Aquatic Insects of Central Virginia: Functional Feeding Groups and the River Continuum

In discussing families of insects in previous entries I've mentioned the phrase "Functional Feeding Group" (FFG) on more than one occasion. Since this is an important ecological concept in the work that we do, I thought I might say a bit more. Combined with the idea of the "River Continuum," it helps us to understand which insects we're likely to find in different parts of rivers and streams.

Let me begin by noting that my information for writing this entry comes from two sources. They are:

1) an article by Richard W. Merritt and Kenneth W. Cummins entitled "Trophic Relationships of Macroinvertebrates," which can be found in F. Richard Hauer and Gary A. Lamberti, eds., Methods in Stream Ecology, Second Edition (Academic Press [Elsevier]: Burlington, MA and elsewhere, 2007), pp. 585-601.

and

2) Glenn B. Wiggins, Caddisflies: The Underwater Architects (Toronto: University of Toronto Press, 2004), pp. 45-47 ("Communities in Streams and Rivers," "the River Continuum Concept," and "Drifting Invertebrates.")

Benthic macroinvertebrates are divided by eating behavior into four different groups. They are:

I. Shredders (sh): (e.g. crane fly larvae, all stoneflies with the exception of the three that are predaceous, and the Lepidostomatid case-making caddis)

II. Collectors -- a group that is subdivided into

A. Filtering Collectors (cf): (e.g. black flies, common netspinners, and brushlegged mayflies)

note: black flies and brushlegged mayflies "filter" particles out of the water using their "whiskers"and hairy legs, respectively; common netspinners pick the food out of their nets.

B. Gathering Collectors(cg): (e.g. midges, all mayflies with the exception of the brushlegged
mayflies, and flatheaded mayflies)

III. Scrapers (sc): (e.g. flatheads, water pennies, riffle beetles, and snails)

and

IV. Predators (pr): (e.g. Perlid [common], Perlodid, and Chloroperlid [Green] stoneflies, the freeliving

caddisfly, and all Odonata (all dragonflies and damselflies).

(To be thorough, I should note that Merritt and Cummins, p. 586, add a fifth group -- the "Piercers-Herbivores" -- but it includes only one taxon, the Micro caddis. The Micro caddis apparently "pierces" algal cells and sucks out the contents. Pretty specialized!)

For a full account of the FFG's of all benthic macroinvertebrates by order and family, I urge the reader to go to the StreamWatch website (http://streamwatch.org/volunteers/forms-and-documents) and open the pdf. on "Tolerance Values."

The concept of the "River Continuum" simply asserts that the numbers of insects in each of these groups will vary with the point of the stream being sampled and the food sources available at that point in the stream. Thus, "Shredders" need leaves and twigs to shred for their food, and they are the dominant group found in the forested headwaters of any watershed (like the site pictured at the top of the page). What have I been finding in leaf packs all fall and winter? Crane fly larvae, Lepidostomatid caddisflies, and large and small winter stoneflies. However, in those same leaf packs, I've found a fair number of common and Perlodid stoneflies -- Predators -- a case of the "eaters" eating the "eaters" (or the eaters becoming the eaten)! Also present in this headwater habitat are the Collectors -- both types of Collectors (e.g. mayflies and black flies). Why? They are there to "pick up the crumbs," so to speak, the fine bits of organic matter left over by the Shredders (and Predators, for that matter). The food consumed by Shredders and Predators is technically known as "coarse particulate organic matter" (CPOM), while that eaten by the Collectors is referred to as "fine particulate organic matter" (FPOM).

As we move further downstream, the stream widens, trees no longer cover the banks dropping their leaves into the water each fall, and the increased sunlight hitting the water leads to an increased growth of algae on the boulders and rocks. All of these factors lead to a decrease in Shredders and an increase in Scrapers -- those insects that eat by scraping their food from rocks (e.g. beetles -- water pennies and riffle beetles -- and flatheaded mayflies). The number of Collectors remains fairly constant as does the number of Predators (though we might start to see more "warm water" predators -- damsels and dragons -- and fewer "cold water" stoneflies).

Finally, as the stream widens into a genuine river, Scrapers too start to diminish, and the Collectors become the dominant group in the stream. But the Predators still remain -- Odonata families by now taking over. StreamWatch samplers all know that we find the greatest number of damsels and dragons in the mainstem of the Rivanna, especially the site near Crofton.

I. Shredders (crane fly larva and Lepidostomatid caddis)