M. Cardillo et al.'s analysis of mammalian extinction biology uses data from the IUCN Red List of Threatened Species [here and below I have replaced the original citations with links]. I believe that conservationists should be much more circumspect than we currently are about conclusions based on such analyses of the Red List.
The Red List is not managed as a database for biological analysis. Many problems are inherent in using threatened species lists for purposes for which they were not designed (Possingham et al., available here). The Red List's categorizations are largely informed guesswork by experts. That guesswork is vital and appropriate, given how little we know of most of the world's species and how little would be done about them if we insisted on full knowledge before action. Nevertheless, what is going to inform guesswork but knowledge of extinction biology?
Consequently, the biology of extinction is being investigated by the use of data that are (properly) fundamentally affected by knowledge of extinction biology. I cannot see that the inevitable circularity is removed by use of, for example, only species categorized on the basis of only population size or rate of decline (Purvis et al., free access; Cardillo et al.): The expert does not know the rate of decline (Blake and Hedges) but does know that large-bodied, slow-reproducing species that live in small geographic ranges are more likely to be threatened than are small-bodied, fast species in large ranges--and so suggests a faster decline for the former.
How can use of Red Lists to investigate extinction biology avoid such circularity, especially when the IUCN does not yet have the resources to make available the data on which the categorizations are based?
Cardillo et al. reply:
Despite Harcourt's concerns about circularity, biological traits do not form part of the process of categorizing species under criterion A. Categorizations of extinction risk are made under explicit, objective, and quantitative criteria (Rodrigues et al.). For example, under criterion A1, a species is listed as Vulnerable, Endangered, or Critically Endangered if the rate of population decline has been 50 to 70%, 70 to 90%, or >90%, respectively, over a period of 10 years or three generations, whichever is longer (IUCN). Rates of decline may be assessed from direct observations or inferred from indirect evidence such as catch statistics or habitat loss. If the latter, there are explicit guidelines for inferring population decline rates from indirect evidence (IUCN), and assessors must present the evidence they have used to conclude that a population has declined by the amount claimed. Before listing, assessments are reviewed by independent Red List Authorities. Species listings are accompanied by a justification giving support for the listing, together with relevant data and references, thus making the process as repeatable and transparent as possible. Species for which too little data exist to assign to an extinction risk category are listed as Data Deficient.Although listing procedures are probably not as rigorous as Cardillo et al. portray them, it is still possible that the Red List data are good enough to learn something from Cardillo et al.'s analysis. But I agree with Harcourt that there remains a suspicion of circularity, and that the same applies to similar studies. Threatened status is no surrogate for extinction, and researchers consciously or unconsciously draw on their ideas about extinction when they make risk estimates for individual species.