Scientists have long puzzled over a seemingly straightforward question: How many species live on Earth?
And now, after nearly three centuries, they have come up with a seemingly straightforward answer: 8.7 million.
But experts say we have identified just a fraction – only 10 per cent – of these species, and that the vast majority are still waiting to be discovered.
Still, the number, according to a new study in PLoS Biology, a journal published by the Public Library of Science, is the most precise estimated total of species in the world. Until now, scientists pegged the figure at three to 100 million – a large range because there was no way to confirm the estimate.
Breaking down the number, 6.5 million species are on land, and 2.2 million are in the ocean. But of those species, 86 per cent of land-inhabitants and 91 per cent of ocean-dwellers have not yet been discovered, described or catalogued, the study reports.
Why it matters?
Lead author Camilo Mora says pinning down the number of species on Earth is a "massive discovery," emphasizing its importance to animal conservation.
"We're losing species," he said, citing climate change, habitat loss and pollution as culprits. "We can only appreciate the magnitude of this loss when we have a reference on how many species are there."
The discovery is also a boon to humans, as well. "The food we eat, the water we drink and the air we breathe – all these things [are] afforded to us by species," Dr. Mora said. "Imagine the possibilities when you know that there are 90 per cent of the species out there that remain to be discovered!"
The estimated 600,000 species of fungi (there are currently 40,000 known species), such as moulds and mushrooms, are a particularly noteworthy finding because they are "very important" for humans, he added, citing fungi's use for things like penicillin and yeast for bread.
How'd they do it?
"We have been working on this for 260 years since the first species was described," Dr. Mora said, referencing Swedish scientist Carl Linnaeus, whose system for classifying organisms launched modern taxonomy in the mid-1700s.
"Unfortunately, the question proved to be so hard, that despite all of this time and all of this work, we haven't been able to provide a number to that basic question of how many species are there. So now for the first time, we have a statistical method that gives a number to it."
He and his colleagues at Dalhousie University refined the estimated total by "identifying numerical patterns" within the taxonomic system, which categorizes life forms in a pyramid-like hierarchy (ranked from the most-specific "species" level up to the most-general "domain" level ).
Researchers found that by using numbers from higher-ranked taxonomic groups, they could accurately predict the number of species.
The future of taxonomy
Dr. Mora has hopes that the new estimate will also bolster interest in the dying field of taxonomy. He recounts the story of an ecologist who suspected a fish he collected in Panama was a new species, but could not verify his hunch due to a lack of resources. It was only 30 years later that a geneticist conducted DNA analysis on the fish, confirming that it was indeed a new species.
"You cannot really have any understanding of the ecology – the role – that a species has [unless] you have a name to them," Dr. Mora said, adding there is a risk that scientists may discover a species that is critical to a functioning ecosystem only after it disappears.
Based on current costs and requirements, classifying all remaining species using traditional methods would cost more than $364-billion (U.S.), with more than 300,000 taxonomists working for 1,200 years, the study suggests. But DNA analysis would reduce the time and money involved in identifying new species.
Countries in Europe and North America invest the most in taxonomy, but the majority of species do not live in these places. Most undiscovered life is below the ocean's surface, in tropical forests or coral reefs.
"How can it be possible that we don't know 90 per cent of the species on this planet?" Dr. Mora said. "Today, we have the technology to go out in space … yet we don't have the technology to go down nine, 10 thousand metres down the ocean's surface to find out what is really down there."
He compares an undiscovered species to a missing engine part, which could cause the engine – the Earth's ecosystem – to stop working. The number illuminates the "magnitude of our lack of understanding or lack of knowledge of planet earth," a deficiency that taxonomy seeks to address, Dr. Mora said.
Among the list of thousands of new species found every year that challenge assumptions about Earth is halicephalobus mephisto, a worm-like species found more than three kilometres into the Earth's crust, which the study says challenged the notion that this "harsh" environment is unique to single-cell organisms. There is also the kiwa hirsuta, a white, fuzzy crustacean-like decapod found 1,500 kilometres south of Easter Island at a depth of 2,200 metres; scientists said it formed a new biological family called kiwaidae.