A Trillion Species?

For many years one of the most famous questions in biodiversity science was "how many species live on earth". Five years ago researchers predicted ∼8.7 million (±1.3 million SE) eukaryotic species globally. Estimating the number of species on Earth has always been a difficult business and some extrapolations are educated guesses. That might explain why estimates of the total number of species vary between 10  and 100 million but 250 years of taxonomic study has formally described just about 1.8 million of them.

The big unknown is microbial (prokaryotic) life. With the advent of high-throughput sequencing numbers and estimates for microbial taxa skyrocketed by orders of magnitude and researchers are starting to think really really big:

...this period of discovery has uncovered few, if any, general rules for predicting microbial biodiversity at scales of abundance that characterize, for example, the ∼10¹⁴ cells of bacteria that inhabit a single human or the ∼10³⁰ cells of bacteria and archaea estimated to inhabit Earth. Such findings would aid the estimation of global species richness and reveal whether theories of biodiversity hold across all scales of abundance and whether so-called law-like patterns of biodiversity span the tree of life.

The realization that microorganisms were significantly under-sampled caused an explosion in new microbial sampling efforts over the past several years, including the collection of human-related microorganisms by the National Institutes of Health's Human Microbiome Project; marine microorganisms by the Tara Oceans Expedition; and aquatic, terrestrial and host-related microorganisms by the Earth Microbiome Project. 

Two US scientists now combined microbial, plant and animal community datasets from government, academic and citizen science sources, resulting in the largest compilation of its kind. Altogether, these data represent over 5.6 million microscopic and non-microscopic species from 35,000 locations across all the world's oceans and continents, except Antarctica.

We suspected that aspects of biodiversity, like the number of species on Earth, would scale with the abundance of individual organisms. After analyzing a massive amount of data, we observed simple but powerful trends in how biodiversity changes across scales of abundance. One of these trends is among the most expansive patterns in biology, holding across all magnitudes of abundance in nature.

Scaling laws, like those discovered by the colleagues, have been used to accurately predict species numbers for plant and animal communities on smaller scales, e.g. the number of species scales with the area of a landscape. The researchers focused on the question if aspects of biodiversity scale with something as simple as the abundance of organisms. Their message is that the relationships are not only simple but powerful, resulting in the estimate of upwards of 1 trillion species.

These results suggest that identifying every microbial species on Earth is an almost unimaginably huge challenge. To put the task in perspective, the Earth Microbiome Project has so far cataloged about 6 million bacterial taxonomic units (genus or species level taxa). 

Of those cataloged species, only about 10,000 have ever been grown in a lab, and fewer than 100,000 have classified sequences. Our results show that this leaves 100,000 times more microorganisms awaiting discovery - and 100 million to be fully explored. Microbial biodiversity, it appears, is greater than ever imagined.