Species Richness Gradients Along Elevational Transects

Abstract

Elevational gradients in species richness -- among the oldest and most contested patterns in ecology -- show highly variable shapes across taxa and mountain ranges, ranging from monotonically declining to hump-shaped profiles, with the relative contributions of geometric constraints (mid-domain effect), climatic drivers, productivity, and area effects remaining debated. This study surveyed vertebrate (mammals, birds, reptiles, amphibians) and invertebrate (butterflies, beetles, grasshoppers) species richness at 42 elevational bands (200 m intervals from 200 to 3,000 m) across six complete European mountain transects (Swiss Alps, Pyrenees, Apennines, Carpathians, Cantabrian Mountains, Sierra Nevada) using standardised community sampling protocols, recording 2,840 species-elevation records across all taxa. Hump-shaped richness profiles (mid-elevation peaks at 800-1,400 m) were dominant for all vertebrate groups (mean R2 = 0.74 +/- 0.08 for quadratic elevation model) and butterflies (R2 = 0.68 +/- 0.10), while beetles and grasshoppers showed monotonic declines (R2 of linear model > R2 of quadratic in 84% of transects). Mid-domain effect (MDE) models explained 28.4 +/- 8.4% of variance in richness gradient shape; actual temperature (R2 partial = 0.48 +/- 0.08), productivity (NDVI; R2 partial = 0.38 +/- 0.08), and habitat area (R2 partial = 0.24 +/- 0.06) explained substantially more. Cross-transect comparison revealed significant mountain range effects: Mediterranean transects (Sierra Nevada, Apennines) showed peaks at lower elevation (840-960 m vs. 1,200-1,400 m in Alpine and Pyrenean transects), attributed to the upward displacement of the thermal optimum in warmer mountain climates. Climate warming projections (SSP2-4.5 by 2050) predict upward peak shifts of 184-248 m per degree of warming in all taxa, with potential local extinction of summit fauna in four of the six transects where summit habitats would be eliminated at predicted warming rates.