How many species are there?

How many species do we share our planet with? It’s such a basic and fundamental question to understanding the world around us.

It’s almost unthinkable that we would not know, or at least have a good estimate, what this number is. But the truth is that it’s a question that continues to escape the world’s taxonomists.

An important distinction is how many species we have identified and described, and how many species there actually are. We’ve only identified a small fraction of the world’s species, so these numbers are very different.

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. Species that we have identified and named.

The IUCN Red List tracks the number of described species and updates this figure annually based on the latest work of taxonomists. In 2021 it listed 2.13 million species on the planet. 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.

These figures – particularly for lesser-known groups such as plants or fungi – might be a bit too high. This is because some described species end up being synonyms – the description of already-known species, simply given a separate name.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.² To give a sense of how large this effect might be, in a study published in Science, Costello et al. (2013) estimated that around 20% of the described species were undiscovered synonyms (in other words, duplicates).³ They estimated that the 1.9 million described species at the time was actually closer to 1.5 million unique 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.

Regardless, we know that any of these figures are an underestimate of the actual number of species. 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.

If we don’t know that certain species exist, we also don’t know that they might have, or will soon, go extinct. Some species will inevitably go extinct before we realise that they existed.

As Robert May summarised in a paper published in Science⁴:

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.

Over decades, researchers have made a number of wide-ranging estimates. As May points out, this ranges anywhere from 3 to 100 million – two orders of magnitude of difference. Most modern estimates fall within a tighter range.

One of the most widely-cited figures comes from Camilo Mora and colleagues; they estimated that there were around 8.7 million species on Earth today.⁵

Mora et al. put an uncertainty of 1.3 million species around this figure. The breakdown of how many of these species are animals; fungi; plants; and other groups is shown in the table. This also shows the split between marine and terrestrial environments. It’s estimated that 2.2 million of these species live in the ocean.

There are also a range of other estimates: Costello et al. (2013) estimate 5 ± 3 million species; Chapman (2009) estimates 11 million; and after reviewing the range in the literature, Scheffers et al. (2012) choose not to give a concrete figure at all.⁶ There is typically strong agreement on the most well-studied taxonomic groups such as mammals, birds, and reptiles. Where most of the disagreement lies is in insects, fungi, and other smaller microbial species. Reaching consensus on such small and inaccessible lifeforms is undoubtedly hard.

How can we even begin to make these estimates? There are several approaches that researchers take.

Mora et al. (2011) – whose estimates are shown in the table – used the fact that there are predictable relationships in higher taxonomic classifications of life, that could be extrapolated to the species level.

Life can be classified at multiple levels: each belongs to a kingdom (e.g. the “Animalia” kingdom – this sorts life into animals, plants, fungi etc.); then a phyla (e.g. “Mollusca” or “Arthropoda” in the animal kingdom); then class; order; family; genus; and finally the species level.

We know much more about the higher taxonomic classifications (kingdoms, phylum; classes) than we do about the specific species-level breakdowns. But, we find that for groups of species that have been well-studied, we find predictable patterns between the higher taxonomic classifications, and estimates at the species-level. Researchers can use these predictable patterns for well-known species and apply them to lesser-known groups.

The honest answer to the question, “how many species are there?” is that we don’t really know. Some estimates span several orders of magnitude, from a few to 100 million. But most recent estimates lie somewhere in the range of around 5 to 10 million.