Food web quantification using secondary production analysis: predaceous invertebrates of the snag habitat in a subtropical river

• Published in: Freshwater biology 2001-03, Vol.46 (3), p.329-346
• Main Authors: Benke, Arthur C., Wallace, J. Bruce, Harrison, James W., Koebel, Joseph W.
• Format: Article
• Language: English
• Subjects: Acroneuria abnormis; Acroneuria evoluta; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Boyeria vinosa; Corydalus cornutus; food web; Fresh water ecosystems; Freshwater; Fundamental and applied biological sciences. Psychology; Megaloptera; Neoperla clymene; Neurocordulia molesta; Odonata; Paragnetina kansensis; Perlesta placida; Plecoptera; predaceous invertebrates; Pteronarcys dorsata; river; secondary production; snag; Synecology; Taeniopteryx lita; USA
• ISSN: 0046-5070; 1365-2427
• DOI: 10.1046/j.1365-2427.2001.00680.x

1. Secondary production was estimated for Plecoptera, Odonata and Megaloptera (mostly large predators) occurring on the snag habitat of a subtropical blackwater river in the southeastern U.S.A. Coastal Plain for 2 years. Production estimates and gut analyses were used in estimating species‐specific ingestion to construct a quantitative food web of the predator portion of the invertebrate assemblage. Neither basal resources (e.g. detritus) nor predaceous vertebrates (e.g. fishes) were considered in this analysis. A discharge‐specific model of snag‐habitat availability was used to convert values per m2 of snag surface to values per m2 of river bed. 
2. These three orders included the major large predators on the snag habitat, as well as two detritivorous stoneflies. The major predators were the hellgrammite (Corydaluscornutus), five perlid stoneflies (Paragnetinakansensis, Perlestaplacida, Neoperlaclymene, Acroneuriaevoluta and Acroneuriaabnormis) and two dragonflies (Neurocorduliamolesta and Boyeriavinosa). The detritivores were Pteronarcysdorsata and Taeniopteryxlita. 
3. Total predator production was high, but varied from only 7.1 to 7.4 g dry mass (DM) m−2 y−1 of snag surface (2.4–2.7 g DM m−2 y−1 of river bed) over two years. Corydalus was the largest predator and had the highest production (2.8–3.1 g DM m−2 of snag surface). The most productive stoneflies were Perlesta (0.7–1.0 g DM m−2 of snag surface) and Paragnetina (1.0–1.3 g DM m−2 of snag surface). The most productive dragonfly was Neurocordulia (0.7–1.9 g DM m−2 of snag surface). Production of the non‐predaceous stoneflies was 1.0–2.3 g DM m−2 of snag surface. Production values per m2 of river bed were 2–3.5 times lower than the values per m2 snag surface. 
4. Measurement of ingestion fluxes within the predator portion of the food web showed that predaceous invertebrates were primarily supported by chironomid and mayfly prey. However, the greatest consumption of chironomids and mayflies was by omnivorous hydropsychid caddisflies, which had a considerably higher production than the larger predators. There was a hierarchy of feeding with Corydalus as top predator consuming all other groups, followed in order by dragonflies, stoneflies and hydropsychids. Although the feeding hierarchy suggested the presence of four predatory trophic levels within the invertebrate assemblage, calculations of trophic position indicated there were less than two. With primary consumers (e.g. midges) having a trophic po