title: Omnivory and grazer functional composition moderate cascading trophic effects in experimentalFucus vesiculosushabitats.
abstract: We tested the relative strength of direct versus indirect effects of an aquatic omnivore depending on the functional composition of grazers by manipulating the presence of gastropod and amphipod grazers and omnivorous shrimp in outdoor mesocosms. By selectively preying upon amphipods and reducing their abundance by 70-80%, omnivorous shrimp favoured the dominance of gastropods. While gastropods were the main microalgal grazers, amphipods controlled macroalgal biomass in the experiment. However, strong predation on the amphipod by the shrimp had no significant indirect effects on macroalgal biomass, indicating that when amphipod abundances declined, complementary feeding by the omnivore on macroalgae may have suppressed a trophic cascade. Accordingly, in the absence of amphipods, the shrimp grazed significantly on green algae and thereby suppressed the diversity of the macroalgal community. Our experiment demonstrates direct consumer effects by an omnivore on both the grazer and producer trophic levels in an aquatic food web, regulated by prey availability.
authors.affiliation: Department of Marine Benthic Ecology and Evolution, Centre for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands.
authors.forename: Britas Klemens
authors.initials: BK
authors.lastname: Eriksson
authors.affiliation: Department of Marine Benthic Ecology and Evolution, Centre for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands.
authors.forename: Christiaan
authors.initials: C
authors.lastname: van Sluis
authors.affiliation: Department of Marine Benthic Ecology and Evolution, Centre for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands.
authors.forename: Katrin
authors.initials: K
authors.lastname: Sieben
authors.affiliation: Department of Botany, University of Stockholm, 106 91 Stockholm, Sweden.
authors.forename: Lena
authors.initials: L
authors.lastname: Kautsky
authors.affiliation: Department of Botany, University of Stockholm, 106 91 Stockholm, Sweden.
authors.forename: Sonja
authors.initials: S
authors.lastname: Råberg