Yes, diversity is beyond richness, and includes a component called
evenness. Evenness refers to the degree to which the individuals in a
sample are distributed across the species present. If a sample
contains the same number of each of the species included, that sample
has maximum evenness compared to samples with varying numbers of each
species. The idea is that a sample with, say 996 of one species and 1
each of each of another 4 species is almost the same as one that has
simply 1000 of one species, whereas a sample that has 200 of each of
five species is quite different from the unispecific sample of 1000.
Now, does all this mean anything, or is it just an exercise?
Some ecologists have attempted to use the concept of diversity to
consider why some systems might be more resistant to degradation, or
more resilient to degradation, or more capable of surviving
degradation, or related matters, than other systems. Others have used
diversity as a stand-in for state of degradation, almost as, well, an
index, usually presumed to be inverse to degradation. But of course,
very simple systems may not be degraded, as I mentioned in my earlier
post. One thing that a fair number of ecologists have agreed on is
that biotic diversity is strongly correlated with habitat diversity in
undegraded systems, and that agreement is the result of a good many
data sets. A classic early paper (1970s) in this field showed that the
increase in species diversity with stream order is a correlate of
habitat complexity. That was actually a starting point for James Karr,
one of the co-authors of that work, to develop the IBI.
Some other ecologists, like Hurlbert as mentioned by Bruce, have
questioned whether "diversity" has any meaning whatsoever. Some have
simply advocated using richness alone when one is interested in these
sorts of things.
There are lengthy discussions of these matters in ecology texts these
days, and a whole series of competing mathematical ways of calculating
diversity based on the number and distribution of species in a sample.
The effort in developing such devices seemed to peak in the sixties of
the last century, and application of them may have peaked soon after
that. But maybe engaging in such matters is why we get so little done,
and the natural world is falling apart while we dither? If you are
interested further, consult any of the excellent laboratory manuals for
a graduate level general ecology course (actually they are introduced
in undergraduate courses for that matter) like Southwood, or Krebs.
At any rate, literally thousands of reports have included diversity
indices as a part of their data set.
David L. McNeely, Ph.D., Professor of Biology
Langston University; P.O. Box 1500
Langston, OK 73050; email: dlmcneely at lunet.edu
telephone: (405) 466-6025; fax: 405) 466-3307
home page http://www.lunet.edu/mcneely/index.htm
"Where are we going?" "I don't know, are we there yet?"
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