The B-IBI scoring system is a quantitative method for determining and comparing the
biological condition of streams. We currently use the Puget Sound Lowlands B-IBI,
which can be calculated three different ways based on the taxonomic resolution of
macroinvertebrate data: Species-Family, Species-Genus, and Family.
Each of the B-IBI scoring methods is composed of the 10 metrics below, except for
the "Family" version which uses only 5 of the metrics. Each individual metric is
given a score of 1 through 5, with higher numbers given to conditions representative
of streams unaltered by anthropogenic influence.
These metrics are then added together for the single, integrated overall B-IBI score.
The overall B-IBI score is associated with one of the following biological condition
categories. Details on the condition value ranges for each scoring methods and their
metric can be found here.
B-IBI Biological Condition Categories
Modified from Karr et al. (1986) by Morley (2000).
| Biological Condition | Description | 10‑50 B‑IBI | 0‑100 B‑IBI |
| Excellent | Comparable to least disturbed reference condition; overall high taxa diversity, particularly of mayflies, stoneflies, caddis flies, long-lived, clinger, and intolerant taxa. Relative abundance of predators high. | [46, 50] | [80, 100] |
| Excellent/Good |
| (44, 46) | |
| Good | Slightly divergent from least disturbed condition; absence of some long-lived and intolerant taxa; slight decline in richness of mayflies, stoneflies, and caddis flies; proportion of tolerant taxa increases | [38, 44] | [60, 80) |
| Good/Fair |
| (36, 38) | |
| Fair | Total taxa richness reduced - particularly intolerant, long-lived, stonefly, and clinger taxa; relative abundance of predators declines; proportion of tolerant taxa continues to increase | [28, 36] | [40, 60) |
| Fair/Poor |
| (26, 28) | |
| Poor | Overall taxa diversity depressed; proportion of predators greatly reduced as is long-lived taxa richness; few stoneflies or intolerant taxa present; dominance by three most abundant taxa often very high | [18, 26] | [20, 40) |
| Poor/Very Poor |
| (16, 18) | |
| Very Poor | Overall taxa diversity very low and dominated by a few highly tolerant taxa; mayfly, stonefly, caddis fly, clinger, long-lived, and intolerant taxa largely absent; relative abundance of predators very low | [10, 16] | [0, 20) |
Learn about the
bracket notation we're using.
Description of Metrics
Unlike the B-IBI, biological conditions are not explicitly associated with individual metrics. Nevertheless, throughout the site you will see metric scores color coded like the B-IBI. This allows us to compare, at a glance, how a particular metric contributes to the overall B-IBI, as well as how the metric is varying across a region or across time.
- Total Taxa Richness
- Ephemeroptera (Mayfly) Taxa Richness
- Plecoptera (Stonefly) Taxa Richness
- Trichoptera (Caddisfly) Taxa Richness
- Intolerant Taxa Richness
- Clinger Taxa Richness and Percent
- Long-Lived Taxa Richness
- Percent Tolerant *
- Percent Predator
- Percent Dominance
Total Taxa Richness
The biodiversity of a stream declines as flow regimes are altered, habitat is lost,
chemicals are introduced, energy cycles are disrupted, and alien taxa invade. Total
taxa richness includes all the different invertebrates collected from a stream site:
mayflies, caddisflies, stoneflies, true flies, midges, clams, snails, and worms.
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Ephemeroptera (Mayfly) Taxa Richness
The diversity of mayflies declines in response to most types of human influence.
Many mayflies graze on algae and are particularly sensitive to chemical pollution
(e.g., from mine tailings) that interferes with their food source. Mayflies may
disappear when heavy metal concentrations are high while caddisflies and stoneflies
are unaffected. In nutrient-poor streams, livestock feces and fertilizers from agriculture
can increase the numbers and types of mayflies present. If many different taxa of
mayflies are found while the variety of stoneflies and caddisflies is low, enrichment
may be the cause.
