Sunflower sea star chemical cues locally reduce kelp consumption by eliciting a flee response in red sea urchins

Predator loss can cause shifts in ecosystem state, especially when accompanied by changes in the behaviour of its prey. The recent decline of predatory sunflower sea stars <i>Pycnopodia helianthoides</i> and a coincident a decline of kelp forests across the northeastern Pacific raises questions about their role in kelp forest ecosystem maintenance and recovery. While <i>P. helianthoides</i> may support kelp forests by consuming herbivorous sea urchins, less is known about their non-consumptive effects, especially in degraded habitats. Here, we ask: How do the non-consumptive effects of this predator vary among prey species, and what are the emergent effects on community-level grazing pressure in a degraded habitat when kelp is reintroduced? We conducted a field experiment, where we deployed kelp blades in urchin barrens at discrete distances from caged <i>P. helianthoides</i> and control cages for 24 h. We demonstrate a reduction in the density of red sea urchins <i>Mesocentrotus franciscanus</i> nearest the sunflower star and no effect on green sea urchins <i>Strongylocentrotus droebachiensis</i>. Our results suggest the chemical cue of <i>P. helianthoides</i> elicits a localized landscape of fear of approximately 15 m² that suppresses grazing, and that the non-consumptive effects of <i>P. helianthoides</i> on sea urchin behaviour may be important for kelp restoration.