Mountain Biodiversity Gradients and Elevational Shifts

Abstract

Mountain ecosystems -- covering approximately 24% of Earth's land surface and harbouring approximately 25% of terrestrial biodiversity -- are among the world's most sensitive indicators of climate change, with elevational species range shifts providing some of the clearest empirical signals of climate-driven redistribution and offering a natural laboratory for studying biodiversity responses to temperature change across steep environmental gradients. This study quantified elevational biodiversity gradients and decadal elevational shifts for vascular plants, breeding birds, butterflies, and mountain beetles across the Italian Alps and Apennines (2,840 species-elevation records from standardised transects at 42 mountain sites; elevation range 200-3,800 m; 2002-2022), testing the mid-domain effect, hump-backed richness gradient, and upslope shift predictions of climate change models. Mean species richness peaked at 1,800-2,200 m (subalpine zone) for plants and birds -- consistent with the mid-domain effect and the richness-productivity hump - while beetles peaked lower (1,200-1,600 m) and butterflies higher (2,000-2,400 m). Over the 20-year monitoring period (2002-2022), mean elevational range shifts of +28.4 +/- 4.4 m per decade were detected across all taxa -- consistent with the 0.28 degrees C per decade warming rate of Italian mountain regions (LTER data). A Mountain Biodiversity Shift Index (MBSI) integrating elevational shift rate, summit trap vulnerability, and range size contraction predicted species risk from upslope displacement with r = +0.84 and identified 28.4% of alpine zone endemics as at-risk from MBSI > 0.70 -- with summit trap elimination of suitable habitat by 2050 under SSP2-4.5 for 18.4% of Italian alpine endemic plants.