Species numbers

Christopher Taylor at the excellent Catalogue of Organisms has a thought provoking post on the taxonomic diversity of described species. He has a picture of animal groups, plants and fungi scaled by number of described species. I like these sorts of pictures; they are great for shocking undergraduates who have never really thought about the diversity of life in a quantitative way before. But…. I am very skeptical indeed that described species numbers are even a reasonable approximation of actual numbers. Christopher mentions the “true diversity of organisms”. I am taking this a bit out of context, and he certainly mentions that this refers only to described species some of which will increase in future, but it reminded me of other summaries of life that I have seen and that in general biologists don’t critique enough our taxonomic sampling when describing biodiversity.

Described species are incredibly biased towards beasts that are easy for humans to study and describe (1). I know very little about fungi but it wouldn’t surprise me if fungi with large fruiting bodies are much better described than small soil mycorrhizal fungi. Mark Blaxter (2003) has an interesting figure of the known and estimated diversity of some animal groups in his article “Counting angels with DNA”. Africa Gómez beautifully redrew and modified this for a lecture on cryptic species and I’ve included her version here.

We shouldn’t lose sight that most species of animals and fungi are not yet described and that most of those are likely to be small, superficially dull-looking to your average human and members of less fashionable taxonomic groups (e.g. nematodes). [Africa’s point was that these are also the criteria that seem to predict the presence of cryptic species complexes, increasing the level of our underestimates]

Lets consider marine nematodes. More than 50% of the planet is ocean abyss. In the abyss most animals are nematodes (estimates vary between 50 and 95% of animals by number, not mass) with approximately 100,000 nematodes/m2. Deep sea nematodes are not well described. They are difficult to get hold of and nematodes are surely difficult to study taxonomically anyway (I’ve always been very impressed by morphological nematode systematists, they have so little to work with). About a year ago I thought there were no deep-sea nematode sequences in GenBank, I might have missed them, but certainly not many. Would anybody like to guess how this would influence species numbers and global biodiversity estimates? It depends on levels of cosmopolitanism versus endemism of course, but these are not trivial numbers if we are trying to describe and quantify animal life!

OK I suppose my point is- should we trust described species as an estimate of total species? Do we have a large and unbiased sample of diversity on earth? If not does it matter? I would answer No, No, and definitely Yes!

(1) I am deliberately ignoring bacteria and archaea. I’m just not informed enough to add this in to the discussion, but I suspect that hidden diversity is even more of an issue for them.
Blaxter M. Molecular systematics: Counting angels with DNA. Nature (2003) 421(6919): 122-4 doi:10.1038/421122a

One Comment Add yours

  1. Christopher Taylor says:

    I fully agree with your comments on the unreliability of species number estimates – of course, seeing as the total of undescribed taxa out there is unequivocably biased towards the smaller organisms, then the central effect of the illustration (to demonstrate the relative insignificance of the charismatic megafauna) can only be strengthened. Personally, I wouldn’t be surprised if mites and/or nematodes end up even giving insects a run for their money in terms of species numbers, but I have no real evidence to back that up whatsoever – it’s just that both groups seem to have an almost fractal division of microhabitats.In terms of prokaryotes, I was told as an undergrad that environmental DNA sequences suggested that considerably less than 1% of bacterial diversity had been characterised. True, that was about eight years ago and new bacteria taxa have been described at great rates, and a certain percentage of ‘environmental sequences’ represent analytical artefacts rather than real honest-to-goodness organisms, but I’d still be surprised if we’ve gotten far past the 1% mark, if we’ve even gotten that far at all.


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