Species Diversity

Scientists have discovered between 1.5 and 1.75 million species. The number of species varies for four reasons. First, scientists don't agree on what constitutes a new species. Second, some descriptions of species are found in older scientific publications which may or may not be accurate. Third, there is always the possibility of human error when scientists are counting species. Fourth, some species are very difficult to identify.

Scientists believe that the number of species they've found are only a small amount of the total number of species that actually exist. Scientists estimate that the total number of species could be between 3.6 million to 117.7 million, though 13 to 20 million is the most likely.

The difference 3.6 and 11.7 million is a large one, isn't it? The reason for the difference is that we're still learning about organisms like bacteria and fungi, so the estimates are different. Over time, scientists learn more and the estimates get better and better.

Besides knowing how many species there are on Earth, you should know many are in each category. The table below will show you what the distribution is like. Don't worry if you don't know what the taxon mean. What you want to pay attention to is the number of species and the percentage of the total number of species columns.)

TABLE 1: Estimated Numbers of Described Species, Based on Lecointre and Guyader (2001)

* The total number of described species is assumed to be 1,747,851. This figure, and the numbers of species for taxa are taken from LeCointre and Guyader (2001).

Taxon	Taxon Common Name	Number of species described*	Percentage of the Total Number of Species
Bacteria	True bacteria	9021	0.5
Archaea	Archaebacteria	259	0.01
Bryophyta	Mosses	15000	0.9
Lycopodiophyta	Clubmosses	1275	0.07
Filicophyta	Ferns	9500	0.5
Coniferophyta	Conifers	601	0.03
Magnoliophyta	Flowering plants	233885	13.4
Fungi	Fungi	100800	5.8
"Porifera"	Sponges	10000	0.6
Cnidaria	Cnidarians	9000	0.5
Rotifera	Rotifers	1800	0.1
Platyhelminthes	Flatworms	13780	0.8
Mollusca	Mollusks	117495	6.7
Annelida	Annelid worms	14360	0.8
Nematoda	Nematode worms	20000	1.1
Arachnida	Arachnids	74445	4.3
Crustacea	Crustaceans	38839	2.2
Insecta	Insects	827875	47.4
Echinodermata	Echinoderms	6000	0.3
Chondrichthyes	Cartilaginous fishes	846	0.05
Actinopterygii	Ray-finned bony fishes	23712	1.4
Lissamphibia	Living amphibians	4975	0.3
Mammalia	Mammals	4496	0.3
Chelonia	Living turtles	290	0.02
Squamata	Lizards and snakes	6850	0.4
Aves	Birds	9672	0.6
Other		193075	11.0

Most public attention is focused on the biology and ecology of large, charismatic species such as mammals, birds, and certain species of trees (e.g., mahogany, sequoia). However, the greater part of Earth's species diversity is found in other, generally overlooked groups, such as mollusks, insects, and groups of flowering plants.



Sources: http://cnx.org/content/m12174/latest/ (CC BY)