Omnivory simultaneously counteracts and short-circuits cascading predator effects in a marine food web

Trophic cascade theory predicts that variation in top predators propagates to lower trophic levels in a linear food chain.  Omnivory, however, reticulates food webs, and potentially mitigates or halts cascading predator effects.  In a reticulated quadritrophic saltmarsh food web, we used step-wise reduction of two omnivores to elucidate how omnivory modifies trophic cascades.  When the top omnivore (killifish) was significantly reduced, the intermediate omnivore (grass shrimp) foraged more actively on consumers (infauna), thereby offsetting the release from direct predation by killifish on infauna so that infaunal abundances remained unchanged.  We suggest that grass shrimp exhibited a trait-mediated compensatory predation response thereby inhibiting cascading effects on lower trophic levels.  When both omnivores were removed, both infauna and primary producers (benthic microalgae – BMA) increased because of weak herbivore-plant interactions and the short-circuiting effect of omnivory (i.e., grass shrimp directly grazed on BMA).  Our results illustrate that omnivory can disrupt a trophic cascade in a low diversity, algae-based food web by both preventing top-down cascading effects and by shorting the food chain.  By countering cascading predator effects, omnivory may stabilize ecosystems against activities such as overfishing.