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

Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada.
PLoS Biology (Impact Factor: 9.34). 08/2011; 9(8):e1001127. DOI: 10.1371/journal.pbio.1001127
Source: PubMed


The diversity of life is one of the most striking aspects of our planet; hence knowing how many species inhabit Earth is among the most fundamental questions in science. Yet the answer to this question remains enigmatic, as efforts to sample the world's biodiversity to date have been limited and thus have precluded direct quantification of global species richness, and because indirect estimates rely on assumptions that have proven highly controversial. Here we show that the higher taxonomic classification of species (i.e., the assignment of species to phylum, class, order, family, and genus) follows a consistent and predictable pattern from which the total number of species in a taxonomic group can be estimated. This approach was validated against well-known taxa, and when applied to all domains of life, it predicts ~8.7 million (± 1.3 million SE) eukaryotic species globally, of which ~2.2 million (± 0.18 million SE) are marine. In spite of 250 years of taxonomic classification and over 1.2 million species already catalogued in a central database, our results suggest that some 86% of existing species on Earth and 91% of species in the ocean still await description. Renewed interest in further exploration and taxonomy is required if this significant gap in our knowledge of life on Earth is to be closed.

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    • "I N T R O D U C T I O N A significant portion of the Earth's biodiversity (an estimated 25% of the total number of species on Earth) is comprised of marine species (Mora et al., 2011). These have evolved mechanisms to survive in an extremely different and hostile environment compared with land in terms of light, salinity and pressure. "
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    • "The question of whether expansionist[2,30]or more constrained[15,16,18,31]patterns of diversification characterise the evolution of life on Earth is among the most contentious macroevolutionary questions[10,28,29,32,33]. Its answer has substantial implications for the origins and future of the enormous scope of extant biodiversity (estimated at 2–8 million species[34]), and for assessing whether equilibrial processes inferred from the study of island biogeography are applicable to global spatial scales and geological time spans26272832]. Animal diversity on land is especially high, comprising 75%–95% of multicellular species on Earth[35,36], and evolved in significantly less time than did the lower diversity of marine animals[11]. This observation has been used to justify an " emerging consensus " that species diversification on land was essentially exponential, irrespective of the evidence for constrained diversification in the marine realm[10,11]. "
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    ABSTRACT: How did evolution generate the extraordinary diversity of vertebrates on land? Zero species are known prior to ~380 million years ago, and more than 30,000 are present today. An expansionist model suggests this was achieved by large and unbounded increases, leading to substantially greater diversity in the present than at any time in the geological past. This model contrasts starkly with empirical support for constrained diversification in marine animals, suggesting different macroevolutionary processes on land and in the sea. We quantify patterns of vertebrate standing diversity on land during the Mesozoic-early Paleogene interval, applying sample-standardization to a global fossil dataset containing 27,260 occurrences of 4,898 non-marine tetrapod species. Our results show a highly stable pattern of Mesozoic tetrapod diversity at regional and local levels, underpinned by a weakly positive, but near-zero, long-term net diversification rate over 190 million years. Species diversity of non-flying terrestrial tetrapods less than doubled over this interval, despite the origins of exceptionally diverse extant groups within mammals, squamates, amphibians, and dinosaurs. Therefore, although speciose groups of modern tetrapods have Mesozoic origins, rates of Mesozoic diversification inferred from the fossil record are slow compared to those inferred from molecular phylogenies. If high speciation rates did occur in the Mesozoic, then they seem to have been balanced by extinctions among older clades. An apparent 4-fold expansion of species richness after the Cretaceous/Paleogene (K/Pg) boundary deserves further examination in light of potential taxonomic biases, but is consistent with the hypothesis that global environmental disturbances such as mass extinction events can rapidly adjust limits to diversity by restructuring ecosystems, and suggests that the gradualistic evolutionary diversification of tetrapods was punctuated by brief but dramatic episodes of radiation.
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    • "The global ocean houses an enormous variety of life. In total, the oceans support an estimated 2.2 million eukaryotic species (Mora et al., 2011), of which science has described some 220,000 (WoRMS Editorial Board, 2013). A key question is whether consistent 'rules' constrain the distribution of this life across the variety of different organisms and habitats examined here, and if so, whether they result in consistent large-scale patterns of biodiversity. "

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