MEPS 581:33-44 (2017)  -  DOI: https://doi.org/10.3354/meps12323

ABSTRACT: Although herbivores often interact indirectly via induced trait changes in food resources, ecologists are just beginning to understand the diversity of these interactions in multi-species communities. For instance, while herbivore identity is often important for eliciting trait changes, we know little about how the strength of induced responses varies with herbivore species. This variation is important, as it may affect interactions between herbivore species and, in turn, herbivore populations, community diversity, and the effectiveness of primary producer responses. To investigate herbivore-specific differences in the strength of induced defenses, we exposed the seaweed Silvetia compressa (hereafter Silvetia) to grazing by isopods (Idotea wosnesenskii, hereafter Idotea), snails (Tegula funebralis, hereafter Tegula), or both herbivores together. After 2 wk, we compared relative tissue palatability by offering each herbivore a choice between conspecific-grazed tissues and either (1) non-grazed tissues, (2) heterospecific-grazed tissues, or (3) tissues grazed by both herbivores. Both species preferred non-grazed over conspecific-grazed tissues, confirming induced changes in palatability. When we directly compared tissues induced by isopods and snails, Tegula-grazed tissues were less palatable than Idotea-grazed tissues, suggesting that Tegula elicits stronger Silvetia responses than Idotea. When offered a choice between heterospecific-grazed and non-grazed tissues, both herbivores responded to heterospecific grazing. Thus, while both herbivores decrease Silvetia palatability, the strength of the response varies with grazer identity, which we would have not predicted had we only compared grazed and non-grazed tissues. Our results suggest that in communities where herbivores elicit defenses of different strength, interspecific herbivore interactions may be asymmetric, depending on which seaweed phenotypes have been induced.

KEY WORDS: Seaweed-herbivore interactions · Inducible defense · Macroalgae · Asymmetric competition