Conspecific mortality and resource availability drive a trait-mediated cascade by modifying sea urchin grazing behavior.

Predator risk effects on prey can cascade through food webs and influence ecosystem state via trait-mediated indirect effects (TMIEs). Predicting when these interactions occur and whether they produce community-level effects requires understanding the cues prey respond to, how responses vary with prey attributes and environmental context, and how these components interact. In kelp forest ecosystems, changes in sea urchin foraging behavior in response to food availability or predators can cause state shifts from forests to urchin barrens. It is less clear what cues of predation risk urchins respond to, how these cues interact with food availability, and their effects on kelp mortality. We conducted a field experiment in Monterey Bay, California using cages containing refuge habitat with live kelp (Macrocystis pyrifera) and purple sea urchins (Strongylocentrotus purpuratus) to test for separate and combined effects of a predator cue (red rock crab, Cancer productus), a damaged conspecific cue (crushed urchins), drift kelp, and urchin satiation on urchin exposure (i.e., use of locations exposed to predators), grazing, and kelp mortality. While predator presence did not affect urchin behavior, the damaged conspecific cue reduced urchin exposure (74%), grazing (48%), and kelp mortality (58%). Urchin satiation also decreased the number of exposed grazing urchins (96%) and consumption of live kelp (84%). Adding drift kelp nearly prevented kelp consumption regardless of the damaged conspecific cue, overriding its effect. These findings show predation risk cues can interact with resource availability to modify prey behavior, demonstrating how a trait-mediated indirect effect may cascade to alter ecosystem state.