Compound impacts of sea urchin grazing and marine heatwave thermal stress drive kelp collapseResistance, Resilience and Phase Shifts

Jiaxin Shi¹, Scott Bennett¹, Jules Kajtar², Neil Holbrook¹

¹University of Tasmania, Australia - ²National Oceanography Centre, UK

In the face of global warming, reliable estimation of species and ecosystem responses to intensifying warming extremes are crucial. For marine organisms, these responses are shaped by multiple biotic and abiotic forcings. Characterising interactions is essential to understand whether combined drivers are likely to exacerbate or mitigate species’ responses. Kelp forests, vital to marine biodiversity and ecosystem services, are increasingly threatened by marine heatwaves (MHWs) as our climate warms. Based on 30 years of reef monitoring data from around Australia, we advance a hybrid model showing that warm-edge kelp forests exhibit higher critical limits but also greater vulnerability to collapse in response to MHWs. In addition to the direct impact, MHWs also pose indirect impacts on kelp via modifications to herbivory stress, particularly from sea urchin grazing. Reef observations along the east coast of Australia revealed that when urchin density is above 2.7 urchins/m2, kelp collapse is inevitable regardless of MHW intensity. Sea urchin grazing is the dominant driver of kelp collapse. It exacerbates the stress from MHW maximum intensity, leading to intensified degradation through compound impacts. Higher densities of sea urchins significantly lower the threshold at which MHWs trigger kelp collapse. Managing sea urchin populations emerges as a powerful strategy to increase the resilience of kelp forests against MHWs. This study enhances the understanding of kelp response to a changing climate and offers valuable insights for effective risk management.