Estimating the number of species not yet described and their characteristics: the case of Western Palaearctic dung beetle species (Coleoptera, Scarabaeoidea)

To determine the degree of completion of Western Palaearctic dung beetle species inventory, as well as to ascertain some of the main geographical and morphological characteristics associated with the probability of new species description, the available taxonomic and biogeographic information of the three Scarabaeoidea families that are associated with dung was analyzed. An asymptotic fit of the cumulative number of described species is used to estimate the approximate number of species not yet described. Variation partitioning and hierarchical decomposition techniques were used to explore the influence of body size, geographical range size and geographical location variables on the process of species description. Results indicate that the inventories of Western Palaearctic Scarabaeidae and Geotrupidae species are almost complete, but that around 16% of Aphodiidae species have yet to be described. As the joint effect of geographical distribution and location is the main determinant of the species description rate in this family, it was predicted that new dung beetle species would be found in smaller areas of the southern and eastern Western Palaearctic region. The species body size accounted for a negligible fraction in the variation of the year of description. Both the geographical range size and the geographical location are essential variables negatively correlated with the year of species description, whose pure and combined effect can account for high percentages in the year of description variation (from 41 to 56%). The location of most of the taxonomic workforce in areas where broader-ranged generalist species prevail in the assemblages is the probable reason for this pattern.