Just a week ago (8/23/2011) it was announced by scientists at Census of Marine Life that the new, best estimate of the number of eukaryote species on Earth is 8.7 million, distributed as shown here in a graphic from ScienceDaily.
(Some group the Chromista among the Protozoa. All Chromista are photosynthetic, and colored golden or beige by pigments in addition to Chlophyll a and c, which are green.)
One detail in the announcement is that the estimate is "plus or minus 1.3 million". That is just enough that, if the estimate is a tad low, it could reach as high as 10 million species. I think it very unlikely that the estimate is high, and in fact, I tend to believe that 8-10 million is still quite an underestimate.
A few times (I don't have the references), scientists have fogged an entire tree in a rain forest with insecticide, then meticulously counted and classified every insect that fell out. There are typically 300 species just of beetles, and most of these are reported as "endemic to the tree". That is a dry way of saying that each such experiment caused the extinction of 100-200 species of beetle, because the entire species lived in that single tree.
How many trees are there in a typical acre of rain forest? A hundred or so? Can it be that every square mile of Amazon or Congo forest is host to nine or ten million beetle species? I think it quite possible. Then there are the ant species, and there could be ten or twenty species of little ant that are "endemic" to a single tree. There's another million animals species per square mile.
Yes, I think 8.7 million is still a bit of an underestimate.
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