ABSTRACT
The question of how many species inhabit Earth has been the focus of scientic curiosity for centuries. Recently this has been presented as a conservation issue, an important unknown factor in planning actions to protect global biodiversity (Bickford et al., 2007). There are other accessory benets to pursuing the project to catalogue and understand life on Earth, but at the root it is primarily an intellectual grand question, the number and variety of species that exist is the type of information that we would generally like to know about how the world works. At the turn of the 21st century, quantitative estimates for global eukaryotic diversity ranged over several orders of magnitude, from 106 to 108 living species (May, 1988, 2010). Several recent analyses have produced apparently congruent estimates at a number between 5 and 8 million species (Mora et al., 2011; Costello et al., 2013), but there is no good evidence that large-scale species richness estimates are genuinely converging at a precise or robust answer (Caley et al., 2014). The true number of living species is meanwhile slowly shifting downward as species are driven to extinction by anthropogenic activities.
