Link strength in lake littoral food webs: Net effects of small sunfish and larval dragonflies

To show how predaceous fish and dragonflies affect benthic community structure, we conducted enclosure experiments in the littoral zone of Bays Mountain Lake, Tennessee. A 'natural' benthic assemblage was subjected to all combinations of 2 densities of 3 predator treatments - small sunfish, 0 or 4/m²; large dragonfly larvae, 0 or 15/m²; and dragonfly eggs at 2 densities, 90 or 900/m². Treatments were assigned randomly in each of 6 spatiotemporal blocks. Net effects of predation over 4 mo show that small sunfish had 'strong' effects (>50% reduction of densities) on triclads, large daphnid cladocerans, and snails, and 'moderate' effects (50% > reduction > 25%) on small dragonflies and ostracods; all these effects were statistically significant, except the one on ostracods. Large dragonflies had moderate non-significant effects on triclads and ostracods. There were no significant increases in prey density associated with fish predation; but chydorid cladocerans and midge larvae showed 'weak' non-significant increases that might be caused by 'indirect effects' of fish predation on invertebrate predators or grazers. There were only 2 significant interaction terms indicative of 'higher-order interactions': Fish X Dragonfly on the large daphnid Simocephalus, and Fish X Egg Density on snails, which were associated with relatively low probabilities. In both cases, the net effect of dragonflies was to reduce prey densities more when fish were present.