How Do Scientists Know We’ve Only Discovered 14% of All Living Species?

Scientists claim that we have discovered only 14% of all living species on Earth. How did they arrive at that number? If we haven’t discovered them, how do we know how many are there? The following four explanations should give you a good understanding about it.

1.

There have been many different estimates given for the total number of species on planet Earth. Some estimates are mere educated guesses by experts, while others are more grounded in statistics.

A famous estimate was provided by Terry Erwin, an entomologist working for the Smithsonian Institute. He sampled beetles from the Amazon basin by pumping insecticides into large rainforest trees and catching the dead insects that rained down into nets (this method is now called ‘fogging’). Using these samples, he observed that many species of beetles were only found within a single species of tree. By sampling lots of different species of tree, he found that on average, each species of canopy tree had roughly 160 species of beetle that were only found on a single tree species. So then, estimating that there are about 50,000 species of canopy trees, he simply multiplied 160 x 50,000 to come up with 8 million.

Since it is relatively well known that beetles make up approximately 25% of all described species on Earth, he then multiplied 8 million x 4 to come up with 32 million. This estimate received a lot of attention because of how large it was. It also received quite a lot of criticism, given the extrapolations that he used. For example, his estimate of 50,000 Amazon tree species is likely too high, and the number of endemic beetles per tree species is also highly variables from one tree species to the next. Today, most scientists think the Erwin estimate is probably too high.

There have thus been many other estimates provided by different groups over the years. A good one that comes to mind is described in a paper by Mora et al. 2011. The authors identify an important relationship that helped them to derive an accurate estimation of global species diversity. That is, there tends to be a linear relationship between the log number of taxonomic units found within different taxonomic hierarchies (i.e., from species, to genus, to family, to order, etc.). While we have a poor idea of the total number of species on Earth, we do have very good estimates for the total number of genera and families, etc. So, using these numbers, the authors simply plotted the number of taxonomic units found within all hierarchies above the species level (i.e., from genera to phylum). Using the linear model obtained from this procedure, they extrapolated their data to the species level and found the model to land on the number 8.7 million. Given the fact that about 1.2 million species have been described, 1.2/8.7 = 14%, bringing us to your original question.

This number is widely regarding as being a fairly accurate estimation of global species richness. Most biologists expect this number to be somewhere between 6 and 12 million now. However, it is important to point out that these estimates ignore microbes! We really don’t have a clue what the diversity of prokaryotes looks like, so they are largely left out of these types of estimations. Advances in genomic sequencing will hopefully help us get closer to an answer, but we are still in the very early stages of developing techniques for describing microbial diversity.

2.

It’s basically extrapolating from data. One way of finding new species is (nowadays, less invasive methods are preferred) to go to the Amazon (or any other biodiverse ecosystem) and find a large tree (which shouldn’t prove much of a challenge), spread a large sheet beneath the tree and then gas the whole tree to send every (formerly) living thing flying down onto your nice big sheet. You can then easily classify every animal. Scientists would then find that a large percentage of the animals collected were previously unknown species. This process would be repeated on several other trees in the area, with similar results. From this, we can tell that there are a whole lot of species we don’t know about yet.

3.

There is a concept called the “species discovery curve” or “species accumulation curve” that helps to visualize this: 
https://terrestrialecosystems.com/species-accumulation-curves/
If you’re still discovering a lot of species, you are necessarily part of the steeper curve, and as you keep coming across known species, you are part of the flatter part of the curve.

4.

So viruses infect other organisms and use the host’s replication machinery to reproduce. And the recent consensus is that pretty much everything harbor viruses. So if we guesstimate that there are about 8.7 million species of eukaryotes on earth (ranging from simple protists to us), and each species of eukaryotes harbor about 10 species of viruses, that’s a lot of viruses.

We currently know about 5k virus species (classified) there is a lot more that we know that aren’t classified, but it’s no where near the total number that’s out there. And this is a conservative estimate, because it’s not including subspecies of eukaryotes or different populations separated by geographical regions.

In addition, bacteriophages (bacteria-infecting viruses) aren’t accounted for either in that estimation. Im assuming that these viruses are found in invertebrates (think insects, spiders, ticks, worms etc.) in aquatic environments (marine and freshwater) and unexplored forests.

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