Direct and Indirect Effects of Predation by Common Eiders and Abiotic Disturbance in an Intertidal Community

Abstract

Predation and disturbance have been well studied in intertidal communities. However, the impact of vertebrate predators, particularly waterfowl, has been only infrequently determined in long‐term intertidal studies. Using predator exclusion cages and simulated abiotic disturbance, I studied the direct and indirect effects of predation by Common Eiders (Somateria mollissima) on an intertidal mussel bed community in Passamaquoddy Bay, New Brunswick, Canada. Eiders fed heavily on blue mussels (Mytilus edulis), the dominant invertebrate present, reducing their abundance in “+duck” (exposed to predation) relative to “−duck” (exclosure) plots by nearly 50% within eight months of initiation of the experiment. Based on counts and estimated consumption rates, eiders appeared to be responsible for most predation observed in this system. Although ducks fed heavily on the dominant intertidal invertebrate, they had little effect on species diversity or richness in the community. However, relative abundance of the most common invertebrates did vary. In particular, exclusion of eiders led to an increase in abundance of dogwhelks (Nucella lapillus) one year into the experiment. Whelks, in turn, fed heavily on mussels under cages and obscured the longer‐term effect of ducks in the system. This indirect effect appears to be an example of asymmetric exploitation competition, with ducks influencing the food supply of whelks, but whelks having little effect on ducks. When ducks were excluded, whelks were released from this competition and acted as compensating predators. No other indirect effects developed following duck exclusion, presumably due to the increased whelk effect. Disturbance, in contrast, did lead to an increase in species diversity, which later returned to predisturbance levels as the community recovered. Predation delayed the recovery of disturbed sites, because ducks began feeding in these plots before mussel abundance had completely rebounded. Disturbance, while initially deterring predation somewhat, ultimately allowed the effects of predators to persist longer. This interaction of predation and disturbance resulted from compensatory growth of mussels under exclosures in disturbed sites. Mussels protected from eider predation grew quickly after disturbance and rapidly became larger than the preferred prey for whelks. As a consequence, whelks did not feed as heavily on mussels in disturbed sites as in undisturbed sites, where more mussels were of the preferred size, and the observed effect of ducks on mussel biomass persisted. This experiment demonstrates that eiders were significant, and, probably, keystone predators in this system. Eiders directly reduced abundance of mussels, thereby indirectly increasing whelk density in plots where ducks were excluded. Compensatory predation by whelks following duck exclusion blocked other potential indirect effects, thus preventing eiders from having more widespread effects in the system. Such compensatory predation may act to stabilize communities and evidently can occur in re...

Effects of predation by sea ducks on clam abundance in soft-bottom intertidal habitats

“…This study corroborates a growing body of literature indicating the important effects of sea duck predation on intertidal populations and communities (Guillemette et al 1996, Hamilton 2000, Larsen & Guillemette 2000. The large body size, high metabolic rates, and gregarious nature of sea ducks leads to high rates of predation and potentially important effects on prey abundance.…”

“…The presence of sea ducks as a top predator may also affect community dynamics by limiting the abundance of competing predators. Hamilton (2000) found that exclusion of common eiders led to an increase in the abundance of dogwhelks Nucella lapillus, as whelks were likely released from interspecific competition for available mussel prey.…”

“…These energetic demands require sea ducks to consume substantial amounts of food, relative to their body size, to maintain a positive energy balance (Goudie & Ankney 1986, Guillemette et al 1992, Guillemette 1998. The high consumption rates of sea ducks have been the focus of recent studies illustrating their ability to substantially deplete prey in wintering habitats; however, almost all of these studies have documented the depletion of mussels in rocky intertidal areas (Guillemette et al 1996, Larsen & Guillemette 2000, Hamilton 2000.…”

The introduced clam Ensis americanus in the Wadden Sea: field experiment on impact of bird predation and tidal level on survival and growth

“…2). Sides were left open to avoid accumulation of drifting macroalgae and to minimize potential cage eVects (Hamilton 2000). Non-covered squares were marked at each corner by a stick projecting 5 cm above the sediment surface.…”

Effects of off‐bottom shellfish aquaculture on winter habitat use by molluscivorous sea ducks

“…The extent of intertidal area, which was identified as an important predictor of surf scoter distribution in the Malaspina Complex, also was an important environmental factor determining surf scoter distribution in Baynes Sound (Ž ydelis et al, 2006) and has been shown to be important for other sea ducks (including Barrow's goldeneye) in other studies (Esler et al, 2000a, b;Hamilton, 2000). Although habitats in the Malaspina Complex and Baynes Sound are very different (steep rocky shores versus broad intertidal soft-sediment flats, respectively), the intertidal zone is known to be the most important foraging habitat for surf scoters, who dig for clams in soft bottoms and take mussels from hard substrates (Lacroix, 2001;Lewis et al, 2007b).…”