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aAverage for 1977-1988.

b Number of papers per number of species per year.

aAverage for 1977-1988.

b Number of papers per number of species per year.

dimensions of species diversity remain uncertain. Understanding is hampered by lack of a consensus about the total number of species that have been named and described, with estimates ranging from 1.4 to 1.8 million species. This probably represents less than 20% of all species on Earth, and with only about 20,000 new species of all organisms being described each year, it seems that most species will remain undescribed for many years unless there is a rapid increase in species descriptions (but see http://www.all-species.org).

About 850,000 to 1,000,000 of all described species are insects. Of the 30 or so orders of insects, four dominate in terms of numbers of described species, with an estimated 600,000 to 795,000 species: Coleoptera, Diptera, Hymenop-tera, and Lepidoptera (Table I). There are almost as many named species of beetle as there are of all other insects added together, or all other noninsects (plants and animals).

There is no complete catalog of names for all organisms, and for many groups it is often difficult to know what has or has not been named and described. It can sometimes be difficult for taxonomists to determine whether a series of individuals constitutes one or several species, or whether a new individual is the same species as others that have been described. On the other hand, a species may be described more than once. A taxonomist in one part of the world may not realize that a given species has already been described from elsewhere. Some species are so variable that they are described many times. For example, the ladybeetle, Adalia decempunctata has more than 40 synonyms. This species has many color morphs, and at various times during the last 200 years different taxonomists have given names to the color morphs without realizing that they were all one species. The level of such synonymy in some groups of organisms may be extremely high: (e.g., 80 and 35% synonymy for Papilionidae and Aphididae, respectively).

The question of how many species in total there are on Earth, including undescribed species, also remains a mystery. In 1833 the British natural historian John Westwood estimated that there might be some 20,000 species of insects worldwide. Today it is recognized that there are about this number of insect species in Britain alone. Estimates for how many species there are on Earth have continued to rise, and still it seems that the answer cannot be provided to within a factor of 100. Groups such as birds, large mammals, and some woody plants are well known, and estimates of their global numbers of species can be made with a fair degree of confidence. However, the scientific rationale for almost all estimates of global numbers of species for the remainder taxa, including insects, is surprisingly thin. Although estimates for global numbers of all species, from bacteria to vertebrates, vary from as low as 2 million to more than 100 million, much evidence seems to support estimates on the lower end of this scale: 5 to 15 million species.

Much of the recent literature on global species estimates has focused on insects and in particular on tropical forest insects. Until the 1980s most entomologists thought that there might be about 2 to 5 million insect species on Earth. However, Terry Erwin of the Smithsonian Institution in 1982 calculated that there are 30 million species of tropical arthropods alone, based on his knockdown insecticide fogging samples of beetles from the canopy of Central American tropical forests. He sampled 1200 species of beetles from the canopy of a single species of tree in Panama and suggested that 13.5% of these (162) must be specific to that tree. He arrived at his total of 30 million by suggesting that (1) all 50,000 species of tropical tree had the same level of insect host specificity, (2) beetles represented 40% of canopy arthropods, and (3) the canopy is twice as rich in arthropods as the ground. Others have since criticized all the steps in Erwin's calculation, suggesting that he overestimated the relative proportion of ground to canopy species, the relative proportion of beetle species to other groups of insects and, perhaps most important of all, the number of species that are host specific to a given species of tree. Another argument Stork and others have proposed is based on well-known insect faunas such as those for Britain and for butterflies. There are some 22,000 insect species in Britain and 67 of these are butterflies. It is also estimated that there are 15,000 to 20,000 species of butterflies in the world. Therefore, if the ratio found in Britain of butterfly species to all other insect species is the same for the whole world, this would indicate that there are 4.9 to 6.6 million species of insects on Earth. These and other analyses indicate that lower estimates (5—10 million insect species worldwide) may be realistic.

One of the reasons so few species have been described is that there are few taxonomists, and most of these are in the developed world. For example, 80% of insect taxonomists are found in North America and Europe. Another critical factor is that most of the type specimens on which species names depend are found in European and (to a lesser extent) North American museums.

It may seem that a great deal is known about the biology, distribution, and threatened or nonthreatened status of insects. In practice, this is far from the truth. For well-known insect faunas, such as those of Britain and other areas of Europe, virtually all species (but, surprisingly, not all) have been described. Even so, distribution maps for these species are often extremely poor, and the data used are often based on records more than 50 years old. For other parts of the world, particularly tropical regions, knowledge of the biota is largely nonexistent. Rarely are there even species lists for some of the better known groups, let alone taxonomic keys and field guides to identify these and other less well-known insects.

Much of the information on the distribution and biology of species is housed in the museums, herbaria, and libraries of developed countries. Some of this information is on index cards. There is now a growing effort to place information associated with specimens in the collection into electronic databases and to make this information readily available. Similarly, the biology and conservation status of the vast majority of insect species remain unknown. For this reason the International Union for the Conservation of Nature's (IUCN) Red Data Books on the threatened status of organisms are mostly limited to groups of large vertebrates and higher plants.

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