Environmental specialization decouples geographic range and habitat occupancy in Ragala ucuquirana-branca (Sapotaceae) across the Amazon Basin

Resumo

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) and the environmental filters constraining its realized distribution. Material and methods – We compiled 72 occurrence records from herbarium specimens and forest monitoring plots across the Amazon Basin. Species distribution models were developed using an ensemble framework integrating climatic, edaphic, topographic, and land cover variables across multiple algorithms. Model performance was evaluated using threshold-independent and threshold-dependent 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 emerged as the most influential predictor, followed by water vapor pressure, minimum temperature of the coldest month, and the cation exchange capacity. Although the species exhibits a broad Extent of Occurrence of ca 1.48 million km2, its Area of Occupancy is highly restricted, revealing a spatially aggregated distribution. This pattern, together with severe fragmentation among subpopulations and environmentally constrained suitable habitats, supports a preliminary conservation assessment of Endangered. Suitable habitats are concentrated in Central Amazonia and adjacent portions of Northwestern and Southwestern Amazonia, whereas marginal conditions predominated across Eastern and especially Southeastern Amazonia. Conclusion – 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 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.

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