American Bison - Buffalo Field Campaign

historically dependent on a combination of bison wallows and
prairie dog colonies for nesting sites. These areas were also
utilised by ferruginous hawks (Buteo regalis) and long-billed
curlew (Numenius americanus) (Knopf 1996). Brown-headed
cowbirds (Molothrus ater), also called buffalo birds, occurred
in association with bison throughout central North American
grasslands prior to the introduction of livestock (Friedman 1929).
Cowbirds feed on insects moving in response to foraging bison
(Goguen and Mathews 1999; Webster 2005). Grasshopper
species richness, composition, and abundance are strongly
influenced by interactions between bison grazing and fire
frequency (Joern 2005; Jonas and Joern 2007).
Bison facilitated dispersal of the seeds of many plant taxa as a
result of the seeds becoming temporarily attached to the bison’s
hair (Berthoud 1892; Rosas et al. 2008) or via passage through
the digestive tract (Gokbulak 2002). Peak passage rate for seeds
was 2 days following ingestion (Gokbulak 2002).
Horning damage to trees along grassland borders is effective
in slowing invasion of trees into shrub and grassland plant
communities or in extending the existing grassland into the
forest margin. Bison within YNP rubbed and horned lodgepole
pine (Pinus contorta) trees around the periphery of open
grasslands to the extent that some were completely girdled
(Meagher 1973). Similarly horning by wood bison in the MBS
has resulted in completely girdled white spruce stands on
the periphery of mesic sedge meadows and willow savannas
(N.C. Larter, personal observation). Several authors (Campbell
et al. 1994; Coppedge and Shaw 1997; Edwards 1978) have
suggested that bison, in combination with other factors such as
fire and drought, significantly limited the historic distribution of
woody vegetation on the Great Plains.
A decomposing bison carcass initially kills the underlying plants,
but subsequently provides a pulse of nutrients, creating a
disturbed area of limited competition with abundant resources
that enhances plant community heterogeneity (Towne 2000).
Carrion from dead bison is an important food resource for both
grizzly and black bears (Ursus americana) as well as scavenging
birds such as bald eagles (Haliaeetus leucocephalus), ravens
(Corvus corax), and black-billed magpies (Pica pica).
6.2.1.2 Contemporary habitat use, nutrition, and foraging
The bison is a ruminant with a four-chambered stomach and
associations of symbiotic microorganisms that assist digestion
of fibrous forage. On lower quality forage, such as grasses
and sedges, bison achieve greater digestive efficiencies than
domestic cattle, but on high quality forages such as alfalfa, the
digestive efficiency of bison and cattle converge (Reynolds et al.
2003). Contemporary studies of plains bison habitat selection
in North American grasslands are limited to confined herds
artificially maintained at varying densities (Table 6.1)—some of
which may differ markedly from pristine conditions (Fahnestock
and Detling 2002).
Herbivores, including bison, respond to gradients in forage
quality and quantity. Hornaday (1889) described a highly
nomadic foraging strategy, where plains bison seemed to
wander somewhat aimlessly until they located a patch with
favourable grazing. A bison herd would then remain and graze
until the need for water motivated further movement. This
account contrasts with more recent studies of bison foraging,
which have found that plains bison actively select more
nutritious forages, and forage in a highly efficient manner that
satisfies their nutritional needs and compliments diet selection
by sympatric herbivores (Coppock et al. 1983a; 1983b; Hudson
and Frank 1987; Singer and Norland 1994; Wallace et al. 1995).
Spatial variation in forage quality and quantity results from
natural gradients in soil moisture, soil nutrients, fire, and other
disturbance, as well as from the impacts of foraging by bison.
Bison exploit variations in forage quality and quantity at all
scales; from selecting small patches of highly nutritious forages
on prairie dog towns, to undertaking long-distance migration in
response to seasonal snowfall or drought.
The following review of bison habitat interactions is based upon
North American ecoregions identified by Ricketts et al. (1999)
and aggregated by Sanderson et al. (2008).
Plate 6.3 Plains bison bull cratering in snow to forage. Photo:
Yellowstone National Park.
American Bison: Status Survey and Conservation Guidelines 2010 43