DIRECT AND INDIRECT EFFECTS OF PREDATION AND PREDATION RISK IN OLD-FIELD INTERACTION WEBS

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 o ne species modifies how two other species interact-called trait-mediat ed indirect effects. I report on field experiments that evaluated how grass and herb biomass in old-field interaction webs was influenced in directly by a spider carnivore through its interactions with a general ist and a grass-specialist grasshopper species. I manipulated interact ion pathways between the spider and the plants using different combina tions of the grasshopper species. I changed the modality of predator-p rey interactions to isolate density-mediated from trait-mediated effec ts using natural spiders (predation spiders) or spiders that were prev ented from subduing prey by mouthpart manipulation (risk spiders). I f ound that indirect effects were stronger in speciose, reticulate food webs than in linear food chains owing to a trait-mediated effect, a di et shift by herbivores in response to predation risk. Spiders alone di d not have significant effects on grasshopper densities in the field e xperiments, removing any possibility of density-mediated indirect effe cts. The study illustrates that ecologists should not underestimate th e importance of behavioral ecology in determining community-level inte ractions.