Cici Zhang in Popular Science:
Mountains aren't just beautiful: these locales also tend to host some of the richest diversity of species on the planet. We’ve known this for a long time—ever since Alexander von Humboldt, the Prussian geographer and naturalist, first climbed up the Andes in the 18th century. But nobody has really figured out why.
One popular hypothesis goes like this: the reason why mountains have so many different species is that, as mountains are uplifted by colliding tectonic plates, the process creates more environments, and therefore more opportunities for new species to adapt to them. However, this hypothesis never had any explicit quantitative testing until now, according to a recent study published in the Proceedings of the National Academy of Sciences.
Many other studies have looked at the diversity of one single plant group or another, and results seemed to support the popular hypothesis. “That claim is often made. The hypothesis often incorporates the narratives of these studies, but it's never been explicitly tested” across time and space, through quantitative comparison, says study co-author Richard Ree, Associate Curator of Botany at Chicago’s Field Museum.