~9 - 52 million species
||Khan S et al., Overview on the Role of Advance Genomics in Conservation Biology of Endangered Species. Int J Genomics. 2016 2016: 3460416. doi: 10.1155/2016/3460416 p.2 left column bottom paragraphPubMed ID28025636
|| U. S. Rawat and N. K. Agarwal, “Biodiversity: concept, threats and conservation,” Environment Conservation Journal, vol. 16, pp. 19–28, 2015 link  Mora C et al., How many species are there on Earth and in the ocean? PLoS Biol. 2011 Aug9(8):e1001127. doi: 10.1371/journal.pbio.1001127 PubMed ID21886479
||Primary source  abstract: "Here [investigators] show that the higher taxonomic classification of species (i.e., the assignment of species to phylum, class, order, family, and genus) follows a consistent and predictable pattern from which the total number of species in a taxonomic group can be estimated."
||P.2 left column bottom paragraph: "Today’s biodiversity about 9.0 to 52 million species is the result of billions of years of evolution, shaped by natural phenomena, and forms the web of life of which we are an integral part and upon which we are so fully [primary sources]."