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.
We evaluated the potential for between-mudflat dispersal of individuals of the burrow-dwelling amphipod Corophium volutator. We first estimated the distances travelled by measuring vertical distribution of swimmers in the water column and their activity in relation to the tidal cycle. We found amphipods were located high over the substratum (up to 4 m) with little vertical structure, and swimming occurred during periods of maximum water velocity (large peak during flood tide). Based on the behaviour of amphipods and information on hydrodynamics, we estimated that an individual could move between 0.4 and 14.4 km toward the upper part of the bay in a single swimming event. This distance is sufficient to allow travel from one mudflat to the next in only a few swimming steps. To identify potential migration routes, we also sampled along a shore (covered in a narrow band of mud) between 2 major mudflats. Patterns in density suggested there was a presence of a travelling wave of individuals, which is consistent with predictions made from their swimming behaviour. We evaluated the possibility that C. volutator in the upper Bay of Fundy are found in one or several metapopulations, where the large and discrete mudflats are connected by the dispersal of individuals. We propose a refined, spatially structured population model (modified from terrestrial metapopulation models) applicable to soft sediment environments in marine intertidal zones, in which several groups of a few intertidal flats are connected by the unidirectional movement of the amphipods. The dynamics will largely be determined by the detailed spatial arrangement of intertidal flats and corridors, as well as by the details of the hydrodynamic conditions and behaviour of organisms. 449: 197-209, 2012 are patchily distributed and exchange individuals, especially through planktonic larval stages. Consequently, such populations are increasingly viewed as metapopulations in the marine ecological literature, although it may not be the most appropriate approach in many cases (Grimm et al. 2003). Marine metapopulations that resemble their terrestrial counterparts (a large number of discrete populations undergoing colonization or extinction dynamics) appear to be rare; examples include copepods in rocky intertidal pools (Dybdahl 1994) and, potentially, giant kelps in shallow coastal waters (Reed et al. 2006). A set of conditions (reviewed in the Discussion) is required for metapopulation models to be more informative than other population models, and these should be assessed before adopting a metapopulation approach (Hanski 1997, Grimm et al. 2003. In systems where a metapopulation approach can be appropriate and informative, marine ecologists will need to incorporate structural differences of the marine environment into the existing body of theory (e.g. spatial arrangement of suitable habitat, means of dispersal of organisms; Kritzer & Sale 2006) before applying these models. KEY WORDS: Marine metapopulation · Soft sediment · Dispersal · Corridor · Coro...
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