Ecoscience: Snail Darters and the Importance of Ecosystems

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Anne and Paul Ehrlich liken the importance of ecosystems and the individual life forms within them to the rivets that hold together a plane: losing any one might not cause a wing to fall off, but each lost rivet weakens the whole.

Many people seem to wonder whether ecologists have gone off
the deep end in the struggle to protect endangered species.
We are often asked, “Isn’t it preposterous that the
existence of such an insignificant fish as the snail darter
should be allowed to stop a public works project like the
TVA’s Tellico Dam, a structure in which millions of
taxpayers’ dollars have already been invested?”

Our answer is always the same: The Tellico Dam should have
been stopped dead in its tracks even if it had
a very important and worthwhile project (which,
according to much testimony, it wasn’t). In fact, even if
the TVA had followed the letter of the law instead of
trying to evade existing legislation, the dam could not have
been allowed to destroy that one little species of fish.

As many of you are aware, the ecological systems of our
planet constitute a gigantic life-support apparatus that
must function smoothly if our civilization is to persist.
We mentioned a few of the “public service!’ functions in
our column on the tropical forests, but let us reiterate the importance of ecosystems:

They maintain the quality of the atmosphere, help to
stabilize weather, provide food from the sea, control the
vast majority of potential crop pests and carriers of human
disease, generate and maintain soils, recycle nutrients
essential to agriculture, and dispose of our wastes.
Perhaps even more important, however, such systems also
preserve a vast genetic “library” on which we can draw
for–among other things–new domestic plants and animals,
spices and medicines, organisms for scientific research,
antibiotics, and so on.

Now, if everything about the life-support apparatus were
understood, all would be well. We would be able to predict
the consequences of fiddling with any part of it, and in
particular to understand what would happen if any one
element were removed from the ecosystem.

We know that many, if not most, ecosystems are quite robust, that they will recover from various sorts of insults,
and even from the extinction of populations and whole
species! The fossil record reveals that the extinction of
populations and species has been a regular feature of the
evolutionary process on this planet for hundreds of
millions of years. In the past century many
extinctions have been observed that have not, at least in
the short term, seemed to have major deleterious
effects on their respective ecosystems.

But this is not a valid reason to believe that Homo
needn’t be concerned about endangered species.
Human knowledge of the properties of specific
ecosystems and ecological structures in general is
extremely limited. Something can be said about their
general properties–such as the way energy flows
through them, or the manner in which materials cycle in
them–but very little is understood of the details of how
ecosystems function and, in particular, of the factors that
make them stable or unstable. Biologists are just beginning
to learn about the vast amount of variability present
within and between natural populations, and the
significance of that variability is still in dispute. It’s
not yet clear how much the coevolutionary interactions
between organisms in natural communities have led to
stability in these communities, nor do we know how often
such interactions may destabilize an ecosystem and lead to
dramatic changes in its structure.

Scientists have not failed to ponder these questions. It’s just that the interactions of the many millions of
kinds of earth’s organisms with each other and with the
physical aspects of their environment are extraordinarily
complex. Although certain principles of ecosystem
organization can be clearly perceived, in
most cases ecologists are not in a position to predict accurately
what will happen to a given system in the face of certain
kinds of disturbances–such as the destruction of

There are weak analogies to this inability to forecast the
future in the complex realm of the social sciences. For
example, political scientists can tell you many things
about the organization of the United States government, but
they find it virtually impossible to foretell the precise
consequences of apparently minor disturbances such as the
Watergate break-in … or even to forecast exactly the
results of elections (Although these scientists are
fortunate enough to be able to ask individual people how
they’re going to vote. An ecologist can’t ask a
snail darter what role it plays in the grander scheme of

It’s often contended that endangered species
shouldn’t be allowed to stand in the way of “progress” simply because it’s impossible to predict the
consequences of the destruction of such plants or animals.
For instance, Roger Starr (a member of the editorial board
of The New York Times) has argued that humanity
need not worry about driving populations of species to
extinction, since–as noted above–extinction has long been
a feature of evolution on this planet. Starr reasons that
if extinction didn’t disrupt ecosystem functioning in the
past, it won’t do so now. This superficially cogent
argument is utterly fallacious for two reasons:

First, the rate of extinction–because of habitat
destruction, poisoning, and exploitation–is vastly greater
today than the rates that have characterized most of the
billions of years of evolutionary history. For example, the
extinction rate for birds and mammals between 1630 and 1975
appears to have been 5 to 50 times as high as in the
distant past, and the rate over the next 20 years is
projected to be 40 to 400 times “normal.”

This snowballing rate of extinction will no longer allow
the losses to be made up, as they formerly were, by
speciation (the evolutionary process that generates organic
diversity and develops new species). After all, even though
extinction is a natural process that has always accompanied
speciation, Homosapiens inherited a species-rich
planet with fully functional ecosystems. Historically,
extinction was more than balanced by speciation.

Thus, the second reason that the “no- need- to- worry- about- extinction” argument is misleading has to do with the
future of speciation. Throughout geological time, the
splitting process of evolution has continually created new
kinds of organisms to be “tested” by the environment. It’s
currently believed by most evolutionists that most
speciation occurs when populations isolated from one
another are subjected to very different environments and,
thus, to different selection pressures. (Natural selection
is simply the differential reproduction of genetic types,
or genotypes. The genotype best suited for
reproduction in one environment may not be the best in
another environment.)

