The Heyday of the Environmental Movement, 1960–1979 119
DOCUMENT 98: Lorus J. Milne and Margery
Milne on the Balance of Nature (1961)The zoologists Lorus and Margery Milne were disturbed by largescale efforts to eliminate pestiferous animals,
insects, and plants. They recognized that even nuisance species often prove useful and that their wholesale
destruction would upset nature’s precarious balance. The Milnes’ review in the New York Times of Rachel
Carson’s Silent Spring [see Document 100] helped to draw attention to another writer who was also concerned
about the consequences of efforts to eliminate pests.Often we forget the contrast between man’s
one-crop fields and nature’s endless variety. One
crop is the ultimate in imbalance, and must be
defended constantly. We overlook, too, how
many animals do both good and bad to man.
The prairie dog’s taste for foliage conflicts with
ou[r] interests, but its liking for grasshoppers is
a help. Nor is it easy to realize how small an area
is home to many native animals. If man removes
as weeds all except his crop plants, creatures
with no special liking for the economic vegeta-
tion may eat it rather than travel to remote sup-
plies of more palatable kinds. Other animals are
driven off, upsetting still more the complex bal-
ance of undisturbed prairie.
Perhaps we tend to overlook our natural allies
through unfamiliarity with modern situations
where a true balance can be found. One of these
was discovered recently, not on a prairie but in
California’s avocado orchards. So perfect is bio-
logical control in this cropland that each year less
than one per cent of the total avocado acreage
needs pest-control treatment. Yet insect enemies
are present, unnoticed although ready to attack.
An experiment tried near San Diego proved how
quickly they can respond to opportunity.
For a period of eighty-four days, entomolo-
gists removed by hand every helpful parasitic
and predatory animal they could find in one por-
tion of a single tree. Within this trial period, cat-
erpillars of one kind multiplied so rapidly that,
to save the leaves, it was necessary to destroy the
insects one by one. Two other avocado enemies
throve until it seemed that similar action might
have to be taken against them. But all of this
change occurred only in the experimental por-
tion of the tree. Other branches of the same avo-
cado and other trees in the same grove presented
no pest problem. Man’s natural allies there con-
tinued their efficient control—at no cost to the
growers.
On prairie ranch land, a balance can be
reached if livestock are managed carefully. Prai-
rie dogs cease to be a menace if a good cover
of grass is present—neither too much nor too
little. Too much grass is a sign of failure to uti-
lize the land economically for cattle, although
underuse of this kind can displace prairie dogs
entirely. Too little grass leads to more prairie
dogs, and often they are blamed unfairly for
the barren ground produced by overgrazing by
man’s animals.
Fortunately, the ranchers are coming to
understand the land they supervise. Some of
them now recognize an obligation both to leave
it in better condition than they found it, and to
retain for the future the aesthetic values to be seen
in native plants and animals. As this philosophy
spreads, so too will a place for a few bison and
prairie dogs. With the land once more in balance,
man’s own economy will also be in order.Source: Lorus J. Milne and Margery Milne, The Balance of
Nature (New York: Knopf, 1961), pp. 19-21.