• According to International Union for Conservation of Nature (2004), more than 1.5 million species of plants and animals have been described so far. Robert May have estimated the global species diversity to be around 7 million.

• Species inventory is more complete in temperate than in tropical countries and hence, more species are yet to be discovered in the tropical area.

• Biologists make a statistical comparison of temprate-tropical species richnes of an intensively studied group of insects and extrapolate this ratio with other groups of plants and animals to come up with a gross estimate of total species on the planet.

• This conventional taxonomic method is not suitable for estimating microbial species.