Grasslands - Mountains and Plains: The Ecology of Wyoming Landscapes - page 103

stock production is the primary management objective.
often they are shot or poisoned even today, when their
population sizes are a small fraction of what they were
in the 1800s. Various studies have described how prairie
dogs have little or no adverse effects on livestock weight
gain, and that long-term benefits for forage productivity
outweigh what appear to be adverse effects. 77 As noted,
research has shown that the nutritive value of forage
on prairie dog towns is higher than in much of the rest
of the landscape, apparently because nutrient cycling
is facilitated by the activities of so many animals in a
small area. there may be a need to control prairie dog
populations in some areas, but doing so has become
highly controversial.
this issue recently came to a head on the thun-
der Basin national Grassland, where federal man-
agers are trying to increase the size of a black-tailed
prairie dog town so that it can sustain a population of
the endangered black-footed ferret. Appropriately, the
managers are collaborating with adjacent landowners,
encouraging them to allow some prairie dog towns on
their property as well—at least in small colonies. 78 Some
ranchers have consented to this plan, but only if they
can shoot or poison nearby dog towns where they are
not wanted—whether on private or public land. Shoot-
ing prairie dogs, however, does not make sense to those
who ask: How can poisoning black-tailed prairie dogs
be justified when their benefits to the ecosystem are so
well known and they have been petitioned for protection
under the endangered Species Act?
the dark cloud of sylvatic plague—a bacterial dis-
ease carried by fleas spread by rodents and introduced
to north America with rats in the 1800s—complicates
matters, because it spreads rapidly in prairie dog towns
and can cause local extinction of the rodents in a year
or two. that has happened numerous times in the past
40 years. 79 insecticides have been used to kill the fleas,
with considerable success—though with the usual
expense and undesired effects of adding pesticides to
the ecosystem. For two reasons, sylvatic plague could
foil the best-designed plan for restoring the ferret in
portions of its native habitat: the ferrets' food supply
is diminished, and the ferret itself is susceptible to the
disease. Vaccinating the ferrets against the disease has
shown some promise, but there are too many prairie
dogs for that to be practical.

Ground squirrels are another group of small mam-
mals that can affect vegetation through herbivory and
burrowing. they do not form towns as prairie dogs do,
but their populations fluctuate between nearly zero and
a hundred or more per acre. the causes of ground squir-
rel population fluctuations are still not well known.
Another common feature of the plains and basins
are the mounds created by harvester ants ( Pogono-
myrmex occidentalis ). 80 these seed-eating ants are some-
times viewed as destructive, because they denude areas
around their dome-shaped mounds (fig. 6.14). An esti-
mated 6 million mounds existed in the Wind River
Basin in the 1970s. the common reaction to such num-
bers is to lament the loss of forage for livestock. Yet, like
other burrowing animals, ants mix and aerate the soil,
and the vegetation surrounding denuded areas is often
more vigorous, most likely because of improved water
and nutrient availability caused by ant burrowing and
defecation. the absence of plant growth on the mound
is probably compensated for by the increased growth
nearby (see fig. 6.14).
Disturbance by burrowing mammals and ants also
adds patchiness to the grassland mosaic—above- and
belowground. the disturbances lead to continual
change in the grassland community as species adjust
to different environmental conditions. the native
flora and fauna are well adapted to such disturbances;
their diversity is high in large part because of them.
As in other ecosystems, however, the effect of people
has often been to modify the frequency of grassland
disturbances or to increase their intensity, resulting
in a more homogeneous landscape with lower spe-
cies diversity. in many areas, the natural disturbances
have been replaced with new ones, like plowing over
large areas, for which native species are not adapted.
thus, it is little wonder that introduced weedy species,
originating in eurasian landscapes with a much longer
history of agriculture, have done so well in American
croplands.

Invasive Plants

native grasslands have been invaded in some places
by a variety of introduced plants that have predictable
effects: crowding out native species, altering habitat
and food availability for wildlife and livestock, reduc-