| Summary: | I investigated interactions between Common Eiders (Somateria mollissima) and the intertidal invertebrate community in Passamaquoddy Bay, New Brunswick. I determined that young ducklings were dependent on rockweed (Ascophyllum nodosum), a perennial macro alga, to obtain food, as indicated by their feeding methods and timing, but that feeding by older ducklings was unaffected by tide level. Commercial harvest of rockweed may therefore be detrimental to the youngest ducklings, but have little effect on older birds. I also found that care of ducklings constrained female behaviour and habitat use. This may be a local effect caused by very high predation pressure exerted on ducklings at this site. Adult eiders were size-selective predators of blue mussels (Mytilus edulis), and preferences varied depending on season and prey availability. Selection of prey corresponded with predictions of the shell mass minimization hypothesis and also with the risk-averse foraging hypothesis. Using a series of predator exclusion cages, I found that in mussel beds, ducks had little effect on species richness or diversity, but significantly reduced total biomass by feeding primarily on blue mussels. Biomass effects persisted throughout the experiment in sites subjected to abiotic disturbance, but disappeared in undisturbed sites within a year. Exclusion of ducks led to an increase in dogwhelks (Nucella lapillus) under natural site cages, and by feeding on blue mussels they obscured the effect of eider predation. This indirect effect is an example of compensatory predation by a formerly redundant predator. Effects of eider predation on rockweed bed invertebrate biomass were small, and again duck predation did not influence species richness or diversity. Harvest of rockweed had no effect on invertebrate abundance, and little effect on predation by ducks. Differences in results between the two areas can be attributed to habitat heterogeneity and relative abundance of whelks. Although the effect on biomass was not measurable, predation by ...
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