Grasslands - Mountains and Plains: The Ecology of Wyoming Landscapes - page 100

Rather than consider the effects of grazing and
burning separately, ecologists now sometimes recom-
mend a patch-burn grazing system, where the grassland is
exposed to both livestock grazing and relatively small,
patchy burns in the same management unit, enhanc-
ing the probability of higher biodiversity because of the
more heterogeneous landscape that results. Large herbi-
vores are attracted to the recently burned area because
of more palatable and nutritious forage there, for rea-
sons just described. Yet nearby, some birds and small
mammals thrive on the tracts of unburned grassland
that remain. 60

and shortgrass prairie could be used for crop produc-
tion in a manner similar to the tallgrass prairies to the
east. the consequences were staggering dust storms. A
subsequent drought occurred from 1952 to 1955, but
by that time land management practices had improved
and blowing dust was less severe.
After the end of an extended drought, the recovery
of mixed-grass prairie may require several years or more,
with some species reinvading from the shelter of prickly-
pear cactus. 64 Livestock managers do not usually appreci-
ate cacti, but spiny plants enable other plants to survive.
Forage availability increases dramatically after cactus are
burned, partly because of greater accessibility without
the cactus spines, but also because plant growth is higher
without competition from the cactus. Still, pricklypear
cactus can help maintain biological diversity.
With time, many of the original species become re-
established following extended drought. Seedling estab-
lishment is more probable with increased precipitation,
but new shoots also sprout from surviving rhizomes.
Research suggests that some perennial natives survive
droughts in a dormant or near-dormant state, even for
periods of 5-7 years. Above-average rainfall for 13 years
changed a shortgrass prairie dominated by blue grama
to a mixed-grass prairie with needle-and-thread grass,
prairie junegrass, and western wheatgrass as well as
blue grama. 65 Predictably, livestock forage increased
considerably.

Drought

the continental climate of the western Great Plains
has great fluctuations in annual precipitation, and it's
common to have several consecutive years with well-
below-average amounts of rain and snow. Predictably,
plant growth and rangeland carrying capacity are much
lower during droughts. Just as important, plant cover is
reduced and shifts in species composition may occur.
Some research suggests that shifts in species com-
position commonly attributed to heavy grazing could
be the result of drought. 61 common Wyoming plants
that sometimes increase in abundance during dry peri-
ods include blue grama, Sandberg bluegrass, prickly-
pear cactus, and threadleaf sedge. those that decrease
include junegrass, little bluestem, needle-and-thread
grass, red threeawn, silver sagebrush, and western
wheatgrass. one of the most successful species during
drought is Sandberg bluegrass, but even it declines with
extended drought. 62 Species that survive or increase do
so partially because of reduced competition from less
drought-tolerant species.
the changes associated with drought are most likely
reduced plant growth, biomass, and cover. Plant cover,
for example, decreased from 69 to 2 percent during a
drought in eastern colorado, and from 28 to 2 percent
in southeastern Montana. 63 Such drastic changes might
be partially the result of grasshopper outbreaks, which
tend to coincide with drought years.
Unfortunately, the drought years of the 1930s (1933-
1939, especially 1934) followed a 50-year period of grass-
land plowing, spurred on in some areas by large markets
for wheat and the erroneous notion that mixed-grass

Grasshoppers

Drought disturbances are frequently accentuated by
outbreaks of grasshoppers—arguably the most impor-
tant insect in western grasslands (fig. 6.12). 66 the causes
of outbreaks during drought are not well understood,
but some research suggests that bacterial and fungal
grasshopper diseases are more common under rela-
tively moist conditions, limiting grasshopper survival.
it would be of great practical significance to know that
population sizes are determined by the weather, but
Gordon Watts and his associates cautioned that, even
if there is a climatic correlation with grasshopper abun-
dance, “there can be no assertion as to whether it is
a direct action of weather on the grasshopper's physi-
ology, an indirect action on the food plants, a differ-
ential effect on their predators, parasites, and diseases,