INDIRECT EFFECTS AND HABITAT USE IN AN INTERTIDAL COMMUNITY - INTERACTION CHAINS AND INTERACTION MODIFICATIONS

Species may indirectly affect populations of other species in two basic ways. Interaction chains result from linked direct interactions between species pairs. Interaction modifications, emergent effects of a multispecies assemblage, result when a third species changes how a pair of other species interacts. Experiments and patterns of abundance in the upper zone of a rocky intertidal community provide examples of both types of indirect effects. By consuming limpets (Lottia digitalis), bird predators indirectly enhance acorn barnacle (Balanus glandula) abundance on horizontal rock benches because limpets dislodge or inadvertently consume young acorn barnacles, an example of an interaction chain. Because L. digitalis has a light-colored shell that is similar in color to barnacle shells, barnacles alter the bird-limpet interaction by changing the ability of birds to find limpets, an example of an interaction modification. Distinguishing between interaction chains and interaction modifications is important because interaction chains can be predicted with only a knowledge of pairwise species interactions while descriptions of interaction modifications require a knowledge of all species combinations involved, precluding a priori prediction. By classifying empirically determined indirect effects, the degree to which indirect effects can be predicted from a knowledge of pairwise interactions can be determined, and the mechanisms that cause interaction modifications can be identified to aid in future prediction.