Do Shapes of Altitudinal Species Richness Gradients Depend on the Vertical Range Studied? The Case of the Himalayas

We analyzed elevational species richness gradients (“decline”, “increase”, “unimodal”, or “bimodal”) in the Himalayan range using data from 157 publications covering both plants and animals. Our study tested the hypothesis that unimodal gradients, explainable by the geometric mid-domain effect, dominate in the mountains, while decreasing or increasing gradients result from studies that only examined limited sections of the full altitudinal range. Multivariate canonical correspondence analysis was applied to associate gradient shapes with altitude ranges, geographic locations, and the taxa studied. Our results show that, across taxa, most Himalayan altitudinal gradients exhibit a unimodal shape, with diversity peaks at approximately 2500 m a.s.l. for plants and 2200 m a.s.l. for animals. The gradient shapes were primarily influenced by three interrelated predictors: vertical range, maximum elevation, and mean elevation. Studies from the world’s highest mountain range suggest that surveys encompassing substantial portions of the elevational range tend to produce hump-shaped gradients, while incomplete sampling leads to declining or increasing species richness patterns.