Caging manipulations were performed in two marine soft-bottom communities to test two nonexclusive hypotheses: (1) polychaete abundance changes in cages are caused by the presence or absence of particular predators and competitors; (2) polychaete changes in cages are caused by animals responding to cage-induced habitat modifications, especially sediment deposition and erosion.The significant changes in the polychaete community beneath a variety of cages installed on a highly wave-exposed sand bottom could not be explained by the presence or absence of predatory demersal fishes. This result was obtained despite the exclusion of fish which normally consume large numbers of infaunal polychaetes.A caging experiment similar to Woodin's (1974) was performed in the channel of Elkhorn Slough using comparable cages and similar exposure period. In Woodin's experiments, the exclusion of a sedentary tube builder was accompanied by an increase in a mobile deposit feeder. This was interpreted as a competitive release. In our experiment, no sedentary species inhabited the channel and none were excluded; nevertheless, this same mobile species increased inside the cages. Although the importance of potential predators and competitors was not documented in the caging experiments, all of the sand-flat and slough caging results are consistent with the hypothesis that animals respond to sedimentary habitat modifications created by cages. This hypothesis is not considered in most caging experiments in marine soft-bottom communities.
The importance of grazing food chain for energy flow and production in three intertidal sand bottom communities of the northern Wadden Sea
