Grasslands - Mountains and Plains: The Ecology of Wyoming Landscapes - page 96

“as devoid of grass as the streets of new York.” 38 Pre-
sumably the cause was livestock abundance, perhaps in
combination with drought. A hundred years later, the
grasslands of this area appeared to have recovered. the
rate of recovery may be slow or rapid, depending on the
plant species present at the time, weather conditions,
the history and nature of grazing pressure on the site,
and livestock management during the time of recovery.
Some plant ecologists attribute pricklypear cactus
abundance to excessive livestock grazing, but Lewis and
clark observed in 1805—long before the first domestic
livestock—“the prickly pears are so abundant that we
could scarcely find room to lye [sic].” 39 Various investi-
gators have observed how cactus increases with drought
and decreases following wetter than normal years, pos-
sibly because of greater susceptibility to insects. there
is little or no evidence that cactus biomass increases
because of grazing pressure alone. 40
travelers in Wyoming often wonder whether the
grasslands and shrublands are similar to those observed
prior to the arrival of europeans. the photographic
record suggests that only a few changes have occurred,
but it is not known whether all the plant species are still
present. Were some plant species driven to extinction
by livestock grazing during the 1880s? Such questions
are difficult to answer. ecologists Richard Mack and
John thompson concluded in 1982 that Great Plains
plants are more tolerant of domestic livestock grazing
than are plants in the Great Basin to the west. they
attributed this difference to a much larger population
of bison on the Great Plains for thousands of years, with
the result that plants there evolved in the presence of
substantial grazing pressure. 41 Long-term studies south
of cheyenne suggest that changes in annual rainfall
affect plant cover and species composition far more
rapidly than livestock grazing does, and though heavy
grazing may cause declines in the abundance of some
palatable species, moderate grazing (defined as 50 per-
cent removal of plant biomass) did not cause a decline
in range condition. 42
the mechanism by which heavy grazing affects
grassland vegetation is generally well known and was
described, perhaps most succinctly, by nebraska ecolo-
gists John Weaver and Frederick clements in 1938:

quickly throws the advantage in competition to the
side of the latter. Because of more water and light,
their growth is greatly increased. they are enabled to
store more food in their propagative organs as well as
to produce more seed. the grazed species are corre-
spondingly handicapped in all these respects by the
increase of the less palatable species, and the grasses
are further weakened by trampling as stock wan-
der about in search of food. Soon bare spots appear
that are colonized by weeds or weed-like species.
the weeds reproduce vigorously and sooner or later
come to occupy most of the space between the frag-
ments of the original vegetation. Before this condi-
tion is reached, usually the stock are forced to eat
the less palatable species, and these begin to yield to
the competition of annuals. if grazing is sufficiently
severe, these, too, may disappear unless they are
woody, wholly unpalatable, or protected by spines. 43

Lincoln ellison, who in 1960 provided one of the
early comprehensive reviews of the effects of grazing on
grasslands, recognized that north American rangelands
have been subjected to grazing for millennia. He made
the following observation when comparing fires, plow-
ing, and grazing:

in arid lands, fires may be rare because the fuel is
eaten by grazing animals, denudation by plowing
may be sporadic because of marginal returns and
recurrent drought; but overgrazing, although caus-
ing less complete denudation in any one season, is
important because it is widespread year after year. 44

in some areas, rangelands probably are still recovering
from the adverse effects of livestock in the late 1800s
and early 1900s, prior to the advent of modern manage-
ment practices.
notably, not all grasslands respond to grazing pres-
sure in the same way. Daniel Milchunas, William Lauen-
roth, and their associates in 1988 compared grasslands
worldwide over a broad range of climatic conditions and
concluded that those in eastern Wyoming—in a semi-
arid environment and with a long history of grazing by
such native ungulates as bison—are tolerant to graz-
ing pressure. in contrast, the taller grasslands of more
humid climates, and grasslands in areas with no history
of long-term grazing, change rather dramatically with
the introduction of livestock.

the more palatable species are eaten down, thus
rendering the uneaten ones more conspicuous. this