin America and the Caribbean: A Working Classification of Terrestrial Systems (Josse et al. 2003).

Developed by NatureServe, its natural heritage member programs, and The Nature Conservancy, this classification is a systematic way of describing and assessing ecological diversity. It fills the need for national and international classification standards for ecological communities, while allowing for classification at a scale fine enough to be used to understand, manage, and protect natural resources on a site-by-site basis. Although the standards can be applied to define terrestrial ecological communities anywhere in the world, the great majority of the classification currently consists of vegetation types that occur in the United States, and adjacent areas of Canada and Mexico.

International Vegetation Classification

The International Vegetation Classification (IVC) (formerly called the International Classification of Ecological Communities or ICEC) is based on vegetation as it currently exists on the landscape. Landforms, soils, and other features are not directly considered as part of the classification criteria. Because of conservation objectives, classification efforts focus on natural and semi-natural types of vegetation, and NatureServe Explorer reports only these types. However, the system can be used to classify any vegetation as it currently exists across the landscape, including heavily human-influenced types, and these may be reported in the future.

The classification system has won broad acceptance within the NatureServe network as well as among our partners. For example, the Federal Geographic Data Committee has accepted it as the standard approach to be used by all U.S. federal agencies (http://biology.usgs.gov/fgdc.veg/standards/vegstd-pr.html). In North America, the IVC consists of the the U.S. National Vegetation Classification (USNVC) and the Canadian National Vegetation Classification (CNVC). The IVC has also been developed for the Caribbean region. Further developments are under way through various Latin American projects.

Standard References

Vascular plant names used in naming and describing ecological communities generally follow:

Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T. and C.A. Meacham. Synthesis of the North American flora [computer program]. Version 1.0. North Carolina Botanical Garden: Chapel Hill, NC.

Standard references for nonvascular plant names used in naming and describing ecological communities are the same as those listed for the nonvascular plant species presented in NatureServe Explorer.

Defining the Classification Hierarchy

The International Vegetation Classification consists of a seven-level hierarchy, with lower, finer levels nested into progressively coarser levels. The finest level, an association, corresponds to an "element of biological diversity," although the next higher level (alliance) may also be considered an element in cases where associations have not yet been defined within the alliance.

Three interrelated criteria-species composition, structure, and habitat-conceptually define an association: it represents plant assemblages that exhibit similar total species composition and vegetation structure and that occur under similar habitat conditions. The association concept encompasses both the dominant species (those that cover the greatest area) and diagnostic species (those found consistently in some vegetation types but not others) regardless of whether they are large trees or diminutive understory plants. This means associations can reflect a greater ecological specificity than can a "cover type" or other type based solely on the dominant species of the upper stratum.

Alliance and association definitions are either adopted from compatible local classification systems, especially natural heritage program classifications, or determined through field studies in which new vegetation information is collected and analyzed. Most associations are defined through a mixture of quantitative analysis of vegetation data (such as plot data) and review of qualitative, descriptive information about vegetation types. In every case, it is critical to have structured peer review by ecologists experienced in the regions being described.

A Summary of the Classification Hierarchy
Please refer to the standard reference (Grossman et al. 1998), for complete information.

Understanding Ecological Community Names

Plant species that are dominant (cover the greatest area) and diagnostic (found consistently in some vegetation types but not others) are the foundation of alliance and association names. At least one species from the dominant and/or uppermost stratum is included in each name. The following guidelines apply to alliance and association names:

• A hyphen ("-") indicates species occurring in the same stratum.
• A slash ("/") indicates species occurring in different strata.
• Species that occur in the uppermost stratum are listed first, followed successively by those in lower strata.
• Order of species names generally reflects decreasing levels of dominance, constancy, or indicator value.
• Parentheses around species name indicate species less consistently found either in all associations of an alliance, or in all occurrences of an association.

Alliance names include the class (e.g., "Forest," "Woodland," "Herbaceous") in which they are classified, followed by the word "alliance" to distinguish them from associations. The lowest possible number of species is used for an alliance name, up to a maximum of four.

Examples of alliance names:

• Pseudotsuga menziesii Forest Alliance
• Fagus grandifolia - Magnolia grandiflora Forest Alliance
• Pinus palustris / Quercus spp. Woodland Alliance
• Andropogon gerardii - (Calamagrostis canadensis, Panicum virgatum) Herbaceous Alliance

Association names include the class in which they are classified. The lowest possible number of species is used in an association name. Up to six species may be necessary to define types with very diverse vegetation, relatively even dominance, and variable total composition.

In cases where diagnostic species are unknown or in question, a more general term (such as "Prairie Forbs" is currently allowed as a "placeholder." An environmental or geographic term (for example, "Northern"), or one that is descriptive of the height of the vegetation ("Dwarf"), can also be used as a modifier when such a term is necessary to adequately characterize the association. When confidence in the circumscription of the association is low, the name is followed by the term "[Provisional]".