Human intervention, however, usually seems to work
against the process of speciation. For example, it
seems likely that in the next half-century the rain
forests of South America will be reduced to a few scattered
preserves. If this happens, most of the larger organisms in
those limited areas will become
extinct in a relatively short time as a result of natural population fluctuations. In
order to minimize this loss of species, attempts will
almost certainly be made to move animals from preserves
where they are abundant to other protected areas where
those species are scarce. However, this process of moving
organisms around–which has already begun with zoos
and nature-preserve breeding programs–reduces or
eliminates the isolation between the preserves, thus acting against the speciation mechanism. Although speciation may
still be able to occur if selection pressures are strong
enough (and if the preserves are maintained for thousands
of years!), its rate will, at best, be greatly reduced.

And, in terms of a human time scale, the development of new
species has always been very slow–slow enough that the
splitting of an evolutionary line of animals has not been
observed in the hundred-plus years since Darwin, although
extinctions have been numerous during that same

So the snail darter, the Furbish lousewort, the mountain
gorilla, the blue whale, and all of the other endangered
species can be viewed as symbols of a much more general
impoverishment of earth’s living biota. Beyond them,
hundreds of thousands of unsung species and–probably
equally important–millions of genetically distinct
populations have either gone extinct in the past few
decades or will do so in the next few.

As we indicated, it’s not possible to predict the
consequences or even to observe the vast majority of
these extinction events. But it is easy to predict their
eventual cumulative effect: the breakdown–first regional
and then global–of the all-important life-support functions
of natural ecosystems.

To make an analogy: Suppose you saw a group of people
prying rivets out of the wing of an airplane you were about
to board. Imagine, also, that you didn’t know the exact
details of the airplane’s construction, but were aware that
the loss of some rivets wouldn’t necessarily cause
the wing to fail. Would this make you relaxed about the
prospect of flying in that airplane? Would you be satisfied
if you asked one of the people to stop prying out a rivet
and that person simply said, “You can’t prove that the loss
of this rivet will fatally weaken the wing, so it’s
perfectly all right for me to take it out”? Would you be
relieved if he said, “Don’t worry … see, I’ve just taken
out the rivet and the wing hasn’t fallen off”?

You would, of course, have to be insane to fly in that
airplane after such a conversation. Needless to say, the
treatment of that imaginary wing bears considerable
resemblance to the present treatment of the life-support
systems of Spaceship Earth–except that we have no
option as to whether or not we’ll fly on her!

The organisms being exterminated are very much like the
rivets in the airplane’s wing. It’s virtually impossible to
prove that the removal of any individual or any group will fatally
weaken the ecosystem. And the lack of catastrophic failure
immediately following the removal is no guarantee that
disintegration won’t result in a later period of

Yet the vast majority of Homosapiens go
merrily on their way popping off rivets without the
vaguest notion of the probable consequences of such
behavior. But we can, at least, be sure that those
consequences will not be pleasant. They can be expected to
include the collapse of fisheries, enormous pest outbreaks,
epidemics, the advance of deserts, erosion, the exhaustion
of freshwater sources, the progressive failure of high yield
agriculture and forestry, and climatic changes that may
induce catastrophic famines.

The avoidance of such events seems to be a powerful
argument for taking extreme precautions to prevent an
increase–or even a continuation–of current extinction
rates. Rather than exterminating other organisms more
rapidly, we should be searching hard for ways both to slow
that rate of loss and to ensure the continuation
of the speciation process at normal levels.

Of course, there are also compassionate and esthetic
arguments for protecting what are–as far as we know for
sure–the only other living beings in the universe. As the
great French anthropologist, Claude Levi-Strauss, once
said: Any bug that is sprayed with pesticides is “an
irreplaceable marvel equal to the works of art which we
religiously preserve in museums.” There is some hope that
we might persuade others of the esthetic value of such
organic diversity, especially if they can be made more
intimately familiar with the beauty and complexity of
organisms. But the compassionate argument hinges upon such
things as upbringing, values, and emotions. Those
adults who do not now accept this sort of reasoning are
unlikely to be converted. As strong as esthetic
arguments may be to many people, we believe that the battle
to save organic diversity can only be won in today’s
society on the most practical of grounds.

It’s against this backdrop that we take our
stand with the snail darter. True, the extermination of
this small fish alone will probably not precipitate an
ecosystem collapse. But no more purposeful extinctions
by humanity can be permitted
. A firm line must
be drawn
NOW! Because, sooner or later, a Tellico Dam
or its equivalent will threaten every population
of nonhuman organisms on the face of this planet. Sooner or
later, each body of water will be needed as a source or a
sink for some human activity that will be lethal to its
natural occupants. Perhaps, indeed, every species will be
found to occupy a piece of land that could be plowed,
grazed, mined, logged, paved, or otherwise disrupted by
someone who sees a profit in it.

The hour has come for humanity to recognize that its fate
is inextricably tied up with the snail darters of our
planet … to recognize that ever-increasing numbers of
human beings, each eternally trying to co-opt a larger
share of the earth’s resources, endanger not only the
other species of our planet but humanity itself. 

Paul Ehrlich (Bing Professor of Population Studies and Professor of
Biological Sciences, Stanford University) and Anne Ehrlich (Senior
Research Associate, Department of Biological Sciences, Stanford) are
familiar names to ecologists and environmentalists everywhere. As well
they should be. Because it was Paul and Anne who–through their writing
and research–gave special meaning to the words “population,”
“resources,” and “environment” in the late 1960’s. (They also coined the
term coevolution, and did a lot to make ecology the household word it
is today.) But while most folks are aware of the Ehrlichs’ popular
writing in the areas of ecology and overpopulation (most of us–for
instance–have read Paul’s book The Population Bomb), far too few people
have any idea of how deeply the Ehrlichs are involved in ecological
research (research of the type that tends to be published only in
technical journals and college textbooks). That’s why it pleases us to
be able to present these semi-technical columns by
authors/ecologists/educators Anne and Paul Ehrlich. 

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