How Many Species Are There?: Revisited

How many species are there? Today, this question transcends the discipline of taxonomy, where it was perhaps first asked by Westwood (1833) and more recently by Sabrosky (1952), reaching across science through conservation biology to the media, even to religion (Cobb 1988). Gaston's article in this volume (pp. 283-296) makes another attempt at trying to guess the nearimponderable. Determining the number of species is like reaching for the stars; there is no way with the data available today that we are even going to get reasonably close. Perhaps what is more important now is not so much the numbers, but how we go about data gathering and making analyses necessary to estimate the order of magnitude with a significant level of confidence. Westwood (1833), in a pre-Darwinian approach to the results of creation, began with the number of species described by Linneaus and compared rates of new descriptions and those species known to be new in the collections. His estimates are thus based on the products of taxonomists doing their job rather than species living in nature. This approach continued to be perused until 1982, and it now has been resurrected in the present volume. Westwood also introduces the idea of relative abundance among groups as a source of data for biodiversity estimates; but with so few species known from so few areas in the year 1833, his conclusions are hopelessly naive. Metcalf (1940) also used the number of described species and the rates of new descriptions each year to arrive at an estimation of the number of species. Then he attempted to establish ratios of Homoptera to other insect taxa to arrive at an estimation of species numbers. Sabrosky (1953) relied too on published species descriptions for his estimates, much the same as Metcalf. White (1975) used taxonomists' products to generate species accumulation curves thinking he was obtaining an asymptote that reflected biodiversity estimates for beetle families. On the other hand, as pointed out by May (1988), a totally different agenda was set up by me (see Erwin 1982) for asking pertinent questions in a scientific way, that is, hypothesis testing using data on how arthropod communities are assembled in nature and the numbers of species in these assemblies. I might add to May's insights here by again (Erwin 1988) pointing out that the estimate of 30 million species in my 1982 paper was set up as a testable hypothesis, not, as Gaston puts it, a "claim" that there are this many species. Each step of my process can be independently tested, weaknesses found and corrected, etc., and these tests (host specificity, relative species abundance between beetle families, canopy versus undercanopy species, etc.) are now underway in my laboratory at the Smithsonian Institution with enormous new collections gathered to ask the questions in the right way. There will be more on this in a subsequent article in this journal. Gaston, however, takes another approach. He decided that, in addition to the literature approach of Westwood and others, the specialists in various taxonomic groups should be consulted to give their opinions on how many species remained to be described in their taxonomic specialties. He then added the number of species already described in the literature to arrive at a total number. The total of these figures is used to refute the number of species that was suggested by my field-gathered data and subsequent analysis. Gaston's Figure 2 summarizes his findings. What is wrong with this method? The data base is nonscientific; it cannot be tested, therefore it cannot be used to test any other methods. Gaston's data base is an anecdotal one that rests on the experience and collections of various workers. How did his experts get their data to give him the estimates? We don't know. Since all those listed are good taxonomists, we can guess that they based their estimates on their own collections (mostly institutional), the collections they borrowed and visited in the past, and their own field experiences, but how they arrived at the numbers to give Gaston is unexplained. One source, P. Hammond of the BMNH, for example, told me in 1986 that