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Global Diversity of Insects:
The Problems of Estimating Numbers.

Ebbe S. Nielsen
Laurence A. Mound
Australian National Insect Collection, CSIRO Entomology, GPO Box 1700, Canberra ACT 2601, Australia

The class Insecta is the most species-rich of all major groups of living multicellular organisms. Any meaningful assessment of the diversity of life on earth depends on estimates of both the number of named insect species and the number of insect species that are living but are yet unnamed or even undiscovered.

Common sense might suggest that the number of described species would be a statistic that science would have available. However, no single compilation exists of the names of described insect species, so the total number remains a matter of conjecture. Indeed, for most groups of insects, apart from the Diptera (Evenhuis 1989), published lists of species names are not readily available, despite a recent surge of interest in computer listings. The production of lists of described taxa should have high priority for insect taxonomic science, whether for a local fauna, such as the Lepidoptera of Australia (Nielsen and others 1996), or for the worldwide fauna of a particular group, such as Geometridae (Scoble 1999). Such lists provide some measure of what has been achieved at a given time. More important, they can be a means of stimulating further studies and of attracting research funding in other aspects of biology (Mound 1998). However, within the taxonomic community, tradition remains biased toward the production of scholarly nomenclatural catalogs, with details of type material that are useful only to other specialist taxonomists. Our use of the term checklist implies a product that can be used as the starting point for investigations into biological diversity by the biological and conservation community in general.

As a result of the lack of checklists, available estimates of the numbers of described species often differ widely. Indeed, at times it is difficult to understand

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precisely what an author means by a figure for the number of species in an insect group. The numbers may represent all published species-group names; only technically available species-group names (excluding, for example, nomina nuda); currently accepted valid species, excluding synonyms but with or without names of subspecies; or the estimated currently extant species, including undescribed or even undiscovered species. In recent accounts, the estimated number of named species varies from 751,000 to 950,000, and the estimated number of living species ranges from 1 million to 100 million (Hammond 1994). The numbers we quote in table 1 presumably suffer from similar problems, but we have attempted to clarify the situation whenever possible.

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A further problem is that “species” are not comparable units throughout the Insecta, thus at times rendering comparisons potentially misleading (Vane-Wright 1992). Although the “species” is the most commonly used unit of biodiversity, organismal diversity cannot be measured objectively solely by differences in the number of species (Hawksworth and Kalin-Arroyo 1995). It is widely agreed that the species number is the most important measure that we have, but we cannot regard it as a standard unit in any statistical sense. The existence of sibling species of Diptera that are distinguishable only through examination of their chromosomes has been well known for many years. In other groups of insects, DNA methods are increasingly demonstrating genetic differences that many authors interpret as evidence of different species. In some ways, we face the same problem that plagued Alfred Russel Wallace and Charles Darwin, in attempting to distinguish units within biological systems that sometimes appear to exhibit almost continuous variation. The peaks of ecological and evolutionary adaptation that we call species can modulate and move in space and time in response to varying pressures of selection. In conservation, it is this ability to change and adapt that we need to protect, not merely the units that we use to measure diversity.

The estimates we give of the number of named species, particularly the total diversity within each order of insects, clearly depend heavily on the bibliographic efficiency and practical experience of individual taxonomic specialists. May (1990) expressed concern that no full published list exists of all described taxa of insects. However, no individual working taxonomist has a particular requirement for such a list. Moreover, most taxonomists who work on insects in the major orders study only a small subset of any major group and thus have little requirement for even a checklist of all the available names within an order or major family. Given that some genera of insects, for example within the Ichneumonidae and Staphylinidae, include more than 1,000 species each, it is scarcely surprising that individual taxonomists have not had the resources to produce or maintain such massive checklists.

Estimates of the possible number of living insect species originate essentially from two sources. One source is the few taxonomists who have experience with very large collections, usually coupled with field experience in areas of high biological diversity. In this case, the data will have been produced haphazardly and over a long time, albeit on a wide front, and the estimate is based on the frequency with which novelties appear in collections. The second source is ecologists who are interested in estimates of species richness. In this case, the data come from intensive sampling of restricted areas over a restricted period followed by extrapolation of these numbers into unsampled areas. Not surprisingly, these techniques yield rather different estimates. The first, which is essentially a species-accumulation curve, is related to the acquisition policy of institutes and the distribution patterns of species. This method will underestimate the total number of species through any failure to score fully the many species in large genera that are difficult to distinguish because they are represented only by single individuals. The second method is concerned with the numbers of species that can be found at a single point, and any assumptions of local endemicity or host specificity when extrapolating from these data will tend to overestimate the total number.

