MadSci Network: Cell Biology

Re: # of Species

Area: Cell Biology
Posted By: Kelleen Flaherty
Program: Biology
Date: Dec 11th, 1995

This was a great question! I've never heard it phrased that way before. It took a little digging through a bunch of major references and some addition, but I have a figure I'm reasonably confident about, and I think it will be useful in subsequent investigations I may undertake.

In terms of biomass, the autotrophs MUST outnumber the heterotrophs or we would all die. Look in any biology textbook at an energy pyramid or a pyramid of biomass: by the time you move from primary producer to primary consumer to secondary consumer to top carnivore, with the associated loss of energy at each step, you end up with something like an entire field of corn supporting one or two top carnivores. HOWEVER, in terms of NUMBER OF SPECIES, at least described species, the heterotrophs have the autotrophs beat. Of course, depending on who you read, there are between 1 and 30 million species of living things, only a fraction of which have been described. Here is the fraction I have come up with:

Animals: ~1 million species (~5% of which are vertebrates and 85% of which are arthropods)
Fungi: ~100,000 species
Protozoa (heterotrophic protists): ~12,000 species
Plants: ~266,000 species
Autotrophic protists (algae, diatoms): ~25,000 species
Bacteria: ~5000 species
So, this gives us a final score of heterotrophs: 1,112,000 to autotrophs: 291,000 or roughly 4:1 in favor of the heterotrophs, in terms of DESCRIBED, EXTANT SPECIES NUMBERS. Of course, that leaves out the bacteria - I suppose you could split those in half. Bear in mind that there are several hundred thousand EXTINCT SPECIES, which may not bear on the question. Also bear in mind, that these are DESCRIBED species: some scientists claim that the reason there are so many arthropods is because that is what we have chosen to sample and identify; not because there are more of them than anything else (sampling error, in other words). So, which is there more of? In terms of biomass: autotrophs. In terms of NAMED species: heterotrophs. Hope this answers the question!

Kelleen Flaherty, Department of Biology, Washington University

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