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Classification of Macroinvertebrates
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In addition to richness (e.g., total number of taxa) and composition metrics (% mayflies), macroinvertebrates can also be classified according to:
- functional feeding groups (e.g., % Filterers, % Grazers and Scrapers)
- habit/behavior characteristics (e.g., Number of Clinger Taxa or % Clingers). , and
- tolerance levels (e.g., % intolerant taxa)
Functional Feeding Groups
Functional feeding groupsprovide a better understanding of food web relationships (see below). They include:
Beetle larve
- Predator – feed on other invertebrates (e.g., dragonflies)
- Omnivore – generalist feeders able to feed on both dead and living organic matter (e.g., crayfish)
- Collector – consume fine pieces of organic matter (e.g., leaf
fragments or other material on the stream bottom). This group can be
further divided into:
- Filtering Collectors (or Suspension feeders)– (e.g., clams)
- Gathering Collectors – (e.g., caddisfly larvae)
- Scraper/Grazer – feed on attached periphyton located on submerged underwater surfaces (e.g., snails)
- Shredder – consume coarse organic matter such as leaves (e.g., sowbugs)
- Piercer – feed by piercing the tissues of other organisms (true bugs - hemiptera)
Beetle larve
Feeding measures or trophic dynamics encompass functional feeding groups and provide information on the balance of feeding strategies (food acquisition and morphology) in the benthic assemblage. Examples involve the feeding orientation of scrapers, shredders, gatherers, filterers, and predators. Trophic dynamics (food types) are also included here and include the relative abundance of herbivores, carnivores, omnivores, and detritivores. Without relatively stable food dynamics, an imbalance in functional feeding groups will result, reflecting stressed conditions. Trophic metrics are surrogates of complex processes such as trophic interaction, production, and food source availability (Karr et al. 1986, Cummins et al. 1989, Plafkin et al. 1989). Specialized feeders, such as scrapers, piercers, and shredders, are the more sensitive organisms and are thought to be well represented in healthy streams. Generalists, such as collectors and filterers, have a broader range of acceptable food materials than specialists (Cummins and Klug 1979), and thus are more tolerant to pollution that might alter availability of certain food. However, filter feeders are also thought to be sensitive in low-gradient streams (Wallace et al. 1977). The usefulness of functional feeding measures for benthic macroinvertebrates has not been well demonstrated. Difficulties with the proper assignment to functional feeding groups has contributed to the inability to consider these reliable metrics (Karr and Chu 1997). [See the example indicators below].
| Metric | Definition | Predicted response to increasing perturbation |
|---|---|---|
| % Grazers and Scrapers | Percent of the macrobenthos that scrape or graze upon periphyton | Decrease |
| % Omnivores and Scavengers | Percent of generalists in feeding strategies | Increase |
| % Ind. Gatherers and Filterers | Percent of collector feeders of CPOM and FPOM | Variable |
| % Gatherers | Percent of the macrobenthos that "gather" | Variable |
| % Predators | Percent of the predator functional feeding group. Can be made restrictive to exclude omnivores | Variable |
| % Shredders | Percent of the macrobenthos that "shreds" leaf litter | Decrease |
Habitat and Behavior Designations
Habitat and behavior designations detail the functionality of the organism (e.g., the way it moves or searches for food). Habitat designations include:
- Clinger – able to remain stationery on bottom substrates in flowing waters
- Climber – feed in submerged aquatic vegetation (SAV) by climbing
- Sprawler – can be found on both the surface of SAV and substrates
- Burrower – feed on fine organic matter while buried in sediments of lakes and streams
- Swimmer – can control the direction and velocity of their movementsDiver - able to swim from the surface to the bottom of the water column
Habit measures are those that denote the mode of existence among the benthic macroinvertebrates. Morphological adaptation among the macroinvertebrate distinguishes the various mechanisms for maintaining position and moving about in the aquatic environment (Merritt et al. 1996). Habit categories include movement and positioning mechanisms such as skaters, planktonic, divers, swimmers, clingers, sprawlers, climbers, burrowers. Merritt et al. (1996) provide an overview of the habit of aquatic insects, which are the primary organisms used in these measures. Habit measures have been found to be more robust than functional feeding groups in some instances (Fore et al. 1996).
| Metric | Definition | Predicted response to increasing perturbation |
|---|---|---|
| Number of Clinger Taxa | Number of taxa of insects | Decrease |
| % Clingers | Percent of insects having fixed retreats or adaptations for attachment to surfaces in flowing water. | Decrease |
Tolerance/Intolerance measures
Tolerance levels refer to the organisms ability to tolerate various forms of stress such as low dissolved oxygen levels, high amounts of siltation or salinity, or varying amounts of toxic chemicals. Tolerance levels can be emperically derived by observation of the occurrence or predominance of certain taxa in specific water quality conditions.
Tolerance/Intolerance measures are intended to be representative of relative sensitivity to perturbation and may include numbers of pollution tolerant and intolerant taxa or percent composition (Barbour et al. 1995). Tolerance is generally non-specific to the type of stressor. However, some metrics such as the Hilsenhoff Biotic Index (HBI) (Hilsenhoff 1987, 1988) are oriented toward detection of organic pollution; the Biotic Condition Index (Winget and Mangum 1979) is useful for evaluating sedimentation. The Florida Index (Ross and Jones 1979) is a weighted sum of intolerant taxa (insects and crustaceans) found at a site (Beck 1965) and functions similarly to the HBI (Hilsenhoff 1987) used in other parts of the country. The tolerance/intolerance measures can be independent of taxonomy or can be specifically tailored to taxa that are associated with pollution tolerances. For example, both the percent of Hydropsychidae to total Trichoptera and percent Baetidae to total Ephemeroptera are estimates of evenness within these insect orders that generally are considered to be sensitive to pollution. As these families (i.e., Hydropsychidae and Baetidae) increase in relative abundance, effects of pollution (usually organic) also increase. Density (number of individuals per some unit of area) is a universal measure used in all kinds of biological studies. Density can be classified with the trophic measures because it is an element of production; however, it is difficult to interpret because it requires careful quantification and is not monotonic in its response (i.e., density can either decrease or increase in response to pollution) and is usually linked to tolerance measures. [See the example indicators below]
| Metric | Definition | Predicted response to increasing perturbation |
|---|---|---|
| No. of Intolerant Taxa | Taxa richness of those organisms considered to be sensitive to perturbation | Decrease |
| % Tolerant Organisms | Percent of macrobenthos considered to be tolerant of various types of perturbation | Increase |
| % Dominant Taxon | Measures the dominance of the single most abundant taxon. Can be calculated as dominant 2, 3, 4, or 5 taxa. | Increase |
| No. Intol. Snail and Mussel species | Number of species of molluscs generally thought to be pollution intolerant | Decrease |
| % Sediment Tolerant organisms | Percent of infaunal macrobenthos tolerant of perturbation | Increase |
| Hilsenhoff Biotic Index | Uses tolerance values to weight abundance in an estimate of overall pollution. Originally designed to evaluate organic pollution | Increase |
| Florida Index | Weighted sum of intolerant taxa, which are classed as 1 (least tolerant) or 2 (intolerant). Florida Index = 2 X Class 1 taxa + Class 2 taxa | Decrease |
| % Hydropsychidae to Trichoptera | Relative abundance of pollution tolerant caddisflies (metric could also be regarded as a composition measure) | Increase |
For more information, please see http://www.epa.gov/owowwtr1/monitoring/rbp/app_b-1.html. Most of the information on this page was taken directly, or modified, from http://www.epa.gov/owow/monitoring/rbp/ch07b.html#Section%207.4]