The Nature Writers of Texas

The best nature writing from the newspaper, magazine, blog and book authors of the Lone Star State . . .

Sunday, October 26, 2003

How many Animal Species exist Worldwide?
Ro Wauer, October 26, 2003, The Victoria Advocate, © 2003

There is an old saying that "little fish have bigger fish that feed on them and bite 'em, and big fish have still bigger fish, and so, ad infinitum." That quote can easily refer to the animal kingdom with its amazing diversity and huge numbers. It is like a great pyramid of life, with the greatest numbers of animal species at the bottom, supporting lesser numbers toward the top. Vertebrates sit at the top, with mammals, including we humans, near the summit.

But have you ever wondered about the total numbers of animal species that exist on planet Earth? Although more than a million have been described, the actual number may be closer to 10 million. Described insects alone make up the largest number at about 751,000 species, but Smithsonian Institution entomologist Terry Erwin estimates there may be 30 million species of tropical insects alone, a far cry from our current understanding. A fascinating indication of how little we know about our world. Edward Theriot, director of the Texas Memorial Museum and president of the Association of Systematics Collections, is quoted as saying: "We don't even know enough to tell you what we don't know."

After insects, the next most numerous kinds of animals are the noninsect arthropods at about 123,400 species. These are the crustaceans such as spiders, mites, and scorpions (about 35,000), centipedes (about 2,000), millipedes (about 7,000), as well as crabs, lobsters, crayfish, and shrimp (about 30,000). Next in numbers are the mollusks at about 110,000 species. These include snails, whelks and slugs that comprise 80,000; clams, oysters, mussels and scallops that account for about 15,000, chitons about 700, octopuses and squids about 400, and tooth or tusk shells about 350.

The protozoa, tiny single-celled organisms such as amoebas, are next with about 30,800; fishes and lower chordates with about 28,800; and earthworms with about 12,000 species. The fishes include about 23,000 species, divided into three groups: lampreys and hagfish; sharks, rays, and skates; and the bony fish, such as sturgeon, trout, perch, and other modern forms. Birds are next in numbers with about 9,000 species worldwide, followed by reptiles, including snakes, lizards, turtles and crocs, with about 6,300 species.

Echinoderms are next with about 6,100 species. These are the starfish, brittle stars, sea urchins, sea dollars, sea cucumbers, and such, all have tube feet and occur only in marine environments. They are followed by the mammals with about 4,500 species, and lastly are the amphibians, the frogs, toads and salamanders, with about 4,200 species.

Vertebrates, the mammals, birds, reptiles, amphibians, and fish, those species with a backbone, number only about 44,000 species in total. They are but a tiny part of the animal kingdom. Yet we mammalian humans apparently are the only ones that seem to care about numbers and categories; we constantly strive to find more. New species of whales, monkeys, deer, and birds, as well as far more forms of invertebrates, have been discovered in recent years. In future years, it is not unimaginable that the number of animal species on Earth could increase by a factor of 10 or more.

Yet, all the while, human activities throughout our planet are seriously affecting our animals. Pollution of all types is causing noticeably declines in amphibians, a group of animals that well may be our "canary in the cage." Land clearing for developments, grazing, and agriculture, also is leading to dreadful losses and declines. We know so little about the abundant species being lost. Their value for medicines, as one example, will never be known.


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