Examples of association names:

• Abies lasiocarpa / Vaccinium scoparium Forest
• Metopium toxiferum - Eugenia foetida - Krugiodendron ferreum - Swietenia mahagoni /Capparis flexuosa Forest
• Rhododendron carolinianum Shrubland
• Quercus macrocarpa - (Quercus alba - Quercus velutina) / Andropogon gerardii Wooded Herbaceous Vegetation

U.S. National Vegetation Classification (USNVC)

The USNVC development began in the late 1980s and a first approximation was published in 1998 (Grossman et al. 1998, Anderson et al. 1998). The most current units of the USNVC are now maintained on the NatureServe Explorer website. The USNVC is strictly a jurisdictional subset of the IVC; that is, all units defined and described for the U.S. contain range-wide information, as best as that can be determined through the review process. The USNVC is being developed by NatureServe and its natural heritage member programs in partnership with the Federal Geographic Data Committee Vegetation Subcommittee (FGDC), the Ecological Society of America Vegetation Classification Panel (see Jennings et al. 2003)) and federal partners, many of whom are represented on the Subcommittee and the Panel.

Standard References for the USNVC

Grossman D.H., Faber-Langendoen D., Weakley A.S., Anderson M., Bourgeron P., Crawford R., Goodin K., Landaal S., Metzler K., Patterson K.D., Pyne M., Reid M., and Sneddon L. 1998. International classification of ecological communities: terrestrial vegetation of the United States. Volume I, The National Vegetation Classification System: development, status, and applications. The Nature Conservancy: Arlington, VA.

Anderson, M., P. Bourgeron, M. T. Bryer, R. Crawford, L. Engelking, D. Faber-Langendoen, M. Gallyoun, K. Goodin, D. H. Grossman, S. Landaal, K. Metzler, K. D. Patterson, M. Pyne, M. Reid, L. Sneddon, and A. S. Weakley. 1998. International classification of ecological communities: terrestrial vegetation of the United States. Volume II. The National Vegetation Classification System: list of types. The Nature Conservancy, Arlington, Virginia, USA. 502 p.

Jennings, M., O. Loucks, D. Glenn-Lewin, R. Peet, D. Faber-Langendoen, D. Grossman, A. Damman, M. Barbour, R. Pfister, M. Walker, S. Talbot, J. Walker, G. Hartshorn, G. Waggoner, M. Abrams, A. Hill, D. Roberts, and D. Tart. 2003. Guidelines for describing associations and alliances of the U.S. National Vegetation Classification. The Ecological Society of America, Vegetation Classification Panel, Version 3.0 November 2003. 100 pp. (+ Appendices)

Canadian National Vegetation Classification (CNVC)

The CNVC development began in 2000, when partners in Canada, together with NatureServe and the Network in Canada, agreed to collaborate on a framework and principles compatible with the IVC (Alvo and Ponomarenko 2003). The potential for integration of existing provincial and territorial classification information into the CNVC and IVC was described by Ponomarenko and Alvo (2001). Forest and woodland associations are being developed first, led by the Canadian Forest Service's Canadian Forest Ecosystem Classification (CFEC) project, which began in 1998. The Canadian Forest Service and NatureServe Canada are key partners in the overall CNVC development, along with provincial forest ecosystem classification programs and the CDC network. Typically, Canadian units are being developed first, with subsequent correlation to the IVC.

Standard references for the CNVC

Ponomarenko, S., and R. Alvo. 2001 Perspectives on developing a Canadian classification of ecological communities. Canadian Forest Service, Information Report ST-X-18E, Ottawa. 50 pp.

Alvo, R, and S. Ponomarenko. 2003. Vegetation Classification Standard for Canada Workshop: May 31-June 2 2000 (Hull, Quebec). Canadian Field Naturalist 117(1):125-139.

Development Status of the International Vegetation
Classification and NatureServe Explorer Data

In North America, the classification is in continuing development, with many associations remaining to be defined. The primary focus of the classification effort to date has been the contiguous U.S. and Hawaii (the USNVC), with increasing activity in Canada (the CNVC). The completeness of ecological community inventory and classification work varies widely among states and regions.

Significant refinements to the classification and to natural heritage data are anticipated in the near term with further integration of local and state classification work from Alaska, California, and Canadian provinces. "Underclassified" portions of the U.S. interior southwest and adjacent Mexico will also be areas of future classification refinement.

In general, more information is available for Forest, Woodland, Shrubland, and Herbaceous classes than for Dwarf-shrubland, Nonvascular, and Sparse Vegetation classes. Shortgrass prairie vegetation and many riparian types have not been consistently classified. In addition, the degree of classification confidence for upland types is generally higher than for wetland types. The classification of communities that occur as vegetation complexes will also require additional research and analysis.

