American Bison - Buffalo Field Campaign

woodland caribou (Rangifer tarandus caribou), white-tailed deer
(Odocoileus virginianus), mule deer and possibly stone sheep
(Ovis dalli). Similarly, wood bison are exposed to the full suite
of predators including wolf, grizzly, black bear, wolverine (Gulo
gulo), cougar (Felis concolor), lynx (Felis lynx), and coyote (Canis
latrans). Wolf predation is an especially important mortality factor
for northern bison (Carbyn et al. 1993; Larter et al. 1994; Van
Camp 1987). Furthermore, wood bison movements are generally
not impeded by fences or other land uses.
6.2.2.2 Contemporary habitat relationships, nutrition, and
foraging
Wood bison of the Nahanni population in the south-west
Northwest Territories must cross the Liard River as it bisects
the bison range for its entirety. Animals of both sexes and
all age classes frequently make river crossings (Larter et al.
2003) making them susceptible to group mortality during
spring ice breakup and rapid snowmelt. Bison use of sedges
associated with wet meadows and lakes in winter also makes
them susceptible to mass mortality when groups fall through
weak ice. A total of 177 animals drowned in the MBS after
breaking through the spring ice of Falaise Lake (Gates et al.
1991). Abnormally high January 2009 temperatures (+12° C)
affected ice conditions which likely caused the drowning of
up to 13 animals of the Nahanni wood bison population (N.C.
Larter, unpublished data). Spring flooding, notably at WBNP, has
caused thousands of bison deaths (Fuller 1962).
Fire, especially in the northern boreal region may improve
foraging habitat for bison and, in some areas of the Northwest
Territories, prescribed burning has been used as a management
tool for habitat enhancement (Chowns et al. 1997). However, fire
may play less of a role in maintaining lowland meadows than
sporadic flooding (Quinlan et al. 2003).
6.2.2.2.1 Northern forests
Bison at WBNP and Slave River Lowlands (SRL) utilised mixed
woodlands and aspen and poplar stands interspersed with
meadows in summer, and upland meadows, lowland floodplains,
and delta marshes in winter (Soper 1941). They feed primarily
on graminoids (Table 6.2) with two genera, slough sedge (Carex
atherodes) and reedgrass (Calamagrostis spp.), making up most
of the annual diet (Reynolds et al. 1978). Willows were 8% of
the summer diet (Reynolds et al. 1978). Bison selectively graze
stands of slough sedge characterised by a biomass level that
would probably minimise daily foraging time (Bergman et al.
2001).
6.2.2.2.2 Subarctic boreal forests
Bison exhibit sex-specific differences in habitat selection with
females found in mesic sedge meadows 55% of the time
in winter (compared to males, 38%) and willow savannas
77% of the time in summer (compared to males, 48%), even
though these two plant communities combined constitute
only about 5% of the area (Larter and Gates 1991; Matthews
1991). Both sexes utilised the most abundant coniferous
forest in proportion to its availability during autumn (Larter and
Gates 1991). Bison frequent areas where frozen lakes, ponds,
oxbow lakes, and disturbed sites provide winter access to
forage. The bison diet varied seasonally from a more diverse
combination of graminoids and woody plants or forbs in summer
to approximately one-third lichens and one-third grasses in
autumn, to almost exclusively graminoids in winter (Table 6.2).
Such feeding patterns were consistent with selection for plants
with relatively high available nitrogen (Larter and Gates 1991)
and to enhance short-term energy consumption (Fortin et al.
2002). This feeding pattern may also be attributed to dedicating
time to avoid insect harassment, scanning for predators,
maintaining thermal balance, or social interactions (Bergman et
al. 2001).
In the Nahanni population of south-west Northwest Territories,
bison utilise horsetails (Equisetum) in summer (Larter and Allaire
2007), a forage that is high in nitrogen, but also high in silica. The
high silica causes rapid tooth wear, resulting in teeth wearing out
10 years earlier than in other areas.
6.2.2.3 Habitat and dietary overlap
There is little dietary overlap between wood bison and the
various ungulate species that share its range. Competition with
moose may occur in the Northwest Territories, where the bison’s
diet has a high browse component. Bison and boreal caribou in
Northwest Territories/Yukon Territory both eat lichens, although
during different seasons. Caribou use lichen as a diet staple
in winter, whereas bison use of lichen is in autumn, when they
disperse into the more forested habitats (Larter and Gates 1991).
Fischer and Gates (2005) concluded that food competition
between caribou and bison was low in winter.
6.3 Demographics
The abundance of the free-ranging populations of plains and
wood bison, so iconic for North America, likely fluctuated
considerably by location and through time. These fluctuations
were probably driven by a sequence of density-dependent
population regulatory factors (Eberhardt 1977; 2002; Fowler
1981; 1987; Gaillard et al. 1998); reduced survival of subadults,
delayed age of first reproduction, decline in the reproductive
rate, and increased adult mortality. This sequence was
undoubtedly set back by density independent events such as
episodic droughts and severe winters. Droughts and dry seasons
in general were characterised by wildfires, which, on occasion,
killed bison (Dary 1989; Isenberg 2000). Winters with deep
snow and warming periods, resulting in ice crusting on top of
American Bison: Status Survey and Conservation Guidelines 2010 47