Extreme marine heatwaves drive divergent kelp forest trajectories and alternative stable states

Jeremie Bauer¹Julio Lorda²Rodrigo Beas-Luna^2*Luis Malpica-Cruz²Alicia Abadía-Cardoso²Andrea Paz Lacavex³Jorge Olmos^1*

• ¹Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Ensenada, Mexico
• ²Universidad Autonoma de Baja California - Campus Ensenada, Ensenada, Mexico
• ³University of California Santa Cruz, Santa Cruz, United States

Kelp forests are experiencing extreme declines globally, yet local social-ecological factors may mediate their responses to climate perturbations. Using a combination of long-term field surveys (2016–2023) and satellite data (2004–2023), we examined the regional effects of the 2014–2016 MHWs regime on kelp forest ecosystems along a 600 km latitudinal gradient in Baja California, Mexico. We documented three distinct geographical subregion trajectories: (1) a remarkable resilience of giant kelp Macrocystis pyrifera, at its thermal limits in the southern subregion, recovering 95% of the historical canopy area despite experiencing the highest temperatures; (2) understory-dominated state in the mid-subregion, with a 96% decrease in M. pyrifera canopy, where understory brown macroalgae filled the ecological niche left by declining giant kelp, maintained by moderate herbivore densities; and (3) a shift to an urchin barren alternative stable state in the northern subregion, characterized by a 2,485% increase in herbivore densities from 2016 to 2023 —dominated by purple sea urchin Strongylocentrotus purpuratus (44%)— and a 95% reduction in giant kelp canopy area compared to pre-warming baseline. The southern subregion's resilience was associated with high predators' (10.26 ± 5.63 per 60 m²) and low herbivores' (25.20 ± 8.89 per 60 m²) densities, dominated by turban snails Megastraea spp. (83%) with S. purpuratus absent. In contrast, the northern subregion's shift from kelp forests to urchin barrens coincided with low predator abundances (5.05 ± 2.32 per 60 m²), potentially linked to fragmented fisheries management. Our findings challenge the assumption that range-edge populations are inherently more vulnerable to warming and underscore how local biological factors and management approaches can either enhance or compromise ecosystem resilience to climate stress. These divergent trajectories suggest that while temperature acts as an initial driver of change, the persistence of alternative states is governed by ecological feedback involving predator-herbivore dynamics, herbivores' assemblages, and management contexts, providing insights for developing climate-adaptive conservation strategies under intensifying global environmental change.