Re: NANFA-L-- Diversity Indicies

Irate Mormon (archimedes at
Thu, 29 Sep 2005 16:27:46 -0400

Quoting Bruce Stallsmith <fundulus at>:

> The trick to, and differences between, various types of diversity indices is
> to measure how even the species distributions are. If you have 100 fish in
> your sample, and these represent 10 different species, how do you describe
> it if you have 10 individuals of each species in your sample, or maybe you
> have 91 individuals of one species and one individual of each of the other 9
> species? This gets into ideas of evenness, and what are truly common
> species, and how sensitive your statistical measure is to the presence of
> species represented by only one individual in a much larger sample.

Does evenness matter if you are measuring diversity (whatever that is)? I can
see how you could build an index for species richness, or species evenness, but
when you try to combine both, what exactly do you get? I guess if you collect
2000 fathead minnows and one stickleback out of a baitshop tank, that sample is
somehow less diverse than if you collect 1000 fatheads and 1001 sticklebacks.
BUT if you give more weight to the fatheads in the first example, because maybe
that one stickleback really wasn't supposed to be in there, then you are
interpreting the data and extrapolating from it, rather than merely describing
(quantifying) it. A fine point perhaps, but what you are actually doing is
building prejudice into the index. And maybe that's exactly what you want to
do? I guess it would help if I knew what diversity was.

Now, your point about having a number to give to bureaucrats, THAT I understand.
They can point at the number and say, "Hey, that meets our guidelines according
to section C, subparagraph IIa: 'Laws Governing Timelords'" without actually
concerning themselves with what it means.


"Mos' peoples is intell'gent" - Mudbone
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