Yellowstone's Northern Range - Greater Yellowstone Science ...

WOODY VEGETATION
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horn diet, 23 percent of the mule deer diet, 4
percent of the elk diet, and 0.1 percent of the bison
diet during the winters 1985-1988. But the
declines were restricted mostly to the Wyoming
subspecies, as well as to the mountain subspecies
of big sagebrush; these declines occurred mostly on
rolling upland topography. Sagebrush on lower
slopes, swales, and drainages, which is mostly the
basin subspecies, was as tall and vigorous as it was
across most of the northern range. Thus, detailed
study at three exclosures in the lower area and five
exclosures in the upper area, as well as extensive
foot and horseback reconnaissance of the rest of the
northern range, indicates that big sagebrush is
suppressed by ungulates and declining on about 3
percent of the range in the park, and is either
holding its own or increasing on the other 97
percent.
Mehus (1995) found that on the northern
range outside the park, big sagebmsh was "the
most significant item in mule deer diets, averaging
33 percent of the diet across 9 sites." It is not
possible to attribute this high level of winter use of
sagebrush to the elk population; Mehus (1995)
found that big sagebrush averaged 3 percent of the
diets of elk in the same area, and winter use of
sagebrush was equally high in the 1960s when elk
and pronghorn were reduced to about 25 percent of
their late-1980s numbers (Singer and Renkin
1995). It should be noted that because elk are so
much more abundant than pronghorn on the
northern range, even very low consumption rates of
sagebrush by individual elk could result in a large
cumulative amount of sagebrush consumed. But
again, it must be pointed out that competition
between these native ungulate species is not in
itself proof that something is wrong.
Declines in big sagebrush in the BLA will
reduce the area's potential as winter range for mule
deer and pronghorn. Mehus (1995) concluded that
"because big sagebrush is a critical cover and
browse species for wintering ungulates in the study
area [the northern range north of the park], habitat
management should focus on protection of these
habitat types. Fire negatively influences nonsprouting
browse species like big sagebrush that
are already declining under intense browsing
pressure." Conversely, Clark (1991) looked at the
effect of fire on seed germination and concluded
that sagebrush seed germination was stimulated by
temperatures of 122°F (50°C). Subsequent work
has shown that sagebrush is re-establishing well on
most of the areas burned in the 1988 fires (D.
Despain, U.S. Geol. SUTY., pers. commun.).
Houston (1982) noted that the BLA was a
later addition to the park that had been heavily used
by livestock from before the turn of the century
until 1932. Houston (1982) and Singer and Renkin
(1995) also suggested that at times the BLA has
hosted unnatural and artificially high numbers of
ungulates "due to animal avoidance of hunting
outside the park" (Singer and Renkin 1995). The
historical presence of livestock and the continued
influences of hunting on animal movements must
be considered when evaluating trends in sagebrush
on the BLA. For example, Mehus (1995) attributed
declines in sagebmsh on the BLA to high
ungulate numbers and concluded that it will be
necessary to reduce herbivore numbers "for browse
species to persist even at their cuuently reduced
abundance," but Houston (1982) interpreted the
decline of big sagebrush in the BLA area as a
return to more pristine conditions following the
removal of livestock in the 1930s, because heavy
livestock grazing promotes big sagebrush. Thus
there is some question if the decline in sagebrush
on the BLA is a departure from prehistoric conditions
or an artifact of human activities in the area in
the period of early white settlement.
RESEARCH
RECOMMENDATIONS:
SAGEBRUSH
Future research on sagebrush should focus
primarily on the BLA. Continued high levels of
human activity in and near the BLA, such as bison
management operations at and near Stephens
Creek, continued year-round pronghorn use in the
BLA, and both livestock grazing and hunting just
outside the park, can affect all native ungulates and
their use of the BLA in the future. The reintroduction
of wolves to Yellowstone National Park may
affect prey species numbers (Singer 1991a), and