Climate-driven range shifts across Subantarctic and Antarctic terrestrial and marine niches in sheathbills (Chionis).

Rapid environmental changes in Subantarctic and Antarctic regions pose significant threats to fauna dependent on interconnected terrestrial and marine ecosystems. Here, we investigate the ecological niches and potential distribution dynamics of two sister species of sheathbills, Chionis albus and C. minor, which range from South America to the Antarctic Peninsula (C. albus) or are restricted to some Subantarctic islands (C. minor). Ecological niches were estimated in environmental space through a principal component analysis-based modelling approach, while species distribution models provided geographically explicit projections; together, these complementary methods allowed us to assess niche dynamics. Projections for 2070 under two climate scenarios (SSP1-2.6 and SSP5-8.5) show that C. albus occupies a broader ecological niche, with high stability in both terrestrial (99%) and marine (94-95%) habitats, and when combined, 95% of its range remains stable with low extinction (≤1%) and moderate expansion (4.2-4.3%). In contrast, C. minor shows lower stability (83-96%), higher extinction potential (up to 6%) and greater expansion variation (2.8-13.6%). However, when habitats are combined, extinction zones decrease to near zero under both scenarios. Our findings underscore the vulnerability of island species to environment-driven range shifts and show that integrating terrestrial and marine habitats provides more conservative and robust risk projections.