Life on Earth is ultimately a mystery. Even more of a riddle is why there are so many mountains rich in species.

LONDON, 16 September, 2019 − Danish ecologists have begun to wrestle with one of life’s great unsolved puzzles: why does the world have so many ranges of mountains rich in species?

This is not just a question for the intellectual high ground. As many as a million species of amphibian, fish, bird, reptile, mammal, insect or plant could be threatened by climate change and the destruction of forest habitat by human action this century.

But forests – if conserved and protected – could play a vital role in mitigating climate change, and researchers have repeatedly found that undisturbed forests hold the greatest levels of biodiversity, and conversely that biodiversity is important to the stability of the great forests.

But when biologists look more closely at the challenge of explaining biodiversity, they are confronted by something unexpected. The richest landscapes on the planet are the tropical and subtropical mountain chains. And the richest of all are the northern Andean chain.

This stretch of soaring peaks and woodland valleys is the most species-rich of all, with 45,000 kinds of flowering plant, 44% of which are found only in that region.

“Mountains, with their uniquely complex environments and geology, have allowed the continued persistence of ancient species deeply rooted in the tree of life”

There are huge concentrations of living things in the highlands of China’s Sichuan and Yunnan, the East African Highlands and the mountains of New Guinea. These contours of ridge and valley occupy only 25% of the inhabited continents, but they are home to 85% of amphibians, birds and mammals.

And of this population of vertebrates, more than half are found only in mountain ranges. To mark the 250th anniversary of the birth of the great explorer Alexander von Humboldt, scientists have dubbed this question “the Humboldt enigma”.

In 1799 Humboldt began a five-year voyage of discovery through Latin America, and made history by mapping the way vegetation changed with altitude on Mt Chimborazo in Ecuador.

“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,” said Carsten Rahbek of the University of Copenhagen and Imperial College London, who led the research, published in the journal Science.

“Mountains are simply too rich in species, and we are falling short of explaining global hotspots of biodiversity.”

Search for principles

Professor Rahbek was one of a team that, five years ago, measured changes of colour in butterflies and dragonflies that could be linked to changes in European temperatures in a world of global heating.

That is, evolution seemed to be responding to environmental change. Scientists call this sort of research macroecology: the search for the principles behind change, rather than the details of change.

There could hardly be a bigger macroecological question than one that concerns the location of the richest concentrations of life’s variety. Climatic variation – including the shifts in temperature with altitude – is clearly a factor.

Geology – because mountains are where bedrock tends to be most exposed – emerges as another factor in the two papers in Science.

Open question

Professor Rahbek describes the studies as testament to the pioneering science of Humboldt more than two centuries ago. The Humboldt enigma, for the moment, remains an open question.

Conservation scientists know that climate change and habitat destruction driven by human behaviour threatens the bewildering richness of life on Earth. But they still don’t know quite why life on Earth is so bewilderingly rich, and especially why it is so rich in relatively confined hotspots.

“The global pattern of biodiversity shows that mountain biodiversity exhibits a visible signature of past evolutionary processes,” Professor Rahbek said.

“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.” − Climate News Network