Mountain ecosystems are often interpreted through the lens of the ‘sky island' model, where high‐elevation habitats function as isolated archipelagos. However, this model's applicability to massive, topographically complex mountain ranges where highlands are continuous and lowlands are fragmented remains untested. We examined microbial taxonomic and functional diversity along the extensive elevational gradients of the Hengduan Mountains to evaluate how biogeographic processes vary across elevation. By integrating multi‐omic data from four characteristic vertical ecosystems within a dispersal‐selection framework, we systematically assessed microbial taxonomic and functional patterns and drivers. Contrary to the classic sky island expectation, we found that high‐elevation areas showed neither the greatest beta diversity or dispersal limitation, nor the highest distance‐decay rates. Instead, lowland river valleys exhibited the strongest island‐like biogeographic signatures, with pronounced taxonomic and functional divergence strongly governed by spatial distance. In contrast, high‐elevation communities were shaped by environmental filtering (i.e. climate and soil), leading to convergence. These findings reveal a reversal of the traditional model in large mountain massifs: rather than mountaintops, it is the fragmented lowland valleys that act as ‘inverse sky islands', where heterogeneous habitats and drainage networks impose primary biogeographic isolation. Our work redefines the elevation‐centric paradigm of mountain biogeography and proposes a more integrative framework that accounts for the important role of lowland isolation in shaping biodiversity patterns in complex montane systems.
Zhang et al. (Thu,) studied this question.