How many species are there?

That is such a great question, which many biologists are still trying to answer.

The only way to start to answer it is to break it down a little. First, we can divide it into (a) how many species are already known to science (those already found and have details recorded in a scientific paper), and (b) how many species there are alive on earth anywhere (as before, but including those no human has ever collected for science and formally recorded).

If we just consider those that have been described and recorded, the numbers are huge. Here, i'm only able to say much about known multicellular creatures (generally those visable to our eyes without microscopes, and made up of many cells working together, =  multicellular Eukaryotes).

Here are some quite current numbers:
Plants (flowering, grasses, conifers and ferns, etc) total - 248.000 species.
Algae (green brown and red, mostly marine) - 27.000 species.
Fungae (mushrooms, molds, etc) - 68.000 species.
Vertebrates and allies (furry mammals, birds, fish and strange sea squirts) - 43.000 species.
Arthropods (spiders, mites, crabs, prawns, flies, beetles, wasps, etc) - 1048.000 species.
Other invertebrates (worms, snails, sea shells, tube worms) - 116.000.

Basically, the 'creepy crawleys' have the highest numbers of described species (these are the arthropods = jointed leg creatures, including insects). Arthropods make up about 66% of described species, and this is why most of museums are filled with many many thousands of insects and similar 'bugs'.

In total, the answer is about 1550,000 or 'one thousand five hundred and fifty thousand' described species. Often, the decision to describe a newly collected creature as a new species can be very difficult to make. It is often slow, and requires careful comparisons of the new creature with older descriptions of known species, and with other specimens kept in museums all around the world. Sometimes, experts disagree whether something is completely new, or further evidence gives a different conclusion. For example, two male birds might look the same but have different songs, or feed on different things. Looking at the birds bodies alone might not suggest they are different species, as they have the same bone shapes, colors, sized beaks, etc. But evidence from their behavior and lifestyles (which might be found out much later) suggest they are separate species. So, even though my numbers are for described species, they are estimates. Even if no new discoveries are added, the numbers can go slighly up or down as scientists re-compare creatures already known to one another and might come to new conclusions about the numbers.

Now, finally. Those were just numbers of described species. There are many places in the world where there are lots more species that haven't yet been described. These are mostly in warm tropical regions (like rainforests), or in the deeper parts of oceans. Saying that, biologists have a good idea that very few large mammals remain un-described in the entire world, as most are large, obvious and need big areas of land to live. But, for other groups like fungi or arthropods, the numbers of un-described species are much larger. For insects (only the six legged arthropods [925.000 described) conservative estimates suggest there are a total of 2.000.000 species (2 million) of these alone, while the biggest estimates suggest 30.000.000 insects alone (30 million!!).

So, not counting the ones waiting to be found or written about, I think an ESTIMATE of 'one thousand five hundred and fifty thousand' DESCRIBED species is about right. But remember, that is just MULTICELLULAR species, there are many many many more described and un-described species of single celled tiny creatures on earth today..... but the numbers and types are really difficult to describe. It is even difficult to tell how many single-celled species are even living in a liter of sea water, but its a huge number!!

Dear Charlie,

You may not realise it, but you have asked a question that continues to cause arguments between scientists. It may seem like a very simple issue – after all, what good can we be at conservation or taxonomy if we don’t even know how many species there are? – but it turns out to be a tricky issue.

The first problem is deciding what we’re going to call a species. Most zoologists follow the Biological Species Concept – that to be a species, a group of organisms have to be able to interbreed, by being similar enough to mate and living in the same place. This idea works well for most animals, but stumbles with plants that are often able to crossbreed and form hybrids, and falls completely flat when you include fungi and bacteria. Really, the problem is that the idea of a species is an entirely human concept, not a natural one, and what works for one group of organisms doesn’t necessarily help with others.

Stuart is right that there are a range of estimates of the number of species, from the lowest ones, based on the rate at which we're identifying new ones (3-5 million species), up to some ridiculously high estimates (30 million plus). Most biologists think the truth lies somewhere in the middle, maybe around 8-10 million. The biggest group of species of all organisms are beetles, mostly herbivorous ones (which feed on plants), mostly in the canopies of tropical rain forests. We have so far discovered only a small fraction of these.

Will we ever count them all? I think not, for three reasons. Firstly, we can’t all decide on what the word species means. Secondly, many are going extinct all the time. Lastly, if we did manage to count them all, it would take so long that new species would probably have evolved, making it impossible to keep track!

Your question becomes very important in the light of conservation. We hear in the news about the number of species going extinct every day – you might want to think about how accurate these figures are. No-one doubts that the extinction crisis is happening, but the actual numbers of species are uncertain. Also, you might want to ask yourself whether all species are equal and as important as each other. Is a different type of slug as important as a giraffe? These are the kinds of decisions that conservationists have to make all the time, and I believe these are far more important than worrying about the accuracy of the big numbers.

All the best,

Dr Markus Eichhorn