Species - Numbers of Species

Numbers of Species

Bearing in mind the aforementioned problems with categorizing species, the following numbers are only a guide. Based on various discussions from the first decade of the new millennium, counts can roughly be broken down as follows:

Number of prokaryotic species, Domain Bacteria

• This number is very difficult to assess, but the discussed range varies from tens of thousands to billions; most recent approaches and studies appear to favor the larger magnitude number. Smaller numbers arise from assumptions based on a plateauing of identification of new species (which has technical explanations other than that fewer species remain to be identified). Larger numbers address the fact that success in culturing bacteria has only been achieved in half of identified Bacterial phyla (where lack of success in attempts to culture a bacterial isolate limits abilities to study and delineate new species), and address the difficulty of applying traditional botanic and zoologic definitions of species to asexually reproducing bacteria (where more modern sequencing and molecular approaches support higher species tallies).

Number of prokaryotic species, Domain Archaea

• As a further microbial Domain, the issues and difficulties of Domain Bacteria also pertain to any counting of species of Archaea, all the more given their various extreme habitats. As the Wikipedia article on the Archaebacteria notes (Classification, Species section), the "classification of archaea into species is also controversial" as they also reproduce asexually (likewise eliminating applicability of species definitions based on interbreeding), and face the same difficulties associated with organism isolation and culturing (see citations for Bacteria, above). Archaebacteria have been shown to exhibit high rates of horizontal gene transfer (resulting from a bacterial cognate of sex), including between organisms quite separate based on genomic analysis. As the Archaea article notes, "urrent knowledge on genetic diversity is fragmentary and the total number of archaean species cannot be estimated with any accuracy" ... though like Domain Bacteria, the number of cultured and studied phyla relative to the total is low (as of 2005, less than 50% of known phyla cultured). Taken together, very high numbers of unique archaebacterial types are likely, as in the case of Domain Bacteria.

Number of eukaryotic species

This number has historically varied from a few million to about 100 millions. However these higher numbers, which were based on the potential deep marine and arthropod diversity, are now considered unlikely. The total number of eukaryotic species is likely to be 5 ± 3 million of which about 1.5 million have been already named.

Some older estimates for various eukaryote phyla are (including some updated numbers):

• As many as 1.5 million fungi;
• 3,067 brown algae
• 17,000 lichens;
• 321,212 plants, including:
  • 10,134 red and green algae
  • 16,236 mosses,
  • 12,000 ferns and horsetails,
  • 1,021 gymnosperms,
  • 281,821 angiosperms;
• 1,367,555 non-insect animals, including:
  • 1,305,250 invertebrates
    • 2,175 corals
    • 85,000 mollusks
    • As many as 1.1 million arachnids, including ~1 million mites and ~100,000 other arachnids
    • 47,000 crustaceans
    • 68,827 other invertebrates;
  • 63,649 vertebrates
    • 31,300 fish,
    • 7,093 amphibians,
    • 9,768 reptiles,
    • 9,998 birds,
    • 5,490 mammals;
• As many as 10–30 million insects;

At present, organisations such as the Global Taxonomy Initiative, the European Distributed Institute of Taxonomy and the Census of Marine Life (the latter only for marine organisms) are trying to improve taxonomy and implement previously undiscovered species to the taxonomy system, although current knowledge covers only a portion of the organisms in the biosphere and thus does not enable a complete understanding of the workings of the environment. Humankind is also currently wiping out undiscovered species at an unprecedented rate, which means that even before a new species has had the chance of being studied and classified, it may already be extinct.