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Wetland Biological Assessments and HGM Functional Assessment

United States
Environmental Protection
Office of Water
Office of Wetlands, Oceans
and Watersheds (4502-F)
July 1998

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]
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.

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.

Relevant Sections (§) of the Clean Water Act (CWA)

CWA §303 (water quality standards):
Water quality standards are state or tribal laws or regulations that, at a minimum, define: (1) the water quality goals of a water body (designated uses), (2) the limits or conditions that, if met, will generally protect water quality goals (criteria), and (3) provisions to protect waterbodies (antidegradation provisions) [See Fact Sheet 7]. States and tribes can use biological assessment methods to develop numeric biological criteria that quantitatively describe the condition of wetland plant or animal assemblages found in minimally disturbed wetlands.

CWA §401 (water quality certification):
Under CWA §401, states and tribes have the authority to certify that federally permitted or licensed activities that may result in a discharge to a waterbody, such as those requiring CWA §404 permits, comply with their water quality standards. If proposed activities will violate their water quality standards, then states and tribes can deny or condition the permits.

CWA §404 (dredge and fill permits):
The U.S. Army Corps of Engineers and U.S. EPA administer a program for permitting the discharge of dredged or fill material in "waters of the U.S.," which, by definition, include wetlands. The HGM approach to functional assessment estimates the change in functioning induced by alteration of a wetland, either positive or negative. Negative effects (i.e, reductions in sustainable levels of functioning) are normally determined in association with dredge-and-fill permits. The permit review process could use output from an assessment as one tool to determine if the project results in significant degradation. Output from HGM models can be used to determine the amount of positive effects (i.e., increases in sustainable levels of functioning) associated with compensatory mitigation requirements, normally through restoration of previously altered wetlands of the same type. Although the HGM approach was designed initially for use in the CWA §404 program, the output of assessments is not constrained to any particular statutes, federal or otherwise.

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.


(Also see Fact Sheets 2, 7, 8)

  • Establishing appropriate narrative and numeric biological criteria for wetlands as part of state water quality standards.
  • Assessing wetlands to determine if they are meeting water quality standards.
  • Evaluating performance of wetland restoration activities at improving the ability of wetlands to support and maintain wetland plant and animal assemblages.
  • Administering CWA §401 water quality certification programs.
  • Tracking condition of wetlands as part of CWA §305(b) water quality reports to Congress.
  • Evaluating impacts of projects that degrade wetland ecosystems, including the comparison of project alternatives. Projects include those related to CWA §404 dredge-and-fill permits, the Swampbuster provision of the Food Security Act, or other relevant projects that seek to detect significant alterations of wetland ecosystems through an analysis of change in functions.
  • Evaluating restoration projects designed to improve wetland conditions by estimating changes in functioning over time.

COMMENTS: Although designed for different purposes, both approaches are flexible and have multiple applications.

Key Steps in Developing Assessment Method
  1. Classify wetlands into biologically distinct classes. Can use a variety of classification techniques (e.g., ecoregions, HGM classification, Cowardin, etc.) or some combination.
  2. For each wetland class, select wetlands across a gradient of human disturbance from minimally impaired reference wetlands to severely degraded wetlands.
  3. Select one or more assemblages (e.g., macroinvertebrates, vascular plants) to monitor.
  4. Directly measure attributes of the selected assemblage (e.g., taxa richness, community composition) and corresponding chemical and physical data in the wetlands.
  5. Identify metrics, which are attributes which show an empirical and predictable change in value along the gradient of human disturbance (Fact Sheet 5). Combine metrics into an Index of Biological Integrity (IBI). Test and validate IBI. If more than one assemblages are measured, then each should have its own IBI.

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.

  1. Classify wetland by geomorphic setting for the purpose of partitioning natural variation, thus allowing variation by impacts to be more easily detected within a regional subclass.
  2. Develop a profile for the wetland subclass that characterizes it according to its geology, hydrology, biogeochemistry, plant and animal communities, and typical alterations that have occurred historically. This profile, in addition to identifying functions characteristic of the subclass, should be assembled by an interdisciplinary group of professionals (fields of hydrology, geomorphology, soil science, plant and animal community ecology, ecosystem ecology, etc.) familiar with the region and the technical literature.
  3. Identify reference standard wetlands from a subset of reference sites that are relatively unaltered or natural, and characterize these sites by estimating or measuring indicators and field variables that will be used to develop models which relate the measurements to functions.
  4. Develop scales for variables that distinguish the reference standard wetlands from those that are degraded.
  5. Combine variables into HGM models of functions. Test and validate HGM models. After models have been tested and validated, users will be able to quickly apply the models to wetlands that have been proposed for alteration or restoration.
  6. Properly constructed and tested HGM models of functions for a specific subclass will quantify differences and similarities between a wetland that is being sampled and reference standard wetlands. The models will also be useful in predicting changes that will result from proposed alterations to the site.

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.

Presentation of Assessment Results
  • summary IBI score.
  • narrative description of overall biotic condition in comparison to reference wetlands of the same region and wetland type.
  • numerical value of each metric.
  • narrative description of metric in comparison to reference wetlands of the same region and wetland type.
  • no overall, summary score.
  • index value of each function in comparison reference standard of wetlands in same reference domain and HGM class or subclass.
  • index value of each variable with supporting narrative describing estimates and measurements.
(See last page for definitions of HGM reference terms)

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.

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:

reference domain
the geographic extent of a wetland subclass
reference wetlands
all sites within the reference domain, regardless of their condition
reference standard sites
a subset of reference wetland sites that are judged to be least altered
reference standards
conditions exhibited by reference standard sites that are reflective of characteristic levels of functioning
site potential
the best conditions that can be achieved on a site within local constrains of land use, etc.
project target
level of functioning negotiated for enhancement, restoration, or creation)
project standards
performance criteria or specifications to guide activities toward project target)

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