International Terrestrial Ecological System Classification

NatureServe and its natural heritage program members, with funding from The Nature Conservancy, have completed a working classification of terrestrial ecological systems in the coterminous United States, southern Alaska, and adjacent portions of Mexico and Canada. NatureServe's terrestrial ecological system classification defines groups of plant communities that tend to co-occur within landscapes with similar ecological processes, substrates, and/or environmental gradients. They are intended to provide a "meso-scale" classification unit that is readily mappable, often from remote imagery, and readily identifiable in the field. Approximately 600 terrestrial ecological system units are described in a comprehensive classification for the lower 48 United States and adjacent Canada (Comer et al 2003). This number compares with some 1,800 alliances and over 5,000 associations in the same area.

Standard References

The standard reference for the terrestrial ecological system classification for the United States is:

Comer, P., D. Faber-Langendoen, R. Evans, S. Gawler, C. Josse, G. Kittel, S. Menard, M. Pyne, M. Reid, K. Schulz, K. Snow, and J. Teague. 2003. Ecological Systems of the United States: A Working Classification of U.S. Terrestrial Systems. NatureServe, Arlington, Virginia.

Vascular plant names used in describing ecological systems generally follow:

Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T. and C.A. Meacham. Synthesis of the North American flora [computer program]. Version 1.0. North Carolina Botanical Garden: Chapel Hill, NC.

Standard references for nonvascular plant names used in describing ecological systems are the same as those listed for the nonvascular plant species presented in NatureServe Explorer.

Defining the Ecological System classification

A terrestrial ecological system is defined as a group of plant community types that tend to co-occur within landscapes with similar ecological processes, substrates, and/or environmental gradients. A given terrestrial ecological system will typically manifest itself in a landscape at intermediate geographic scales of 10s to 1,000s of hectares and persist for 50 or more years. Therefore, these units are intended to encompass common successional pathways for a given landscape setting.

By plant community type, we mean a vegetation classification unit at the association or alliance level, where these are available in the International Vegetation Classification (IVC) and its U.S. or Canadian components, the USNVC and CNVC (Grossman et al. 1998, Alvo and Ponomarenko 2003), or, if these are not available, other comparable vegetation units. IVC associations are used where available to describe the component plant communities of each terrestrial system. Ecological systems are defined using both spatial and temporal criteria that influence the grouping of associations. Associations that consistently co-occur on the landscape therefore define biotic components of each ecological system type.

A set of diagnostic classifiers is used to describe the critical ecological variables that support the co-occurrence of plant communities. This multi-factor approach to classification integrates knowledge of natural disturbances such as fire and flooding along with environmental settings, such as soil and bedrock features or wetland characteristics. Variables describing environmental gradients include local climate, hydrologically defined patterns in coastal zones, arid grassland or desert areas, or montane, alpine or subalpine zones.

Ecological processes include natural disturbances such as fire and flooding. Substrates may include a variety of soil surface and bedrock features, such as shallow soils, alkaline parent materials, sandy/gravelling soils, or peatlands. Finally, environmental gradients include local climates, hydrologically defined patterns in coastal zones, arid grassland or desert areas, or montane, alpine or subalpine zones.

NatureServe's ecological system concept is defined as follows:

1. The unit is explicitly scaled to represent
      a. spatial scales of 10s to 1000s of hectares.
      b. temporal scales of 50 to 100 years
2. The variability in the system is explicitly described in terms of a consistent list of abiotic and biotic criteria and by linking ecological systems to plant community types (associations and alliances of the NVC) that describe the biotic community variation within the system.
3. Long-term sustainability and local stability are to be considered by mapping and evaluating the occurrence of ecological systems at the local site and the regional level.
4. Population processes not formally included as explicit system dynamics, but through knowledge of the component plant communities, the major plant species and their dynamics within the systems can be described. Additional work could formalize the roles of additional biotic elements such as invertebrates and vertebrates.

Meso-Scale Ecosystems

Our conceptualization of terrestrial ecological systems includes temporal and geographic scales intermediate between those commonly considered for local stand and landscape-scale analyses, which can range from 50 to 1,000s of years and 10s to 1,000s of hectares. These "meso-scales" are intended to constrain the definition of system types to scales that are of prime interest for conservation and resource managers who are managing landscapes in the context of a region or state. More precise bounds on both temporal and geographic scales take into account specific attributes of the ecological patterns that characterize a given region.

