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Developing an Index of Biological Integrity
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United States
Environmental Protection
Agency |
Office of Water
Office of Wetlands, Oceans
and Watersheds (4502-F) |
EPA843-F-98-001e
July 1998 |
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Wetland Bioassessment Fact Sheet 5
One method to assess biological integrity of wetlands is to develop an
index of biological integrity (IBI) for an assemblage of wetland plants
or animals. An IBI is made by combining several biological indicators,
called metrics, into a summary index. A well-constructed IBI that can
allow scientists to: (1) measure condition, (2) diagnose the type of stressors
damaging a wetland's biota, (3) define management approaches to protect
and restore biological condition, and (4) evaluate performance of protection
and restoration activities.
FOUR STEPS TO CREATE AN IBI
1
Select an Assemblage |
2
Test and Evaluate Metrics |
3
Combine Metrics into an IBI |
4
Test and Validate IBI |
1 Select an Assemblage
An assemblage is a group of plant and animals that are combined to form
a larger group. Common wetland assemblages include:
- VASCULAR PLANTS
- AMPHIBIANS
- BIRDS
- ALGAE
- MACROINVERTEBRATES (snails, insects, clams, crayfish, etc.)
2 Test and Evaluate Metrics
A metric is a measurable component of a biological system with an empirical
change in value along a gradient of human disturbance Scientists can measure
many biological attributes of wetlands such as the diversity of amphibians
or the number of pollution-tolerant insects. Some of these attributes
will provide valuable information about biological integrity and other
attributes will not. The goal is to identify metrics, which are
attributes that show an empirical and predictable change in value along
a gradient of human disturbance. The gradient of human disturbance can
represent the amount of logging, agriculture, development, impervious
surfaces, or other land use or activity in a watershed, or some combination
of land use, depending on the purpose of the bioassessment. An example
of a metric is taxa richness of macroinvertebrates (the number of taxa
of insects, snails, clams, crayfish, etc.). Several states have found
that macroinvertebrate richness decreases as a wetland is degraded by
human activities.
(Figure 1). For illustrative purposes, Figures 1-5 were developed
using hypothetical data, but are based on figures that were provided by
Dr. James Karr (University of Washington). As Figure 1 shows, there is
a clear response to increasing human disturbance and this attribute could
be used as a metric.
In contrast, total abundance of macroinvertebrates is often more dependent
on natural environmental variability of wetlands and does not show a reliable
change in response to human disturbance (Figure 2). As Figure 2
shows, there is no clear response to increasing human disturbance and
this attribute would not be useful as a metric. In these two examples,
total taxa richness of macroinvertebrates could serve as a metric and
total abundance could not.
3 Combine Metrics into an IBI
Typically, an IBI is formed by combining at least 7 metrics from one
biological assemblage. One approach of combining metrics into an IBI is
to assign scores of 1, 3, or 5 to the metrics according to how they respond
to human disturbances. For example, the diversity and richness of macroinvertebrate
taxa may consistently decrease with increasing human disturbance (Figure 3).
In this case, we could assign a score 1 to indicate poor conditions, 3
to indicate moderate conditions, and 5 to indicate minimally impacted
conditions (Figure 3). Another metric, the relative abundance
of tolerant taxa [(number of tolerant individuals in sample) / (total
number of individuals in sample) x 100], may increase with increasing
human disturbance (Figure 4). In this case, a wetland dominated
by tolerant taxa would receive a low score and a wetland with a small
percentage of tolerant taxa would receive a high score .
If 10 metrics were scored in this manner, then the scores could be added
together to form the index of biological integrity (IBI) with potential
scores ranging from 10 (maximally impacted) to 50 (minimally impacted).
The IBI scores should form a relatively straight line when plotted against
the gradient of human disturbance (Figure 5). Sometimes there
will be scores that are far from the line which should be investigated.
More often than not, an outlier is either the result of (1) misclassifying
the wetland or (2) a stressor, such as acid mine drainage, that is damaging
the wetland biota and was not captured by the gradient of human disturbance.
4 Test and Validate IBI
After developing the IBI, the scientists would then test the IBI to see
if it accurately detects the effects of human disturbances on the biological
assemblage. One approach is to (1) randomly split the data into two halves,
(2) develop the IBI on one half of the data, and (3) test the IBI on the
other half of the data. The results should be similar. Scientists can
also test the IBI on more than one gradient of human disturbance. For
example, the scientists may first develop the IBI with a gradient such
as the percent of a watershed that is logged. During subsequent years,
they could test the same IBI across another gradient of human disturbance,
such as percent of watershed with impervious surfaces or distance of wetlands
to nearest road or farm field. Some metrics will consistently show clear
patterns regardless of the type of human disturbance used on the X axis.
After testing and validating the index, they could directly measure the
health of similar wetlands without having to measure every attribute.
They would only have to measure the ten metrics and some basic chemical
and physical characteristics of the wetlands to help diagnose the type
of stressor(s) damaging wetlands and to develop plans to reduce the impacts.
When reporting results of a bioassessment, the IBI score should always
be accompanied by a narrative description of the overall site condition,
scores of the individual metrics, and a narrative descriptions of each
metric as compared to conditions found in reference wetlands of the same
type and region.
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