Water: Monitoring & Assessment
Wetland Bioassessment Basics
- What are biological assessments?
- What is biological integrity?
- What is an Index of Biological Integrity (IBI)?
- How is an IBI developed for wetlands?
- What other methods are used in biological assessments?
- What is the difference between biological assessments and functional assessments?
What are biological assessments?
Below are photographs of two wetlands. The one on the right has obviously been damaged by human activities. The one on the left looks like it is healthier. But is it really healthy? Some human activities can damage a wetland without leaving clearly visible signs. For example, some herbicides and pesticides kill tadpoles, dragonflies, and other wetland creatures. How can we measure the health of a wetland and be sure that it really is healthy?
To evaluate the biological integrity or "health" of wetlands, scientists participating in BAWWG are developing biological assessment (bioassessment) methods. To conduct a bioassessment, a scientist visits a wetland and collects information about the number of different kinds of organisms and what types of organisms are living there. The scientist also collects information about the habitat quality, water level, and chemistry to support the biological information. The scientist then analyzes the information and compares the wetland's biological information to reference conditions. This is similar to the way a human doctor would collect information about a patient, such as blood pressure and temperature, and compare it to a known range of condition. If the values are too high or too low, then the doctor knows that the patient is sick. Similarly, the wetland scientist can compare the biological information to a known range of conditions and determine the health of the wetland.
Bioassessments are based on the premise that the community of plants and animals living in a wetland will reflect the health of a wetland. When a wetland is damaged, the diversity of animals and plants often decreases and the composition of species changes. Typically, the organisms that are intolerant to human disturbances die and organisms that are more tolerant to the disturbance make up a larger proportion of the individuals. For example, the farmed wetland shown above will most likely have fewer kinds of plants and animals than the healthier wetland and will be dominated by organisms that can tolerate poor environmental conditions. After examining an assemblage of plants or animals in wetlands ranging from high quality to poor quality, scientists can use this known range as a measuring stick to estimate the relative health of other wetlands. The bioassessment results will show if a wetland is damaged in any way.
Organism assemblages used in wetland bioassessment
What is biological integrity?
The goal of the Clean Water Act is to maintain and restore the chemical, physical, and biological integrity of the Nation's waters, including wetlands. But what is biological integrity?
Biological integrity is commonly defined as "the ability to support and maintain a balanced, integrated, and adaptive community of organisms having a species composition, diversity and functional organization comparable to those of natural habitats within a region" (Karr, J. R. and D. R. Dudley. 1981. Ecological perspectives on water quality goals. Environmental Management 5: 55-68).
A more detailed discussion of biological integrity can be found on the Key Concepts for Using Watershed Biological Indicators page of the Biological Indicators of Watershed Health site.
What is an Index of Biological Integrity (IBI)?
Most wetland biological assessment (bioassessment) projects use an index of biological integrity (IBI) to evaluate the health of wetlands. An IBI is somewhat similar to the economic indicators used to evaluate the condition of our economy. An IBI combines multiple indicators of biological condition, called metrics, into an easy-to-understand index value. The value can be compared to reference values and help managers assess the relative health of individual wetlands. The IBI concept was originally developed by stream ecologists to assess the biological condition of streams (see Key Concepts for Using Watershed Biological Indicators). Although sampling methods and metrics are different for wetlands than for streams, the general IBI framework can be adapted for wetlands as well as lakes, estuaries, and terrestrial systems.
Some states--Maine, for example--use multivariate statistics instead of IBIs.
How is an IBI developed for wetlands?
The first step in creating an IBI is for researchers to classify wetlands and select reference sites. Wetlands occur in many climatic, hydrologic, and landscape conditions; as a result, the community composition and diversity of an assemblage will naturally vary between wetland types. The goal is to group wetlands such that the diversity and composition of an assemblage within groups is minimized and the variation between groups is maximized.
Bioassessment projects typically group wetlands by using one or more existing classification systems, such as ecoregions or hydrogeomorphic (HGM) classes. Classification is an iterative process. Researchers often start with one or more methods and then lump or split as needed to end up with an appropriate number of groups of biologically distinct wetlands. For example, when the Montana Department of Environmental Quality developed its IBI, the state used ecoregions as a first tier and then further separated wetlands by landscape position and other characteristics, such as acidity and salinity. The state later found that it could lump the wetlands of two ecoregions because of their biological similarity.
