Variation in the Vulnerability of Prey to Different Predators: Community‐Level Consequences

Midge larvae (Diptera, Chironomidae) that weave filamentous algae into retreats of tufts, are dominant primary consumers in a river food web. In a previous study, 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. To examine the mechanisms underlying these dynamics, we compared the vulnerability of tuft—weaving midges (naked or in algal tufts) to fish and predatory invertebrates, in field and laboratory experiments. 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. We suggest that the vulnerability of tuft—weaving midges to invertebrate predators, and their relative invulnerability to fish, sets the stage for trophic cascades observed in the system. 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 consumer, which, when released from predation, can suppress plants.