There are several good reasons bugs are such a cool way to tell how healthy a stream is. For one thing, they are relatively easy and inexpensive to collect. But there's more...
Nutrient Cycle. Bugs play a crucial role in the stream nutrient cycle. If bug populations are suffering it will affect the whole ecosystem.
Pollution Tolerance. Some insects are tolerant of pollution, whereas others are not. The presence or absence of tolerant and intolerant types can indicate the condition of the stream. For example, the order Plecoptera, or Stoneflies, are very sensitive to pollution, so their absence in a stream can signal a problem.
Population Fluctuations. Because many bug life cycles are short (sometimes one season in length), we can detect population fluctuations in a short period of time. Population fluctuations might indicate that a change (positive or negative) may have occurred in the stream.
When we want to monitor a stream, we choose a sampling location typical of the stream in general. Using a Surber sampling net, we collect the bugs on the stream bottom. Then we preserve the bugs in an alcohol solution and send the preserved critters to a lab where they are identified and counted.
King County uses a method called the Benthic Index of Biological Integrity, or B-IBI, as a "report card" for measuring the health of the benthic bug community and for the stream ecosystem as a whole. The B-IBI is composed of ten "metrics." Metrics measure different aspects of stream biology, including the diversity of bug species, number of bugs, presence of bugs that are tolerant and intolerant to pollution, reproductive strategy, feeding ecology, and population structure. Using a mathematical calculation, we derive a raw value for each metric from which a value of 1, 3, or 5 is assigned. We use a score of 5 to indicate little or no degradation, a score of 3 to indicate moderate degradation, and a score of 1 to indicate severe degradation. All ten metric scores are added together to get a value ranging from 10 to 50.
The ten metrics tested and developed for the Pacific Northwest are listed below. Click on a metric for its description.
