How many species?

A basic question, for which we still don't have a good answer. Lately, we've seen numbers like 30 million thrown around routinely as a possibility. A new study appearing in American Naturalist suggests the number is likely considerably lower than that. The sticking point is in our knowledge of tropical arthropods, which without question make up the lion's share of remaining, undescribed species. Previous studies have suggested that there might be tens of millions of unknown insect species in the tropics. The new study, led by Andrew Hamilton from the University of Melbourne, uses applied probability modeling to estimate, to estimate that there is a 90% likelihood that there are between 2 and 7 million tropical arthropods, with the estimate centering at 3.7 million. After throwing in reasonable estimates for undescribed species in other groups (and ignoring the bacteria, which are something of a taxonomic black hole right now), they come up with a best estimate of about 5.5 million species on the planet. That's a heckuva lot less than most of the estimates we've seen over the last couple of decades. The new estimate still means, however, that some 70% of arthropods remain unknown.