Puget Sound Stream Benthos

About the Benthic Index of Biotic Integrity (B-IBI)

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 ConditionDescription10‑50 B‑IBI0‑100 B‑IBI
ExcellentComparable 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]
(44, 46)
GoodSlightly 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)
(36, 38)
FairTotal 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)
(26, 28)
PoorOverall 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 PoorOverall 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.

  1. Total Taxa Richness
  2. Ephemeroptera (Mayfly) Taxa Richness
  3. Plecoptera (Stonefly) Taxa Richness
  4. Trichoptera (Caddisfly) Taxa Richness
  5. Intolerant Taxa Richness
  6. Clinger Taxa Richness and Percent
  7. Long-Lived Taxa Richness
  8. Percent Tolerant *
  9. Percent Predator
  10. 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. Return To Top

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. Return To Top

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. Return To Top

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. Return To Top

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. Return To Top

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. Return To Top

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. Return To Top

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.

Return To Top

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. Return To Top

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. Return To Top