After classifying wetlands, researchers select sampling sites across a gradient of human disturbance for each wetland group. Because no standard gradient of human disturbance yet exists, some projects have tried to quantify disturbance by using surrogates such as the percentage of impervious surfaces or agriculture in a watershed. Other projects have tried using qualitative indices of human disturbance that incorporate a combination of watershed and wetland information. Regardless of the gradient used, it is crucial to select wetlands that range from minimally impaired reference sites to severely degraded wetlands, and everything in between. Hand picking sites is most appropriate for developing bioassessment methods because random samples often fail to provide sufficient range in condition.
Once sites are selected, sampling can begin. Typically, researchers sample at least two assemblages—commonly this can include algae, amphibians, birds, fish, macroinvertebrates, and vascular plants. An IBI is developed independently for each assemblage. Researchers then measure many biological attributes of each assemblage (e.g., number of individuals, number of taxa) in search of attributes that show clear patterns of response to human disturbance. Most attributes will form scatter plots when plotted against a human disturbance gradient. For example, abundance, density, and production attributes typically form shotgun patterns because they are naturally variable. However, some attributes will show clear patterns, these are the metrics. Figures 1 through 3 are examples of attributes that Ohio EPA might use as metrics (see the Case Study for Ohio Environmental Protection Agency). Richness metrics (e.g., number of taxa) and relative abundance metrics (e.g., number of tolerant individual/total number of individuals) are often the most dependable. Ratios or sums of attributes (e.g., number of midges/number of dragonflies; mayflies + caddisflies + dragonflies) can hide valuable signals, be difficult to interpret, and are often more variable than the individual attributes (Karr, J.R., and E.W. Chu. 1999. Restoring Life in Running Waters: Better Biological Monitoring. Island Press, Washington, D.C.).
After graphically and statistically analyzing the data, researchers select 8–12 metrics that show empirical and predictable changes along a gradient of human disturbance. A well-constructed IBI will contain a number of metrics sensitive to chemical alterations, such as nutrient enrichment, and others sensitive to physical or biological alterations, such as the introduction of exotic species.
One approach of combining metrics into an IBI is to assign scores of 1, 3, and 5. For example, a metric with a low score would indicate degraded condition and a metric with a high score would indicate more pristine condition. If ten metrics are scored in this manner and then added together, then the resulting IBI would range from 10 (severely impaired) to 50 (minimally impaired). The IBI score should form a relatively straight line when plotted against a gradient of human disturbance. Wetlands that fall far from the line should be investigated. These outliers are often the result of either: (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.
Perhaps the greatest benefit of an IBI is that it summarizes and presents complex biological information in a format that is easily communicated to managers and the public. Most people can relate more easily with plant and animal IBIs than with complex statistical calculations or some of the more abstract chemical and physical wetland functions. While an IBI score is helpful for quickly communicating the overall condition of a wetland, most of the valuable information lies in the individual metrics. When reporting bioassessment results, the IBI score should be accompanied by (1) a narrative description of overall biotic condition in comparison to reference wetlands of the same region and wetland type, (2) numeric values of each metric, and (3) narrative descriptions of each metric in comparison to reference conditions of the same region and wetland type.
What other methods are used in biological assessments?
In addition to IBIs, some states are using advanced statistics in the biological assessments. Maine DEP uses multivariate statistics to evaluate the condition of the state's streams and rivers. Maine DEP recently started a pilot project in the Casco Bay Watershed to develop biological assessment methods utilizing multivariate statistics. In addition, advanced statistics have been commonly used to evaluate algal communities. Maine DEP and Montana Department of Environmental Quality both use statistical methods to examine the algal assemblage in their wetlands.
What is the difference between biological assessments and functional assessments?
Functional assessments, such as the Hydrogeomorphic Approach (HGMA), are designed to estimate the functions that wetlands provide, such as water storage, nutrient cycling, and wildlife habitat. While bioassessments are designed to evaluate wetland health, functional assessments are primarily designed to inform management decisions involving the dredge and fill of wetlands and restoring wetlands to compensate for wetland losses. Functional assessments tend to focus on the physical structure and habitat features of a wetland.
Used independently, functional assessments are not appropriate for estimating wetland health because they do not adequately evaluate the condition of wetland biological communities. In addition, functional assessments will not detect damage to wetlands caused by many subtle stressors, such as toxic chemicals. Some bioassessment projects, however, use some form of functional assessments to gather habitat information. This information is very valuable to confirm that a wetland is classified properly and to help identify potential stressors damaging wetlands. Wetland Bioassessment Fact Sheet 6, compares bioassessments and HGM.
Additional information about wetland bioassessment methods is found on the Bioassessment Methods page.