Kingdom and Major Subdivision

Common Name

No. of Described Species

Totals

Mollusca

Mollusks

50,000

 

Echinodermata

Echinoderms (starfish and relatives)

6,100

Arthropoda

Arthropods

 

Insecta

Insects

751,000

Other arthropods

 

123,161

Minor invertebrate phyla

 

9,300

989,761

Chordata

Tunicata

Tunicates

1,250

 

Cephalochordata

Acorn worms

23

Vertebrata

Vertebrates

 

Agnatha

Lampreys and other jawless fishes

63

Chrondrichthyes

Sharks and other cartilaginous fishes

843

Osteichthyes

Bony fishes

18,150

Amphibia

Amphibians

4,184

Reptilia

Reptiles

6,300

Aves

Birds

9,040

Mammalia

Mammals

4,000

43,853

TOTAL, all organisms

 

 

1,392,485

aCompiled from multiple sources.

in a few well-studied groups such as the vertebrates and flowering plants. If insects, the most species-rich of all major groups, are included, I believe that the absolute number is likely to exceed 5 million. Recent intensive collections made by Terry L.Erwin and his associates in the canopy of the Peruvian Amazon rain forest have moved the plausible upper limit much higher. Previously unknown insects proved to be so numerous in these samples that when estimates of local diversity were extrapolated to include all rain forests in the world, a figure of 30 million species was obtained (Erwin, 1983). In an even earlier stage is research on the epiphytic plants, lichens, fungi, roundworms, mites, protozoans, bacteria, and other mostly small organisms that abound in the treetops. Other major habitats that remain poorly explored include the coral reefs, the floor of the deep sea, and the soil of tropical forests and savannas. Thus, remarkably, we do not know the true number of species on Earth, even to the nearest order of magnitude (Wilson, 1985a). My own guess, based on the described fauna and flora and many discussions with entomologists and other specialists, is that the absolute number falls somewhere between 5 and 30 million.

A brief word is needed on the meaning of species as a category of classification. In modern biology, species are regarded conceptually as a population or series of populations within which free gene flow occurs under natural conditions. This means that all the normal, physiologically competent individuals at a given time are capable of breeding with all the other individuals of the opposite sex belonging



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