Rarity

[Dennis Paulson]

>>From Kunin & Gaston's book it appears that there is no standard
>definition of rarity. This could pose a problem for comparative
>studies, so Gaston argues for the adoption of a standard definition
>which is simple and broadly applicable. He suggests classifying 25%
>of species in any assemblage with the lowest abundances or smallest
>range sizes as rare, regardless of how the parameters of abundance
>and range size are measured.
>
>Any comments?
>
>Craig Hilton-Taylor

Yes. My immediate response is that I think 25% is too large a proportion of
an "assemblage" to classify as rare. A number closer to 10% seems more
reasonable to me. I've been involved in thinking about bird abundance for
quite a few local and some larger-scale lists, and here we use a whole
array of terms--abundant, common, fairly common, uncommon, and rare, even
down to casual and accidental, which refer mostly to species out of range
so of lesser interest in an ecological context. Some have suggested even
more modifiers (very abundant, very common, very rare, etc.). In the
recently published Birds of British Columbia, there is an attempt to
standardize these terms more or less quantitatively, and some other bird
books have done so as well. Faunal/floral works on other taxa seem to avoid
the issue, for the most part, and I would be interested to learn if there
are exceptions to this.

I'm also working on various dragonfly lists, using at least a 3-level
system of common, uncommon, and rare, and, although I haven't tried before
to quantify such levels in this way, if I had to use numbers, I would
propose that the most common 50% of the species in the community
(assemblage) = common, least common 50% = uncommon, except least common 10%
= rare. I would think "rare," as I wrote before, means you don't have a
very high likelihood of encountering the species when you go out looking
for it. Thus I would restrict the term to a rather small percentage of the
group. Again, this is just as arbitrary as that rather daunting array of
Nature Conservancy definitions, but much simpler, which appeals to me.

Clearly, as biologists we are interested in the whole array of abundance,
from most common to least common, while as conservation biologists we're
interested only in the far end of the array.

Dennis Paulson, Director                           phone 253-756-3798
Slater Museum of Natural History                 fax 253-756-3352
University of Puget Sound                       e-mail dpaulson at ups.edu
Tacoma, WA 98416
http://www.ups.edu/biology/museum/museum.html