Marine heatwave and keystone predator loss drive broad-scale decline and hinder recovery of a rocky intertidal kelp.

Human activities are increasingly driving the co-occurrence of multiple ecological stressors, resulting in interactive and cumulative impacts that can reshape ecosystem dynamics and accelerate population declines of climate-sensitive species. Here, we use over two decades of rocky intertidal monitoring data from 17 sites spanning over 1200 km of coastline to assess how two unprecedented stressors-a multiyear marine heatwave and the disease-driven loss of a keystone predator (Pisaster ochraceus)-impacted populations of the canopy-forming intertidal kelp Postelsia palmaeformis. We show that Postelsia experienced rapid and severe declines during the 2014-2016 northeast Pacific marine heatwave, with an average population decline of 50%, multiple site-level extirpations, and particularly striking losses in the southern portion of the species' geographic range. Concurrently, Pisaster declines triggered mussel bed expansion into habitats previously occupied by Postelsia, further inhibiting kelp recoveries. Our findings reveal how converging stressors can drive persistent, broad-scale ecological shifts through both direct and indirect pathways. These results also highlight the critical role of long-term, spatially extensive monitoring in detecting and understanding global change impacts and provide a foundation for guiding Postelsia conservation and restoration efforts.