Drastic Biodiversity Transition as a Function of Rainfall Patterns: From Coastal Drylands to Amazonian Rainforests

Abstract

Aim By moving beyond traditional biome‐centric approaches to sample continuously across a massive ecotone, we investigate how species richness, community composition, and dominance of woody plants change across a strong longitudinal precipitation gradient and identify thresholds in water availability that drive shifts in community assembly between tropical dry forests and rainforests. Location A longitudinal precipitation gradient within and beyond the Marañon Valley, a biogeographic corridor linking Pacific dryland forests, Andean highland forests, and Amazon rainforests in northern Peru. Time Period Contemporary forests sampled between 2016 and 2020. Major Taxa Studied Woody tree species (angiosperms). Methods We modelled species richness, community composition, and species dominance in response to climatic and edaphic factors using floristic data from the first National Forest Inventory in Peru. Results Alpha diversity increased from the coastal drylands to the Andes, peaking at a 1299 mm annual precipitation threshold. Beyond this threshold in the Amazon, climatic factors no longer explain diversity patterns. Species composition shifted gradually, exhibiting sharp regional differences at 1265 mm. Species dominance patterns changed markedly along the longitudinal axis, from locally dominant, stress‐adapted species in drylands to regionally widespread low‐dominant species in the Amazon Forest. Main Conclusions Our findings indicate that major transitions in woody plant biodiversity are tightly linked to rainfall patterns up to a critical threshold, beyond which other factors likely govern community structure, highlighting the vulnerability and potential reorganization of these ecosystems under ongoing climatic change. Ultimately, this demonstrates that assessing the landscape as a continuum, rather than exploring isolated biomes, is crucial for capturing the non‐linear environmental thresholds that redefine community assembly rules on a global scale.

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