Art Weil asked how estimates of extinction rates are made. This is a good
question.

Some groups are relatively well known, such as birds. Most birds are active
during the day, and there are hordes of amateur and professional
ornithologists to study them. It is very unlikely that any new species of
birds will be discovered in North America this year; we have an accurate
species count. We also know which birds have historically become extinct,
and in some cases we even know the day when they became extinct, so we have
a good idea of how many species of birds became extinct per century, or per
decade, in North America. The situation is similar in other parts of the
world, though new birds are still being discovered in places like New
Guinea. These numbers can be graphed and projected into the future. That is
one source of statistics on extinction rates.

Other groups are not quite as well known as the birds. Mammals are mostly
active at night, and new species of bats and mice are still discovered
occasionally. But it is only in very remote places that new species of
large mammals can still be discovered, e.g., the jungles of southeastern
Asia. The same statistical methods can be applied to mammals as for birds,
but with a greater possibility of error. If the discovery rate for mammal
species is plotted on a graph, it shows a steep rise early on, then it
flattens out as almost all mammals have been discovered. A similar
situation might be demonstrated for freshwater mussels.

We have a rough idea of how many species are threatened or endangered in
North America, and how many have been lost in the last few decades, and
where. If a knowledgeable person says, "We've lost (say) one-third of the
species of endemic land snails in Hawaii and are likely to lose another
third in the next few years," it is an estimate based on hard statistics.
(Incidentally, I just made up the 1/3 figure for purposes of discussion; I
don't know the actual numbers for Hawaii.)

Some groups are not well known at all. New species of insects are named by
the thousands every year, particularly from the tropics. If the discovery
rate for insect species is graphed, it doesn't show any sign of slowing
down at all. Evidently, there are a LOT more insects out there to be
described for the first time. And marine mollusks too, I might add. How can
we estimate their number?

This isn't easy. One method is to list all the species known from an area,
then sample the area intensively and see what proportion is new. Then
correct the total number of insect species in a larger region by the same
proportion. The estimated numbers rose steeply a few years ago when an
enterprising biologist wrapped whole trees in plastic and gassed them to
kill every insect. The number of new species living in the tree canopy
turned out to be phenomenally high. Biologists who formerly estimated the
total number of insect species as 1 or 2 million now talk about 10 or 20
million. The range of error is correspondingly great.

Now, it is known that much of the world's diversity lies in the tropical
forests. It is also known that many plants and animals live only in small
parts of this forest, or live so widely scattered that they require vast
stretches of forest (compare the peregrine falcon of North America, for
instance). It is usual for a square kilometer of land to have no two trees
alike, if I remember right, a situation that is absolutely amazing to
Europeans and North Americans. And other organisms, from insects to fungi,
often live on only one part of one species of tree, so this system is
highly vulnerable to species loss if large tracts are cut down. Well, large
tracts are being cut and burned, and much of this land is being turned into
farmland and pasture instead of returning to forest. The idea among
biologists is that if half the forest is destroyed, then more than half of
its species will be destroyed. And that's why they say things like,
"Thousands of species will be destroyed forever before we even have a
chance to see them for the first time," which admittedly sounds strange at
first!

And that's where you get some of the latest estimates of extinction rates.
And that is why Kurt Auffenberg is haunted by the idea that so many species
of land snails will vanish without being recorded, and why he makes it his
business to collect and record as much as he can. That is why Marlo
Krisberg has been trying to document the species of one area of Florida
while collecting very little. That is why Doug Shelton spends some of his
own time to look for species of freshwater mussels that may not be extinct
yet and will need protection to survive, photographing mussels and
returning them to their bed.

What can be done? The quality of the environment can be improved: less
pollution, more effective ways of production, more efficient use of
materials per person. Refuges can be set aside for whole forests and reefs.
Zoos and botanical gardens can save a few plants and animals. Seed banks
can be maintained, especially for the most important agricultural species.
Expeditions can be financed to collect and record the organisms that live
in threatened areas, and comprehensive monographs can be compiled on the
organisms of particular regions. The dispersal of organisms to new habitats
can be discouraged. Individuals can contribute money, or grow rare
organisms in greenhouses, or document the fauna of their favorite area, or
vote.

But in the long run, there will have to be fewer people, or we will have a
lot fewer kinds of plants and animals on this planet, and that does include
shells. We are still in the beginning of this great crisis of extinction,
and we can make a difference if we act now. Later will be too late.

Andrew K. Rindsberg
Geological Survey of Alabama