Effects of predation by common eiders (Somateria mollissima) in an intertidal rockweed bed relative to an adjacent mussel bed

Abstract.

Little is known about the effect of commercial rockweed (Ascophyllum nodosum) harvest on intertidal community structure, and on interactions among species. Using predator-exclusion cages and by simulating rockweed harvest, we studied the combined effects of predation by common eiders (Somateria mollissima) and harvest on an intertidal invertebrate community associated with rockweed. Predation had no effect on invertebrate species richness, and only a short-term (<1 year) negative effect on total invertebrate biomass. Most of this was due to a decline in common periwinkle abundance in controls relative to exclosures. Eiders had no clear effect on blue mussels (Mytilus edulis), their preferred prey, probably because predation by dogwhelks (Nucella lapillus), which were likely the more important predator in this system, more than compensated for the absence of duck predation. Rockweed harvest had no overall effect on the invertebrate community, but did reduce the effectiveness of ducks as predators during summer, when ducklings and associated females fed on invertebrates at the top of the rockweed canopy. We conclude, by comparing results with a similar experiment conducted in the adjacent mussel bed, that both ducks and whelks are major predators of blue mussels, but that their relative importance differs in the two areas, probably as a result of differences in habitat dimensionality and heterogeneity.