MEPS 465:137-153 (2012)  -  DOI: https://doi.org/10.3354/meps09897

ABSTRACT: The outer Bay of Fundy, Canada, hosts rocky intertidal communities often dominated by beds of blue mussels Mytilus edulis, which support vertebrate and invertebrate predators at different times of the year. Strong predation by ducks in this system opens the possibility of a trophic cascade whereby ducks substantially reduce mussel density, opening space for other species. However, previous work has shown no long-term cascading effect; dogwhelks Nucella lapillus appeared to compensate for duck exclusion by consuming excess mussels. To quantify this compensation, the related effects of other invertebrate predators and the temporal effects on predation, we conducted 2 exclosure experiments in Passamaquoddy Bay, St. Andrews, New Brunswick, Canada, which were initiated at different times, one in spring and the other in fall. Exclosures excluded ducks and allowed variable access to different invertebrate predators within the community. As predicted, ducks had a substantial effect in both cases, reducing mussel density in areas where they could feed. The timing of duck exclusion was crucial in determining the outcome of experiments. When ducks were excluded before their fall migration, the full range of mussel sizes remained present within the system and whelks congregated in exclosures, compensating for duck exclusion. When exclosures were set up after duck foraging had commenced, the remaining mussels were generally larger. In this case, green crabs Carcinus maenas compensated but whelks did not. We recommend that researchers carefully consider the timing of manipulative studies to ensure that results are not artefacts of seasonal variation in predator activity.

KEY WORDS: Temporal effect · Compensation · Trophic cascade · Blue mussel · Common eider · Dogwhelk · Green crab