We found 2,207 cryptic species reports (CSR) under 771,931 studies that were suitable to detect cryptic species. Log10 number of CSR were correlated with the log10 of estimated number of described species in different metazoan taxonomic groups (Fig. 1, R2 = 0.53, F1,18 = 20.69, P = 0.0002). Deviations from the regression line, termed CSR taxon variation, are presumably composed of differences in study intensity and taxonomic practice in the respective research community, true differences among taxonomic groups and random error. In order to assess the impact of differential research intensity, we regressed the log10 number of studies on the log10 number of described species in the respective taxonomic group; the residuals were used as a measure of taxonomic study bias (R2 = 0.64, F1,18 = 8.63, P = 0.0088). This parameter explained part of the CSR taxon variation (Fig. 2, R2 = 0.47, F1,18 = 15.67, P = 0.0009). Assuming that the numbers of CSR are proportional to the true number of cryptic species, their distribution is nearly homogeneous across the taxa analysed. Only six groups fell outside the 95% confidence intervals: Mammalia, Amphibia and combined smaller Arthropoda classes exhibited an excess of CSR, while Bivalvia, Arachnida and combined smaller Mollusca classes exhibited a deficit of CSR. At least for vertebrates, we suspect that these results might result from taxonomic inflation [2]. Overall, differential taxonomic practice in the various research communities seemingly exerted no major impact on the results.
Applying the same procedure to the classic biogeographical regions revealed a marginally significant correlation between the number of CSR and the proportion of described species in the respective region (Fig. 3, R2 = 0.59, F1,5 = 7.34, P = 0.0423). Study intensity of biogeographical regions was independent of estimated metazoan species richness (R2 = 0.41, F1,5 = 3.41, P = 0.1242). However, regressing the residuals against each other, the same nearly neutral pattern emerged (Fig. 4, R2 = 0.89, F1,5 = 40.44, P = 0.0014), which contradicts the view that tropical regions harbour relatively more cryptic species [1].
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