Grasslands - Mountains and Plains: The Ecology of Wyoming Landscapes - page 98

Similarly, Krueger found that pronghorn used prairie
dog colonies preferentially, perhaps because of a higher
number of forbs there. 50
Such interactions among bison, pronghorn, and
burrowing animals surely must have been widespread
prior to the arrival of europeans. Because of the prairie
dog and other burrowing animals, north American
bison may have been more abundant, along with their
predators (grizzly bears and wolves). one study found
more herbivorous insects on rangeland grazed by cattle
than on rangeland that had not been grazed for a long
period, and another study found more belowground
invertebrates in the soil of prairie dog towns than away
from the towns. 51
the effects of large herbivores have attracted the
attention of most grassland scientists, yet the effects of
the abundant belowground herbivores, small as they
are, should not be ignored. For example, in an experi-
mental study, a nematocide was applied to kill all the
nematodes in the study area. 52 Plant growth declined.
Apparently, microbial grazers improve nutrient avail-
ability for vascular plants just as their much larger
aboveground counterparts do.

50

Cause of fire in 1960:
Human Lightning

40

30

20

10

0

APR MAY JUN JUL AUG SEP OCT

Fig. 6.10. number of recorded human- and lightning-caused
fires in the grasslands and shrublands of the Powder River
Basin north of Douglas, Wyoming, in 1960. Adapted from
Komarek (1964).

Fire

Prior to the advent of fire suppression, prairie fires
occurred frequently because of the highly flammable
leaves and stems that rapidly accumulate. Without fire,
and if grazing intensity is low, the standing biomass
is gradually flattened to become part of the mulch on
the soil surface. once fires were ignited, plumes of
smoke would have been visible. the fires would have
burned for weeks or months until extinguished by wet
weather, or until an effective fire break was reached
(such as a sufficiently wide river or ridge, or an area of
inadequate fuel to sustain the fire). Fires might have
been less common during extended droughts, largely
because of less fuel. 53
the source of ignition was usually lightning strikes,
which start fires in grasslands as they do in forests.
An annual average of 10-25 lightning-caused fires
per 1,000 square miles has been reported for differ-
ent parts of the northern Great Plains. 54 Lightning has
been documented as the major cause of prairie fires
in the Powder River Basin north of Douglas (fig. 6.10).

Most occur during July and August, when fuels are
drier and thunderstorms more frequent. thunder-
storms may produce enough rain to extinguish such
fires, but lightning strikes with no or little rain are
common. 55
native Americans also started fires, sometimes
to facilitate their hunting. 56 in 1985 George Gruell
reviewed 145 historical accounts of fire by 44 observers
in the Rocky Mountain region. He concluded that fires
set by indians were most common in the lowlands and
that they could have been annual events in some areas
(although probably the same tract of grassland did not
burn in two consecutive years). For much of the Great
Plains, mean fire intervals were from 2 to 25 years, with
the longer intervals in areas of rough topography. 57
Grassland fires in central nebraska occurred at intervals
of 4-6 years from 1850 to 1900, and every 15-30 years
in western nebraska. Wyoming grasslands probably
burned less frequently owing to slower fuel accumula-
tion in a more arid climate (fig. 6.11).