Study suggests there could be more species on Earth than stars in our galaxy

There may be as many as one trillion species on Earth, the vast majority of which are microorganisms like the ones pictured here(Credit: Sofia Vini)

Biologists at Indiana University believe we may have only discovered a mere thousandth of a percent of the species on our planet. To put a number on it, that means the 10 million or so species identified so far represent a drop on the ocean of as many as one trillion total species. As a point of comparison, consider that current estimates suggest that our home galaxy the Milky Way may contain between 100 and 400 billion stars. This is a big number, with possibly big implications.

The new estimate draws on big data analyses which suggest that biodiversity operates on an exponentially sliding scale. At the level of biological complexity within us big mammals, there's not a huge number of organisms. But by the time you get down to the microbial level – to microorganisms like bacteria – the number balloons.

To complete their study, the scientists took tens of thousands of existing records to assemble a dataset of more than 5.6 million species (both micro and non-microorganisms) from 35,000 locations around the world – spanning all continents and oceans except Antarctica. By analyzing this data, they found that many previous estimates of biodiversity significantly under-sampled microorganisms – the populations of which have been scrutinized more deeply in recent years.

Biologists and ecologists have what they call scaling laws. These are rooted in the idea that the number of species counted will typically increase with the area over which you count them. The scaling laws generally also follow the principle that the count will increase more rapidly over a very small scale – say, if you go from one square inch to one square foot – than at larger scales, thanks to the way in which habitats tend to change slowly.

Defining these laws correctly is tough, however, and getting a precise count is next to impossible because new species constantly emerge while others go extinct. But the Indiana University researchers were able to use their huge dataset to observe trends in how species abundance – the number of individual organisms per species – scales with biodiversity.

Through studying these sorts of relationships they estimated "that Earth is home to as many as one trillion microbial species."

And that staggering number seems even larger when you consider that fewer than 10 million microorganisms have been identified, of which around 10,000 have been grown in a lab and fewer than 100,000 have been classified by scientists. It'll be next to impossible to classify the remaining 999 billion plus species, though perhaps the bigger takeaway here is that if accurate the estimate implies that life on Earth is diverse beyond anything ever imagined.

A paper describing the study was published in the Proceedings of the National Academy of Sciences.

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