Effects of Host-Dependent Niches and Biotic Constraints on Climate Change Driven Range Shifts in Anemonefish

We investigated how obligate mutualisms constrain species distributions under climate change, challenging the assumption that biotic interactions are negligible at macro-scales. By integrating host sea anemone distributions into Species Distribution Models for 17 anemonefish species, we found that host availability is a primary determinant of the realised niche, especially for specialists. Under future warming (SSP5-8.5), host immobility creates a biotic constraint, causing fish ranges to lag significantly behind their climatic potential. This mismatch generates over 3.2 million km2 of climatically suitable but ecologically inaccessible ocean. Furthermore, specialist anemonefish species with the narrowest niches face the highest climate velocities while being constrained to the most dispersal-limited hosts. These findings indicate that climate-only assessments underestimate extinction risk. Conservation should shift to a host-first management strategy to prevent the collapse of these mutualisms. Scientific Significance StatementClimate change assessments often assume species can freely track their preferred temperatures, ignoring the critical species they rely on for survival. We demonstrate that for obligate mutualists like anemonefish, the future is defined not just by where they can swim, but by where their host sea anemones can persist. Our models reveal that millions of square kilometers of ocean will become climatically perfect for fish but devoid of the hosts they need to survive. This mechanism of range loss disproportionately threatens specialist species. Our findings highlight the need to prioritise the conservation of immobile partner species, as their failure to migrate effectively traps their mobile symbionts in degrading environments.