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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).
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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.
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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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- 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.
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- 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.
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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 |
- Classify wetlands into biologically distinct classes. Can
use a variety of classification techniques (e.g., ecoregions,
HGM classification, Cowardin, etc.) or some combination.
- For each wetland class, select wetlands across a gradient
of human disturbance from minimally impaired reference wetlands
to severely degraded wetlands.
- Select one or more assemblages (e.g., macroinvertebrates,
vascular plants) to monitor.
- Directly measure attributes of the selected assemblage (e.g.,
taxa richness, community composition) and corresponding chemical
and physical data in the wetlands.
- 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.
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- 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.
- 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.
- 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.
- Develop scales for variables that distinguish the reference
standard wetlands from those that are degraded.
- 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.
- 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.
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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. |
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.
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- 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).
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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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COMMENTS: |
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