In a paper in the October 2001 issue of Conservation Biology , Gomez de Silva and Medellin (2001) studied the problem of incomplete species lists in conservation and macroecology. They based their study on the observation that “studies of species diversity, macroecology, and conservation are usually based on lists of species, but lists found in the scientific literature vary in completeness.” They concluded that “realistic results may be produced in macroecological and conservation studies only if they are based on reasonably complete species lists.” Here, I make the point that there is an even more fundamental issue with respect to species lists that is in fact known, but whose effects are increasingly underestimated, especially in conservation: even complete species lists may fail to produce realistic results, and thus lead to wrong decisions, if they do not include estimates of the relative abundances of the species. In ecology and conservation, conclusions or management decisions are often based on comparisons of species occurrences in different habitats or locations. These comparisons, in turn, are based on the assumption that species lists reflect the ecological character and value of