Our latest paper (here) in the prestigious journal Nature Communications is about global patterns of plant diversity. Species richness is usually described with alpha diversity (number of species in a plot), beta diversity (change of species among plots) and gamma diversity (regional to continental number of species). This paper is about alpha diversity.
The number of species found in a plot is related to the plot size. As any chosen plot size is arbitrary, the question was to check how alpha diversity would change with different plot sizes (also called grain) and how the alpha diversity measured at different grains would be distributed over the world. The number of species was also correlated to climate, soil and topography.
Global maps of alpha diversity were produced from more than 170,000 plots worldwide. The number of species in these plots was extracted at three different scales (say small, medium and large).
The maps of alpha diversity worldwide classically show an increase of local diversity from the temperate to the tropical regions. However, the three grains do not exactly produce the same maps, which shows that alpha diversity is not accumulated at the same rate from region to region. The African forests have high coarse grain richness whereas Eurasian temperate forests have high fine grain richness. The cause of these differences are still unknown and among the potential candidates are: history, species assembly rules, selection.
India’s plant alpha diversity in general is intermediate with a regular accumulation of species from fine to coarse grain. The North-East and a small part of the Himalayas have a very high alpha diversity at small grain but not at coarse grain. Alpha diversity hotspots are small or non-existent even though the gamma diversity is intermediate in the Western Ghats and the assemblage unique due to endemics.
It should be noted that non-forest plots appear as data deficient in north India: is it because data could not be found online or because there are too few studies on vegetation?