A statistical approach to estimate soil ciliate diversity and distribution based on data from five continents

A total of 359 soil samples collected from five continents (Africa, Asia, Australia, Europe and South America) were investigated for the presence/absence of soil ciliate species. Merging records by species identity, we have compiled a master data list (species by sample matrix). In the list, a total of 964 soil ciliate species (644 described and 320 undescribed) are recorded. The species distributions within the 359 samples and across the five continents are examined. The frequency distribution of the species over samples is used for global diversity estimation. A statistical ACE (abundance‐based coverage estimation) model which links observed data to unseen species is applied to assess regional and global soil ciliate species richness. The model, whose reliability was tested by its power to predict the number of new species in additional samples from Africa, may resolve the controversial issue on global species diversity of soil ciliates. Although an accurate point estimate is not feasible due to severe undersampling, the statistical model enables us to obtain a minimum regional diversity and global species diversity. A consistent finding over all five continents is that at least half of the species diversity is still undiscovered. Our model also yields a global soil ciliate diversity of at least 1900 species, that is, doubles the number of currently known species, and thus diversity is relatively high. This is consistent with the finding of Foissner, who used a probability‐based method. Soil ciliate distributions between continent pairs are analyzed by adjusted abundance‐based similarity/overlap indices. These new indices account for the effect of unseen species and also reduce the bias generated by undersampling. The adjusted abundance‐based Jaccard (or Sørensen) index shows that there is about 30% (18% for Sørensen) dis‐similarity between any two continents, supporting the moderate endemicity model. The results are discussed with respect to protist species distribution, that is, whether they are cosmopolitan or of restricted distribution.