Mountains are the home to a disproportionately large number of species, says S.Ananthanarayanan.

The biodiversity found in mountains is a major factor in the environmental stability of the surrounding lowlands, and the earth itself. This is because mountains act as junction points of the water cycle, distributing water and nutrients, and as a storehouse of species that are able to survive when things get difficult in the plains

Charles Darwin, the author of our understanding of how species arise, was struck by the great variety of life forms on the earth. Even if it was God who created living things, why did He need so many? Darwin set out on his legendary voyage around South America in 1831-36 and put down his work, The Origin of the Species, in 1859. But the questions that he asked, in substance, had been asked by Alexander von Humboldt, a polymath, botanist, geologist, scientific adventurer, who had travelled extensively in South America and the Amazon between 1799 and 1804

As 14th September 2019 was the 250th anniversary of Humboldt's birth, Science, the journal of the American Association for the Advancement of Science, carries two papers that examine a related question – of how is that so much of the biodiversity of the earth is concentrated in the mountains? Mountains cover only 25% of the earth's land area. But they are home to 85% of the species of amphibians, birds and mammals found on the earth.While the wealth of species in mountains is seen to arise from the diversity ofmountain terrain, and the rapid changes seen in mountains, current ideas of how species arise have not been able to account for the extremely high concentration of different species, particularly in tropical mountains.In a sense, the question of why there is so much diversity of species on earth has morphed into, "why is such a large part of it found packed in the mountains?"

Carsten Rahbek, Michael K. Borregaard, Alexandre Antonelli,Robert K. Colwell, Ben G. Holt, David Nogues-Bravo,Christian M. Ø. Rasmussen, Katherine Richardson, Minik T. Rosing,Robert J. Whittaker, Jon Fjeldså, Bo Dalsgaard, Naia Morueta-Holme David Nogues-Bravo, from the University of Copenhagen, University ofSouthern Denmark,Imperial College London, University of Oxford, Royal Botanic Gardens, Kew, Surrey Gothenburg Global Biodiversity Centre Sweden, University of Connecticut,and Colorado Museum of Natural History, in the two papers in the journal, examine the mechanism that could lead to this mystery, a question known as 'Humboldt's enigma'.

Professor Carsten Rahbek, lead author of both papers, says, "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 biodiversitymodels, based on contemporary climate, cannot explain: mountains are simply too rich in species, and we arefalling short of explaining global hotspots of biodiversity."

The main reasons that the studies identify is that mountains, being the products of tectonic upheavals and affected by erosion or climatic processes, undergo comparatively frequent changes. They also have varying topography and can present diverse and changing environment, at a rate many times that of non-mountainous areas. These changes result in splitting of habitats and the separation of species, leading to evolutionary adaptation, and this can lead to exponential multiplication. "These biological processescreate a shifting balance between speciationand extinction, in which mountains mayact as “cradles” (areas of especially rapid speciesorigination), “museums” (areas of especially longtermpersistenceof species), and “graves” (areas with especially high rates of extinction) for biodiversity," one of the papers says.

"People often think of mountain climates as bleak and harsh",Michael K. Borregaard, co-leader of the paper says, "but the most species-rich mountain region in the world, the Northern Andes, captures, for example, roughlyhalf of the world's climate types in a relatively small region--much more than is captured in nearby Amazon, aregion that is more than 12 times larger". This is particularly so, he says, in the tropics, where a point of high altitude can have what looks like Arctic conditions, and within a short distance, there could be the range of temperatures associated in the space from the Equator to the Poles. "A tropical mountain has more climatezones than a temperate mountain of similar height," the second paper says.

The normal dynamics of generation of species include movement of poulations, by dispersal or shifting to a different range, specialisation, or niche evolution, persistence of a hardy species, and extinction. These processes, the paper says, are not able to account for the number of species. It is by simulation, using models that take into account realistic climatic changes in past geological ages, that better results have come, the paper says. More accurate models of past climate conditions as well as models of interaction of organisms and environment are allowing us to understand how populations changed and guided evolution, the paper says.

The concept that geology and biology are intimately connected, the 'unity of nature', was the message in Humboldt's major work, Kosmos (1845-51). A. R. Wallace, British explorer, geographer, anthropologist, and biologist, had (in 1880) used the relationship to confirm that there was a series of Ice Ages from how a group of related species was distributed over islands in the Indo-Pacific. But, so far, there has always been too little data or the statistical methods were not good enough to put all biological and geological processes into a model of diversity of mountain species and evolution, the paper says.

It is now, with sophisticated technology, that we are able to reconstruct the sequence of geological changes. And with techniques of DNA analysis to connect with movements or changes of population and genetic patterns, we may create models that help us better understand how diversity and distribution of life over the earth came to be.

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