I'm new to the shell hobby and this site, so please bear with me if I say
anything too far out.  I consider myself more of a natural history buff than
a specialist in anything.   Andrew's point on rarity at the edges of a
species distribution is important.  I have an idea that such individuals (all
living things) are the genetic pioneers of their species.  They are the
individuals that are more likely to carry the genetic code that makes the
species adaptable to changing conditions.  So they are important to the long
term survivability of their species when conditions change either naturally
or assisted by man.
Another point to think about in the debate of  harvesting "rare" versus
"common" species (of any creature, for any reason) is that a species may be
common in an area, but it may be a keystone species and severe changes to
it's population at that location could mean changes for other creatures that
depend on them for say,  food.  Here's an example that I made up (so don't
get hung up on the specifics) - say there is a mud-living gastropod that is
common in a certain bay and lives there all year in huge numbers, but maybe
during the months of October and March large flocks of some migrating birds
depend on massive quantities of these individuals to sustain them through
their migration.  It wouldn't help them if depleted populations rebounded in
the summer, nor would it help them if a nearby bay not on their route had
large numbers of the species.  This is all a figment of my imagination that
is meant only to ilustrate that things in nature may be co-dependent in ways
that we might not be easily able to discern.
What does this mean for the hobby of shelling (that I admit I know nothing
about)?  I'm not sure, but if one of our goals is to regain our ability to be
"in touch" with nature, then these are some of the things we should consider
and make our judgments accordingly.
 
Andrew K. Rindsberg wrote:
 
> I liked Wes Thorsson's notes on rarity. I'd just like to point out that
> virtually any species is "rare" at the edges of its distribution. There,
> even the collection of a few specimens could conceivably damage the local
> population. The more collectors know about shells before they collect
> (including their natural distribution), the better.
>
> As to the word "rare", in botanical Latin it meant "sparse", hence
> "sparsely distributed", hence "uncommon". For example, a tree with widely
> spaced branches might be said to have "rare" branches. We have some rare
> birds in Conch-L.
>
> Andrew K. Rindsberg
> Geological Survey of Alabama