Restricted occupancy and environmental specialization of Ragala ucuquirana-branca (Sapotaceae) across the Amazon Basin

Background and aims – Understanding the distribution of Amazonian tree species is hindered by sparse occurrence data and strong environmental heterogeneity. Species with wide geographic ranges but restricted habitat occupancy challenge climate-only explanations of distribution patterns. Here, we investigate the environmental drivers shaping the distribution of Ragala ucuquirana-branca (Sapotaceae, Chrysophylloideae), a canopy tree native to the Amazon Basin, to disentangle the roles of environmental filters in constraining its realized distribution.Material and methods – We compiled and spatially validated 72 occurrence records from verified herbarium specimens and long-term forest monitoring plots collected between 1941 and 2025 across the Amazon Basin. Species distribution models were developed using an ensemble framework that combined multiple algorithms and integrated climatic, atmospheric, edaphic, topographic, and land cover variables. Model performance was evaluated using threshold-independent and threshold-dependent accuracy metrics, and habitat suitability was projected across the Amazon Basin.Key results – Model performance was consistently high, indicating robust discrimination between suitable and unsuitable environments. Soil-related predictors collectively accounted for the largest share of model importance, exceeding climatic and other environmental variables. Soil pH in water (Phh2o) emerged as the most influential predictor of distribution, followed by atmospheric water vapor pressure (Vapr), minimum temperature of the coldest month (Bio6), and the cation exchange capacity (Cec). Although the species exhibits a broad Extent of Occurrence of approximately 1.48 million km2, its Area of Occupancy is highly restricted, revealing a spatially aggregated distribution. Suitable habitats are concentrated in the central and western Amazonia, with strong fragmentation toward the eastern and southern margins of the basin.Conclusion – Our results demonstrate that the distribution of R. ucuquirana-branca is strongly constrained by edaphic conditions and atmospheric stability rather than by broad climatic gradients alone. This decoupling between geographic range size and effective habitat occupancy highlights pronounced ecological specialization and potential vulnerability. More broadly, the study underscores the importance of incorporating soil and atmospheric filters into assessments of species rarity, distribution, and conservation priorities in hyperdiverse tropical forests.