Marine heatwaves drive range contraction and alternative states of kelp forests at their warm limit

Marine heatwaves are transforming ecosystems, yet their role in driving alternative states—and the conditions that enable these transitions—remains poorly understood. Using 30 years of satellite and underwater data, we assessed the impact of the 2014–2016 Pacific marine heatwaves on giant kelp forests ( Macrocystis pyrifera ) at their warm range limit in Mexico. By 2016, 88% of forests were lost, with limited recovery by 2023, including an 80 km range contraction at the southern edge. Surveys revealed three alternative states: replacement by heat-tolerant palm kelp ( Eisenia arborea ) in warmer regions; urchin barrens due to predator overfishing; and, unexpectedly, persistent giant kelp near the southern limit where high temperatures coincide with low human pressure. Pre-existing conditions, such as high urchin and palm kelp densities, shaped these outcomes. These findings show that responses to marine heatwaves are shaped by local ecological and human contexts, requiring tailored climate-adaptation strategies to promote resilience.