Extinction Events

The most dramatic cause of extinction is direct
endangering, usually by overexploitation. And the
classic example is the fate of the passenger pigeon. This
fascinating, graceful, pretty bird–with a slate blue
back and deep pink breast–didn’t coo like a dove, but
produced “shrieks and chatters and clucks.” Its greatest
claim to fame was the enormous size of its populations …
in fact, it may well have been the most abundant bird that ever existed. John James Audubon observed a flock
that passed overhead for three days in a row, sometimes at
the incredible rate of more than 300 million birds an hour.
Alexander Wilson–co-founder, with Audubon, of American
ornithology–estimated that another flock contained
two billion birds! The passage of such huge
numbers in flight created a roar of wings that could
actually be heard half a dozen miles away!

These pigeons, which originally lived throughout eastern
North America, fed on the fruits of forest trees,
especially acorns and beechnuts. They nested in narrow
colonies as much as 40 miles long and several miles across.
Their droppings in roosting areas piled up inches thick,
killing all the vegetation below the trees and–in
time–the trees themselves.

This country’s early settlers had no trouble, as you’d
imagine, adding the passenger pigeon to their diets. The
nesting grounds were so crowded that adult birds were
always being injured or killed, and succulent squabs were
constantly knocked out of their nests … so a hungry
person could simply wander through a colony and pick
up dinner.

As the human population increased, however, two things
began to happen: First, railroads pushed through the
wilderness and enabled professional hunters to ship the
birds to centers like New York. Second, the great
oak and beech forests in which the birds nested began to be
cleared.

Profit hunters devised ingenious ways of killing the
pigeons in large numbers. They suffocated entire flocks by
burning grass or sulfur under their roosts. The birds
were also batted down with long sticks, blasted with shotguns, fed grain soaked in alcohol and picked up dead drunk, or netted (after which their heads were crushed with
pincers). One trapping device depended upon a decoy
bird–with its eyes sewed shut–tied to a perch
called a “stool” (“stool pigeon” thereby became a part of
the language).

The disappearance of the species was startlingly rapid.
After the Civil War, so many millions of the birds were
shipped from the Middle West to New York that live pigeons
were even used as targets in shooting galleries. But the
huge flocks were by then already gone from the coastal
states and–by the 1880’s–were dwindling
everywhere. In 1878 one hunter shipped some three million
birds from Michigan, the passenger pigeon’s final
stronghold. The last wild bird was seen in that state just
11 years later, and the final captive pigeon died in the
Cincinnati Zoo in 1914. Her name was Martha.

A Delicate Balance

Economic extinction (reduction of the passenger pigeon
population to a level where hunting was no longer
profitable) preceded biological extinction. The last birds
in the wild were not killed by Homo sapiens , but
perished as a result of “natural causes.” Habitat
destruction was not a major factor, however, because even
today large areas of forest suitable for the pigeons exist
in the eastern United States. Apparently the ability to
form huge flocks was essential to the birds’ survival. Once
their populations became too small to maintain sufficiently
large breeding colonies, nesting failures, inbreeding, and
mortality from predation must have escalated and pushed the
species to extinction.

Other species, too, are known to have been wiped out by
direct human intervention. The dodo, a flightless bird that
once inhabited Mauritius, was eliminated a couple of
centuries ago by early European settlers. Its story is not
as well documented as is that of the passenger pigeon,
but the fact is commemorated in the phrase “dead as a
dodo.” Furthermore, at least it’s suspected that
thousands of years ago–at the end of the last glacial
epoch–Stone-Age hunters played a major role in the
extermination of many species of large mammals, including mammoths, woolly rhinos, and ground sloths. Today,
numerous other species are endangered as a result of human
persecution. Elephants are slaughtered for their ivory;
rhinos for their horns (thought by some cultures to have
aphrodisiac qualities); whales for their meat;
cheetahs, snow leopards, and other cats because they
threaten livestock; and antelopes for “sport.” Sea
turtles are chased for their shells, and their nests are
robbed of eggs. Collectors pay large sums for rare fishes,
amphibians, reptiles, and especially birds. And, closer to
home, the “cactus craze” has become so extreme that many
species are faced with extinction. Even national parks
are being stripped of these key desert plants.

