How many species have been identified on earth?

How many species are there in the world?

Answer by Liz Osborn (© Current Results Nexus): 
Scientists have described over 1.7 million of the world's species of animals, plants and algae, as of 2010.
The list below gives the number of species known in the world for each major category of animals and plants.
The numbers most accurately represent all living species of mammals, birds and coniferous plants. Only for those groups have scientists almost completely identified all the world's species.
Biologists have yet to describe many species of plants, invertebrate animals and lichens. So the number of these species known to science increases substantially every year.
The greatest species diversity exists among insects, which account for one million of the earth's species known to science. Mammals make up one of the smallest groups, with just 5,490 members.
Altogether the earth's oceans, lakes, continents and islands support over 62,000 identified species of vertebrate animals and 320,000 species of plants.

How many species are known to currently exist in the world.

Category	Species	Totals
Vertebrate Animals
Mammals	5,490
Birds	9,998
Reptiles	9,084
Amphibians	6,433
Fishes	31,300
Total Vertebrates		62,305
Invertebrate Animals
Insects	1,000,000
Spiders and scorpions	102,248
Molluscs	85,000
Crustaceans	47,000
Corals	2,175
Others	68,827
Total Invertebrates		1,305,250
Plants
Flowering plants (angiosperms)	281,821
Conifers (gymnosperms)	1,021
Ferns and horsetails	12,000
Mosses	16,236
Red and green algae	10,134
Total Plants		321,212
Others
Lichens	17,000
Mushrooms	31,496
Brown algae	3,067
Total Others		51,563
TOTAL SPECIES		1,740,330

The species totals do not include domestic animals such as sheep, goats and camels. Nor do they include single-celled organisms such as bacteria.

Reference

The World Conservation Union. 2010. IUCN Red List of Threatened Species. Summary Statistics for Globally Threatened Species. Table 1: Numbers of threatened species by major groups of organisms (1996–2010)

Source: http://www.currentresults.com/
Answer by explorebiodiversity.com: 
This is a very good question, but the truth is, its very hard to know, or even estimate. Currently there are about 1.4 million species described. Yet, we're probably far from being close to the actual number of species.

Take this into consideration. In a famous study conducted in Panama, 19 trees were "fogged" with insecticide and the dead were collected as they fell through the canopy. In this study, nearly 1,200 species of beetles alone were collected. Of those, 80 percent were not known to science. While it may be dangerous to extrapolate numbers like these to other places, it gives at least a high estimate of the number of species that could exist on earth - that high estimate being around 100 million species. A low estimate is 2 million. The best estimate might be around 10 million. But even if that’s the case, it means we've only known about a small fraction of what is presently there.

Many of the species that we're discovering live in areas that are not often studied. Take the sea floor for instance. Hydrothermal vents along the floor provide a type of chemical energy for bacteria. These bacteria use this energy like plants use the energy of the sun. They then form the base of a giant food web thousands of feet below the surface. Only until recently have we been able to discover this new habitat. Now scientists believe there could be as many as a million species just in on the ocean floor.