species diversity gradient problem

Robert H. Cowie rhcowie at BISHOP.BISHOP.HAWAII.ORG
Tue Jun 22 08:43:43 CDT 1999

In looking at species richness of land snails in the Hawaiian Islands, and
more widely in the Pacific, I used a statistical technique called path
analysis in an attempt to disentangle the effects of collecting effort from
island area and a bunch of other variables. Path analysis has subsequently
been criticized (but not destroyed) in the literature, but you might care
to look at the following to see what I did:

Cowie, R.H. 1996. Variation in species diversity and shell shape in
Hawaiian land snails: in situ speciation and ecological relationships.
Evolution 49(6)[1995]: 1191-1202.

Cowie, R.H. 1996. Pacific island land snails: relationships, origins, and
determinants of diversity. In: The origin and evolution of Pacific island
biotas, New Guinea to eastern Polynesia: patterns and processes (Eds.
Keast, A. & Miller, Scott E.), p. 347-372. SPB Academic Publishing, Amsterdam.


At 05:31 PM 6/22/99 +1000, Mike Crisp wrote:
>Currently I am dealing with a similar problem to that of Peter Schuchart
>(below).  I am trying to relate species richness and endemism of plants in
>Australia to climatic variables, latitude, etc.  Our data are scored in 1
>degree grid cells, and we too have a problem of sampling artifacts, eg the
>deserts are not well explored.  If sampling were not an issue, regression
>is an obvious method to use, but it is very sensistive to sampling
>artifacts.  Moreover, Peter Schuchart indicates that his search effort is
>directly related to the variable he is testing, ie depth.  Separating these
>in his data will be a big problem, maybe impossible.
>There is a well known inverse relationship between species richness and
>latitude, and a large literature about this.  These folk must have had to
>deal with sampling problems.  See, eg, Brown, J. H., and Lomolino, M. V.
>(1998). 'Biogeography.' 2nd edn. (Sinauer Associates: Sunderland,
>Massachusetts.), chapter 15, and references therein.
>I too would be interested in how other Taxacomers have dealt with the
>'search effort' problem.
>Mike Crisp
>Peter Schuchart wrote:
>>dear collegues
>>could anyone give me some hints on publications
>>with studies comparable to my case decribed below?
>>I have identified all marine hydroids down to 2400 m depth from
>>a collection made in a defined region. I would now like to
>>know how species diversity decreases with depth.
>>However, there are two ways of analysing this, and I don't know
>>which one is more appropriate.
>>First, I can define equal depth intervals of 100 m and then
>>count the species present in the individual intervals. But this will
>>lead to unequal sample sizes (unequal search effort).
>>The other solution is to define depth intervals which have
>>the same number of samples taken. This will lead to rather unequal
>>I assume, that species diversity is both dependent on
>>search effort and area (= depth interval).
>>Both approaches give a decreasing species number with depth,
>>but the first method gives a much more dramatic result.
>>Any idea?
>Dr Michael D. Crisp
>Reader in Plant Systematics
>Division of Botany and Zoology
>Australian National University
>Canberra ACT 0200 Australia
>Phone + 61(0)2 6249 2882
>Fax   + 61(0)2 6249 5573
>E-mail mike.crisp at anu.edu.au
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 Robert H. Cowie, Ph.D.
 Department of Natural Sciences
 Bishop Museum
 1525 Bernice Street
 Honolulu, Hawaii 96817-2704

 Phone: (808) 848 4118
 Fax:   (808) 847 8252
 Web:   http://hbs.bishop.hawaii.org/cowie.html
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