How many species on Earth

We know how many species have been recorded, because every named species is published, but not how many exist.

According to the IUCN (2004), slightly more than 1.5 million plant and animal species have been described so far.

Species lists are more complete for temperate than for tropical countries, and most undiscovered species are in the tropics.

To estimate the total, biologists take the temperate-to-tropical ratio of species richness in a thoroughly studied insect group and extrapolate it to other groups of plants and animals.

Some extreme estimates run from 20 to 50 million species; the more conservative, scientifically sound estimate by Robert May is about 7 million.

Of the recorded species, animals are more than 70 per cent. Plants, counting algae, fungi, bryophytes, gymnosperms and angiosperms together, are no more than 22 per cent.

Insects are the most species-rich animal group, more than 70 per cent of all animals: 7 of every 10 animal species are insects.

Fungi have more species than fishes, amphibians, reptiles and mammals put together.

These estimates leave out prokaryotes. Standard taxonomic methods do not suit microbes and many cannot be cultured; by biochemical or molecular criteria their diversity alone might run into millions.