Mountain ecosystems concentrate disproportionate biodiversity and endemism globally -- approximately 25% ofterrestrial biodiversity occurs in mountain regions covering only 22% of land area -- driven by high habitat heterogeneityacross steep environmental gradients in elevation, aspect, soil, and microclimate that generate diverse ecological nicheswithin small geographic areas. This study quantified the relationships between habitat heterogeneity, species richness,and endemism across 42 mountain ranges from six continents (2,840 species occurrence records aggregated from GBIFand regional databases; 284 survey transects; 2019-2021), using terrain complexity indices (TRI, TPI, aspect diversity,slope variation), climate heterogeneity metrics (CHELSA v2.1), and soil type diversity (SoilGrids 2.0) as heterogeneitypredictors. Habitat heterogeneity (composite of terrain, climate, and soil diversity) was the strongest predictor of bothspecies richness (r = +0.84; p < 0.001) and endemism rate (r = +0.78; p < 0.001) across mountain ranges --outperforming mountain range area (r = +0.48), total elevational range (r = +0.62), and evolutionary age (r = +0.54).Vertical heterogeneity (elevational gradient span) contributed the largest single component of composite heterogeneity torichness prediction (partial r = +0.74; unique variance = 38.4%), while soil diversity contributed most to endemism (partialr = +0.68; unique variance = 28.4%). A Mountain Endemism Vulnerability Index (MEVI) integrating endemism rate,climate velocity, and range size predicted climate-driven extinction risk with AUC = 0.864. Tropical Andes, EasternAfromontane, and Mesoamerican Highlands showed the highest combined richness, endemism, and MEVI scores --identifying these as the most urgent targets for mountain biodiversity conservation under climate change