Cryptic animal species are homogeneously distributed among taxa and biogeographical regions

Abstract

Background: Cryptic species are two or more distinct but morphologically similar species that were classified as a single species. During the past two decades we observed an exponential growth of publications on cryptic species. Recently published reviews have demonstrated cryptic species have profound consequences on many biological disciplines. It has been proposed that their distribution is non-random across taxa and biomes.

Results: We analysed a literature database for the taxonomic and biogeographical distribution of cryptic animal species reports. Results from regression analysis indicate that cryptic species are almost evenly distributed among major metazoan taxa and biogeographical regions when corrected for species richness and study intensity.

Conclusion: This indicates that morphological stasis represents an evolutionary constant and that cryptic metazoan diversity does predictably affect estimates of earth's animal diversity. Our findings have direct theoretical and practical consequences for a number of prevailing biological questions with regard to global biodiversity estimates, conservation efforts and global taxonomic initiatives.

Figure 1

The log₁₀ of cryptic species reports (CSR) as a function of the log₁₀ number of described species in the respective taxon. Deviations from the regression line represent CSR taxon variation. Dashed lines represent 95% confidence intervals.

Figure 2

Regression of CSR taxon variation on taxon study bias for 19 metazoan taxa. Dashed lines represent 95% confidence intervals.

Figure 3

The log₁₀ of CSR as a function of the log₁₀ number of described species in the respective region. Deviations from the regression line represent CSR region variation. Dashed lines represent 95% confidence intervals.

Figure 4

Regression of CSR taxon variation on biogeographical region study bias. Dashed lines represent 95% confidence intervals.