How many species are there on Earth?

Abstract

How many species are there in the marine or terrestrial realms in the world? Biologists have tried to answer this question for the past half century. However, different estimations are made of the number of species, using different methods, estimates range from 500000 to 100000000 species on earth. There is no accurate estimate and the estimations of the total number of species on earth do not converge yet. However, the number of species of higher plants, freshwater fishes, amphibians, reptiles, birds and mammals are relatively clearly ascertained. Most botanists agree there are 30000000– 3500000 plant species on earth. FishData (2016) reported 33200 fish species that have been scientifically described, of which 14000 are freshwater species. The American Museum of Natural History (2016) recorded 7493 amphibians in the world. Reptile Data Base (2016) recorded 10272 reptile species. BirdLife International (2016) reported 10426 bird species. Wilson and Reeder (2005) reported 5436 mammalian species in 2005. Adding the new species reported worldwide since then, IUCN (2016) reported there are 5515 mammalian species excluding domestic animals on earth. Why can we not get an accurate count of species number on earth? The reasons lie in differences in the definition of species, taxonomic methods, standards of species used by different taxonomists and the research scope and depth in different taxa or in different regions on earth. The species is a disputed concept, which has evolved since Darwin’s time, yet we still cannot reach a universal definition of the concept. In the past, taxonomists used morphorlogical traits to classify species, later numerical taxonomic methods and cladisitic methods were used. Now molecular phylogeny is widely in use in taxonomy, and more phylogenetic species are discovered with this method. Taxonomy is an empirical science. Taxonomists study different goups, and even different taxonomists studying the same group may hold different opinions on the standard of species, reflecting that often taxonomists cannot reach agreement on whether a species is valid, whether to split a species or to lump several taxa into a single species. Taxonomists have done siginficant work on describing species in mammals, birds, reptiles and amphibians, but little work on, e.g., nematodes. Most biodiversity studies are concentrated in deveploped countries like USA and UK while the biodoversity-rich counrties like Ecuador, Indonesia, Madagascar and Peru are neglected. Most leading biologistss are working in academic institutions and universities located in developed countries, who dominate the conservation science policy making bodies such as IUCN/SSC specialist groups, Red List Committee and IUCN Endangered Species Redlist Techanical Working Group (Wilson et al. 2016). Thus, most species remain undescribed. If one day, people can carry out thorough studies on all taxa in all habitats on earth when taxonomists studying different taxa cannot standardize their definition of species, or even taxonomists studying the same taxa cannot standardize their definition of species, we still cannot get an accurate census of number of species on the earth. Nevertheless, it would be logic to estimate the number of species on the earth.