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Plecoptera (Stonefly) Taxa Richness
Stoneflies are the first to disappear from a stream as human disturbance increases.
Many stoneflies are predators that stalk their prey and hide around and between
rocks. Hiding places between rocks are lost as sediment washes into a stream. Many
stoneflies are shredders and feed on leaf litter that drops from an overhanging
tree canopy. Most stoneflies, like salmonids, require cool water temperatures and
high oxygen to complete their life cycles.
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Trichoptera (Caddisfly) Taxa Richness
Different caddisfly species (or taxa) feed in a variety of ways: some spin nets
to trap food, others collect or scrape food on top of exposed rocks. Many caddisflies
build gravel or wood cases to protect them from predators; others are predators
themselves. Even though they are very diverse in habit, taxa richness of caddisflies
declines steadily as humans eliminate the variety and complexity of their stream
habitat.
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Intolerant Taxa Richness
Animals identified as intolerant are the most sensitive taxa; they represent approximately
5-10% of the taxa present in the region. These animals are the first to disappear
as human disturbance increases.
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Clinger Taxa Richness and Percent
Taxa defined as clingers have physical adaptations that allow them to hold onto
smooth substrates in fast water. These animals typically occupy the open area between
rocks and cobble along the bottom of the stream. Thus they are particularly sensitive
to fine sediments that fill these spaces and eliminate the variety and complexity
of these small habitats. Clingers may use these areas to forage, escape from predators,
or lay their eggs. Sediment also prevents clingers from moving down deeper into
the stream bed, or hyporheos, of the channel.
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Long-Lived (Semi-Voltine) Taxa Richness
These invertebrates require more than one year to complete their life cycles; thus,
they are exposed to all the human activities that influence the stream throughout
one or more years. If the stream is dry part of the year or subject to flooding,
these animals may disappear. Loss of long-lived taxa may also indicate an on-going
problem that repeatedly interrupts their life cycles.
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Percent Tolerant
Tolerant animals are present at most stream sites, but as disturbance increases,
they represent an increasingly large percentage of the assemblage. Invertebrates
designated as tolerant represent the 5-10% most tolerant taxa in a region. In a
sense, they occupy the opposite end of the spectrum from intolerant taxa.
Change Notices
On September 1, 2012, we changed ways of calculating this metric.
In our source material, percent tolerance is stated to be "The total number
of tolerant individuals counted in each replicate, divided by the total number of
individuals in that replicate, multiplied by 100. Chironomids are not included in
this metric." There are two different ways to interpret 'not including chironomids'.
For example, if we had a sample with 500 organisms, 50 of which were 'tolerant'
and 250 of which were chironomids (none of which are classified as 'tolerant'),
percent tolerance could be calculated two ways:
1) 50 tolerant organisms/500 total organisms = 10% or
2) 50 tolerant organism/(500 total organisms - 250 chironomids) or 50/250 = 20%.
This website previously had been calculating tolerance the second way; however Leska Fore who
was part of the 10-50 B-IBI Puget Lowland calibration team has checked her notes
from the 1990s and has confirmed that we should be making the first calculation
and keeping chironomids in the denominator for the total number of organisms.
In many cases, this will increase the tolerant score (low percentages get high scores).
At a maximum, the tolerant score could go from 1 to 5, and thus slightly increase
the overall B-IBI scores by 4 points.
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Percent Predator
Predator taxa represent the peak of the food web and depend on a reliable source
of other invertebrates that they can eat. Predators may have adaptations such as
large eyes and long legs for hunting and catching other animals. The percentage
of animals that are obligate predators provides a measure of the trophic complexity
supported by a site. Less disturbed sites support a greater diversity of prey items
and a variety of habitats in which to find them.
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Percent Dominance
As diversity declines, a few taxa come to dominate the assemblage. Opportunistic
species that are less particular about where they live replace species that require
special foods or particular types of physical habitat. Dominance is calculated by
adding the number of individuals in the three most abundant taxa and dividing by
the total number individuals in the sample.
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