Primates–monkeys, gibbons, chimps, and
gorillas–are much sought for medical research. (The
standard practice in collecting primates is to shoot a
group of adults and then capture the young!) Unhappily, the
research is much too often poorly designed and utterly
useless. And saddest of all, only a small proportion of the
specimens gathered for experiments or collections ever
survive the journey.

Cruel Evictions

In spite of all this, the second major cause of
extinction–habitat destruction–is much more
important than the first. For every species and population
endangered because of direct human action,
thousands are threatened by the inadvertent loss
of suitable places in which to live. Often these are
relatively obscure organisms–insects and other
invertebrates, small plants, and so on–whose jeopardy
or eventual passing goes completely unmarked.

For example, the plight of the Bay checkerspot butterfly is
typical of many temperate-zone organisms. The
insects occupy patches of serpentine grassland in the San
Francisco Bay area, where the food plants required by their
caterpillars grow. Long-term research has shown that the
separate populations of this insect are subject to periodic
extinction by drought, and it’s suspected that other
colonies may occasionally be wiped out by the fires that
maintain the local chaparral habitat.

But before the Bay Area was developed, the losses were not
catastrophic. The vacant “islands” of serpentine grass
would be repopulated, after a decade or so, by migrant
individuals from a nearby island where the butterflies had
survived. Thus, a shifting checkerboard of populations
persisted.

Things began to change late in the last century, however,
as the human population of San Francisco and its
suburbs grew. Gradually, islands of serpentine became sites
of human habitation. The entire process accelerated rapidly
following World War II, as servicemen returning from the
Pacific decided to settle in beautiful California. More and
more checkerspot colonies disappeared under homes, shopping
centers, fast food outlets, and freeways.

Then, during the mid-1970’s, a drought pushed additional
populations to extinction. As a result, the Bay checkerspot
is now endangered. The few remaining populations are
probably too far from the “empty” islands for migrant
individuals to repopulate them before extinction overtakes
the still existing colonies.

Habitat destruction, of course–in the form of the
conversion of its river home into a lake behind the Tellico
Dam–is the sole threat to the snail darter. The
conversion of virgin land to cities, farms, and pasture is
decimating the rich flora of South Africa’s Cape region,
threatening an orchid in Russia, and destroying a
beautiful buttercup in New Zealand.

The Destruction Grows

The list is long and depressing. All the fish
populations in some 300 Adirondack lakes have been
exterminated by acid rains. Siberian tigers are
disappearing as their forest home is cleared. Desert
animals and plants in the American Southwest are being
destroyed by offroad vehicles (a carefully driven dirt bike
degrades an acre of land in a 20-mile journey … a
four-wheel-drive vehicle, more than three acres in the same
distance). Rats, goats, burros, mongooses, and various
other animal and plant “weeds” that have been moved around
by humanity so degrade habitats that native
species–especially on islands–go extinct
wholesale.

All over the globe, at an ever-increasing pace, populations
of all sorts of organisms are being paved over, plowed
under, grazed away, flooded out, stripmined, poisoned,
invaded, and–above all–destroyed as forests are
cleared. This last cause of extinction is especially
serious because of the mounting assault on tropical rain
forests, which may contain two-fifths or more of
all the Earth’s species. If current trends
continue, most of this enormous reservoir of life’s
diversity will be gone early in the next century … and
our spaceship may have lost enough rivets to make the
survival of society impossible!

For more details on the way other organisms are forced
to extinction and a discussion of ecosystem services and
the consequences of their loss, see Extinction by P.
Ehrlich and A. Ehrlich (Random House, 1981) upon which this
column is based.

In November 1981 Anne and Paul Ehrlich, and their
friend and coauthor John Holdren, will join MOTHER EARTH NEWS for a
unique South Pacific Seminar nn the idyllic islands of
Bora Bora and Raïa-téa.

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.