What is Humboldt's Enigma and what does it mean for India?

News Excerpt:

The earth’s tropical areas receive more sunlight, so they should be the most biodiverse places. Mountains defy this rule.

About Humboldt's Enigma:

  • Explorers and naturalists have been finding the factors leading to biodiversity  concentration for centuries.
  • One was Alexander von Humboldt (1769-1859) – a polymath who recorded observations on various natural phenomena across the fields known today as geography, geology, meteorology, and biology.
    • From his various studies, Humboldt suggested there was a relationship between temperature, altitude, and humidity on the one hand and the occurrence patterns of species – or their biodiversity – on the other. 
  • Two centuries later, a group of biogeographersscientists who explore the relationship of diversity with geography – used modern tools to take another look at the drivers of biodiversity.
    • They proposed their own version of the link between biodiversity and mountains and called it Humboldt’s enigma.

What is Humboldt’s enigma?

  • The world’s tropical areas receive more energy from the Sun because of the earth’s angle of inclination. 
  • So, the tropics have greater primary productivity, facilitating greater diversity: more ecological niches become available, creating more complex ecosystems and greater biological diversity.
  • The proponents of Humboldt’s enigma have held that the earth’s tropical areas alone don’t contain all the biodiverse regions and that many areas outside the tropics are highly biodiverse. These places are mountains.
    • Indeed, while we expect diversity to decrease away from the tropics, mountains have been an important exception. 
    • This is the essence of Humboldt’s enigma.
  • A simple way to think of Humboldt’s enigma in India is to consider the biodiversity in our tropical areas south of the Tropic of Cancer, passing through Madhya Pradesh and Chhattisgarh. 
    • These areas are supposed to be the most diverse in the country. 
    • The Western Ghats plus Sri Lanka biodiversity hotspot lies in this zone.
    • However, the eastern Himalayas are much more diverse.

What drives biodiversity?

  • The history of the earth, its geography, and the climate are the main drivers of mountain diversity. 
  • Different biodiversity at different locations results from changes in how these factors have intermingled over time and space.
  • Mountains host two processes that generate biodiversity. 
    • First, geological processes, like uplifts, result in new habitats where new species arise, so the habitats are ‘cradles’. 
    • Second, species on some climatologically stable mountains persist for a long, so these spots are ‘museums’ that accumulate many such species over time.
      • Coastal tropical sky islands (mountains surrounded by lowlands), like the Shola Sky Islands in the Western Ghats, are a good example. 
      • Here, old lineages have persisted on the mountain tops as climates and habitats fluctuated around them in the lower elevations. 
      • This is why some of the oldest bird species in the Western Ghats, such as the Sholicola and the Montecincla, are housed on the Shola Sky Islands.
    • Sometimes, the same mountain can be a cradle for some species and a museum for others, depending on the species’ ecologies.
  • Another critical force in biodiversity formation is geology. The foundations on which mountains are erected often differ from those on which low-elevation regions rest.
    • Scientists have found that the more heterogeneous the geological composition of mountains is, the more biodiverse they are. 
    • Around the world, all mountains with high biodiversity also have high geological heterogeneity, especially in the tropics. 
    • We also know plants are influenced by the type of soil, which depends on the type of rocks in that area. 
      • So, high geological heterogeneity often produces unique mountain habitat patches within similar climate regimes and promotes diversification.

Challenges regarding finding the pattern of biodiversity richness:

  • An important limitation of scientists’ attempts to explain biodiversity patterns is the lack of fine data on where species occur.
  • There is a lack of research
    • In India, in particular, several areas are under-studied. 
    • We can’t expect to understand a place’s true biodiversity without using modern genetic tools.

Initiatives to tackle the above gaps:

  • National Mission on Himalayan Studies
  • National Mission for Sustaining the Himalayan Ecosystem 
  • National Mission on Biodiversity and Human Wellbeing 
  • They need to be strengthened and bolstered by the will to support basic research on diversity.
  • There is a need to use modern genetic tools.

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