MEPS 141:295-302 (1996)  -  doi:10.3354/meps141295

The aim of this study was to determine whether experimental artifacts have the potential to bias comparisons of relative predation intensity among different soft-sediment estuarine habitats. A field experiment was used to test (1) whether predation intensity on the infaunal clam Mercenaria mercenaria (L.) differed between shallow-subtidal and intertidal sand flats, (2) whether predation on clams differed between intertidal sand flats and vegetated habitats [the edge of Spartina alterniflora (Loisel) salt marshes] of similar elevation, and (3) whether the outcome of these comparisons was influenced by the experimental method used. For mobile prey, some restraint on prey movement is typically required to compare predation intensity among different habitats. This is most often achieved by tethering prey. Tethering and other experimental interventions may induce different experimental artifacts in estimates of predation intensity in different habitats, which would invalidate between-habitat comparisons (Peterson & Black 1994: Mar Ecol Prog Ser 111:289-297). I used combinations of 2 types of prey restraint, tethers and buried fences, to test for possible interactive effects of habitat and prey restraint on prey mortality. Tethered or untethered clams were deployed within fenced or unfenced field plots within each habitat type, in a factorial design, and were exposed to the natural assemblage of predators for 1 wk. Within each habitat type, both tethering and fencing increased the proportion of clams recovered live after 1 wk compared to the treatment in which no prey restraints were used. Significantly more live clams were recovered in the intertidal sand flats than in the subtidal sand bottoms within field plots enclosed with buried fences, suggesting lower predation intensity in the intertidal than in subtidal sand flats. In contrast, there was no significant difference in proportions of live clams between subtidal sand bottoms and intertidal sand flats within unfenced plots. These results were obtained with both tethered and untethered clams. Significantly more clams were recovered live in intertidal sand flats than in intertidal salt marshes of similar elevation in all treatment combinations, suggesting lower predation intensity in unvegetated than in vegetated habitats. This result was unexpected because habitat structural complexity is known to decrease predation rates in other shallow marine habitats, such as rocky intertidal habitats and seagrass beds. Results of this field experiment indicate that prey restraints can bias not only estimates of predation intensity within a single habitat type but can also bias between-habitat comparisons of predation intensity, probably by causing experimental artifacts of different magnitudes in each habitat. The actual magnitude of experimental artifacts in each habitat type can only be estimated with direct observations of predator-prey interactions in the field.

Bivalves · Clams · Experimental artifacts · Fencing · Intertidal · Mercenaria mercenaria · Predation · Marine soft bottoms · Tethering