Predation risk indirectly enhances survival of seaweed recruits but not intraspecific competition in an intermediate herbivore species

1. Non-consumptive predator effects are an important driver in predator-prey interactions. While indirect effects of predation risk shape interspecific species interactions, e.g. in fuelling trophic cascades, knowledge about non-consumptive predator effects on intraspecific interactions among intimidated prey is scarce. 2. In laboratory experiments, we tested whether predation by male and female green crabs (Carcinus maenas) is stronger on small than on larger specimens of the periwinkle Littorina littorea, an important herbivore species in intertidal habitats of the temperate zone. Moreover, we asked whether size-specific differences in predation-induced mortality reflect as a stronger effect of predation risk (caused by nearby consuming crabs) on the foraging behaviour of more threatened periwinkles and ultimately on the recruitment success of the brown seaweed Fucus serratus. Furthermore, predator activity and prey abundance was measured in the field and the efficacy of predation risk on the survival of seaweed recruits was assessed in three field experiments with two different set-ups (open plots and crab exclosures). 3. Male crabs consumed smaller periwinkles more often than larger conspecifics, while females refused periwinkles as food. In summer, 80-90% of periwinkles in the field attained a size at which crab predation was minimal in laboratory assays. The observed up to 15-fold decrease in the consumption of seaweed recruits by periwinkles in the presence of nearby consuming crabs in laboratory assays was, however, independent of the size of periwinkles. Predation risk effects from the laboratory were only confirmed in field experiments using crab exclosures but not with open plots, suggesting an artefact due to trespassing by wild crabs in the open plot set-up. 4. Synthesis. Predation risk may not change intraspecific competition for food in L. littorea. Yet, risk effects on foraging behaviour of periwinkles that experience a low threat of real predation indicate that non-consumptive predator effects may affect prey population dynamics more strongly than consumptive predator effects. Thus predation risk effects may be indirectly beneficial for the survival of basal species like perennial seaweeds, as experimental evidence from field and laboratory experiments indicates.