10.6. Functional feeding groups
Although aquatic insects are used widely in the context of applied ecology (section 10.5) it may not be possible, necessary, or even instructive, to make detailed species- level identifications. Sometimes the taxonomic framework is inadequate to allow identification to this level, or time and effort do not permit resolution. In most aquatic entomological studies there is a necessary trade-off between maximizing ecological information and reducing identification time. Two solutions to this dilemma involve summary by subsuming taxa into (i) more readily identified higher taxa (e.g. families, genera), or (ii) functional groupings based on feeding mechanisms (“functional feeding groups”).
The first strategy assumes that a higher taxonomic category summarizes a consistent ecology or behavior amongst all member species, and indeed this is evident from some of the broad summary responses noted above. However, many closely related taxa diverge in their ecologies, and higher-level aggregates thus contain a diversity of responses. In contrast, functional groupings need make no taxonomic assumptions but use mouthpart morphology as a guide to categorizing feeding modes. The following categories are generally recognized, with some further subdivisions used by some workers:
- shredders feed on living or decomposing plant tissues, including wood, which they chew, mine, or gouge;
- collectors feed on fine particulate organic matter by filtering particles from suspension (see the chapter vignette of a filter-feeding black-fly larva of the Simulium vittatum complex with body twisted and cephalic feeding fans open) or fine detritus from sediment;
- scrapers feed on attached algae and diatoms by grazing solid surfaces;
- piercers feed on cell and tissue fluids from vascular plants or larger algae, by piercing and sucking the contents;
- predators feed on living animal tissues by engulfing and eating the whole or parts of animals, or piercing prey and sucking body fluids;
- parasites feed on living animal tissue as external or internal parasites of any stage of another organism.
Functional feeding groups traverse taxonomic ones; for example, the grouping “scrapers” includes some convergent larval mayflies, caddisflies, lepidopterans, and dipterans, and within Diptera there are examples of each functional feeding group.
One important ecological observation associated with such functional summary data is the often observed sequential downstream changes in proportions of functional feeding groups. This aspect of the river continuum concept relates the sources of energy inputs into the flowing aquatic system to its inhabitants. In riparian tree-shaded headwaters where light is low, photosynthesis is restricted and energy derives from high inputs of allochthonous materials (leaves, wood, etc.). Here, shredders such as some stoneflies and caddisflies tend to predominate, because they can break up large matter into finer particles. Further downstream, collectors such as larval black flies (Simuliidae) and hydropsychid caddisflies filter the fine particles generated upstream and themselves add particles (feces) to the current. Where the waterway becomes broader with increased available light allowing photosynthesis in the mid-reaches, algae and diatoms (periphyton) develop and serve as food on hard substrates for scrapers, whereas macrophytes provide a resource for piercers. Predators tend only to track the localized abundance of food resources. There are morphological attributes broadly associated with each of these groups, as grazers in fast-flowing areas tend to be active, flattened, and current-resisting, compared with the sessile, clinging filterers; scrapers have characteristic robust, wedge-shaped mandibles.
Changes in functional groups associated with human activities include:
- reduction in shredders with loss of riparian habitat, and consequent reduction in autochthonous inputs;
- increase in scrapers with increased periphyton development resulting from enhanced light and nutrient entry;
- increase in filtering collectors below impoundments, such as dams and reservoirs, associated with increased fine particles in upstream standing waters.
