Earth is home to as many as 8.7 million species, according to a new estimate described as being the most accurate to date. However, the vast majority of species have not yet been identified, and cataloguing them all could take over 1,000 years.
Calculating species numbers
Scientists have been attempting to count and catalogue species for over 250 years, ever since the Swedish scientist Carl Linnaeus devised a taxonomy system – a way of classifying organisms – that is still used today.
In Linnaeus’ system, species are grouped into a hierarchical “tree of life”, in which closely related species are classified in the same genus, these in turn being grouped into families, then orders, then classes, then phyla, and finally into kingdoms, which include the animal, plant and fungi kingdoms.
However, accurately calculating the total number of species on Earth has always been difficult, with current estimates ranging from 3 million to 100 million.
In the new study, published in the journal PLoS Biology, researchers looked at the relationships between species and the broader taxonomic groups to which they belong, focusing on the rates of new discoveries at different levels in this taxonomic ‘tree’. From this, they were able to use predictable patterns to estimate the total number of species in each taxonomic group.
Vast majority of species unknown
The technique was found to accurately predict the number of species in well-known groups, such as mammals, birds and fish. It was then applied to all domains of life, and predicted a total of around 8.7 million species, give or take about a million.
Of these 8.7 million, around three quarters live on land, with only a quarter (about 2.2 million) in the oceans, even though water covers most of the Earth’s surface.
Most species (about 7.8 million) belong to the animal kingdom, with progressively smaller numbers of fungi, plants, protozoa (a group of single-celled organisms) and ‘chromists’ (algae and related microorganisms). The figures did not include the many thousands of species of bacteria.
However, the results also suggest that a staggering 86% of terrestrial species and 91% of ocean species are as yet undiscovered, underlining just how little we still know about the natural world.
“When we think of species we tend to think of mammals or birds, which are pretty well known. But when you go to a tropical rainforest, it’s easy to find new insects, and when you go to the deep sea and pull up a trawl, 90% of what you get can be undiscovered species,” said Dr Derek Tittensor, one of the researchers.
Unknown species becoming extinct
At the current rate at which new species are discovered, the researchers estimated that it would take over 1,000 years to completely catalogue all life on Earth. However, new techniques – for example, DNA ‘bar-coding’, in which species are identified from their DNA – may speed up the process.
The team also said that they know this is unlikely to be a final estimate, and expect others to refine their methods and conclusions in the future. In addition, they warned that many species are likely to become extinct before they are even known.
Speaking about the paper, Professor Jonathan Baillie, director of conservation programmes for the Zoological Society of London (ZSL), said, “I think it’s definitely a creative and innovative approach, but like every other method there are potential biases and I think it’s probably a conservative figure.”
“But it’s such a high figure that it wouldn’t really matter if it’s out by one or two million either way. It is really picking up this point that we know very little about the species with which we share the planet; and we are converting the Earth’s natural landscapes so quickly, with total ignorance of our impact on the life in them.”
Another of the researchers, Dr Camilo Mora, said, “We know we are losing species because of human activity, but we can’t really appreciate the magnitude of species lost until we know what species are there.”
“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?”
Read the original article in PLoS Biology – How many species are there on Earth and in the Ocean?
Liz Shaw, ARKive Species Text Author