Data from: Moving beyond linear food chains: trait-mediated indirect interactions in a rocky intertidal food web

In simple, linear food chains, top predators can have positive indirect
effects on basal resources by causing changes in the traits (e.g.
behaviour, feeding rates) of intermediate consumers. Although less is
known about trait-mediated indirect interactions (TMIIs) in more complex
food webs, it has been suggested that such complexity dampens trophic
cascades. We examined TMIIs between a predatory crab (Carcinus maenas) and
two ecologically important basal resources, fucoid algae (Ascophyllum
nodosum) and barnacles (Semibalanus balanoides), which are consumed by
herbivorous (Littorina littorea) and carnivorous (Nucella lapillus)
snails, respectively. Because crab predation risk suppresses snail feeding
rates, we hypothesized that crabs would also shape direct and indirect
interactions among the multiple consumers and resources. We found that the
magnitude of TMIIs between the crab and each resource depended on the
suite of intermediate consumers present in the food web. Carnivorous
snails (Nucella) transmitted TMIIs between crabs and barnacles. However,
crab–algae TMIIs were transmitted by both herbivorous (Littorina) and
carnivorous (Nucella) snails, and these TMIIs were additive. By causing
Nucella to consume fewer barnacles, crab predation risk allowed fucoids
that had settled on or between barnacles to remain in the community.
Hence, positive interactions between barnacles and algae caused crab–algae
TMIIs to be strongest when both consumers were present. Studies of TMIIs
in more realistic, reticulate food webs will be necessary for a more
complete understanding of how predation risk shapes community dynamics.