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Gaston (1991a) pointed out that few of his taxonomic colleagues who had experience with tropical diversity considered it likely that the group in which they specialized would prove to be larger than the currently described subset by orders of magnitude. Similarly, Hodkinson and Casson (1991) produced a figure for the total worldwide insect fauna of 1.87–2.49 million species on the basis of large collections of Hemiptera made in Sulawesi. In contrast, ecological estimates—such as those by Erwin (1982), Stork (1988, 1993), Kitching (1990), and Recher and Majer (1996)—imply that the world's insect fauna is 30 or more times that of the currently described subset. Current evidence from the major museum collections of sorted and labeled insect species, whether described or undescribed, does not support these larger estimates. Insect taxonomists generally concur that, although there may be as many as 5 million species of insects in the world, there are probably fewer than 10 million.

The suggestion that urgent efforts be made to describe all the world's species of insects leads us to a further series of issues. Even an estimate of 5 million species implies logistical demands that far exceed available resources. Mound (1998) pointed out that the practical problems involved in describing such very large numbers of species have never been considered seriously. These problems include communicating the information to other scientists, the effect on library budgets of a further 8 million pages of descriptions for the minimum of 4 million new species, and the effect of all the new insect material on museum budgets. Wilson (1985) expressed a more positive viewpoint by saying that describing a large proportion of the world's fauna is feasible but with the caveat that this possibility exists only if the priorities of human society change substantially from producing armaments to protecting the biosphere.

Some biologists assert that the study of highly diverse biological systems must be preceded by description of the many species that make up such systems. This is not entirely true, as indicated by the extensive karyotypic studies by M. J. D. White (1982) on species of Australian morabine grasshoppers that even now are undescribed. Similarly, Robinson and Nielsen (1989) gave an account of the Australian fauna of tineid moths, despite the fact that half the 380 known species remain formally unnamed. In both those instances, the species are sorted, labeled, and available for study in the Australian National Insect Collection. The importance of a major collection is the quantity and quality of information that it can contain, including distribution patterns in time and space and biological details, such as host plants and parasites. This information can be made available to biologists and conservation workers, even if not all the taxa are formally named. We certainly are not suggesting a moratorium on describing species of insects, but we suggest that greater thought be given to the question of what benefits will be obtained by describing a much larger fraction of the world's insect fauna.

The question of how science should respond to the problem of such a vast number of undescribed insect species is complex. Gaston (1994) pointed out that although most insect species are tropical, most taxonomic effort continues to be applied to temperate faunas. Mound (1998) indicated that science budgets in tropical countries will need to take a greater share of this burden of description

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in the future, but emphasized that more appropriate responses need to be considered than the ad hoc description of large numbers of species. The interesting scientific problem lies not in the description of all the species, but in why so many species exist. We need to describe formally only the species that we require for our analyses of biologically diverse systems, whether these analyses are ecological or systematic. The activity of describing species is sometimes advocated as providing the building blocks for the rest of biology. However, ad hoc description of new taxa is like the unplanned production of building blocks in the hope that one day they may find a place in our biological building. Can we not find a more rational and effective use of our resources for such a gigantic task?

Gaston (1991b) made the point that better data on the total numbers of species could be obtained by conducting detailed studies of the numbers of both described and undescribed species from a number of specified sites; that is, data should be collected purposefully, with particular objectives. Similarly, Longino (1994) has pointed out the advantages to be gained from a sampling program that has specific long-term objectives. Again, Mound (1998) pointed out that when descriptive taxonomy is incorporated into focused interdisciplinary projects on particular systems or groups, then the whole subject is enriched by data from other biological disciplines. Detailed sampling and interdisciplinary studies then have the objective of facilitating comparisons between sites, seasons, and habitats and thus are relevant to a wider community of scientists. More important, such an approach is based on the view of faunas as dynamic systems, in which processes can be studied, rather than as static systems, in which units need to be described.

As taxonomists ourselves, we find that the absence of an accurate figure for the total number of living insect species does not limit our studies of patterns in structural, behavioral, and geographic diversity. We continue to describe new species when this is relevant to our exploration of interesting patterns in nature, not as part of any program to provide names for the entire insect fauna. Far more important to us are the problems of the origin of insect diversity and of how to maintain this diversity in a rapidly changing world. In this context, we emphasize again the importance of well-curated museum collections and effective access to the information they contain (Nielsen and West 1994), because these tangible and available records of biodiversity facilitate the comparisons between sites and seasons that are valuable to the rest of society.

Table 1 summarizes the number of named species and the estimated total number of species that we consider valid, and table 1 is a brief discussion of each order according to various authors. Our estimated numbers are those we believe to be most accurate, given our current knowledge.

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(table continued on next page)

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Acknowledgments

We are grateful to our many colleagues in Canberra, at the Australian National Insect Collection and Australian Biological Resources Study, for their frequent advice, help, and criticism. C.W. Schaefer, of the University of Connecticut, Storrs, kindly gave us his opinion on numbers of Heteroptera species.

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