Purple sea urchins Strongylocentrotus purpuratus reduce grazing rates in response to risk cues from the spiny lobster Panulirus interruptus

The classical view of trophic cascades is that predators, by consuming herbivores, exert a positive indirect effect on plants. Although this form of trophic cascade has been demonstrated in a variety of terrestrial, aquatic, and marine systems, growing evidence suggests many trophic cascades are driven by anti-predator behaviors in prey. Despite abundant evidence of behavioral responses by sea urchins to predators, there has been little examination of how predation risk may influence urchin grazing rates. To determine if purple sea urchins Strongylocentrotus purpuratus graze less in the presence of predation risk, I monitored individual urchin grazing on the kelp Macrocystis pyrifera in the presence and absence of waterborne cues from damaged conspecifics and the predatory spiny lobster Panulirus interruptus. Sea urchin grazing rates were similar in the presence and absence of damaged conspecifics, but sea urchins exposed to lobster cues, regardless of lobster diet, reduced grazing rates by 44 %. Given that trophic cascades involving herbivorous sea urchins exert an important influence on primary production in kelp forests, these results suggest that predation risk may play an important but under-appreciated role in the dynamics of kelp forest food webs and primary production.