AbstractThe number of species on Earth is one of the most fundamental numbers in science, but one that remains highly uncertain. Clearly, more species exist than the present number of formally described species (approximately 1.5 million), but projected species numbers differ dramatically among studies. Recent estimates range from about 2 million species to approximately 1 trillion, but most project around 11 million species or fewer. Numerous studies have focused on insects as a major component of overall richness, and many have excluded other groups, especially non-eukaryotes. Here, we re-estimate global biodiversity. We also estimate the relative richness of the major clades of living organisms, summarized as a “Pie of Life.” Unlike many previous estimates, we incorporate morphologically cryptic arthropod species from molecular-based species delimitation. We also include numerous groups of organisms that have not been simultaneously included in previous estimates, especially those often associated with...