Covering only 25 percent of the world’s land area, mountains hold more than 85 percent of Earth’s amphibian, bird and mammal species, many of them endemic to mountain environments.
Despite the fact that we are currently experiencing the Earth’s sixth mass extinction crisis, I take great solace in knowing that life here is still amazingly diverse. And, somewhat surprisingly, the hot spots for that extraordinary and baffling biological richness occur in our planet’s mountainous regions—especially those found in the tropics.
We’re accustomed to thinking of mountains as bleak, cold and inhospitable places where not much can thrive. Yet although they cover only 25 percent of the world’s land area, mountains are home to more than 85 percent of Earth’s amphibian, bird and mammal species—many of which are found only in the mountains.
Why are these high places so full of life? It has to do with alpine climates and how these majestic landforms are built.
Alexander von Humboldt’s well-known illustration of the Chimborazo volcano in Ecuador shows which plant species live at various elevations.
From climate to Mount Chimborazo
What determines global patterns of biodiversity has puzzled scientists since the days of Alexander von Humboldt, who, in 1799, set sail on a five-year, 5,000-mile voyage of scientific discovery through Central and South America. His journey through the Andes Mountains—captured by his famous vegetation zonation figure, featuring Mount Chimborazo, a stratovolcano in central Ecuador—canonized the place of mountains in understanding Earth’s biodiversity. Yet, despite two centuries of research, this question has remained unanswered.
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 with data from the disparate fields of earth sciences, evolutionary biology, geology and macroecology. Publishing two papers in the journal Science in September 2019—one titled Humboldt’s Enigma: What Causes Global Patterns of Mountain Biodiversity? and the other Building Mountain Biodiversity: Geological and Evolutionary Processes—the scientists concluded that part of the answer lies in that the climate of rugged, tropical mountain regions is fundamentally different in complexity and diversity compared to adjacent lowland regions. Uniquely heterogeneous mountain microclimates likely play a key role in generating and maintaining high diversity.
The most species-rich mountain region in the world is the Northern Andes. It encompasses, for example, roughly half of the world’s climate types in a relatively small area; much more, in fact, than is held in the nearby Amazon rain forest, a place that is more than 12 times larger.
Chamois are short-horned, goatlike antelopes adapted to living in high, rocky environments. They are endemic to mountain ranges across Asia and Europe.
But there’s another unique feature of mountain climates. Tropical mountains, based in fertile and wet equatorial lowlands and extending into climatic conditions superficially similar to those found in the Arctic, hold—in just a few miles—a gradient of annual mean temperatures as large as that found stretched over more than 6,000 miles from the tropical lowlands at the equator to the Arctic regions at the poles. That’s astonishing.
From mountain-building to channel-cutting
Another part of the explanation for why there is such a great amount of biodiversity in 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.
But just how are mountains built? Scientists have long assumed that as land is pushed faster upward to form a mountain, its height increases in a continuous and predictable way. But recent research published in the journal Nature Geoscience shows that these predictions may stop working for the steepest mountains at a certain point, and many factors may impact their ultimate height, including the erosion of the areas between mountains, which are known as “channels.”
Tropical mountains span a gradient of annual mean temperatures that is as large as that found from the lowlands at the equator to the Arctic regions at the poles—or that cover more than 6,000 miles elsewhere in the world.
For the Nature Geoscience study, researchers analyzed samples from a broad range of mountain landscapes across the tropics, including some in Brazil, Guatemala, Costa Rica, Taiwan and Venezuela, controlling for rock types and climate conditions to make true parallel comparisons. They found that after mountains reach a certain elevation, channels between mountains suddenly become extremely sensitive to subtle changes in their inclines, thereby limiting the height of the mountains above. The scientists added data from hundreds of mountain ranges worldwide and found that they followed a similar pattern: the height, or relief, of the landscape is capped after crossing a threshold driven by channel steepness.
According to the paper’s lead author, the land above a channel is likely being controlled by how quickly a river can cut down, which is the traditional framework by which we have come to understand how the height of mountains varies as a function of climate and the collision of continents. So, the “channel steepness” anomaly is kind of a mystery and is not necessarily what conventional theory might predict. And there might be something about the way in which rivers incise mountains that we just don’t yet comprehend.
From bedrock to biodiversity
Another explanation for why there is so much biological richness in mountains may lie in the interaction between biology and geology. The scientists report a novel and surprising finding: the high diversity in most tropical mountains is tightly linked to bedrock geology—especially in mountain regions with obducted (when the oceanic crust of the edge of a tectonic plate is thrust over the continental crust of the edge of another, adjacent plate), ancient, oceanic crust.
Mountains have allowed the continued persistence of ancient species, such as gentians, which have been evolving for 60 million years or more. They were on Earth long before humans.
To explain this relationship between biodiversity and geology, 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 changes in plants. Special adaptations that allow plants to tolerate these unusual soils, in turn, may touch off 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.
From continued persistence to cradle proliferation
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 astoundingly biodiverse as the Amazon rain forest.
Although I may not be a fan of heights, I am a devotee of mountains. For me, mountains are some of my most meaningful places. And now that I know how full of life they are, they have taken on an added luster—as long as I’m standing at the bottom of them, as one elder New Zealander said to me, and looking up.
Here’s to finding your true places and natural habitats,
Candy
