Non‐consumptive predator effects indirectly influence marine plant biomass and palatability

Summary 1.  Predators can reduce herbivory and increase plant biomass by consuming herbivores, lowering individual herbivore feeding rates, or both. We tested whether the presence of predators increases plant quality by non‐consumptively reducing grazing pressure and thereby weakening the strength of the induced response in plant chemical defences. 2.  We performed a 42‐day outdoor mesocosm experiment in which the herbivorous amphipod Ampithoe longimana was cultured on the brown seaweed Sargassum filipendula in the presence and absence of olfactory cues of its principal fish predator, the pinfish Lagodon rhomboides . The presence of fish cues reduced per capita rates of amphipod grazing by nearly 50%. Over the span of the mesocosm experiment, this per capita reduction in feeding rate yielded at least a 40% lower growth rate of amphipod populations (i.e. r reduced from 1.01 to 0.61). The lower rates of amphipod grazing (overall or per capita) correlated with higher algal biomass. 3.  We pursued a series of laboratory‐based feeding choice assays with naïve amphipods to determine tissue palatability and the plant traits that mediate feeding choices. Tissue from tanks without grazers was more palatable than tissue from tanks with grazers, a pattern of induced plant defences that has been documented previously. Surprisingly, however, plant tissue from tanks with grazers and fish cues was more palatable than tissue from tanks with grazers but without fish cues. All changes in algal palatability were mediated by polar, but not lipophilic metabolites. These results suggest that the non‐consumptive effects of fish predators increases the food quality of Sargassum by weakening the strength of its induced chemical defences. 4.Synthesis . The smell of predators has the potential to regulate herbivore populations and affect the ecological dynamics of plant biomass and chemical defences.