Role of predation in organizing benthic communities of a temperate-zone seagrass bed

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Abstract
Monthly coring for 6 mo. at 9 positions along a transect extending from 15 m within to 15 m outside a North Carolina (USA) seagrass bed revealed higher densities of macrobenthic animals within the vegetated area. In the seagrass, average density of epibenthos was 52 .times. and of infauna 3 .times. the level observed on the adjacent sand flat. Among infaunal guilds, suspension feeders responded most, predator-scavengers less, and surface- and subsurface-deposit feeders least to the presence of seagrass. Caging with 6 mm mesh for successive 2 mo. periods in the seagrass has a small and statistically non-significant effect on macro-infaunal density. In analogous sand-flat experiments, the total infaunal density was substantially higher under full cages than in uncaged controls. Under partial cages, used as cage controls, infaunal densities were indistinguishable from those observed in uncaged controls, implying that predation by large (> 6 mm) epibenthic consumers rather than cage artifacts was the major cause of the response of infauna to caging. Among infauna, only the suspension-feeder guild exhibited a significant cage artifact, which in combination with a large effect of predator exclusion may explain its relatively large response to seagrass. Relative and absolute responses of separate infaunal guilds to caging closely matched their responses to the presence of seagrass. Total infaunal densities inside full cages on the sand flat increased to levels indistinguishable from those observed at the same times in the seagrass bed, implying that predation by large epibenthic consumers was the major cause of the observed between-habitat differences in infaunal densities. Average species diversity (H''), richness (S) and evenness (J'') per core changed along the transect from sand flat to seagrass bed and underneath cages on the sand flat as predicted by a simple species-individual curve. This implies a lack of competitive exclusion where macrobenthic densities were partially released from epibenthic predation. Seining revealed large numbers of epibenthic predators inside seagrass beds and few, if any, on unvegetated flats except at night. Epibenthic predators probably remain inside seagrass beds and other refuges during daylight and restrict their predation on sand-flat infauna to the night, when risk of higher-order predation is reduced. This hypothesis implies that the importance of epibenthic predation on the infauna should decline as a function of increasing distance from the nearest seagrass-bed refuge.

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