Cryptic animal species are homogeneously distributed among taxa and biogeographical regions
© Pfenninger and Schwenk; licensee BioMed Central Ltd. 2007
Received: 13 April 2007
Accepted: 19 July 2007
Published: 19 July 2007
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.
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.
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.
Cryptic species are two or more distinct species that were classified as a single species due to their morphological similarity. Facilitated through technical advances such as PCR and direct DNA sequencing, many phylogenetic, phylogeographic and population genetic studies in the last two decades discovered – frequently without a priori intention – genetically divergent but morphologically cryptic lineages. These discoveries continue to increase exponentially and raise a number of general questions, such as: How frequent are cryptic species? Are cryptic species evolutionary young? Is morphological stasis upon speciation more often found in environmental extremes, such as the tropics, the artic or the deep sea? A recent review proposed that the distribution of cryptic species is non-random across taxonomic groups and biomes, which might have substantial consequences for biodiversity assessments, macroecology, biogeography, conservation management and evolutionary theory . Biodiversity estimates of certain taxonomic groups might be largely underestimated, ecological interactions remain hidden, conservation efforts may be inappropriate and cryptic pathogens, parasites and invasive species might represent unrecognised threads to human health. To tackle these issues, we need more information on the proportion of cryptic species in different phyla and different biomes. Here, we analysed the Zoological Record™ database (1978–2006) for the taxonomic and biogeographical distribution of cryptic metazoan species in relation to the number of described species.
Results and discussion
Our results indicate that the proportion of cryptic species is almost evenly distributed among major metazoan taxa and biogeographical regions when corrected for species richness and study intensity. All users of taxonomic information must consequently be aware of the potential presence of cryptic diversity, regardless of taxonomic group or study area in focus. Further studies will show whether this relation holds for lower level systematic categories and other kingdoms.
Species are cryptic to human perception largely due to the lack of conspicuous differences in outward appearance. Given their homogeneous systematic and geographic distribution, it seems therefore that morphological stasis upon speciation represents an evolutionary constant, independent of phylogenetic relation or ecological circumstances.
Although the true proportion of cryptic species in nature is unknown, our results indicate that it seems to be similar across major metazoan taxa and biogeographical realms. Therefore, global barcoding initiatives , aiming at the exhaustive treatment of selected taxa like birds or fishes could also provide a first glimpse on the real extent of cryptic diversity in all metazoans.
The most important consequence of our unexpected finding is, however, that cryptic metazoan diversity can be treated as random error in biodiversity assessments . There are probably not systematically more cryptic species among insects than in reptiles or in the tropics than in temperate regions. It stresses therefore that we should not preferentially target specific taxonomic groups or regions to detect cryptic species, but rather expect a predictable proportion of cryptic diversity in each metazoan group.
Results of database search in Zoological Record™ (1978–2006) for several metazoan taxa. CSR = cryptic species reports.
Other Arthropoda classes
other Chordata classes
Other Mollusca classes
Other Metazoan phyla
Results of database search in Zoological Record™ (1978–2006) for biogeographical regions. CSR = cryptic species reports.
Australasia + Oceania
We thank M. Stoeckle, M. Perez-Losada, W. Salzburger, G. Carvalho and M. Reid for helpful comments on the manuscript. This study benefited from financial support by ESF EURODIVERSITY project BIOPOOL (nationally funded by DFG; SCHW830/6).
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