The vast majority of stream-dwelling macroinvertebrates live in the riffle areas. Although riffles in a stream may be the result of anything from an uneven bedrock bottom to an aggregation of large boulders, the optimum habitat for macroinvertebrates is a riffle uniformly composed of moderately-sized particles ranging from 10-inch cobbles down to 1-inch gravel. Riffles composed of much smaller or much larger particles provide far fewer living quarters for these organisms. In addition, the constant flow of water that the riffle areas provide offers a continuous and plentiful supply of food in the form of plant and animal matter.
The feeding habits of macroinvertebrates can be divided into four basic categories: shredders, collectors, grazers and predators. The shredders, such as many stoneflies, feed on larger, mostly dead plant materials (such as fallen leaves) called "detritus." These organisms shred the detritus into smaller particles during the feeding process, making it available for the collectors. The collectors, such as most caddisflies and blackflies, feed on the fine organic materials by filtering it from the water and gathering it from the stream bed.
The grazers, such as snails and beetles, roam about the stream bed scraping algae and other organisms from stone and plant surfaces. The predators, represented by damselfly and dragonfly larvae as well as hellgrammites, attack other living organisms and engulf their prey whole or in parts. It is important to understand this interaction between macroinvertebrates to fully realize how the stability of one taxon can be dependent upon the welfare of another. Because of this interdependence, the greater the diversity of organisms, the better the water quality.
Riffle-dwelling macroinvertebrates generally require an environment that has a plentiful supply of oxygen and is free of toxic pollutants, although each is species-specific in its tolerance to low oxygen levels and toxic substances. Generally, most are mildly to highly sensitive to such adverse conditions, with only a small percentage surviving severe oxygen depletion or high levels of toxic substances.
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