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 across 17 sites spanning over 1,200 km of coastline to assess how two unprecedented stressors—a multi-year 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. DOI https://doi.org/10.32942/X2SQ00 Subjects Ecology and Evolutionary Biology, Life Sciences, Marine Biology

climate change, sea star wasting, sea palm, mussel, Postelsia palmaeformis, Pisaster ochraceus, Mytilus californianus Dates Published: 2025-07-21 19:47 Last Updated: 2025-07-21 19:47 License CC-BY Attribution-NonCommercial 4.0 International Additional Metadata Conflict of interest statement: The authors declare no competing interests. Data and Code Availability Statement: This manuscript uses novel data and code that are publicly available at https://github.com/fgerraty/Postelsia_Collapse and archived via Zenodo at the following DOI: https://doi.org/10.5281/zenodo.16241083. Study site names and site coordinates are not included in publicly available data because Postelsia palmaeformis is a protected species and threatened by illegal human harvest. Language: English