Elevational Gradients in Species Richness

Abstract The abiotic and biotic gradients on mountains have enormous potential to improve our understanding of species distributions, species richness patterns and conservation. Here we describe how abiotic factors change with elevation, how flora and fauna respond to these changes and how elevational species richness patterns have been studied to uncover drivers of biodiversity. There are four main trends in elevational species richness: decreasing richness with increasing elevation, plateaus in richness across low elevations then decreasing with or without a mid‐elevation peak and a unimodal pattern with a mid‐elevational peak. We discuss the history of elevational richness studies and overview the various hypotheses thought to be important in richness trends, including climatic, spatial, biotic and evolutionary factors. Key Concepts: Elevational gradients exhibit complex variation in abiotic conditions over short distances. Patterns of elevational species richness follow four common patterns: mid‐elevation peaks, decreasing, low‐elevation plateaus and low plateaus with mid‐elevation peaks. Patterns of elevational species richness vary between taxonomic groups. A combination of water availability and temperature is often found to be related to elevational species richness patterns. No consistent support is found for the importance of area or mid‐domain effects for elevational species richness patterns. Support for the various mechanisms underlying elevational richness patterns tends to be related to the ecology and evolutionary history of the taxonomic group of interest. Elevational gradients are valuable in our task to disentangle the causes behind broad‐scale patterns in biodiversity, and in our quest to understand threats to biodiversity with climatic change.