Variation in the vulnerability of prey to different predators: community-level consequences

Midge larvae (Diptera, Chironomidae) that weave filamentous algae into retreats or tufts, are dominant primary consumers in a river food web. Densities of tuft-weaving midges increased in the presence of large fish. In the absence of large fish, midges decreased as densities of predatory invertebrates built up, and higher standing crops of algae were maintained. When midges were exposed for 1 h in the river to fish, 15 out of 15 midges in tufts survived, while 15 of × 15 naked midges were consumed. Tufts afforded only partial protection to midges exposed to invertebrate predators, however. After 1 h, enhancement of survivorship by tufts was moderately significant for midges exposed to aeshnids, and insignificant for midges exposed to lestids and naucorids. The vulnerability of tuft-weaving midges to invertebrate predators, and their relative invulnerability to fish, may set the stage for trophic cascades. Fish, by consuming small predators, release midges, which graze down algae. The strong effects of fish as fourth-level consumers would not be predicted from their diets, in which algivorous mayflies dominate (>60% of the insect biomass found in each of the two most common fish species). Nevertheless, fish in this food web act as fourth-level, rather than third-level, consumers because of the differential vulnerability of one guild of primary consumers, which when released from predation, can suppress plants. -from Authors