stream ecology primer
physical
water cycle
watersheds
river continuum
basic hydrology
geomorphology
chemical
dissolved gases
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biological
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primary production
decomposition
nutrient dynamics
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autotrophs
macroinvertebrates
fish
 
 
  Macroinvertebrates
 

Much of the aquatic life in streams is composed of benthic macroinvertebrates. This large group of organisms includes clams, crayfish, worms, and insects. As a descriptive term, benthic macroinvertebrate refers to organisms without an internal skeleton, that are larger than 5 microns, and that dwell on the bottom or substrate of a body of water. As a group these organisms process and transform organic matter into sources of food for other organisms. As processors of organic matter benthic macroinvertebrates play a variety of roles that categorize them into functional groups. Shredders are known for consuming large organic particles such as leaves that fall into streams. Grazers and scrapers forage on benthic surfaces, eating the algae that grow there. As pieces of organic matter become smaller, are dislodged, or are converted into feces by invertebrates, they are captured by collectors that filter or trap fine particles as they drift downstream. Predators feed on other benthic macroinvertebrates. In most streams it is expected that areas of more diverse habitat will have a more diverse benthic macroinvertebrate community. Typically, pools and runs are inhabitated by actively filtering collectors such as clams and mayflies. Stationary, passive collectors such as blackflies or netspinning caddisflies are found in riffles where the current carries fine particulate organic matter to them. Wherever leaf litter collects, shredders are likely to be found. This may be in pools or in leaf packs trapped between rocks in riffles and rapids. Predators give clues to their specific habitat by their body form. Dragonfly larvae are suited to pools and slack water near streambanks, while predaceous beetle stonefly larvae inhabit riffles, runs, and rapids. In areas where periphyton thrives, scrapers such as snails and mayflies will be found grazing on substrate surfaces.

In some stream environments the macroinvertebrate community may include hundreds of species from many phyla. Most of these organisms are associated with some type of stable substrate, whether it is part of the channel (e.g., bedrock, cobble, finer sediment) or an obstruction or macrophyte (e.g., fallen trees, snags, roots, and aquatic vegetation) (Hauer, R. and V. Resh, 1996). As the link between organic matter (resulting from primary production) and fish, and because of their common and diverse presence in streams, macroinvertebrates are of key interest in the study of stream ecology and can be used as key indicators of stream habitat and water quality. Changes in physical habitat and water quality affect the distribution and diversity of macroinvertebrates in streams. Monitoring their diversity, abundance, and distribution can offer clues to changes that are occurring over time in streams and rivers.

Phylogeny and Adaptations to Lotic Environments
The most studied group of lotic macroinvertebrates is aquatic insects (Class Insecta). Thirteen orders of aquatic insects occur in North America (Merritt and Cummins, 1996), but only five of these orders are comprised exclusively of aquatic species: the dragonflies and damselflies (Odonata), the stoneflies (Plecoptera), the mayflies (Ephemeroptera), the caddisflies (Trichoptera), and the hellgrammites (Megaloptera). While the other eight orders are comprised mostly of terrestrial species, some of the orders do have a wide representation in aquatic habitats. This is especially true of the beetles (Coleoptera) and the true flies (Diptera) (Merritt and Cummins, 1996). Insects solved the challenges of living in running water many different times over the course of evolutionary time. Among the diverse orders of insects that inhabit running water, problems such as how to obtain oxygen or remain in a stationary position have been solved in a variety of ways. The nuances of insect adaptations to streams illustrate how specialized the habitat requirements of some insects can be. Oxygen is obtained through many different morphological and behavioral adaptations such as aerial tubes, trapping and carrying bubbles, respiratory pigments, highly developed tracheal gill structures, and movements designed to enhance gill aeration (stonefly "push-ups"). The challenge of maintaining position in the current has similarly been met by many different adaptations among insect orders. Silk threads (blackflies), streamlined exoskeletons, suckers, and leg and anal hooks (caddisfllies) all help insects remain stationary in a moving medium.

mayflyA burrowing mayfly with feather-like gills
caddisflyA free-living caddisfly with hooks on its anal prolegs.

The life history of lotic macroinvertebrates also shows adaptations that are specific to running waters environments, including permanently cool temperatures (Arctic or spring-fed streams) or intermittent flow (1st order or desert streams). Survival and reproduction are dependent on adaptations that allow inverbrates to successfully deal with environmental challenges. Examples of strategies that have evolved in answer to challenges presented by lotic environments include dormancy in eggs to withstand drought, timing reproduction to maximize growth opportunity, minimizing competition between trophically similar species through differences in life history events . Variation in the length of life cycles is also apparent in lotic macroinvertebrates even among closely related species. Life cycles may range from several per year (multivoltine) to one cycle per year (univoltine) to one cycle every two or three years (semivoltine). Geographic differences in life history can be seen across the range of a single species. A limnephilid caddisfly is univoltine in the coastal streams of California and Oregon, but semivoltine in the colder mountain streams of Montana (Hauer and Stanford, 1982b).

Hydrologic processes, food resources, nutrient dynamics, riparian vegetation, human disturbance, and other factors affect stream ecosystem structure and function (Allan, 1995). These factors not only differ among different rivers, but also longitudinally within a single watershed. The species composition of macroinvertebrates changes as the stream travels from headwaters to middle reaches to lower reaches (Vannote et al., 1980).

Species adapted to coarse particulate organic matter (shredders) occupy reaches of the stream with a well-developed canopy, while species adapted to fine particulate organic matter (collectors) are more common in reaches where photosynthesis is limited by turbidity. In reaches of the stream that support well-developed periphyton and macrophytes, grazers are more common. In essence, some species occur only in first and second order streams, and are replaced by other species in the middle reaches (3rd - 5th order), and still other species are found only in high order, large rivers. Even within similar functional groups species replacement occurs along the course of a river. For example, the burrowing mayflies found in headwaters reaches will be replaced by other burrowing mayfly species in the lower reaches of the river. The same phenomenon occurs in invertebrate groups other than the insects such as mollusks.



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