Marine heatwaves leap tropical, herbivorous fish into temperate ecosystems and threaten future kelp strongholds

As oceans warm and environmental isotherms shift, so does the distribution of species to track regions that encompass their preferred thermal ranges. Whilst gradual warming is a major driver of this phenomenon, marine heatwaves can also play a role by creating sudden shifts that facilitate longer-term ‘tropicalisation’, indirectly affecting the resilience of temperate kelp forests through heightened herbivory. Predicting the future distribution of temperate kelp forests is key to making robust conservation management decisions, however, current studies do not take discrete climatic events or their indirect effects into account. Here, we incorporate climatic extremes and biotic interactions into species distribution models to help understand future kelp forest persistence, by projecting distributions of tropical, herbivorous fish as they respond to future extreme marine heatwaves. Other studies indicate kelp forests are likely to be limited to the southern coast of Western Australia under gradual warming scenarios, however we show this region will also be highly suitable for tropical herbivores who contribute to the erosion of kelp forest resilience, and promote the persistence of canopy free-states. We demonstrate major differences in fish distributions between Representative Concentration Pathway scenarios (2.6 ‘most optimistic’, versus 8.5 ‘business as usual’), with the latter resulting in the majority (~70%) of temperate Western Australia becoming highly suitable for tropical herbivores. Whilst seemingly inevitable that some degree of kelp forests will be lost from the region over the coming decades, our ability to incorporate climate-mediated indirect impacts into targeted intervention could help direct the trajectory of change towards more desirable outcomes.