As May points out, this ranges anywhere from 3 to 100 million – two orders of magnitude of difference. Over decades, researchers have made a number of wide-ranging estimates. If some alien version of the Starship Enterprise visited Earth, what might be the visitors’ first question? I think it would be: “How many distinct life forms-species-does your planet have?” Embarrassingly, our best-guess answer would be in the range of 5 to 10 million eukaryotes (never mind the viruses and bacteria), but we could defend numbers exceeding 100 million, or as low as 3 million. Some species will inevitably go extinct before we realise that they existed.Īs Robert May summarised in a paper published in Science 4: If we don’t know that certain species exist, we also don’t know that they might have, or will soon, go extinct. The fact that there are so many species that we’ve yet to discover has real consequences for our ability to understand changes in global biodiversity and the rate of species extinctions. Regardless, we know that any of these figures are an underestimate of the actual number of species. If we were to assume this “20% synonym” figure held true, our 2.12 million described species might actually be closer to 1.7 million. 3 They estimated that the 1.9 million described species at the time was actually closer to 1.5 million unique species. (2013) estimated that around 20% of the described species were undiscovered synonyms (in other words, duplicates). 2 To give a sense of how large this effect might be, in a study published in Science, Costello et al. 1 There is a continual evaluation process to remove synonyms (and most are removed eventually), but often species are added at a faster rate than synonyms can be found and removed. This is because some described species end up being synonyms – the description of already-known species, simply given a separate name. These figures – particularly for lesser-known groups such as plants or fungi – might be a bit too high. In the chart we see the breakdown across a range of taxonomic groups – 1.05 million insects over 11,000 birds over 11,000 reptiles and over 6,000 mammals. In 2021 it listed 2.13 million species on the planet.
The IUCN Red List tracks the number of described species and updates this figure annually based on the latest work of taxonomists. Species that we have identified and named.
Before we look at estimates of how many species there are in total, we should also ask the question of how many species we know that we know.
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