Climate change

Business

Diversification (marketing strategy)

Biota

Mathematics

Species richness

Biology

Biodiversity

Marketing

Biodiversity hotspot

Composite material

Global change

Geography

Global biodiversity

Ecology

Materials science

Range (aeronautics)

Geometry

Elevation (ballistics)

Nature

A global study of all bird species in mountainous areas shows that richness decreases predictably with elevation, whereas diversification rates increase. Next to the latitudinal biodiversity gradient, the decline in species richness with elevation is one of the most ubiquitous ecological patterns. Yet consensus about the processes that underlie this gradient is lacking. Ignacio Quintero and Walter Jetz examine the evolutionary underpinnings of elevational gradients in bird species richness in the 46 main mountain systems of the world. They find that across all mountain ranges, species richness decreases linearly with elevation, whereas rates of diversification increase. The findings go against the idea that higher diversification rates support the build-up and maintenance of greater species richness at high elevations, and point to the role that ongoing and recent diversification has in maintaining the highly adapted biodiversity of higher elevations. Mountain ranges harbour exceptionally high biodiversity, which is now under threat from rapid environmental change. However, despite decades of effort, the limited availability of data and analytical tools has prevented a robust and truly global characterization of elevational biodiversity gradients and their evolutionary origins1,2. This has hampered a general understanding of the processes involved in the assembly and maintenance of montane communities2,3,4. Here we show that a worldwide mid-elevation peak in bird richness is driven by wide-ranging species and disappears when we use a subsampling procedure that ensures even species representation in space and facilitates evolutionary interpretation. Instead, richness corrected for range size declines linearly with increasing elevation. We find that the more depauperate assemblages at higher elevations are characterized by higher rates of diversification across all mountain regions, rejecting the idea that lower recent diversification rates are the general cause of less diverse biota. Across all elevations, assemblages on mountains with high rates of past temperature change exhibit more rapid diversification, highlighting the importance of climatic fluctuations in driving the evolutionary dynamics of mountain biodiversity. While different geomorphological and climatic attributes of mountain regions have been pivotal in determining the remarkable richness gradients observed today, our results underscore the role of ongoing and often very recent diversification processes in maintaining the unique and highly adapted biodiversity of higher elevations.

Global elevational diversity and diversification of birds