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

1. Secondary production was estimated for Plecoptera, Odonata and Megaloptera (mostly large predators) occurring ion 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 m(2) of snag surface to values per m(2) 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 (Corydalus cornutus), five perlid stoneflies (Paragnetina kansensis, Perlesta placida, Neoperla clymene, Acroneuria evoluta and Acroneuria abnormis) and two dragonflies (Neurocordulia molesta and Boyeria vinosa). The detritivores were Pteronarcys dorsata and Taeniopteryx lita. 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 m(2) 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 position of 2, Corydalus had a trophic position of only 3.5. 5. A relatively high fraction of invertebrate production was consumed by predaceous invertebrates, ranging from 9 to > 100% for various primary consumer groups, with total consumption representing 52% of total production. Because these estimates do not include vertebrate consumption or emergence, it means that a high fraction of larval mortality is due to predation.