Cascading effects of a top predator on intraspecific competition at intermediate and basal trophic levels

1. Predators can impact competition among prey by altering prey density via consumption or by causing prey to modify their traits or foraging behavior. Yet, differences between these two mechanisms may lead to different cascading impacts on lower trophic levels. 2. Using a crab-snail-barnacle rocky intertidal food chain, we tested the effects of predation risk from crabs (top predators) on intraspecific competition among snails (intermediate consumers) and emergent indirect effects on the density of and competition between barnacles (basal resources). 3. The per capita foraging and growth rates of snails declined with high conspecific density. Predation risk from crabs, which caused even larger reductions in snail foraging and growth, weakened competition among snails, whereas a 45% increase in barnacle density had no detectable effect on snail competition. 4. Intraspecific competition between barnacles, however, depended on the interactive effects of barnacle density, snail density,...