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Research Article

How Many Species Are There on Earth and in the Ocean?

  • Camilo Mora mail,

    moracamilo@hotmail.com

    Affiliations: Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada, Department of Geography, University of Hawaii, Honolulu, Hawaii, United States of America

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  • Derek P. Tittensor,

    Affiliations: Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada, United Nations Environment Programme World Conservation Monitoring Centre, Cambridge, United Kingdom, Microsoft Research, Cambridge, United Kingdom

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  • Sina Adl,

    Affiliation: Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

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  • Alastair G. B. Simpson,

    Affiliation: Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

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  • Boris Worm

    Affiliation: Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

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  • Published: August 23, 2011
  • DOI: 10.1371/journal.pbio.1001127
  • Featured in PLOS Collections

Reader Comments (9)

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Asymptotic nature of higher taxa an artifact?

Posted by MichaelSch on 30 Aug 2011 at 12:11 GMT

The assumption central to all further calculations by Mora et al. (2011) is that species numbers described more or less steadily increase, while numbers of higher taxa reach an asymptote. This makes total numbers of higher taxa predictable, and given that the ratio of species assigned to higher taxa remains stable, the total number of species is predictable.

There is no doubt that discovery of new species will continue, likely on an even higher pace when DNA taxonomy or integrative approaches will be broadly used to diagnose animal species.
However, the asymptotic increase of new higher taxa illustrated by Mora et al (2011) could be an artifact. Centenary taxonomic practice of organizing organismic diversity consequently into hierarchic categories gives way to phylogenetic systems. Phylogenetic systematists broadly agree that higher categories have no intrinsic properties. What makes a clade an order rather than an infraorder or a superfamily? Their relative position to established taxa attributed to a certain rank only. Modern systematists thus may try to present their topologies in a way most compatible to traditional classifications, but usually avoid establishing new higher categories with new names for every newly recognized clade sister to an established e.g. genus, family or order (e.g. Martynov & Schrödl, 2011). And even if new names are established for major newly recognized clades, e.g. for the sake of clarity and recognition by a non-expert public, usually no ranks are assigned to these new clade names (e.g. Jörger et al., 2010; Schrödl et al., 2011). New higher taxa are steadily discovered, but no more flagged as such.

Taxonomic paradigms have changed, slowing down the pace of new higher category names being established and retrievable from public repository systems. The asymptotic nature of higher groups “found” or “recognized” (i.e. defined and named) is either meaningless as are higher ranks (if taken as beings with properties) in my personal opinion, or, as a phenomenon/pattern could be a dramatic underestimation.

Aliens having read Mora et al. (2011) might wonder how humans compare fish families with mollusc or insect families. They might decide on scanning “species” on earth themselves and happily find double or triple numbers …. In case some diversity is left when aliens finally arrive.



References:

Jörger, K. M., Stöger, I., Kano, Y., Fukuda, H., Knebelsberger, T. & M. Schrödl. 2010. On the origin of Acochlidia and other enigmatic euthyneuran gastropods and implications for the systematics of Heterobranchia. BMC Evolutionary Biology 10: 323. doi:10.1186/1471-2148-10-323

Martynov, A. & M. Schrödl. 2011. Phylogeny and evolution of corambid nudibranchs (Mollusca: Gastropoda). Zoological Journal Linnean Society. DOI: 10.1111/j.1096-3642.2011.00720.x

Schrödl, M., Jörger, K., Klussmann-Kolb, A. & N.G. Wilson. 2011. Bye bye “Opisthobranchia”! A review on the contribution of mesopsammic sea slugs to euthyneuran systematics. Thalassas 27: 101-112.

No competing interests declared.