Understanding the individual and interactive roles of consumer species is more than academic when the host plant is a subject of intense conservation interest. In a mesocosm experiment, we compared effects of common invertebrate grazers in San Francisco Bay seagrass (Zostera marina
, eelgrass) beds, finding that some species (a native opisthobranch, Phyllaplysia taylori
; a native isopod, Idotea resecata
; and an introduced gastropod, Ilyanassa obsoleta
) enhanced eelgrass growth through removal of epiphytic algae, as is often predicted for small invertebrate grazers on seagrasses, while one (an introduced caprellid amphipod, Caprella
) had neutral effects. In contrast, the putatively-introduced gammaridean amphipod, Ampithoe valida
, had strong negative effects on eelgrass (in addition to epiphytes) through consumption, as we had previously observed in the field during restoration programs. We tested whether other common grazer species could influence the effects of the eelgrass-grazing Ampithoe
, and found that Idotea
induced production of phenolic compounds and limited eelgrass damage by Ampithoe
, without affecting Ampithoe
abundance. These results have implications for restoration strategies, and contribute to a growing awareness of the importance of trait-mediated indirect grazer interactions through grazer-induced changes in plant traits, providing the first example in a seagrass system.
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