Size-dependent trait-mediated indirect interactions among sea urchin herbivores

Despite their importance in community interactions, nonlethal indirect effects of predators are not well understood in many marine food webs. In this study, I found that within a guild of herbivorous sea urchins, small urchins (Strongylocentrotus droebachiensis and small Strongylocentrotus franciscanus) altered grazing rates in the presence of the predatory sea star (Pycnopodia helianthoides) and were highly preferred by the predator. In contrast, large urchins (adult S. franciscanus) did not significantly alter grazing in the presence of cues from the sea star and, when immobile, were less frequently attacked by the predator. However, the sea star's preference (active predator choice) was obscured by sea urchin mobility, that is, small S. franciscanus was only most preferred when unable to escape. These results suggest that by identifying the relative threat of predation facing guild members and the degree to which individuals transmit trait-mediated indirect interactions, these indirect effects may be predictably incorporated in community interactions.