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A count of life on planet Earth

8.7 million species inhabit Earth, say Dal researchers

- August 24, 2011

The deviled rain-peeper, first discovered in 2008 in the high Andes of southeastern Ecuador. (Alejandro Artega photo)
The deviled rain-peeper, first discovered in 2008 in the high Andes of southeastern Ecuador. (Alejandro Artega photo)

Consider how many species of animals may be on earth – every type of bird, snake, fish, ape. How high do you think you’d have to count?

Scientists have been wondering this for centuries. And now, thanks to work led by Dalhousie researchers, they have a more precise answer to work with than ever before.

Census of Marine Life scientists, led by Camilo Mora of Dalhousie and the University of Hawaii, have published a new paper in PLoS Biology estimating that there are 8.7 million species on Earth, with 6.5 million on land and 2.2 million in the ocean. The number is based on groundbreaking, analytical methods that narrow down earlier estimates.

Age-old taxonomy


In 1758, Carl Linnaeus established a system used to formally name and describe species on land and in the ocean. Since then, roughly 1.25 million have been described and catalogued into central databases. But experts estimated the total number of species could range anywhere between three million and 100 million – a sizable variance for scientists to work with.

To focus this number, lead author Dr. Mora—with co-author Boris Worm, Dalhousie biologist, as well as colleagues Derek P. Tittensor, Sina Adl, and Alastair G.B. Simpson—identified numerical patterns in the taxonomic classification system (domain, kingdom, phylum, class, order, family, genus, species). This allowed researchers to see the numerical relationships between the higher levels (domain and kingdom) and the lower levels (species and genus).

The team discovered that by using numbers from the higher taxonomic groups, like in the domain and kingdom, they were able to accurately predict the number of species. This method was proven correct when it was used to calculate the number of species in many well-researched groups like mammals, fish, and birds.

Work in progress


Although the study done by Dr. Mora’s team has already made international headlines, he thanks those scientists who came before him that paved the way for such research to be done.

“Keep in mind this study has been ongoing for 253 years,” he explains with a chuckle. “We were fortunate that a lot of information from the past had been collected and well documented. That being said, we still had to compile the information, synthesize the information, making sure everything was statistically correct. We definitely had challenges.”

'This was a very difficult study to undertake,” says co-author Dr. Tittensor, a research scientist working for the United Nations Environment Programme World Conservation Monitoring Centre and Microsoft Research. “We tried many different approaches over a period of several years, only to hit a brick wall each time. We had all but given up, but kept out eyes open to possible solutions to the puzzle, and when we started to work on this approach we knew we were on to something.”

Time to shape up


While this study gives the most validated estimation ever published, it also means that a overwhelming number of species have yet to be discovered, described, and catalogued – 86 per cent of all land species and 91 per cent in the ocean.

Dr. Mora says, now more than ever, there is an emphasis on narrowing down the number of species that exist given the number of species that are considered ‘threatened.’

“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 of a host of human activities and influence are accelerating the rate of extinctions,” he explains.

“Many species may vanish before we even know of their existence, of their unique niche and function in the ecosystem, and of their potential contribution to improved human wellbeing.”  

Asks Dr. Worm, “If we do not know—even by an order of magnitude (1 million? 10 million? 100 million?)—the number of people in a nation, how could 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.”

“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,” says Dr. Mora. “Renewed interest in further exploration and taxonomy could allow us to fully answer this most basic question: What lives on Earth?”


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