Direct and indirect effects of predation and predation risk in old-field interaction webs

ABSTRACT Indirect effects emerge when a change in the abundance of one species indirectly affects another by changing the abundances of intermediate species-called density-mediated indirect effects-or they arise when one species modifies how two other species interact-called trait-mediated indirect effects. I report on field experiments that evaluated how grass and herb biomass in old-field interaction webs was influenced indirectly by a spider carnivore through its interactions with a generalist and a grass-specialist grasshopper species. I manipulated interaction pathways between the spider and the plants using different combinations of the grasshopper species. I changed the modality of predator-prey interactions to isolate density-mediated from trait-mediated effects using natural spiders (predation spiders) or spiders that were prevented from subduing prey by mouthpart manipulation (risk spiders). I found that indirect effects were stronger in speciose, reticulate food webs than in linear food chains owing to a trait-mediated effect, a diet shift by herbivores in response to predation risk. Spiders alone did not have significant effects on grasshopper densities in the field experiments, removing any possibility of density-mediated indirect effects. The study illustrates that ecologists should not underestimate the importance of behavioral ecology in determining community-level interactions.