Life on Earth is amazingly diverse, and much of this diversity lies in a rich variety of geographical patterns. What determines these global patterns has been a puzzle for scientists since the days of famed scientists Alexander von Humboldt and Charles Darwin. Yet, despite two centuries of research, this question remains unanswered.
A pair of companion papers published Sept. 13 in Science reveal that mountain regionsâ€”especially those in the tropicsâ€”are hotspots of extraordinary and baffling richness.
To confront the question of why mountains are so biologically diverse, scientists at the Center for Macroecology, Evolution and Climate at the GLOBE Institute of the University of Copenhagen worked to synthesize understanding and data from the disparate fields of macroecology, evolutionary biology, earth sciences, and geology. They were joined by individual collaborators from the University of Connecticut, Oxford University, and Kew Gardens.
Although mountain regions cover only 25% of Earthâ€™s land area, they are home to more than 85% of the worldâ€™s species of amphibians, birds, and mammals, and many of these are found only in mountains. The global pattern of mountain biodiversity and the extraordinarily high richness of tropical mountains, in particular, are documented in the Science papers, which focus on the fact that the high level of biodiversity found on mountains is far beyond what would be expected from prevailing hypotheses.
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.
â€œPeople often think of mountain climates as bleak and harsh,â€� says Michael K. Borregaard, who led the study with Carsten Rahbek. â€œBut the most species-rich mountain region in the world, the Northern Andes, captures, for example, roughly half of the worldâ€™s climate types in a relatively small regionâ€”much more than is captured in nearby Amazon, a region that is more than 12 times larger.â€�
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 the Equator to the arctic regions at the poles, Borregaard explains.
â€œItâ€™s pretty amazing if you think about it.â€�
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.
â€œThe 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,â€� says Rahbek.
Climates of the pastâ€”and their role in generating and maintaining biodiversityâ€”cannot be known with certainty, but these processes can be simulated, says Robert Colwell, emeritus professor of ecology and evolutionary biology University of Connecticut, who worked on the study.
Using a computer model, UConnâ€™s Colwell and doctoral student Thiago Rangel were able to simulate the processes of species origination, persistence, and extinction in South America over the past 800,000 years, through ten cycles of glaciation and warming.
â€œWe found that the high Andes were cradles of species origination, while the eastern Andean slopes were graves of extinction, during warming periods following glacial episodes. But the net effect was a persistent pattern of high richness in the Andes,â€� says Colwell.
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 in most tropical mountains is tightly linked to bedrock geologyâ€”especially mountain regions with upducted, 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 a group leading to speciation in other groups), all the way to animals, and ultimately contribute to the shape of global patterns of biodiversity.
The legacy of von Humboldt â€“ his 250th anniversary
The two papers are part of Scienceâ€™s celebration of Alexander von Humboldtâ€™s 250th birth anniversary. In 1799, Alexander von Humboldt set sail on a 5-year, 5,000-mile (8,000-km) voyage of scientific discovery 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 Earthâ€™s biodiversity.
Acknowledging von Humboldtâ€™s contribution to our understanding of the living world, Rahbek, one of the founding scientists of the newly established interdisciplinary GLOBE Institute at the University of Copenhagen says,
â€œOur papers in Science are a testimony to the work of von Humboldt, which truly revolutionized our thinking about the processes that determine the distribution of life,â€� said Rahbek. â€œOur work today stands on the shoulders of his work, done centuries ago, and follows his approach of integrating data and knowledge of different scientific disciplines into a more holistic understanding of the natural world.â€