Wetland Biological Assessments and HGM Functional Assessment
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United States Environmental Protection Agency |
Office of Water Office of Wetlands, Oceans and Watersheds (4502-F) |
EPA843-F-98-001f July 1998 |
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Wetland Bioassessment Fact Sheet 6
The purpose of this fact sheet is to provide a comparison of a functional assessment method, the Hydrogeomorphic (HGM) Approach, and biological assessments based on an index of biological integrity (IBI). Our intention is not to advocate one particular approach, because each was developed for a different purpose and has many strengths. Rather, our intention is to identify their similarities and differences and to identify ways that the two approaches can be supportive of each other. The functional assessment column was written primarily by Mark Brinson (East Carolina University).
Biological Assessment [Index of Biological Integrity (IBI)] |
Functional Assessment [Hydrogeomorphic (HGM) Approach] |
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Purpose of Assessment | To evaluate a wetland's ability to support and maintain a balanced, adaptive community of organisms having a species composition, diversity, and functional organization comparable with that of minimally disturbed wetlands within a region. The condition of the biota will show if a wetland is degraded by any chemical, physical, or biological stressors and will help scientists diagnose the stressor(s) causing the damage. Biological assessments (bioassessments) also detect intermittent stressors or the cumulative effect of multiple stressors. |
To evaluate current wetland functions and predict potential changes to a wetland's functions that may result from proposed activities. A wetland is compared to similar wetlands that are relatively unaltered. The approach is based on combining variables that are typically structural measures or indicators that are associated with one or more ecosystem functions. Functions normally fall into one of three major categories: (1) hydrologic (e.g., storage of surface water), (2) biogeochemical (e.g., removal of elements and compounds), and (3) physical habitat (e.g., topography, depth of water, number and size of trees). |
COMMENTS: Both approaches evaluate the condition of individual wetlands by comparing them to the conditions found in an established set of reference wetlands. The goal of both approaches is to maintain wetlands in their minimally disturbed conditions and wetlands are only compared to other wetlands of the same type. The definition of reference wetlands is discussed on the last page of this fact sheet. |
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Primary Means of Estimating Conditions | Direct, quantitative measurements of certain attributes of a wetland assemblage (e.g., taxa richness of macroinvertebrates) that show clear, empirical changes in value along a gradient of human influence. Typically, between 8 and 12 of these attributes, called metrics, are combined into an Index of Biological Integrity (IBI) for an assemblage (See Fact Sheet 5). The biological data are related to corresponding physical and chemical data. |
Estimates and some measurements of variables related to wetland functions in comparison to reference standard conditions characteristic of relatively unaltered sites of the same wetland type. Available technical literature, ongoing research, and best professional judgement are used in the development of the assessment method and in its application. |
COMMENTS: Biological assessments can be used to: (1) determine if HGM's field indicators and variables accurately reflect the biotic condition of wetlands, (2) determine the level of spatial and temporal variation in HGM's biotic field indicators and variables, (3) validate or invalidate how HGM model variables are scaled and combined as they relate to ecosystem functions, and (4) detect if selected animal and plant community functions have changed from HGM reference standard conditions. |
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Relevant Sections (§) of the Clean Water Act (CWA) | CWA §303 (water quality standards): CWA §401 (water quality certification): |
CWA §404 (dredge and fill permits): |
COMMENTS: HGM has direct applications for CWA §404 decisions and bioassessments have indirect applications to CWA §404 decisions through CWA §401 water quality certification programs. |
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Applications
(Also see Fact Sheets 2, 7, 8) |
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COMMENTS: Although designed for different purposes, both approaches are flexible and have multiple applications. |
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Key Steps in Developing Assessment Method |
A properly constructed IBI will detect damage of a wetland caused by a variety of chemical, physical, or biological stressors. An IBI will also help diagnose the type of stressor(s) that caused the damage. After the IBI has been tested and validated, scientists can use the IBI to screen wetlands for signs of degradation without having to conduct expensive chemical and physical analyses. If signs of degradation are detected, then the scientists can conduct more extensive biological measurements and chemical and physical tests to determine the stressors impacting the wetland. By understanding how biological assemblages respond to increasing human disturbance, wetland managers can predict how the taxa richness and composition of assemblages may change following alternative development approaches, restoration activities, or conservation measures. |
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COMMENTS: Both methods require the development or refinement of regionally appropriate assessment methods. Wetland ecosystems are the unit of assessment and comparison in both approaches, not individual functions. Under HGM, the score of a variable or function index can never exceed the score of a reference standard wetland. |
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Presentation of Assessment Results |
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COMMENTS: HGM does not use an overall, summary score to compare wetlands. Both approaches use minimally impaired wetlands as their measuring sticks. Both approaches only compare wetlands to other wetlands of the same region and type. For example, both approaches would compare a New England bog only to other New England bogs and a minimally impacted bog would receive the highest score. |
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Method of Classifying Wetlands |
Wetlands occur in many landscape positions with a variety climatic, hydrologic, and soil conditions. As a result, the community composition and diversity of an assemblage (e.g., amphibians) will naturally vary between wetland types. When examining how an assemblage is affected by a stressor, too much natural variation in the data can make it difficult or impossible to detect signs of impairment. Thus, in bioassessments, the purpose of classifying wetlands is to group wetlands with assemblages of similar diversity and composition, and separate those wetlands with assemblages that are not similar. The goal is to avoid comparing apples to oranges. By minimizing natural variation within classes and making sure that wetlands within a class respond similarly to human disturbances, it is much easier to identify signs of degradation. Current wetland bioassessment projects use a variety of classification systems, such as ecoregions and the HGM classification method (See Fact Sheet 4). Researchers often start with a method or a combination of methods and then lump or split as needed based on biological data to end up with classes of biologically distinct wetlands. |
The HGM approach identifies 7 geomorphic settings of wetlands as guidance for the identification of regional subclasses that function similarly (i.e, riverine, depressional, slope, mineral soil flat, organic soil flat, estuarine fringe, lacustrine fringe). Settings differ by dominant sources of water and hydrodynamics (e.g., flow rates and fluctuations of water within the wetland). Local vernacular is preferred in naming regional subclasses as long as it is recognized that vegetation cover types may not vary between some subclasses that are functionally distinct. |
COMMENTS: The HGM classification system can provide a good starting point for biological assessment programs. For bioassessment projects, one option is to classify first by ecoregion and then by HGM class or subclass. Then lump or split these classes as needed based on preliminary bioassessment data. | ||
Definition of Reference Terms | In biological assessments, the terminology for reference conditions is based on the protocols that have been developed for assessing the condition of streams, lakes, and estuaries. From this heritage, a reference site or reference wetland is a minimally impaired wetland that is representative of the expected ecological conditions of a wetland of a particular type and region. The reference sites serve as the measuring stick to determine the integrity of other wetlands. Each biologically distinct class of wetlands has its own set of reference sites. For example, bogs are only compared to other minimally impaired bogs and prairie potholes are only compared to other minimally impaired prairie potholes. When developing an IBI, however, researchers compare the condition of an assemblage (e.g., birds) in reference sites and impaired wetlands that represent a gradient of human disturbance. No term has been developed for the impaired wetlands or for the larger set of wetlands (reference and impaired wetlands). |
The HGM approach identifies a suite of terms to facilitate assessments and recognize ambiguities that often develop in the regulatory environment if terminology is not defined. Only cryptic definitions are given here for expediency, and include:
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