Bas,

Let's step away from shells for just a moment.

Some groups of plants and animals tend to be more widespread than others
because they are more able to disperse. Birds and bats and butterflies have
wings. Did you know that the Painted Lady butterfly is found on five
continents? It flies.

In many cases, an organism can disperse easily in one stage of life: Many
sessile marine animals have planktonic larvae; likewise, some plants have
sticky or winged or floatable seeds.

Contrariwise, some organisms don't disperse readily at any stage of life.
River mussels, and many other river-dwellers, are good at dispersal within
rivers but not between them. Flightless birds can disperse themselves by
walking, but they can't move from island to island very well. The faunas of
old islands and old river systems therefore tend to be more distinctive than
those of younger places: think of Madagascar, or the Alabama-Coosa river
system. The shallow marine shelves around islands can also have distinctive
faunas, including mollusks such as distinctive cones and cowries.

It is now known that hundreds, if not thousands, of bird species that were
each confined to single islands became extinct when Polynesia was settled.
Bones tell the story. These birds were presumably not rare in their original
habitat, but they never existed in very large numbers and they lived in just
one place, so they would have been rare in museum collections if any had
been made at that time. They succumbed to human pressure: They were eaten;
their eggs were eaten; they were outcompeted by alien species; or their
habitat was destroyed. All of which is happening to the remaining bird and
landsnail populations in the Hawaiian Islands today.

If increasing pressure is placed on the narrow marine shelves surrounding
the Hawaiian Islands, then what will happen to the species of mollusks
living there today? This is not a rhetorical question. Would any Hawaiians
or friends of the Islands care to reply?

Best wishes,
Andy

Andrew K. Rindsberg
Geological Survey of Alabama

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