Conservation gaps and spatial priorities in threatened South American ecotones

Introduction: Climate change and land-use conversion are driving unprecedented biodiversity loss across South American ecotones, yet most native plant species in these regions lack formal conservation assessments. Here we present the first comprehensive climate vulnerability analysis for Castela tweedii (Simaroubaceae), an endemic shrub of southern South American ecotonal habitats. Materials and methods: We integrated ensemble species distribution modeling (SDM) (six algorithms, 15-fold cross-validation), Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections under two Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5) for 2050 and 2080, high-resolution land-use data from MapBiomas 2022, landscape fragmentation analysis, and Zonation-based systematic conservation prioritization across the species’ range in southern Brazil, Uruguay, Paraguay, and northeastern Argentina. Results: Our ensemble model projects severe habitat contraction under all scenarios: 38–39% loss by 2050 and 53–64% by 2080, with precipitation-related variables (precipitation of the driest month, precipitation seasonality and precipitation of the wettest month) as the dominant drivers (46.8% combined variable importance). Climate refugia contract dramatically from 29.2% to 15.6% of current habitat between 2050 and 2080, concentrating in the Atlantic Forest–Cerrado transition zone. Critically, 48.8% of climatically suitable habitat retains less than 30% natural vegetation and is dominated by anthropogenic land uses, reducing the effective mesh size by 95.2% compared to climate-only projections, a disparity invisible to traditional SDM approaches. Only 5.5% of suitable habitat overlaps with existing protected areas, far below the Kunming–Montreal 30% target, exposing a severe conservation gap. Our Zonation-based prioritization identifies 108,800 km2 of high-priority areas for reserve expansion and corridor establishment, of which 65.4% retain natural vegetation. Conclusions: These findings provide the first quantitative basis for formal International Union for Conservation of Nature (IUCN) threat evaluation of C. tweedii and demonstrate that climate-only assessments, by neglecting the compounding effect of anthropogenic land-use conversion, systematically underestimate habitat loss for ecotonal species, with direct implications for conservation planning across Neotropical transition zones.