Abstract

Indirect effects of predators on primary producers may arise when predators suppress herbivore abundance and/or alter the behaviour patterns of herbivores in ways that reduce grazing pressure. Most studies highlight the role of predators in reducing herbivore abundance (i.e. density-mediated interactions), while behavioural effects (i.e. trait-mediated interactions) induced by predators are less commonly considered and are often assumed to be of secondary importance. We used a mesocosm experiment to evaluate the consumptive and behavioural effects of 2 species of predatory fishes on amphipod grazers that feed and nest on the giant kelp Macrocystis pyrifera. Only one, Notolabrus celidotus, of the 2 predator species reduced grazer abundance. Although the second predator species, N. fucicola, did not affect grazer abundance directly, it significantly reduced the grazing efficiency of amphipods on blades of M. pyrifera. Our study illustrates how density-and trait-mediated interactions reduce grazer efficiency and provide potential indirect benefits to primary producers. To our knowledge, this is the first experimental evidence of a potential trophic cascade in kelp beds generated by consumptive and nonconsumptive effects. When considered alongside results of other studies that highlight positive effects of macroalgae on reef fish demographic rates, our results provide compelling evidence that mutualistic relationships may exist between kelp and associated predatory fishes.