More diversity in higher planes

Bird populations become less diverse at higher mountain elevations — but they diversify faster, according to new research published in Nature.¹ 

Although previous studies have heavily reported that species diversity peaks at mid-elevations, the new research uses a fresh sampling approach that compensates for what the scientists term as over-representation of wide-ranging species at mid-elevations, finding instead that across all mountain ranges, the richness of bird species decreases in proportion with increasing elevation. 

According to the study, at higher elevations, environmental conditions become more extreme — with larger daily temperature fluctuations, and higher levels of wind and solar radiation.

Mountain ranges play a key role in driving evolution and are considered hubs of biodiversity. They have complex terrains and varied, often highly separate, habitats — enabling the genetic isolation of populations that can lead to the development of new species. At the same time, these qualities make mountains notoriously difficult to study.

In their new study, however, Ignacio Quintero and Walter Jetz looked at the distribution, diversity and evolution of 8,470 different bird species (around 85% of all known bird species) on the world’s 46 major mountain ranges, including across Africa. 

Examining evolutionary relationships among different bird species, the authors found that the more sparsely populated assemblages of species at higher elevations are associated with greater rates of species diversification — flying in the face of the idea that less diverse biota are caused by recent lulls in the rate at which new species arise. 

Instead, it would seem that intense, ongoing processes of species diversification are necessary to support the unique and highly adapted biotas found at higher elevations.

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