Refuges, Biological Disturbance, and Rocky Subtidal Community Structure in New England

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Abstract
The effects of two sources of biological disturbance–predation and sea urchin grazing–on the structure of benthic communities inside and outside beds of the horse mussel, Modiolus modiolus, were examined in the rocky subtidal zone off the Isles of Shoals, New Hampshire, USA. Multivariate analysis revealed three major communities between 8 and 30 m in depth: (1) a Modiolus community; (2) a 30 m community; and (3) an 8—18 m community. At all depths, mussel beds contained significantly higher densities of infauna than did other subtidal habitats. The hypothesis that Modiolus beds provide a refuge from predation for the associated community was tested in five manipulative field experiments. Three members of the mussel bed community, the bivalve Hiatella arctica, the sea urchin Strongylocentrotus droebachiensis, and the ophiuroid Ophiopholis aculeata, were used as experimental prey. Consumption of these prey by a guild of generalist predators was observed outside but not inside mussel beds, which indicates that subtidal Modiolus beds provide a spatial refuge from predation. Deaths from predation were significantly higher at night than during the day for Hiatella but not for Strongylocentrotus. At night, crab (Cancer borealis, Cancer irroratus) and lobster (Homarus americanus) predation accounted for all attacks that were directly witnessed, while fish (Tautoglabrus adspersus, Pseudopleuronectes americanus) predation accounted for 71% of the total prey consumed during the day. Such diel differences in predation corresponded with predator abundance patterns. The sea urchin Strongylocentrotus droebachiensis was the most significant agent of biological disturbance during the 1979—1984 study period. Strongylocentrotus intensively grazed the benthos at 8 m in 1982, causing a 79% reduction (from predisturbance levels) in the mean population density of invertebrates outside the mussel beds. This event served as a natural experiment in which to test the hypothesis that Modiolus beds function as a refuge from severe grazing disturbance. Re—sampling of communities inside and outside mussel beds immediately after the grazing disturbance indicated that the Modiolus community exhibited less change in species composition, dominance, and diversity than communities outside the mussel beds. Such comparisons indicate that Modiolus beds afford protection from severe grazing disturbance for infaunal invertebrates. Long—term photographic monitoring of marked mussel beds at 8, 18, and 30 m depth showed that Modiolus beds at all depths persisted for more than 5 yr. Mortality rates of adult Modiolus were low; however, mortality was highest at the shallow site (8 m). Mussel beds successfully resisted the grazing disturbance which eliminated all other biogenic habitats except those created by crustose coralline algae. Modiolus beds are effective refuges because they persist for many years and resist biotic disturbance. In the subtidal communities examined here, lower levels of disturbance inside mussel beds can account for the abundance and spatial distribution of Modiolus—community species. These results demonstrate the functional significance of mussel beds in cold—temperate subtidal regions where predation and sea urchin grazing are major determinants of community organization.