Passage II: Diversity of species Although much debate on biodiversity focuses on 'species', it is important to note that this is not a standard unit. The way species are defined differs between groups and between taxonomists. However, despite these ambiguities, measures of species richness and distribution are important tools in assessing the state of the environment and the direction and speed of change. The number of described species is now around 1.7 million. The estimated total number of species in existence ranges in order of magnitude from around 10 million to 100 million. The below table summarizes numbers of described species by taxonomic group from the IUCN Red List version 2010.3.