One problem with a discussion of the word "rare" is that it has more than
one meaning. One person can make a convincing argument using one meaning of
the word, and another can make an equally convincing argument using a
different meaning. They are like ships that pass in the night. I will
demonstrate this.
One meaning of "rare" refers to low total number. There are many cases of
plant and animal species that are confined to a single island or lake, like
the dodo, coco-de-mer, and hundreds of species of cichlid fishes. These
species always had low populations. There are many, many such cases of
freshwater mollusks that live only in a single lake or river, or even in
one set of rapids. Land snails can be similarly restricted. Marine mollusks
are harder to pinpoint, but some are apparently restricted to the waters
around a single island, and it is possible that some live on a single reef,
seamount, or oceanic trench. The seafloor is not everywhere the same.
Also, any species that becomes extinct usually becomes rare first. In 1812,
there were billions of passenger pigeons; in 1912, only one. In 1912, the
passenger pigeon was a rare bird.
Another meaning of "rare" refers to low population density. Tigers are rare
because each animal requires a vast amount of territory for hunting. We
could substitute the word "sparse" for "rare" in arguments of this kind for
greater clarity. Many species of deep-sea mollusks are sparsely
distributed, but occur very widely in the ocean, so their total populations
can be fairly high. In this case, the species can be described as "rare"
(because the species is sparse and hard to find) but also "not rare"
(because the species includes many individuals)!
A completely different problem is that people are reluctant to admit that a
resource can be exhausted, or indeed that a situation will change.
"There'll always be an England"... but the British Empire, which seemed
equally solid, has vanished. Where are the 10 lost tribes of Israel? For
centuries, people were reluctant to admit that they were gone, perhaps
dead, perhaps assimilated into other cultures.
Similarly, it came as a profound shock to Western culture when it became
clear that animal species could become extinct. The 18th-century reasoning
went like this: "God created the earth for man, so everything in it must be
useful for man. But if an animal becomes extinct, then part of the creation
(man's tool kit) is missing and God's plan is no longer perfect. That's
impossible. Therefore, we must be mistaken if we think that any species is
extinct. There must be ammonites living in the unexplored tropics, and
mammoths living in the far reaches of North America." The flaws in this
argument are obvious now (for instance, an animal species might be intended
for use for a while and then no longer be needed). When the far reaches of
the world were explored and no mammoths or ammonites were found (though a
few "living fossils" did turn up!), and especially when the dodo, a useful
bird, was killed off, people were shocked, but Western religions survived
the crisis handily. After all, the Bible says nothing about extinction
being impossible; it was later deduction that seemed to indicate this.
Still, the idea persists that "There must be more snails out there" no
matter how many specimens have been collected. Without evidence to back
this feeling up, people are better advised to exercise restraint when they
collect, rather than to assume that there will always be more.
Andrew K. Rindsberg
Geological Survey of Alabama
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