American Bison - Buffalo Field Campaign

If a large population has undergone a recent reduction
(=population bottleneck) in a short period of time (e.g., fewer
than three generations), and is allowed to subsequently
increase in size rapidly and without culling, the resulting
population will probably suffer only small reductions in allelic
variation and heterozygosity (Nei et al. 1975). The same is not
true of the bottleneck effect in small populations, where the
loss of allelic variation and heterozygosity tends to be much
higher; in this case, extra measures must be taken to maximise
the transfer of genetic diversity and minimise the loss of
heterozygosity across generations.
Several strategies can be used to alter the breeding strategy
of a small herd to maximise recruitment rates and genetic
diversity in the calf crops. For instance, attempts can be made to
randomise breeding. Bison are naturally polygamous breeders,
and it may be necessary or desirable to implement a controlled
mating scheme to ensure that a maximum number of males
are breeding with the available females, and to maximise the
transmission of genetic variation across generations. If semen
viability or other reproductive barriers are an issue, artificial
insemination may also be considered.
In some cases, altering the breeding strategy of a herd may
not be sufficient to reverse the effects of small population size
(e.g., Halbert et al. 2005a). In these cases, it may be necessary
to import bison from other herds to improve recruitment rates
and increase genetic variation. As the effects of importing bison
into a small herd can be irreversible and even detrimental, the
ultimate decision to implement this strategy should be made
only after careful consideration, and as a last resort (all issues
discussed in section 9.6.2 should be considered). Furthermore,
options to maximise demographic and genetic impact (e.g.,
importing several males vs. a few females) should be considered
in threatened herds.
9.2.7 Recovering herds from germplasm
introgression
If a bison herd has had an influx of germplasm (genetic
material) from an outside source, including another bison
herd or a related bovid species, the ability to recover the
germplasm of the original herd depends on: 1) the ability to
detect bison containing introgressed fragments, and 2) the
number of generations since the original introgression event.
For instance, if two distinct bison herds are accidentally mixed,
parentage testing would allow for post-mating segregation
of the two herds and their offspring provided that the bison
from each herd are distinguishable (e.g., identification tags
or sufficient genetic differences) and that a limited number of
generations have passed (fewer than three). If more than a few
generations have elapsed since the initial introgression event,
the introgressed segments will become dispersed throughout
92 American Bison: Status Survey and Conservation Guidelines 2010
the genome of the herd (hybrid swarm) and reconstitution of
the original germplasm will not be possible (Allendorf et al.
2001). For example, low levels of domestic cattle introgression
have been detected in many extant bison herds (Halbert 2003;
Halbert et al. 2005b) and can be traced back to human-induced
hybridisation of the two species over 100 years ago; in these
cases, multiple domestic cattle fragments are dispersed so
thoroughly throughout the genome that it is not possible to
detect, much less remove, all introgressed fragments.
9.2.8 Herd size reduction
Bison have a high intrinsic reproductive rate and bison herds
generally grow rapidly (see Chapter 6). Therefore, when
resources are limited, bison herds often exceed the carrying
capacity of their environment and begin to have negative
impacts on other grazers and native plant species. As a
result, most bison herds are subjected to some level of culling
(=periodic removals) to maintain a suitable population size
(Table 9.4). In extreme cases, it may be necessary to remove
a large proportion of the population to meet management
goals. For example, if bison have not been culled from a herd
in several years, the herd may have nearly doubled in size, and
it may threaten the survival of other species. In these cases,
extreme caution should be taken to remove bison in a manner
that will minimally influence herd and germplasm composition
according to the following guidelines. Some discretion is needed
in applying these guidelines. For example, it is important to
avoid social disruption while simultaneously removing animals
from all segments of the population. Managers must carefully
evaluate their goals and the specific situation to achieve the best
outcome (Table 9.4).
9.3 Behaviour: Mating System, Social
Structure, and Movements
Bison behaviour is an index, or reflection, of the conditions
experienced by individuals in a population, and behaviour is
an emergent property of these conditions. For example, the
intensity of competition for mates will be largely determined by
population structure and density, and the ability of the herd to
exploit environmental heterogeneity through foraging behaviours
will be largely be determined by population density and habitat
characteristics. Vertebrates exhibit a remarkable ability to
modify behaviour, including territorial defence, mating system,
or seasonal movement pattern, in response to environmental
factors (Lott 1984). Here, we describe desirable behaviours
related to social structure, mating, foraging, and movements.
Unlike population or genetic composition, behaviours can only
rarely be manipulated directly, and behavioural “adjustments”
must be accomplished by modifying other factors.