Re: NANFA-L-- Diversity Indicies

dlmcneely at
Thu, 29 Sep 2005 13:32:28 -0500

At the risk of irritating those who want to get on to other things:

I think Todd wants to include something like overall rarity (of the
species wherever it might occur), or some sort of "value" for
particular species (a common carp is worth considerably less in most
systems than, say, a golden redhorse), in order to have a score that
reflects some kind of "condition" for the system sampled. Rare species
being common in a place might indicate its condition, or intolerant
species being common might also.

If each species receives exactly the same weight (as they typically do
in diversity indices), then whatever the quality Todd is trying to get
at is, it gets completely lost. IBI is supposed to get around that
problem, but it is so difficult to properly tune for a given location
(white suckers are "tolerant," and receive relatively low scores in the
IBI as originally developed, but in the Ozarks they are generally found
only in high quality streams. How does that work out for the index?)
And it is really confusing when different species are weighted
differently in different places and by different workers. but as Bruce
pointed out, that is necessary, and my white sucker example illustrates
why. In the case that you presented, Irate, where a stream might have
100 species, and the score for a location is simply the decimal
fraction of that total fauna collected at a site, headsprings for
example would have a very low score. If the only point is to express
that fraction, fine, and that value has a name, it is species
richness, "s," usually expressed not as a fraction, but simply as a
whole number. But if the intent is to express some numerical value that
reflects environmental condition, the headspring looks to be bad off,
when in fact it may be supporting least darters, pigmy sculpins, and an
undescribed but exceedingly intolerant stoneroller.

Todd would like to encapsulate something like that, I think.


David L. McNeely, Ph.D., Professor of Biology
Langston University; P.O. Box 1500
Langston, OK 73050; email: dlmcneely at
telephone: (405) 466-6025; fax: 405) 466-3307
home page

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