Global biodiversity is frequently expressed as the total number of species currently living on Earth, i.e., its species richness. Between about 1.5 and 1.75 million species have been discovered and scientifically described thus far (LeCointre and Guyader, 2001; Cracraft, 2002). Estimates for the number of scientifically valid species vary partly because of differing opinions on the definition of a species.For example, the phylogenetic species concept recognizes more species than the biological species concept. Also, some scientific descriptions of species appear in old, obscure, or poorly circulated publications. In these cases, scientists may accidentally overlook certain species when preparing inventories of biota, causing them to describe and name an already known species.
More significantly, some species are very difficult to identify. For example, taxonomically "cryptic species" look very similar to other species and may be misidentified (and hence overlooked as being a different species). Thus, several different, but similar-looking species, identified as a single species by one scientist, are identified as completely different species by another scientist. For further discussion of cryptic species, with specific examples of cryptic frogs from Vietnam, see Inger (1999) and Bain et al., (in press).
Scientists expect that the scientifically described species represent only a small fraction of the total number of species on Earth today. Many additional species have yet to be discovered, or are known to scientists but have not been formally described. Scientists estimate that the total number of species on Earth could range from about 3.6 million up to 117.7 million, with 13 to 20 million being the most frequently cited range (Hammond, 1995; Cracraft, 2002).
The estimation of total number of species is based on extrapolations from what we already know about certain groups of species. For example, we can extrapolate using the ratio of scientifically described species to undescribed species of a particular group of organisms collected from a prescribed area. However, we know so little about some groups of organisms, such as bacteria and some types of fungi, that we do not have suitable baseline data from which we can extrapolate our estimated total number of species on Earth. Additionally, some groups of organisms have not been comprehensively collected from areas where their species richness is likely to be richest (for example, insects in tropical rainforests). These factors, and the fact that different people have used different techniques and data sets to extrapolate the total number of species, explain the large range between the lower and upper figures of 3.6 million and 117.7 million, respectively.
While it is important to know the total number of species of Earth, it is also informative to have some measure of the proportional representation of different groups of related species (e.g. bacteria, flowering plants, insects, birds, mammals). This is usually referred to as the taxonomic or phylogenetic diversity. Species are grouped together according to shared characteristics (genetic, anatomical, biochemical, physiological, or behavioral) and this gives us a classification of the species based on their phylogenetic, or apparent evolutionary relationships. We can then use this information to assess the proportion of related species among the total number of species on Earth. Table contains a selection of well-known taxa.