How Many Leaves on the Tree of Life?

http://www.nytimes.com/2011/09/06/opinion/how-many-leaves-on-the-tree-of-life.html?ref=endangeredandextinctspecies

In 1691, the scientist John Ray estimated that there were 20,000 species of insects. His numbers were significantly off — at least a million insect species have been described so far. But he reached it the way most scientists still do, by extrapolating from the number of already known species. Three centuries later, there is still no scientific consensus on the total number of species. The most rigorous attempt at a statistical analysis of the problem, a recent study led by scientists at Dalhousie University, concludes that there are about 8.7 million species on Earth. The team analyzed the numerical relationship between species, genus, family and order in well-studied life-forms and used that pattern to estimate the number of species in categories of life that haven’t been well studied. Some scientists argue that that almost surely underestimates some lesser-known classes of life.