Density- and trait-mediated effects of fish predators on amphipod grazers: potential indirect benefits for the giant kelp Macrocystis pyrifera

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. densitymediated 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.

Top predators rely on carbon derived from giant kelp Macrocystis pyrifera

“…Previous research has indicated that fishes eat kelp mesograzers and may control their abundance (Davenport & Anderson 2007, Pérez-Matus & Shima 2010. Our results ex pand on this to show that kelp forest fishes, particularly higher trophic level fishes, depend on kelp mesograzers, and thus on kelp itself, for a significant portion of their dietary carbon.…”

In situ warming strengthens trophic cascades in a coastal food web

“…It is most likely explained by the structurally complex granite stone habitat that provided shelter from fish predation, but at the same time appears to have reduced the grazing pressure of gammarids on algae growing on the top part of the stones. Such cascading predator effects on primary producers mediated by changes in herbivore behavior are well known to cause strong trophic cascades in various ecosystems (Werner and Peacor , Peckarsky et al , Pérez‐Matus and Shima , Carr and Boyer ). Consequently, we propose that in situ warming, by increasing predation pressure on, and predation risk for, gammarid herbivores, strengthened a trophic cascade that benefitted the filamentous algae.…”

Macrofauna associated with the brown algae Dictyota spp. (Phaeophyceae, Dictyotaceae) in the Sebastião Gomes Reef and Abrolhos Archipelago, Bahia, Brazil

“…ͳͳ Epifaunal abundance can be affected by fish predation and, by regulating the presence of mesograzers, invertivore fishes may even play a role in macroalgae performance (Moksnes et al, 2008;Pérez-Matus and Shima, 2010;Reynolds and Sotka, 2011). Dictyota is known as a chemically-defended seaweed against fishes (Hay, Duffy et al, 1987;, which turns it into a good choice of habitat for mesograzers (Christie et al, 2003(Christie et al, , 2009Duffy and Hay, 1991;Hay, 1997).…”