MadSci Network: Zoology
Query:

Re: How many different living organisms are there today?

Date: Tue May 30 09:41:39 2000
Posted By: James Cotton, Graduate Student
Area of science: Zoology
ID: 957382903.Zo
Message:

Dear Karen,

An excellent question, because this is a question that biologists have been asking themselves a lot, and can't really agree on an answer for!

When biologists ask this question, they usually ask something like 'how many species are there on earth?', but thats basically just another way of asking the same question, as species are the most important way of diciding up organisms into 'kinds'.

The obvious way to answer the question would be go to all the books and journals where people have written describing new species, or new kinds of living things, and then adding up how many people have found. This is very difficult, because people have been writing about new kinds of living things for a very long time, and there's no single place where they are all listed. Things will slowly get better, with big projects like Species 2000 trying to list all the species we know about. People have tried to work out (basically by asking experts in each group of living things to make an educated guess) approximately how many living things scientists have given names to, however, but there estimates are pretty different from one another. Most people guess around 1 million species..If your interested, here's one set of guesses, taken from a web page at the world resources institute.

I hope you'll be surprised by this. All the big land animals (mammals, birds, reptiles) put together make up only as many species as there are fish, and there are far more insects, spiders, and fungi (mushrooms etc) than these. This is despite the fact that 1) far more scientists work on mammals, birds etc 2) big animals are much easier to find and 3) differences between these big animals are much easier to identify. Clearly something surprising is going on!

This, however, doesn't tell the whole story - there are problems like people giving the same name to more than one different species (synonymy) and people naming the same species twice that make these estimates quite unreliable. Also, nobody thinks that scientists have finished describing and naming all of the living things - there just aren't enough scientists trying to do this (the type of scientist that does this kind of things are called systematists or taxonomists). There are statistical techniques that try to estimate how many species scientists will eventually get around to describing, but these methods can't account for the fact that some organisms will never be found, and, more importantly, they're very inaccurate because they assume that scientists will continue describing and collecting organisms in the same kinds of ways, so that the rate of growth of the number of species known won't change too much.

Many studies have attempted to work out how many species there are on earth by extrapolating from a sample that they believe to include all the species of a particular kind from some location. For example, one study of beetles inhabiting the canopies of trees in tropical rainforest found 1,200 species of beetle in the canopies of 19 trees of a single species. 85% of these were new to science, and the author assumed certain figures for the host-specificity of rain forest insects, he proportion of total insects found in the canopy and the proportion of total canopy fauna that are beetles to get an estimate of 30 million species of tropical forest arthropods alone! Other authors have used similar data sets, or altered these assumptions to get estimates as high as 100 million species!

Many other estimates have also been used. For example, if we assume that all large animals are known, and using the ratio of large:small species in well known groups like butterflies and birds, we can estimate that there might be 10 million terrestrial animals of about 0.2 mm body size.

These sorts of estimates have been particularly popular for arthropods, which are thought to represent a large proportion of all living things, but we know very little about the possible diversity of many other groups. Bacteria, viruses and nematodes are among the leading contenders as 'hyper-diverse' groups which may contain many millions of species about which we know almost nothing. Focusing on bacteria, there is a great deal of evidence that we are able to culture (grow in the laboratory) only a tiny fraction - probably only one or two percent - of all the species that are out there. We've only identified 3,058 species of bacteria, but one well-known study estiamted that there could be 50,000 species in 1g of soil from a Swedish woodland! What is really needed are intensive studies of these little-known groups of living things in particular localities to allow estimates of their overall diversity. Basically, however, we know virtually nothing about the diversity of many potentially very important groups of living things.

These different ways of counting have lead to guesses of between 2 and 100 million species in total, but these are just scientific guesses. I'd personally put my money on a total at the top end of this range, or even higher, but I think the prevailing fashion is towards the lower end of this range! Between 3 and 30 million would probably be a popular figure.

Hope this answers your question - i'm afraid nobody can give a single number to answer it, and probably never will. Feel free to ask again if you want more information on any of the above issues.

Yours, James.

References.

Hammond, P. (1992) 'Species Inventory'. In Global biodiversity : status of the Earth's living resources. Edited by Brian Groombridge.
May, R. M., HOW MANY SPECIES. PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL SCIENCES, 1990, Vol.330, No.1257, pp.293- 304
MAY, R. M., HOW MANY SPECIES ARE THERE ON EARTH. SCIENCE, 1988, Vol.241, No.4872, pp.1441-1449
Erwin, T. L. (1991) How Many Species are There? Revisited. Conservation Biology 5:1-4.


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