Limited evidence for range shift-driven extinction in mountain biota

Mountain biodiversity reorganizes rapidly as species shift upslope to track temperatures. Pervasive species redistribution poses substantial threats to mountain ecosystems, a phenomenon sometimes described as an "escalator to extinction," primarily through mountaintop extinctions, range shift gaps (i.e., rapid shifts of suitable temperatures getting ahead of narrow-range species' upper limits), and lowland biodiversity attrition, yet empirical evidence remains scarce. In this study, our analysis of 8800 records of historical and modern elevational range limits for 440 animal and 1629 plant species revealed little evidence supporting the proposed threats. Observed changes largely fell within random expectations, accounting for geometric constraints. Although delayed mountaintop extinctions point to accumulating extinction debt, concurrent range expansions of both narrow-range and lowland species suggest thermal niche underfilling, processes that collectively drive biotic homogenization across biologically complex mountain ecosystems.