Aquatic invertebrates perform a variety of functions in a stream, based on their feeding habits. The descriptions below are general, which means there are always exceptions to the rule. In headwater streams (orders 1-3), shredders and collectors co-dominate the invertebrate world. In medium-sized streams (orders 4-6), collectors and grazers rule. In large rivers (orders >6), collector aquatic invertebrates (such as gatherers) dominate the scene. There are also aquatic invertebrates that are predators, parasites and omnivores.

Benthic macroinvertebrates are immobile for most of their lives. In their larva and pupa stages, they are stuck underwater, between a rock and a hard place or on the bottom of the stream or river or lake. They only get to fly and mate as adults the last few days of their lives. But as aquatic bugs, they need a place to hang on and hide out, where they can find their favorite food nearby.

Shredders and collectors co-dominate the invertebrate world. Except... There is always an exception. There are more grazers and very few shredders in riffle samples of macroinvertebrates in streams west of the Cascades in Washington. And that is where data collection protocols recommend sampling. But, along the margins and in leaf packs of the streams, there may be more shredders.
stonefly Photo: RN	Shredders process leaves and particulates greater than 1 mm, breaking them into smaller pieces. Habitat: Shredders take advantage of leaves, sticks, and twigs. Areas that can trap and hold this organic matter are attractive to these species.
blackfly Photo: XXX	Collectors ( filterers and gatherers) use food particles smaller than 1 mm. Filterers, like blackflies, are couch potatoes: they sit around and wait for food to come to them. Habitat: Collector-filtering invertebrates need large, stable rocks or logs to hang onto while they wait for food suspended in the water to reach them.
caddisflies Photo: XXX	Gatherers, like some caddisflies, actively seek their supper. Habitat: Collector-gathering invertebrates live in streams with medium-large rocks. Organic plant and animal material that collects among the stream substrate provide their food.
mayfly Photo: BC	Collectors and grazers eat algae that grow on medium-large flat rock surfaces. Portions of streams that have sunlight filtering through will likely have some algal growth due to sunlight penetration. Habitat: Medium sized streams (order 4-6). More sunlight gets in, the water warms up and increased nutrients encourage the growth of algae.