In 1799, Alexander von Humboldt set sail on a 5-year, 8000-km voyage through Latin America. His journey through the Andes Mountains, captured by his famous vegetation zonation figure featuring Mount Chimborazo, canonized the place of mountains in understanding Earth's biodiversity.  

One puzzle for scientists since von Humboldt 250 years ago, and certainly later with Darwin, Wallace, and Mendel, was global pattern of mountain biodiversity, and the extraordinarily high richness in tropical mountains in particular.

Two new papers focus on the fact that the high level of biodiversity found on mountains is far beyond what would be expected from prevailing hypotheses.

"The challenge is that, although it is evident that much of the global variation in biodiversity is so clearly driven by the extraordinary richness of tropical mountain regions, it is this very richness that current biodiversity models, based on contemporary climate, cannot explain: mountains are simply too rich in species, and we are falling short of explaining global hotspots of biodiversity," says Professor Carsten Rahbek, lead author of both review papers published in Science.

Panel A: The global pattern of species richness for mammals, birds, and amphibians (A). The number of species is highest in the tropical parts, especially the mountain regions, of South America, Africa, and Southeast Asia, as shown in red and yellow.  Panel B: In tropical mountains regions the number of species much higher than expected (red colors), by current Diversity hypothesis. That is, the tropical mountains of the Andes, the East African Highlands, the mountains of Sichuan and Yunnan, and the mountains of Papua New Guinea. Figure from Rahbek et al. Science 2019. Credit: Rahbek et al. Science 2019

Part of the answer, these studies find, lies in understanding that the climate of rugged tropical mountain regions is fundamentally different in complexity and diversity compared to adjacent lowland regions. Uniquely heterogeneous mountain climates likely play a key role in generating and maintaining high diversity.

"People often think of mountain climates as bleak and harsh," says study co-leader Michael K. Borregaard. "But the most species-rich mountain region in the world, the Northern Andes, captures, for example, roughly half of the world's climate types in a relatively small region--much more than is captured in nearby Amazon, a region that is more than 12 times larger."

Stressing another unique feature of mountain climate, Michael explains, "Tropical mountains, based in fertile and wet equatorial lowlands and extending into climatic conditions superficially similar to those found in the Arctic, span a gradient of annual mean temperatures over just a few km as large as that found over 10,000 km from the tropical lowlands at Equator to the arctic regions at the poles. It's pretty amazing if you think about it".

Another part of the explanation of the high biodiversity of certain mountains is linked to the geological dynamics of mountain building. These geological processes, interacting with complex climate changes through time, provide ample opportunities for evolutionary processes to act.

"The global pattern of biodiversity shows that mountain biodiversity exhibits a visible signature of past evolutionary processes. Mountains, with their uniquely complex environments and geology, have allowed the continued persistence of ancient species deeply rooted in the tree of life, as well as being cradles where new species have arisen at a much higher rate than in lowland areas, even in areas as amazingly biodiverse as the Amazonian rainforest," says Professor Carsten Rahbek.

From ocean crust, volcanism and bedrock to mountain biodiversity

Another explanation of mountain richness, says the study, may lie in the interaction between geology and biology. The scientists report a novel and surprising finding: the high diversity is in most tropical mountains tightly linked to bedrock geology--especially mountain regions with obducted, ancient oceanic crust. To explain this relationship between geology and biodiversity, the scientists propose, as a working hypothesis, that mountains in the tropics with soil originating from oceanic bedrock provide exceptional environmental conditions that drive localized adaptive change in plants. Special adaptations that allow plants to tolerate these unusual soils, in turn, may drive speciation cascades (the speciation of one group leading to speciation in other groups), all the way to animals, and ultimately contribute to the shape of global patterns of biodiversity.