For many taxonomic groups, inventories are more complete in temperature than in tropical countries. Estimates are made of their number in the tropics on the basis of temperature tropical species richness of some exhaustively studied’ groups like insects. 
According to various calculations by scientists, the total number of species in the world ranges from 20 to 50 million. Robert May, with his conservative and scientifically sound estimate, places the number of global species diversity at about 7 million.