Encyclopedia of Evolution.pdf - Online Reading Center

iodiversity
Biodiversity (as indicated by the number of families of fossilized marine and terrestrial organisms) has increased through geological time. After each of
the five mass extinctions in earth history (five arrows), biodiversity decreased (especially after the Permian extinction, largest arrow) but within a few
million years began to recover and continue its increase. (Redrawn from Benton)
abundant photosynthetic organism in the oceans, was not discovered
until 1988. There is no end in sight for discovering
new insect species. Partly this is because there are so many
of them (350,000 species of beetles and counting!) but also
because they are so hard to see.
Only an expert can recognize many of the distinctions
between closely related species. Flies are often classified using
the arcane science of chaetotaxy, which distinguishes them
on the basis of the arrangements of their bristles. Clearly, one
limiting factor to the discovery of new species is the availability
of experts who can recognize that they are new.
About a million and a half species have been named.
Some biologists estimate that there may be as many as 30
million to 100 million species in the world. Ecologist Terry
Erwin calculated this estimate. He began by using pesticides
to kill all the insects in certain tropical trees and collecting the
insects that fell. By estimating how many of these insects were
unique to certain species of trees, and extrapolating to the
number of tree species, he was able to estimate the number of
tropical insect species. From there he could estimate the number
of insect species in the world, and then of all other species,
using existing proportions of each taxonomic category.
Although his estimate is probably high, it is certain that there
are many more species than those already discovered. Robert
May estimates just under seven million (see table at right).
Estimate of Global Species Diversity
(Excluding Prokaryotes)
Taxon Estimated number of species
Protist kingdom 400,000
Plant kingdom 320,000
Fungus kingdom 500,000
Animal kingdom
Nematodes 500,000
Molluscs 120,000
Arthropods
Crustaceans 150,000
Arachnids 500,000
Insects 4,000,000
Vertebrates
Fishes 27,000
Amphibians 4,000
Reptiles 7,150
Birds 9,800
Mammals 4,800
Others 150,000
Total 6,790,000