Scientists have estimated the total number of species on Earth at 8.7 million (give or take 1.3 million) – one of the most precise calculations ever offered – with 6.5 million species found on land and 2.2 million (about 25 percent of the total) dwelling in the ocean depths. Of these, 86 percent (within the oceans, it’s 91 percent) have yet to be discovered, described and cataloged.
Dalhousie University scientists participating in the Census of Marine Life said they base their new figures on an innovative technique that dramatically narrows the range of previous estimates. Findings of their study appear in the August 23, 2011 issue of PLoS Biology.
Until now, the best estimate of Earth’s species total was based on the educated guesses and opinions of experts, who variously pegged the figure in a range from three to 100 million – wildly differing numbers that scientists questioned because there is no way to validate them.
Swedish scientist Carl Linnaeus created and published (1758) the system that scientists use to formally name and describe species. In the 253 years since, about 1.25 million species – roughly one million on land and 250,000 in the oceans – have been described and entered into central databases. Roughly 700,000 more are thought to have been described but have yet to reach the central databases.
Camilo Mora of the University of Hawaii and Dalhousie University (Halifax, Canada), along with his Dalhousie team, refined the estimated species total to 8.7 million by identifying numerical patterns within the taxonomic classification system. This classification system groups the forms of life in a pyramid-like hierarchy, ranked upwards from species to genus, family, order, class, phylum, kingdom and domain.
Analyzing the taxonomic clustering of the 1.2 million species today in the Catalogue of Life and the World Register of Marine Species, the researchers discovered reliable numerical relationships between the more complete higher taxonomic levels and the species level.
Sina Adl, of Dalhousie, said:
We discovered that, using numbers from the higher taxonomic groups, we can predict the number of species. The approach accurately predicted the number of species in several well-studied groups such as mammals, fishes and birds, providing confidence in the method.
The research team applied the new analysis to all five known eukaryote kingdoms of life on Earth. The study did not include certain micro-organisms and virus types, for example, which could be highly numerous.
The new analysis predicted the following:
1) ~7.77 million species of animals (of which 953,434 have been described and cataloged)
2) ~298,000 species of plants (of which 215,644 have been described and cataloged)
3) ~611,000 species of fungi (molds, mushrooms) (of which 43,271 have been described and cataloged)
4) ~36,400 species of protozoa (single-cell organisms with animal-like behavior, or movement, of which 8,118 have been described and cataloged)
5) ~27,500 species of chromists (including brown algae, diatoms and water molds, of which 13,033 have been described and cataloged)
Total: 8.74 million eukaryote species on Earth.
Within the 8.74 million total is an estimated 2.2 million (plus or minus 180,000) marine species of all kinds, about 250,000 (11 percent) of which have been described and cataloged. When it formally concluded in October 2010, the Census of Marine Life offered a conservative estimate of one million+ species in the seas.
Boris Worm, of Dalhousie, said of the new method:
This work deduces the most basic number needed to describe our living biosphere. If we did not know – even by an order of magnitude (1 million? 10 million? 100 million?) – the number of people in a nation, how would we plan for the future? It is the same with biodiversity. Humanity has committed itself to saving species from extinction, but until now we have had little real idea of even how many there are.
Worm notes that the recently updated IUCN Red List assessed 59,508 species, of which 19,625 are classified as threatened. This means the Red List – the most sophisticated ongoing study of its kind – monitors less than one percent of world species.
Jesse Ausubel, co-founder of the Census of Marine Life, said:
Awaiting our discovery are a half million fungi and molds whose relatives gave humanity bread and cheese. For species discovery, the 21st century may be a fungal century!
Ausubel notes the enigma of why so much diversity exists, saying the answer may lie in the notion that nature fills every niche, and that rare species are poised to benefit from a change of conditions.
Robert May, of Oxford, who wrote a commentary that accompanies the paper, said that the practical benefits of taxonomic discovery are many, citing the development in the 1970s of a new strain of rice based on a cross between conventional species and one discovered in the wild. The result: 30 percent more grain yield, followed by efforts ever since to protect all wild varieties of rice. May wrote:
[Gaining practical benefits] obviously can only be done if we have the appropriate taxonomic knowledge. Given the looming problems of feeding a still-growing world population, the potential benefits of ramping up such exploration are clear.
The question of how many species exist has intrigued scientists for centuries and the answer, coupled with research by others into species’ distribution and abundance, is particularly important now because a host of human activities and influences are accelerating the rate of extinctions. Many species may vanish before we even know of their existence, of their unique niche and function in ecosystems, and of their potential contribution to improved human well- being.
Based on current costs and requirements, the study suggests that describing all remaining species using traditional approaches could require up to 1,200 years of work by more than 300,000 taxonomists at an approximate cost of 364 billion dollars. Fortunately, new techniques such as DNA barcoding are radically reducing the cost and time involved in new species identification.
With the clock of extinction now ticking faster for many species, I believe speeding the inventory of Earth’s species merits high scientific and societal priority. Renewed interest in further exploration and taxonomy could allow us to fully answer this most basic question: What lives on Earth?
Co-author Alastair Simpson said:
We have only begun to uncover the tremendous variety of life around us. The richest environments for prospecting new species are thought to be coral reefs, seafloor mud and moist tropical soils. But smaller life forms are not well known anywhere. Some unknown species are living in our own backyards – literally.
Bottom line: Scientists from the University of Hawaii and Dalhousie University participating in the Census of Marine Life used an innovative analytical technique to estimate a number of species on Earth that is likely the most accurate to date. Published in the August 23, 2011 issue of PLoS Biology, their paper shows that the total number of species on Earth is approximately 8.7 million. Of these, 86 percent have yet to be discovered, described and catalogued.
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