American Bison - Buffalo Field Campaign

extensive seasonal north-south movements from summer to
winter ranges (Seton 1929) on both sides of the Mississippi
River (Garretson 1938; Roe 1970) and from the prairies into the
Parkland (Campbell et al. 1994). Large herds also remained on
the northern prairies throughout winter (Malainey and Sherriff
1996). River valleys were crucial to the survival of bison over-
wintering on the grasslands (West 1995). Plains bison also
undertook seasonal east-west movements from the prairies
to the foothills of the Rocky Mountains in winter (Garretson
1938). Inferences from historical reports of seasonal movement
patterns are confounded by the timing of the account relative to
the impacts of market hunting, establishment of pioneer trails,
and construction of the railroads (Roe 1970). In summer, bison
on the Great Plains moved to water on an almost daily basis,
and on occasion moved from 80 to 160 kilometres over several
days to access water (Dary 1989).
Plains bison currently occupying the YNP spend summer at
higher elevations and move to winter ranges at lower elevations
(Aune et al. 1988; Gates et al. 2005; Meagher 1973; Olexa and
Gogan 2007). These movements are made over a network of
trails, geothermal features, and along the banks of rivers and
streams, or along groomed roadways aligned with natural travel
routes (Bjornlie and Garrott 2001). Adult males are often the first
to pioneer previously unoccupied areas, a behaviour that has
been observed in both wood bison and plains bison (Gates et al.
2005). Yellowstone bison have expanded their range in response
to increased population densities (Taper et al. 2000) exacerbated
by particularly severe winters (Meagher 1989).
Wood bison at Wood Buffalo National Park (WBNP) annually
travel up to 50 kilometres maximum from a centre of activity
(Chen and Morley 2005), and individual wood bison at the MBS
range over areas of 179 to 1,442 km 2 (Larter and Gates 1990).
Wood bison have slowly been expanding their range in the
northern boreal forest. Range expansion is generally initiated
by large males who then seasonally return from the peripheries
of the range to join females and juveniles during the rut (Gates
and Larter 1990; N. Larter and J. Nishi unpublished data).
Subsequently, mixed-sex and groups move into the expanded
peripheral range. Range expansion typically follows periodic
high local population densities (Gates and Larter 1990) and is
density-driven (Gates et al. 2005).
6.2 Ecology
6.2.1 Plains bison
6.2.1.1 Ecological role
Millions of plains bison historically ranged over North America’s
grasslands and functioned as a keystone species (Knapp et al.
1999). They shared this landscape with a variety of other large
42 American Bison: Status Survey and Conservation Guidelines 2010
mammals including pronghorn (Antilocapra americana), elk
(Cervus elaphus), deer (Odocoileus spp.), wolves, and grizzly
bears. At the landscape level, bison served as ecosystem
engineers, both responding to, and creating, heterogeneity.
An estimated 100 million bison wallows had a major effect on
surface hydrology and runoff (Butler 2006). Ephemeral pools of
standing water that persisted in wallows for many days following
spring snow melt or rainstorms (Knapp et al. 1999) supported a
variety of wetland plant species (Collins and Uno 1983; Polley
and Wallace 1986). Similarly, bison wallows provided important
breeding habitat for the Great Plains toad (Bufo cognatus; Bragg
1940) and the plains spadefoot toad (Spea bombifrons; Corn and
Peterson 1996). Bison directly affect vegetation communities
through their grazing, physical disturbance, and by stimulating
nutrient recycling and seed dispersal (McHugh 1958). Such
activities help to maintain meadows and grasslands on which
they, and many other animal and plant species, depend.
In tallgrass prairie, bison grazing of grasses increased soil
temperature, light availability, and soil moisture availability to
forb species (Fahnestock and Knapp 1993). The net result
was beneficial to forbs not eaten by bison (Damhoureyeh and
Hartnett 1997; Fahnestock and Knapp 1993), and may thereby
have been beneficial for other herbivores such as pronghorn.
Bison grazing of short and mixed-grass prairie vegetation
increased the rates of nutrient cycling (Day and Detling 1990),
modified plant species composition (Coppock and Detling
1986) and increased the nutritive value of grasses (Coppock
et al. 1983a; 1983b; Krueger 1986). Locally, bison consumed
forage resources (England and DeVos 1969; Hornaday 1889)
and reduced forage height to levels that facilitate colonisation
by prairie dogs (Cynomys spp.; Virchow and Hygnstrom 2002).
In turn, prairie dog activities enhanced the ratio of plant live:
dead material, crude protein content, and digestibility (Coppock
et al.1983a; 1983b) and thereby encouraged further grazing
by bison over more than 20% of the natural short and mixed
grass prairie (Whicker and Detling 1988). While bison grazing
was independent of pocket gopher (Geomyidae) activities, it
influenced gopher distribution by modifying the distribution and
abundance of patches of forbs used by gophers (Steuter et al.
1995).
Bison grazing, frequently in conjunction with fire and wallowing,
enhanced the grassland heterogeneity necessary to provide
suitable nesting sites for a variety of obligate grassland nesting
bird species (Knapp et al. 1999). Bison grazing, particularly on
recently burned areas, enhances the abundance of breeding bird
species, such as upland sandpipers (Bartramia longicauda) and
grasshopper sparrows (Ammodramus savannarum), in tallgrass
prairie (Fuhlendorf et al. 2009; Powell 2006). Similarly, a number
of bird species endemic to the short and mixed grass prairies
of North America, such as the mountain plover (Charadrius
montanus) and McCown’s Longspur (Calcarius mccownii), were