Biodiversity loss may be a serious environmental problem (I certainly believe it is), but scientists have had a hard time figuring out how many species are disappearing and how fast. One difficulty is the lack of consensus on how many species there are in the first place. Approximately 1.2 million species have been identified, but conventional (and often poorly substantiated) estimates of the total number of extant species have ranged from 3 to 100 million species. But researchers may be closing in on a more reliable estimate. A new study in PLoS BIology estimates the number of species at approximately 8.7 million. As the Washington Post reports, the new study is getting positive reviews, as it represents the most rigorous effort to estimate species numbers based upon what is known about cataloged species to date. As a consequence, this estimate should displace some of the speculative (and unfounded) numbers that have been thrown around in the past.