How many species are there on Earth? Given the effort required to sample habitats and identify species this figure will likely never been known with great certainty. Indeed current rates of discovery may be slower than the current species extinction rate! To make a start ecologists have thus turned to estimates based on extrapolations of well known patterns in the distribution of species, or the rate of taxonomic discovery. Best estimates vary from as few as 3 million up to 100 million species. A recent paper by Mora et al. (2011, PLoS Biology) has come up with a value of ~8.7 million (+/- 1.3 million) eukaryotic species. This is somewhat lower than the previous best estimates of ~15 million species. The authors base their estimate on the predictable increase in the number of taxa with increasing taxonomic resolution (i.e., from phylum, class, order, family, genera and species). Although the number is likely an underestimate (consider the diversity of microorganisms that remain undiscovered) it is the method that has been criticized. Some experts believe that the patterns upon which the estimates are based are more the reflection of the taxonomic enterprise and have little to do with how many species actually inhabit the Earth. Read the piece in the New York Times about this study.