Seasonal variation in effects of multiple predators on an intertidal mussel bed: implications for interpretation of manipulative experiments

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.

Recruitment, abundance, and predation on the blue mussel (Mytilus edulis) on northeastern estuarine rocky shores

“…By consuming crabs, gulls can have indirect trophic impacts on other invertebrates, including M. edulis as well as N. lapillus and L. littorea (Ellis et al 2007). Moreover, seasonal variation in bird predation on mussels (particularly by eiders) has been found to affect the size distribution of mussels remaining in a community, thereby affecting which consumers are most effective in subsequent mussel predation (Boudreau and Hamilton 2012). Given the surprising lack of predation effects we documented in Casco Bay, the few predators we were able to observe, and the important role predation and other sources of mortality play in structuring population dynamics (Persson and De Roos 2013), the identity, roles, and interactions of intertidal predators at these sites warrant further attention.…”

“…Dominant consumers at northwest Atlantic rocky shore sites include gulls, ducks, crabs, whelks, and seastars (Menge 1976, Petraitis 1990, Ellis et al 2005, Boudreau and Hamilton 2012. Here we synthesize what is known about the species-and size-specific effects of these predators on mussel populations.…”

“…Here we synthesize what is known about the species-and size-specific effects of these predators on mussel populations. Boudreau and Hamilton (2012), on mildly to moderately wave-exposed rocky shores in Canada's Bay of Fundy, documented that eider ducks ingested mussels ranging from 2-60 mm in length (i.e., both newly recruited juveniles and adults), whereas crabs tended to forage on 30-42 mm long mussels, and whelks consumed 25-50 mm long mussels, although they preferred individuals closer to 25 mm in length. Hunt and Scheibling (1998), working on wave-exposed rocky shores in Nova Scotia, found that even one of the smallest size classes (!5 mm shell length) of the whelk N. lapillus could significantly affect mussel cover.…”

“…Further observational and experimental studies at these sites are needed to verify this finding, as well as to investigate the relative contributions of individual predatory species on mussel populations, and how those effects may vary geographically. By employing a series of cages similar to Boudreau and Hamilton (2012), and coupling this experiment with more comprehensive intertidal and subtidal surveys, it should be possible to distinguish between species-specific consumer effects of birds, crabs, whelks, and seastars at these sites. Relatedly, addition of another nested set of estuarine sites north of Cape Cod would improve the spatial coverage of the comparativeexperimental study and enable more robust inferences of the regional-scale variation in recruitment, adult abundances, and consumer pressure that we documented.…”

“…Investigations into how key life history traits in mussels (such as physiology, growth, survival, and/or reproduction) vary with exogenous factors including temperature (Helmuth and Hof-mann 2001, Petes et al 2007, Lesser et al 2010, hypoxia (Altieri and Witman 2006), food availability (Menge et al 1997, Leonard et al 1998, and disturbance regimes Levin 1981, Hunt andScheibling 2001) have contributed substantially to understanding how environmental factors influence organismal and population performance, and community dynamics more broadly. Observations and experiments of predator-prey dynamics involving mussels have also yielded important insights, including understanding of how spatial and temporal variation in predation strength and predator identity can shape benthic community structure and functioning (Paine 1966, Robles et al 1995, Navarrete and Menge 1996, Menge et al 2004, Navarrete and Manzur 2008, Boudreau and Hamilton 2012.…”