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

Figure 7.3. Bison
winter range. Since
the early 1980s,
bison have learned of
potential winter
rallges north of the
park boundary,
leading to a variety
of controversial
management actions,
including public
hunting ill the 1980s
and capture, test,
and slaughter
operations more
recently. Map by
Yellowstolle Spatial
Analysis Center alld
Yellowstone Center
for Resources.
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THE NORTHERN RANGE
84
Northern Winter Range
Bison Range
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evidence offood shortages caused by overgrazing
in the park. Meagher (1989a, 1989b, 1996)
evaluated these movements and concluded that
much of the movements, if not all, are associated
with bison learning to use hard-packed snowmobile
roads for ingress and egress to the park. The
grassland studies cited earlier in this document
indicate that the range is not overgrazed.
Meagher's observations further indicate that bison
"roam" regardless of snow depth or forage availability.
Once the migratory habit is learned, as it
now is in the northern range bison herd and near
the west boundary, the bison will quite naturally be
inclined to move to lower elevations in the winter,
just as the elk do. The animals shot by hunters or
by management agencies after leaving the park in
recent years were in good to even excellent
condition as judged by body fat; they were not
starving for lack of food in the park (K. Aune,
Mont. Dept. Fish, Wildl. and Parks., unpubl. data;
M. Meagher, U.S. Geol. Surv., pers. commun.)
They were moving, as bison moved for millennia
prior to the settlement of the west by Europeans.
Kirkpatrick et al. (1996) compared bison
pregnancy rates of the smaller northern herd with
those of the more robust Mary Mountain herd.
They concluded that the northern herd, which was
considered to be below its ecological carrying
capacity (ECC), had higher pregnancy and birth
rates than bison in the Mary Mountain area, which
was considered to be near or at ECC. This was
broadly considered to be physiological evidence of
density dependence and natural regUlation in
Yellowstone bison. DelGuidice et al. (1994), while
not directly addressing the natural regulation
question, provided physiological evidence also
suggestive of the northern bison herd being below
ECC and the Mary Mountain herd being at or near
ECC.
Bison, unlike elk, deer, moose, bighorn
sheep, and pronghorn, have few constituencies to
champion their cause on public or private lands
outside of Yellowstone Park. As noted above, their
population has increased and they have learned