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 specific 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 predation risk cues 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 predation cue (crushed urchins), drift kelp, and urchin satiation on urchin exposure, grazing, and kelp mortality. While predator presence did not affect urchin behavior, the predation cue markedly reduced urchin exposure (74%) and grazing (48%), reducing kelp mortality (58%). Urchin satiation strongly decreased exposed urchin behavior (96%) and grazing (84%). For fed urchins with low levels of grazing, drift kelp further decreased urchin grazing (90%) and nearly prevented kelp loss regardless of the predation cue, thus overriding its effect. These findings show predation cues can interact with resource availability to modify prey behavior and demonstrate how a trait-mediated indirect effect may cascade to alter ecosystem state.