Actually, there's a neat book called "Why Big Fierce Animals Are Rare".
Obviously it would be difficult to maintain an ecosystem consisting of equal
numbers of wolves and sheep.

Similarly, some gastropods are carnivorous and are also specialists in what
they eat and where they live. Even at their acme of abundance, they will
have a lower density than their prey.

In contrast, generalists can be extremely abundant, especially in places
that have lots of food but are difficult to live in, such as estuaries and
marshes. The molluscan fauna of a brackish bay may be limited to half a
dozen species, but those species may be extremely abundant. In Mobile Bay,
Alabama, these are Crassostrea virginica, Rangia cuneata, and Neritina usnea
(or reclivata, depending on which book you consult).

How low a density can a molluscan population have in nature? Well, consider
the major bottleneck: reproduction. If mollusks are too widely separated,
then there will be no next generation. Therefore, mollusks tend to be
clustered into groups that live near one another, as Paul Monfils points
out. And it is only a matter of time before someone discovers those
clusters. Though Abbott was right about the clusters existing, there is no
reason to believe that there will always be more clusters, any more than
there will always be another redwood grove in the next valley.

In temperate countries, we are used to seeing vast numbers of a few species:
oaks, dandelions, robins. Winter helps to keep parasites and predators in
check. In the winterless tropics, plants and animals commonly maintain very
low densities of population, and this helps to keep predation and parasitism
from getting out of control, simply because the preferred prey is hard to
find. For instance, it is common in the Amazon to see no two trees of the
same species out of the first hundred that you examine -- which is
unheard-of in a temperate forest. Conditions underwater are analogous in
reefs. I think we can call species like that "uncommon" or even, in some
cases, "rare".

In some cases, it is obvious that the population is severely limited:
terrestrial snails that live on only one island or mountain, or even one
tree; mussels that live in only one river, or only in a few rapids on one
river; cones that live only in the narrow shelf around one island; marine
mollusks that live only on reefs or submarine canyons. In most cases, it is
not clear what the total range of a species is.

In a "common" species, many individuals can be harvested without endangering
the species as a whole. But if a collector takes too many specimens from a
population that is small to begin with, then that population may die out as
a result. (More often: If a logger or developer destroys a habitat.) That
this has happened with species of birds, plants, butterflies, fishes, and
mammals, we well know. The developer is by far a greater danger to mollusks
than the collector. But in a few cases where a shell is rare, restricted in
range, declining in population due to pollution, and/or very beautiful, then
the collector could be a threat to a species' existence.

So far as I know, no mollusk (freshwater, terrestrial, or marine) has ever
become extinct because of shell collectors, while many species have been
documented by shell collectors. This is a good record to have -- butterfly
collectors can't say that. Let's act responsibly to keep it that way.

Andrew K. Rindsberg
Geological Survey of Alabama