Re: NANFA-L-- Diversity Indicies

Bruce Stallsmith (fundulus at
Thu, 29 Sep 2005 15:52:35 -0400

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.

Of course, the whole IBI concept is a much more ambitious attempt to
standardize comparisons between ostensibly equivalent systems using a
convoluted weighting system for individual species' "worth" and tying that
somehow to their abundance. This is an appealing concept in principle to
many regulators and some scientists, but has various statistical weaknesses
such that many scientists who aren't directly trying to impress regulators
ignore such contrivances.

--Bruce Stallsmith
freshwater mussel diversity in the Tennessee is interesting too!
Huntsville, AL, US of A

>I am also intrigued by the difference between the concepts of diversity and
>richness. I gather that richness simply means number of species, but what
>diversity? Would that be measured by lineage, habitat preference, food
>preference, number of bumps on the tongue, or ... what? And is this a
>_meaningful_ distinction (to wit, "A difference which MAKES no difference
>IS no
>difference.") ?
>Maybe those interested could reply offline, so as to streamline the list?
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