Marine Heatwaves and Reduced Dietary Diversity Weaken Herbivory Control Over the Invasive Seaweed <scp> <i>Rugulopteryx okamurae</i> </scp>

ABSTRACT The brown macroalga Rugulopteryx okamurae , identified as invasive in the Strait of Gibraltar a decade ago, is rapidly expanding across the NE Atlantic and the NW Mediterranean, displacing native assemblages and disrupting coastal ecosystems. However, the capacity of native herbivores to regulate this invasion remains poorly understood, especially under climate‐driven extreme events such as marine heatwaves (MHWs). In this study, we conducted a mesocosm experiment to test the feeding response of the sea urchin Paracentrotus lividus on R. okamurae and two native macroalgae, Dictyota fasciola and Ulva sp., across a simulated MHW gradient. Consumption rates declined significantly with rising temperatures, indicating that warming reduces overall grazing activity, and R. okamurae was always the least consumed species. Feeding preference was consistent across treatments: D. fasciola was the first preference, followed by Ulva sp., and R. okamurae as the last option. Our findings show that R. okamurae is only partially grazed under lower temperature conditions, with consistently lower consumption than native macroalgae, and that rising temperatures progressively suppress overall herbivory, further reducing top‐down control on the invader and facilitating its spread across Mediterranean and Atlantic reefs.