For botanists, the Himalayas have long been regarded as something of a paradise. One of the world’s ‘biodiversity hotspots’, the region is home to over 3000 endemic species of plant, found naturally nowhere else on Earth. These include five endemic families of plants and more than 70 genera. However a good number of our familiar garden species also have their origins here, which raises two interesting questions; why is the flora so diverse and why have such a wide range of species thrived, once introduced, in other parts of the world? Maybe questions best dealt with separately!
The range of climatic conditions within the region is one important cause of diversity – after all, the Himalayas lie not far north of the Tropic of Cancer but the abrupt rise of the mountains, from less than 500 m, to more than 8000 m, means a wide range of ecosystems exist in close proximity to one another. Alluvial grasslands give way to subtropical and temperate broadleaf forests in the foothills and midhills and these, in turn, are replaced first by coniferous forest and finally by alpine meadows. A few cushion plants can survive even in the harsh permanent rock and ice zone at altitudes of 5500 to 6000 m. Aspect, topography and the diverse underlying geology must also be significant.
So where did all these plants come from in the first place? 200 million years ago (Ma), India was part of the super-continent Pangaea.
When Pangaea started to break apart, about 165 Ma, the Indian continental plate drifted rapidly northwards, eventually colliding with the Eurasian plate, some 55 Ma ago.
Though the early fossil evidence is patchy, it suggests that ancestors of the flowering plants (angiosperms) first appeared in wet tropical regions of Pangaea over 200 Ma ago; the late Triassic period. Their spread halted during the arid Jurassic period which followed, but angiosperms then diversified rapidly as the climate improved again in the Lower Cretaceous, 90 million years ago – between Pangaea starting to break up and the collision which gave rise to the Himalayas. By 50 Ma ago angiosperm trees were widespread and dominated many environments. They would have been evolving separately on the Indian and Eurasian continental plates for many millions of years, so the collision must have brought together two quite different groups of species. The radiation of herbaceous angiosperms, in contrast, occurred much later, so perhaps not until after the Himalayas started to form.
This process of mountain building (the Himalayan orogengy) continues today, with the Himalayas still rising at rates of up to 10 mm per year as the sediments of the Indian plate margin are thrust upwards and folded. This uplift goes hand-in-hand with rapid erosion, both processes resulting in the production of new surfaces which organisms such as lichens are quick to exploit. It means that plants have to adapt, admittedly gradually, to changes in altitude as well as in the topography of their surroundings, with all that means for temperature and availability of light, water and mineral nutrients. The aridity of land at the highest altitudes must also give rise to geographical isolation of populations of plants in adjacent valley bottoms. How important are these factors in driving the appearance of new species?