Craig Hilton-Taylor hiltonta at NBICT.NBI.AC.ZA
Fri Aug 1 10:16:23 CDT 1997

In response to the comments from Doug Yanega and Dennis Paulson:

Gaston argues that neither abundance nor range size has any logical
precedence as a measure of rarity, and that they are often not
interchangeable. He suggests that where data are available, analyses
should consider both variables and their interaction (he does not
provide any formula for combining the 2 parameters). The best known
example of this approach is that of Rabinowitz (1981) which was cited
by other contributors. Gaston points out (without any substantiation)
that while the Rabinowitz approach is useful conceptually, it has
limited practical value. I would disagree with this as I have found
it a very useful approach in classifying rare plant species in
southern Africa.

I tend to agree with Dennis' arguments that 25% is too large a
cut-off level for the definition of rarity and that 10% is more
reasonable. Unfortunately Gaston's rationale for the 25% figure is
not presented in the Kunin & Gaston book. I presume it is in an
earlier book: Gaston, K.J. 1994. Rarity. Chapman & Hall, London. I do
not have access to a copy of this, if anyone else has, perhaps they
could check out the rationale behind selecting 25% and not a lower

Stephen Rae said:
>Rarity deals with numbers and distributions, while endangerment is a
>subjective measure of the expectation of survival over time.

The system applied in California may have been subjective, however,
the new IUCN Red List categoriesand criteria are an attempt to
be far more quantitative and to remove subjectivity from such
listings. The new categories and criteria are still far from ideal
and the IUCN is currently conducting a review of the applicability of
the new system. Any comments should be directed to David McDowell at
mail at hq.iucn.org by October 1, 1997.

Craig Hilton-Taylor

Craig Hilton-Taylor
Ecology and Conservation Research
National Botanical Institute
Private Bag X7                        Tel: +21 - 762 1166
Claremont 7735                        Fax: +21 - 797 6903
South Africa                        Email: hiltonta at nbict.nbi.ac.za

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