Bugs are good indicators for the health of a stream
Why do we look towards stream bugs for indications of stream health?
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.
How does King County's program work?
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.
Below are descriptions of the ten metrics tested and developed for the Pacific Northwest.
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.
Mayfly (Ephemeroptera) 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.
Stonefly (Plecoptera) 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.
Caddisfly (Trichoptera) 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.
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.
Clinger taxa richness. 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.
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.
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.
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.
Percent dominance (3 taxa). 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 of individuals collected in the sample.