Bill Frank's happy commentary over the discovery of species new to Peanut
Island, a very thoroughly studied area, inspire me to write a few words and
ask some questions.
Biologists have long been familiar with the fact that, when you're trying to
inventory all the species in an area, you never catch them all on the first
day. How long does it take? Let's look at a case study first.
Suppose we are studying Cretaceous mollusks from a marl formation in New
Jersey. The first day, we spot several species of large oysters, a belemnite
guard (similar to a squid pen), some molds of bivalves and gastropods, and a
few fragments of scallops: let's say 20 species total.
The second day we go over the same area with a five-year-old in tow, and she
finds 15 species we didn't spot before, because they are all tiny and her
eyes are closer to the ground. We also find a few more species that we
didn't notice before because they are rare or inconspicuous. We are also now
sitting on the ground occasionally to really look hard for fossils.
On the third day, we bring a new tool: a hoepick, which has a pick at one
end and a hoe at the other. Now we dig and find that we missed a lot of
fossils that are too fragile to survive long when they weather out of the
marl. In fact, when we examine the surface of the marl, we can now see the
marks where the shells are underneath, but the shells break off in tiny bits
as the rock weathers. We would never have seen them without digging. We also
find a few more species just walking around looking at the ground, but it's
clear that returns are getting slimmer by this method.
On the fourth day, we collect a bulk sample of marl and take it back to the
lab to be washed and screened and examined under a microscope. The species
count goes up, though not as much as we'd like, because the micromollusks
are not well preserved.
Next month, we go back with a small army of high-school teachers, and with
many eyes to scan the ground, we collect a lot of the same things as before,
but also five or six species that are new to the area. It turns out that the
patch of poison ivy that we avoided before is very rich in shells. Of
course, the teacher who didn't recognize poison ivy has been given a quick
rubdown with jewelweed. And the athletic young teacher from Burlington High
who was willing to climb down into the ravine has found a mosasaur skeleton.
Not a mollusk of course, but very welcome.
The number of new species collected by each method continues to decline
every time we try it. over the next decade, we have been to the site about
25 times and have found something new each time, but less and less. It's
time for a new method or new eyes, or maybe just for investing a lot more
time to find a lot fewer species. If we have kept accurate records, we can
plot the numbers of species new to the site over time, and will generally
find that it rises very steeply at first, then slopes bit by bit up to a
flat plateau. When we see that plateau, we know it's time to do something
different, because if we do the same thing again we'll be wasting our time
looking for new species. (This is called a rarefaction curve, incidentally.)
So. Many of you have collected the same site over and over. What's the story
like for living mollusks? How long does it take for an area to become
thoroughly known? What are the maximum numbers of molluscan species in
different areas?
Andrew K. Rindsberg
Geological Survey of Alabama
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