Regulation of Intertidal Food Webs by Avian Predators on New England Rocky Shores

Abstract

Although there is a large body of research on food webs in rocky intertidal communities, most of the emphasis has been on the marine benthic components. Effects of avian predation on highly mobile predators such as crabs, remains practically unstudied in rocky shore ecosystems. The crab, Cancer borealis, is an important component of the diet of gulls (Larus marinus, L. argentatus) at the Isles of Shoals, Maine, USA. C. borealis prey include the predatory gastropod Nucella lapillus L., the herbivore Littorina littorea, and mussels Mytilus edulis L. We hypothesized that gulls reduce abundance of C. borealis in the low intertidal and shallow subtidal, thereby allowing C. borealis prey to persist in high numbers. A study of crab tidal migration showed that C. borealis density nearly doubled at high tide compared to low tide; thus, crabs from a large subtidal source population migrate into the intertidal zone during high tides and either emigrate or are removed by gulls during low tides. Results from a small-scale (1 m2) predator caging experiment in the low intertidal zone indicated that enclosed crabs significantly reduced L. littorea abundance when protected from gull predation. In a much larger-scale gull exclusion experiment, densities of C. borealis increased significantly during low and high tides in exclosures relative to the controls. C. borealis density was inversely correlated with changes in the abundance of two mesopredators Carcinus maenas and Nucella lapillus, and with the space-occupier M. edulis. There was a similar negative correlation between abundance of C. borealis and the change in abundance of the herbivore L. littorea, but the trend was not significant. Mortality of tethered L. littorea was associated with C. borealis density across sites. However, preferred algae did not change in response to L. littorea density during the experiment. Thus, we found suggestive, but not conclusive, evidence for a three-level cascade involving gulls, crabs, and L. littorea. Our studies strongly suggest that gulls, as apex predators, generate three-level trophic cascades in rocky intertidal food webs by preventing the highly mobile subtidal predator, C. borealis, from establishing substantial populations in the low-mid intertidal zone thereby indirectly enhancing densities of two key mesopredators (N. lapillus, Carcinus) and blue mussels (M. edulis).

Biodiversity Effects on Aquatic Ecosystem Functioning – Maturation of a New Paradigm

“…For example, in a seagrass system, grazer richness had effects on system processes only with a predator present . Indeed, three manipulations of predator richness suggest that losing predator diversity from marine systems can have cascading effects on the algal communities (BYRNES et al, 2006;BRUNO and O'CONNOR, 2005;ELLIS et al, 2007). The results of these experiments parallel those of observational studies on functional diversity and predator extinctions.…”

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

“…Crabs can also have indirect effects on mussels through direct predation on other mussel consumers, such as whelks. Crab predation on grazers, such as Littorina littorea (Ellis et al 2007, Perez et al 2009, also has been shown to alter surface rugosity, and thus mussel recruitment, by effecting grazing activity (Petraitis 1990). In addition, ducks and gulls can have both direct and indirect effects on mussel v www.esajournals.org populations.…”

“…For example, gulls prey heavily on crabs in the Gulf of Maine, especially during summer months (Ellis et al 2005). 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).…”

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

“…Robles 1997, Posey et al 2006, Ellis et al 2007. In this experiment, we had multiple exclosure treatments involving different mesh sizes.…”

“…In rocky intertidal communities, invertebrate (Paine 1966, Robles & Rob 1993, Navarrete & Menge 1996, Mowles et al 2011) and vertebrate (Hamilton 2000, Ellis et al 2007, Freudendahl et al 2010) predators can have important effects on community structure. Details of these effects are not always easy to identify in a field setting; species interactions can alter the overall effects of a species and complicate their measurement (Dungan 1986, Trussell et al 2003, Quinn et al 2012.…”