Diagnostic Classifiers

As the definition for ecological systems indicates, this is a multi-factor approach to ecological classification. Multiple environmental factors - or diagnostic classifiers - are evaluated and combined in different ways to explain the spatial co-occurrence of IVC associations (See below). The structure of the ecological systems classification is more "modular" in that it aggregates diagnostic classifiers in multiple, varying combinations, without a specific hierarchy. The focus is on a single set of ecological system types. This is in contrast to, for example, the framework and approach of the IVC. The nested IVC hierarchy groups associations into alliances based on common dominant or diagnostic species in the upper most canopy. This provides more of a taxonomic aggregation with no presumption that associations co-occur in a given landscape. The ecological system unit links IVC associations using multiple factors that help to explain why they tend to be found together in a given landscape due to similar ecological processes, substrates, and/or ecological gradients. Therefore, ecological systems tend to be better "grounded" as ecological units than most IVC alliances and are more readily identified, mapped, and understood as practical ecological classification units. Diagnostic classifiers include a wide variety of factors representing bioclimate, biogeographic history, physiography, landform, physical and chemical substrates, dynamic processes, landscape juxtaposition, and vegetation structure and composition.

Understanding ecological system names

The nomenclature for the ecological systems classification includes three primary components that communicate regional distribution (predominant Ecological Division), vegetation physiognomy and composition, and/or environmental setting. The final name is a combination of these ecological characteristics with consideration given to local usage and practicality.

Biogeographical Divisions: The Division-scaled units typically form part of each classification unit's name. For example, a "Rocky Mountain" ecological system unit is entirely or predominantly found (>80% of its total range) within the Rocky Mountain Division, but could also occur in neighboring Divisions. This nomenclatural standard is applicable to most ecological system units, except for those types that span many several Divisions (e.g., some tidal or freshwater marsh systems), or that are more localized (>80% of the range) within a subunit of the Division (e.g., Colorado Plateau, within the Inter-Mountain Basins Division).

Vegetation Structure and Composition: Vegetation structure (e.g., Forest and Woodland, Grassland), and vegetation composition (e.g. Pinyon-Juniper, mixed conifer) is commonly used in the name of a system. In sparse to unvegetated types, reference to characteristic landforms (e.g., badland, cliff) may substitute for vegetation structure and/or composition. It will typically come after Biogeographical Division, but may come before or after Environment.

Environment: Environmental factors (e.g., xeric, flats, montane) can be used in conjunction with Vegetation Structure and Composition or, on their own, to name system types. This will typically come after Biogeographical Division, but may come before or after Vegetation Structure and Composition.

Examples:
Laurentian-Acadian Pine-Hemlock-Hardwood Forest Cross
Timbers Oak Forest and Woodland Central Appalachian
Limestone Glade and Woodland Southern and Central
Appalachian Cove Forest North-Central Interior Shrub-Graminoid
Alkaline Fen Cross Timbers Oak Forest and Woodland
Western Great Plains Wooded Draw and Ravine Rocky
Mountain Foothill Grassland Chihuahuan-Sonoran Desert
Bottomland and Swale Grassland

Development status of the International Terrestrial Ecological System Classification and NatureServe Explorer Data

Approximately 600 terrestrial ecological system units are described in a comprehensive classification for the lower 48 United States and adjacent Canada (Comer et al. 2003). All of these ecological systems are served here on NatureServe Explorer. Summarizing across the range of natural variation, some 381 ecological systems (63%) are upland types, 183 (31%) are wetland types, and 35 (6%) are complexes of uplands and wetlands. Considering prevailing vegetation structure, 322 systems (54%) are predominantly forest, woodland, or shrubland, 166 systems (28%) are predominantly herbaceous, savanna, or shrub steppe, and 74 systems (12%) are sparsely vegetated or "barren".

Terrestrial ecological systems have also been developed and described comprehensively for Latin America and the Caribbean (Josse et al. 2003). This report summarizes the nearly 700 ecological systems that currently are classified and described in the region, emphasizing the natural portion of the landscape. These ecological systems are not currently available on NatureServe Explorer, but will soon be posted in Spanish on InfoNatura. Summarizing across the range of natural variation, some 477 ecological systems types (69%) are from uplands, 199 types (29%) wetland, and 17 types (2%) are complexes of uplands and wetlands. Considering prevailing vegetation structure, 512 types (71%) are predominantly forest, woodland, or shrubland, and 198 types (28%) are predominantly herbaceous, savanna, or shrub steppe. Seventeen types (2%) are sparsely vegetated.

Ecological system units tend to be readily mapped using areal photographs or satellite imagery, especially when combined with ancillary data, such as soil maps, elevation-derived landform models, and stream or wetland map layers. NatureServe ecological system units have become established as map legends for several federal agencies (e.g. USGS Gap Analysis programs) for new mapping efforts that depict units at fine grain (0.25 hectare) as well as intermediate grain (5 hectare). Given their utility for standardized mapping, the ecological systems classification lends itself to a wide range of mapping activities and subsequent assessments of habitat diversity and landscape conditions.