All the fish in the sea?

Mankind has identified just a fraction of the species on land and in the sea, according to new estimates published on Tuesday - meaning many will go extinct before they can be described and indexed.

Scientists from Dalhousie University in Canada and the University of Hawaii in the US have a new estimate of the number of eukaryotic species - organisms made up of cells with membrane-bound nuclei - on Earth. Their projections suggested that our planet is home to 8.7 million species, with a margin of error of 1.3 million.

Their study, published in the latest edition of the journal PLoS Biology, said marine species accounted for about 2.2 million of that number, though taxonomists have yet to describe and index a staggering 91 percent of the species in the world's oceans.

For terrestrial species, that figure was 86 percent - despite ongoing efforts to categorize new species since Swedish scientist Carl Linnaeus developed his categorization scheme more than 250 years ago.

Only about 6,200 eukaryotes have been described each year over the past two decades, the authors wrote. If that trend persists, it could take over a millennium and hundreds of billions of dollars to classify the rest of the species on Earth.

"Our results also suggest that this slow advance in the description of species will lead to species becoming extinct before knowing they even existed," they said.

Pattern predictions

Finding out how many species are out there is a way of putting the scale and rate of extinction in perspective, lead author Camilo Mora told Deutsche Welle.

"If you have $1 million in your pocket and you lose $1, than losing $1 won't mean much," he said. As the study put it, the 8.7 million figure serves as "reference point" for lost biodiversity, both now and in the years to come.

It also helps us understand more about the complex world around us. Derek Tittensor, one of the study's authors, said via email that humans depend on other species, "for services such as food, crop pollination, carbon sequestration, nutrient cycling and many others."

"We need to know how the ecosystems that we depend upon are organized, and this number tells us how much - or how little - we know."

The authors arrived at this new reference point by examining patterns between taxonomic ranks, from kingdom and phylum all the way down to species.

One key observation showed that well-studied taxonomic groups, such as birds and mammals, display numerical similarities in terms of how many species are assigned to different taxonomic ranks.

For lesser-known groups the number of species remains a question mark, but scientists know more about higher ranks, such as phyla, which are described more completely. The team used patterns observed among better-known groups to predict species numbers for other groups - giving them an estimated global total.

Mora compared the hierarchy to area populations. Individuals live in houses, which are located in neighborhoods, which comprise cities, which are located in countries - much like species fall within a genus, then a family and an order, class, phylum and kingdom, respectively.

Finding out the number of individuals in a country is much easier if one has more reliable information about categories located higher up the hierarchy - for instance, how many cities there are.

The missing pieces

Using this method, researchers predicted that about 7.77 million out of the 8.7 million eukaryotic species on Earth are animals. Yet less than a million of those species have been described and indexed.

By contrast, estimates indicate that more than 70 percent of the planet's plants have been classified.

"That has to do a lot with taxonomic effort and how hard it is to describe a species," Mora said. Plants are static, making them easy to spot, yet animals are mobile - and might even be microscopic.

"Some are located in places that taxonomists don't go all that often," he added.

So what is still out there? According to the study, Earth's most elusive life forms are likely confined to smaller geographic areas or "hotspots," often in the depths of Earth's underwater and underground habitats.

Other differences also remain. Alastair Simpson, one of the authors of the study and a protists expert at Dalhousie University, said what biologists count as a species for microbes is very different than what it is for animals.

"Part of the problem is we know less about eukaryotic microbes," he said, adding that scientists have less taxonomic information to go on.

The numbers debate

Yet within the scientific community, it seems, the count continues. While Mora's research estimated that the world is home to about 611,000 species of fungi, a new study led by botanist Meredith Blackwell suggested there could be more than 5 million.

Philippe Bouchet, a professor at the French National Museum of Natural History in Paris, said there has never been consensus on the number of species on Earth.

"The debate is not over; it should not be seen as the end of the discussion," he said.

Estimates on that front have run between 3 million and 100 million over the years. The projections started out small, but three decades ago researchers started ratcheting up their projections.

"The revolution that came in the 1980s was that suddenly we realized how diverse the tropical rainforests were," Bouchet said. Later on, the estimates were revised downward, to between 5 and 10 million.

"To me, the advantage of this study is that it reaches a result from a completely different approach, and yet falls more or less in the same magnitude of earlier estimates," he added.

A race against time

But as researchers fine-tune their predictions of how many different life forms inhabit the Earth, the clock is ticking: According to the United Nations Environment Programme, experts say we are losing species at a rate of 0.01 to 0.1 percent of the total each year.

Another troubling trend, Bouchet said, is that taxonomy is seen as less and less important by decision-makers in the science world.

"If it is in bad shape in terms of a power struggle for resources, it is because it was perceived as a science belonging to the 19th century," he said. New technologies, such as identifying species via DNA bar coding, have helped change that perception, but the extinction clock continues to tick.

And as species loss continues, Bouchet said, researchers are still "far from closing up the inventory."

Author: Amanda Price
Editor: Holly Fox