Direct and Indirect Effects of Predation and Predation Risk in Old‐Field Interaction Webs

• Published in: The American naturalist 1998-04, Vol.151 (4), p.327-342
• Main Author: Schmitz, Oswald J.
• Format: Article
• Language: English
• Subjects: Biomass; Ecology; Grasses; Herbivores; Herbs; Predation; Predators; Spider webs; Spiders; Trophic relationships
• ISSN: 0003-0147; 1537-5323
• DOI: 10.1086/286122

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.