How Many Animal Species Exist

Aug 25, 2011
John Blatchford

Animal Diversity - Medeis - Wikimedia Commons

A new study suggests that the most life has yet to be discovered, and that most will be gone before we get to it.

There are probably over seven million different animal species on the planet, and most of them have not yet been described. To study them all would take a thousand years, and many will be extinct before we even see them. This all comes from a recent study (PLoS Biology) that attempts to answer the question of how many species there are on Earth.

How Many Species on Earth

Expert (taxonomist) opinion has given a range of numbers in the past, from 3-100 million. The new study (PLoS Biology) puts the figure at around eight or nine million, but guesses that there will be many more if all the bacteria and so on are included.

The researchers point out that we have been best at noticing big animals (and plants) that live on land, and worst with small organisms in remote places. Named land animals exceed named bacteria, but the study also points out that experts use different criteria when naming different types of organism.

The short answer is that we do not know how many different things live on earth at present, but there are lots! As Professor Jonathan Baillie (Zoological Society of London) has said, "it's such a high figure that it wouldn't really matter if it's out by one or two million either way."

Species

The concept of 'species' has been very good at recognising different types of animal (and plant), and the study estimates that there are about eight million animals (with 12% so far described), and three hundred thousand plants of which the majority have been catalogued.

When the smaller (bacteria and their allies) enter the scene the old and familiar concept of exactly what 'species' means can break down. The loose (old) definition of 'species' is about populations of organisms capable of interbreeding and passing on their genes. It comes unstuck when things like bacteria pass genetic material to one another.

If DNA is used as the sole criterion for species differences then we also run into problems. Each individual human has a different genome, and even within our own body cells we have at least two different genetic lines going on (those of the mitochondrion and those of the nucleus).

Classifying organisms might be somewhat arbitrary, but they certainly are different from one another.

What all life on Earth shows is that different kinds of creatures (and plants, fungi, bacteria etc.) rely on one another for survival. Nothing lives outside its ecosystem.

Habitats and Ecosystems

Interdependent collections of living things (an ecosystem) need a place to live (a habitat). Once again, exactly what is studied depends on human definitions of these concepts – is each small part of a wooded area a distinct habitat, or is it the whole wood?

Whatever the definition of 'habitat' it now seems that there is an awful lot of interacting going on between a vast number of living things. Perhaps the best way to understand what is happening is to concentrate more on these interactions than on trying to name everything?

Conservation and Preservation

If all we want to do is save a few 'examples' of a species (much in the way a museum has 'specimens'), then a small number of reserves (each for particular 'specimens') is OK. If, on the other hand, we want to keep the full richness of our planet's wildlife as it is at present then perhaps the focus should be on keeping as many diverse habitats as possible.

I am not suggesting that concentrating on the level of 'species' is wrong in any way – just pointing out that in some (many?) species there are huge differences between the various populations. Killer Whales have well recognised 'clans', and different Chimpanzee populations have unique cultures. Even when we look at the Common Frog in the UK there seem to be many different genetic stocks.

'Saving' a species does not equate with keeping a few alive somewhere – imagine a future zoo with the last small group of humans in it. Would this be a true example of what 'humanity' was?

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