American Bison - Buffalo Field Campaign

Table 9.4 Important considerations for culling bison herds. See section 9.2.8 for explanation.
Genetic diversity When removing a large proportion of a herd, the primary threat to long-term preservation of the herd is a
loss of genetic diversity that can be very difficult, if not impossible, to restore. Therefore, thorough genetic
evaluation (e.g., section 9.2.3), is necessary before, during, and after planned large-scale herd reductions.
The primary genetic considerations should be the overall maintenance of mitochondrial and nuclear diversity,
such that the genetic architecture of the herd is maintained during and after the reduction period. Routine
examination of culled animals during the reduction period will allow for detection—and hopefully correction—
of “biased” removals, such as removal of a sibship or multigenerational family groups. Preferential removal of
related individuals can lead to losses in genetic diversity and effective population size and should be avoided
(Frankham 1995).
Herd composition If, prior to removals, the herd has the desired composition, bison should be removed proportionally from all
age and sex classes to avoid disruption of social behaviours and demographic structure. If the current herd
structure is substantially different from that desired (e.g. section 9.2), animals may be preferentially removed
from certain classes. In the case of disproportional removals, particularly care should be taken to assess and
mitigate the potential effects of removals on social structure and genetic diversity.
Population
substructure
9.3.1 Social structure and spacing
Bison are inherently gregarious and there are many historical
observations of huge bison herds roaming across North America.
Despite the enormous size of some bison aggregations, astute
observers consistently reported a definable population structure
where cows, calves, and immature males formed mixed-sex
groups, and where large bulls tended to form separate, much
smaller groups throughout much of the year. Groups of bulls are
typically smaller than cow-dominated or mixed groups, and bison
bulls have frequently been observed alone (Allen 1876; Berger
and Cunningham 1994; Meagher 1973; Melton et al. 1989). In
winter, the general pattern is one of smaller mixed groups, with
group size increasing to large aggregations that peak in size
during the summer breeding season and then rapidly diminishing
(Berger and Cunningham 1994; Hornaday 1889).
The fundamental social group in bison is thought to consist of
matrilineal groups (Green et al. 1989), although the persistence
of these groups in populations that differ in size and ecological
circumstances is poorly documented (e.g., McHugh 1958). These
general patterns provide a basis for social behavioural guidelines:
1) Bison herds should have the capacity to exhibit seasonal
changes in group size;
Population substructure is likely important in many bison populations (see section 9.2). The presence of
distinct subpopulations should be carefully evaluated prior to large-scale herd reductions and accommodated
in planned reductions.
Time scale Bison should be removed at regular intervals (rather than large, occasional events) to minimise potentially
irreversible impacts on social structure and genetic diversity The exact time period for removals will likely be
different for each situation and will depend on such factors as total herd size, the total number of animals to be
removed, and the resources available (e.g., facilities, manpower).
Assess effects
of management
actions
Before and after management actions are implemented, thorough genetic, health, and demographic monitoring
is necessary to evaluate recovery efforts, and to detect the need for alternative management strategies.
Small populations are especially sensitive to management changes, and comprehensive monitoring may be
necessary for some time to ensure the recovery of such herds. Sections 9.2,1, 9.2.3, and 9.5.2 summarise
information that should be monitored to detect changes in a timely manner. Especially for small herds, the
overall health of the herd should be continuously monitored to detect and treat any heritable or transmissible
diseases that may impede recovery efforts.
2) Average herd sizes will usually be smaller in mountains or
mixed terrain than in open prairie;
3) Old bulls will be observed alone or in small groups during
much of the year;
4) Persistence of matrilineal groups should be facilitated and
activities that divide matrilineal groups should be avoided;
5) Activities (roundups, harvest, visitor disruptions, and so on)
that disrupt social groupings should be avoided. Where
unavoidable, implement carefully to minimize disruptions.
9.3.2 Foraging and movements
Hornaday (1889) described a highly nomadic foraging strategy,
where plains bison seemed to wander somewhat aimlessly
until they located favourable grazing conditions. Bison then
grazed until a need for water motivated further movement. More
recent studies of bison foraging have shown that they actively
select more nutritious forages, and forage in a highly efficient
manner that satisfies their nutritional needs and that frequently
compliments diet selection by sympatric herbivores (Coppock
et al. 1983; Hudson and Frank 1987; Larter and Gates 1991;
Singer and Norland 1994; Wallace et al. 1995). Spatial variation
in forage is produced by natural gradients in soil moisture, soil
nutrients, fire, other disturbances, including foraging by bison.
After massive wildfires swept along the Alaska Highway in NE
British Columbia and the SW Yukon Territory during the early
1980s, bison continued extensive use of recovering areas 15
years later (Larter et al. 2007). Bison serve as an ecosystem
American Bison: Status Survey and Conservation Guidelines 2010 93