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Data and code from: Thermal niche breadth shapes regional elevational range sizes of tropical-like mountain stream insects: Evidence for Janzen's tropical mountain passes hypothesis

Zhou, Tingting1; Martín-Devasa, Ramiro2; Chen, Xiaodan1; Wang, Haojie1; Tan, Yuxin1; Liu, Zhifa3; Han, Bo-Ping1; Wang, Lu1; Liu, Zhenyuan1; Soininen, Janne1

Published Jul 13, 2026 on Dryad. https://doi.org/10.5061/dryad.9cnp5hr0h

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Abstract

The core of Janzen’s tropical mountain passes hypothesis is that relatively stable annual temperature variation limits species’ thermal tolerance breadth, which, in combination with steep elevational temperature gradients, constrains the elevational distributions of these species. In addition to thermal niche breadth, elevational range size was shaped by species’ position in thermal niche space, ecological dominance (e.g., local abundance), and functional traits. However, the relative importance of these factors and the pathways through which they jointly influence elevational ranges remain unclear. Here, we explored the determinants of regional elevational ranges of stream insects across 52 non-disturbed reaches along a 1400 m elevational gradient in tropical-like Nanling Mountain, South China. We used phylogenetic linear regression and relevant hierarchical partitioning to determine the relative roles of niche characteristics, local abundance and functional traits, while controlling for phylogenetic effects. Environmental niches were further divided into thermal and local habitat components, with their breadth and position quantified. We then performed piecewise structural equation modeling (SEM) to quantify the pathways through which niche characteristics and traits influence elevational ranges. We found that elevational ranges were positively associated with local abundance and niche breadth, but negatively associated with niche position. Phylogenetic hierarchical partitioning showed that niche characteristics were the primary drivers of elevational ranges, with thermal niche breadth being the most important driver. SEM analyses further revealed that thermal niche breadth was the primary direct driver of elevational ranges, with its indirect effect mediated by local abundance being comparatively minor. Overall, our results provide support for Janzen’s hypothesis in a tropical-like mountain stream system, revealing that thermal niche breadth is a primary driver of elevational ranges. This finding suggests that stream insects in these tropical-like climates are sensitive to climate change due to their narrower thermal tolerance and limited distributions, and should be conservation priorities as warming accelerates.