Until now, estimates of the world’s species ranged from 3 million to 100 million. Five academics from Dalhousie University in Halifax, Nova Scotia, refined the number by compiling taxonomic data for roughly 1.2 million known species and identifying numerical patterns. They saw that within the best-known groups, such as mammals, there was a predictable ratio of species to broader categories. They applied these numerical patterns to all five major kingdoms of life.


Source: PLoS Biology report “How many species are there on Earth and in the Ocean?” | Todd Lindeman/The Washington Post August 23, 2011
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