How do scientists extrapolate the total number of species on earth ?

Explanation

There are two methods to estimate and extrapolate the number of species on earth
(i) The primary method used by the scientists to extrapolate the number of species on earth is the estimation rate of discovery of new species.
(ii) Total number of species can also be extrapolated by the statistical comparison of the tropical and temperate species richness of exhaustively studied groups of insects. The ratio is then extrapolated with existing species of plants and animals to predict the gross estimate of the number of species on earth.