Multimetric Analysis: How B-IBIs work
multivariate analysis

How it works * Calculating B-IBIs * Using B-IBI info

Oregon B-IBI metric scoring sheets (.pdf)

How does the B-IBI work?

Before a B-IBI can be calculated for a stream, the index needs to be adapted for the watershed or region. Adapting the index is often carried out by state agencies, universities, or macroinvertebrate consultants, but you don't have to be a professional to to it. There are three major steps in adapting the B-IBI.

1. attribute testing
2. metric selection
3. scoring criteria development

Unless you're starting from scratch and able to develop scoring criteria, you don't necessarily need to be involved in the first three steps of this process. Once scoring criteria are available, then all you have to do is calculate the metric values for your stream and calculate a B-IBI score from those values.

Attribute testing
When you collect macroinvertebrates, you're collecting an assemblage that represents the entire macroinvertebrate community. Different characteristics of that assemblage (total abundance, number of mayfly taxa, relative abundance of the taxon with the most individuals) will respond differently to changes in human influence. Only those that respond predictably can be included in a multimetric B-IBI.

The process of determining which characteristics respond predictably is called attribute testing. For instance, the graph on the left (click to enlarge) shows the relationship between the total number of organisms collected (abundance) and percent impervious area (a surrogate for human influence). You might think that when conditions are poorer, fewer organisms can live in a stream, but the truth is that macroinvertebrates can be either abundant or scarce in the best of streams or in the most degraded of streams. Because no clear pattern of response is revealed, this attribute cannot be considered a metric.

Clingers are organisms adapted to hanging on in an environment of flowing water. We might expect that clingers would be adversely affected by changes in flows, sedimentation, or growth of slimy green algae. The graph to the right shows that the community attribute clinger taxa richness responds predictably to increasing human disturbance. The graph shows that we can be confident that poorer conditions in the stream mean fewer clinger taxa, i.e. clingers respond predictably to increasing human influence, and can therefore be called a metric.

You may notice that the data points do not exactly follow the line. What matters in the development or adaptation of a B-IBI is that you can see a distinct pattern. Usually, more of the spread is related to our inability to accurately measure human influence than our ability to measure the biological characteristics. Some streams may have urban growth scattered throughout, but be well buffered by intact riparian zones. Other streams may have an intact watershed but two or three point source impacts in the headwaters. In both cases, the biological condition of the stream will not be what you might expect from percent impervious area as a sole measure of human influence.

Metric selection
B-IBIs are generally based on 10 or 12 metrics, and most developed in the Pacific Northwest have been developed for 10. Metrics used by the Oregon DEQ and outlined in the Oregon Plan Water Quality Monitoring Guidebook include:

• Taxa Richness - This is the total number of invertebrate taxa identified from the sample.
• Mayfly Richness - This is the total number of mayfly taxa identified from the sample.
• Stonefly Richness - This is the total number of stonefly taxa identified from the sample.
• Caddisfly Richness - This is the total number of caddisfly taxa identified from the sample.
• Sensitive Taxa - This is the number of taxa identified that are known to be very sensitive to stream disturbance. The list of taxa that qualify as "sensitive" are listed in Appendix F of the Oregon Plan's Water Quality Monitoring Guidebook.
• Sediment Sensitive Taxa - Some taxa are known to be very sensitive to inputs of fine sediment. The presence of one or more of these taxa indicate that fine sediments are probably not a major concern.
• Modified HBI - "HBI" stands for Hilsenhof Biotic Index. This is an index of a taxon's sensitivity to organic enrichment that typically occurs as a result of excessive nutrient inputs. Index values for individual taxa range from 1 to 10. Low scores indicate high sensitivity (found only in waters with low organic enrichment). High scores indicate low sensitivity (tolerant of waters with high organic enrichment). HBI index values for each taxa are listed in the taxa list for Oregon streams in Appendix F of the Oregon Plan's Water Quality Monitoring Guidebook.
• % Tolerant Taxa - This is the percent of the invertebrate community made up of taxa tolerant to disturbance. Taxa counted as "tolerant" taxa are listed in Appendix F of the Oregon Plan's Water Quality Monitoring Guidebook. Divide the abundance of tolerant organisms by the total number of organisms sorted from the sample, and multiply by 100.
• % Sediment Tolerant Taxa - This is the percent of the invertebrate community made up of taxa tolerant to fine sediments. Divide the abundance of sediment tolerant taxa by the total number of organisms sorted from the sample, and multiply by 100.
• % Dominant (single taxon) - This is the total abundance of the single most abundant taxon in the sample divided by the total number of organisms sorted from the sample, multiplied by 100. A high percent of a single taxon indicates some disturbance has likely occurred to the invertebrate community.

Two other metrics commonly used in the Pacific Northwest are:

• Clinger richness - This is the total number of taxa identified from the sample that are known to be specially adapted to hanging onto substrates in flowing waters.
• % Predators - This is the percent of the invertebrate community made up of taxa that prey on other animals (mostly other macroinvertebrates but some also feed on small amphibians and fish). Divide the abundance of predators by the total number of organisms sorted from the sample, and multiply by 100.

Scoring criteria development
Most metrics respond linearly to changes in human influence and can just be divided roughly into thirds with the poorest sites receiving a score of 1, the middle sites receiving a score of 3, and the reference sites and other healthy streams receiving a score of 5. In the case of the clingers, streams where 0 to 8 clinger taxa were found receive a score of 1. When 9 to 17 clinger taxa were collected, the stream receives a value of 3, and where more than 18 were found, the stream receives the highest value of 5 for the clinger metric.

Sometimes a metric will respond in a nonlinear fashion. One example is when a metric value is high at reference streams, drops very rapidly, then remains low through much of the range of human disturbance. When a metric response is clearly nonlinear, natural break points in the graph should be used for assigning scoring criteria.

Once scoring criteria have been adapted, they can be tested against an independent data set to make sure they accurately reflect the effects of human activity on the biological condition of streams in that region.

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