As estimated by Robert May, what is the total number of species present on earth?

Robert May was a theoretical ecologist, scientific promoter, and politician. Robert established the field of theoretical ecology, and developed influential and widely quoted hypotheses in population biology by pioneering a mathematical approach to ecology.

Given that the tropics contain an overwhelmingly large proportion of the undiscovered species, biologists compare the temperate-tropical species richness of an extensively studied group of insects and extrapolate this ratio to other animal and plant groups to arrive at a gross estimate of the total number of species on the planet.

Some estimates range from 20 to 50 million species, but Robert May's more conservative and scientifically valid estimate puts the world species diversity at around 7 million.