Olguin-Jacobson, C. | bioRxiv Science

bioRxiv2025

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.

ecology↗

bioRxiv2024

Recovery mode: Marine protected areas enhance the resilience of kelp species from marine heatwaves

Marine protected areas (MPAs) can promote population recovery from climate change impacts by reducing local stressors, such as fishing. However, with extreme climatic events such as marine heatwaves (MHWs) increasing in frequency and duration, it remains unclear whether MPAs enhance recovery following these acute perturbations, and how recovery varies across species and ecological traits (e.g., sedentary vs mobile species). We used 16 years (2007-2022) of kelp forest monitoring data in Isla Natividad, Baja California Sur, Mexico, to assess the impact of the 2014-2016 MHWs on fish and invertebrate communities. Then we evaluated the impact and recovery from the MHWs of economically and ecologically important invertebrate species inside and outside two fully protected marine reserves. We found that the 2014-2016 MHWs, which were the most intense and persistent ever observed in Isla Natividad, impacted invertebrates but not fish biomass. Marine reserves did not confer resistance to the MHWs, however, reserves did enhance the recovery of some species after the MHWs. Inside marine reserves, abalone (Haliotis spp.) and wavy turban snail (Megastraea spp.) (benthic sedentary invertebrates) recovered to pre-heatwave biomass after two years and spiny lobster (Panulirus interruptus) (benthic mobile invertebrate) after four years. Outside the reserves, abalone recovered after three years, while the other two species never recovered. The warty sea cucumber (Apostichopus parvimensis) population collapsed after the MHWs and never recovered inside nor outside the reserve. Remarkably, abalone biomass had an outstanding and sustained recovery inside reserves, with a 5.6-fold increase in biomass after the MHWs, which was over three times higher than the recovery reported outside the reserve. Our analysis of long-term monitoring data shows that marine reserves cannot prevent adverse impacts from extreme climatic events but can enhance species recovery following these events. Benefits conferred by marine reserves, however, are species-specific and may be limited to species with limited dispersal and localized population dynamics.

ecology↗