Less diversity could add greater urgency to the extinction crisis facing 100 species per million.
A new estimate puts the number of species on Earth at around 6 million -- far fewer than once thought.
Seventy percent of species on the planet have yet to be identified and named.
Assessing the number of species on Earth could help to project extinction rates.
There may be far fewer species on Earth than scientists thought, according to new calculations.
If accurate, the new estimate -- that there are some 6 million species, rather than 30 million to 100 million or more -- could add urgency to the extinction crisis. About 100 species per million are currently disappearing from the planet, and scientists predict that the extinction rate is on its way to 10 times that level, far beyond what experts say is sustainable for ecosystems.
"We need to know how many there are to know how many we're losing," said Andrew Hamilton, an ecologist at the Melbourne School of Land and Environment in Australia. "Ecosystems might be more sensitive to change because there are fewer species out there."
Our planet is home to about 50,000 species of vertebrates -- a group that includes mammals, amphibians and reptiles. We have about 400,000 species of plants. And while there is wiggle room in those numbers (mostly as genetic techniques redefine what it means to be a species), the biggest source of unknowns lies within the arthropods, which includes insects, spiders and mites.
Arthropods make up 80 percent of all known species (not including bacteria, which are not usually included in studies like these), and most arthropods live in the tropics. Scientists have named about 855,000 arthropod species, but that is just a small fraction of what's out there.
The first stab at estimating actual arthropod numbers came in 1982, when scientists counted the number of creatures living on a single species of tropical tree and multiplied that by the total number of tree species. They came up with an estimate of 30 million species on Earth.
Using the same kind of method, though, other researchers produced estimates ranging from 10 million to more than 100 million species. As researchers realized the extent of what they didn't know, the discussion gradually petered out.
In the new study, Hamilton and colleagues used a type of mathematical model that allowed them to include how certain or uncertain they were about various assumptions. For example, with a wealth of data from Papua New Guinea, they knew that some species of trees hosted far more species of arthropods than others.
Based on their model, the researchers reported in The American Naturalist with 90 percent certainty that the number of arthropods on Earth is between 2 million and 7 million, with the number most likely falling around 3.7 million. That translates to between 4 million and 9 million total species of organisms that share the planet.
Some 70 percent remain to be discovered. About a quarter of all species are probably beetles.
The new study is a solid step forward, but there are still a lot of unknowns, said Rob Dunn, a biogeographer at North Carolina State University in Releigh. In recent genetic studies, for example, scientists have looked closer at certain moths, flies and other insects, only to find that one species is actually multiple species -- sometimes on the order of 20 or more. He thinks the true number of species on Earth could be three or four times higher than the new estimate.
"We probably still don't have a single hectare of forest where we know all the species anywhere on Earth," Dunn said. "It means that if we step back and think about what's changing on Earth, most of the effects of those changes are likely to be due to species that don't yet have names. Our ignorance, in general, is still huge."
That inability to determine absolutely the number of species on the planet makes extinction rates difficult to assess. "We are extinguishing the species we pay attention to -- the birds and mammals -- at rates 100- or 1,000-fold greater than what would be occurring in the absence of our actions," Dunn added. "What that means for the great majority of smaller, less often studied and unnamed species is essentially anyone's guess."