American Bison - Buffalo Field Campaign

9.2.2 Herd-level population and
genetic management
For many conservation herds, the most frequent and contentious
decisions will concern herd level management, especially
population control. Key decisions address how many animals to
maintain, which ones to remove, and how often to remove them,
when to add animals, and where to source them. This section
provides advice for active population management at the herd
level—guidelines for establishing a new herd, maintaining the size
of an existing herd, reducing the size of a herd that has become
much too large, and how to deal with known genetic issues.
9.2.2.1 Soft release procedures
Bison may need to be moved to supplement an existing herd,
or to establish a new herd. In such cases, the use of a “soft”
release process should be considered in virtually all cases.
Soft releases typically involve placing animals in a (usually
large) holding facility prior to full release. Holding bison in a
large pen may increase their tendency to remain in the area of
release and establish some degree of site fidelity. American
Prairie Foundation, for example, held bison for one month in a
large corral prior to release on the American Prairie Reserve in
Montana. An additional benefit of a soft release procedure is the
effective quarantine and associated ability to monitor and more
easily re-capture animals if any health issues become apparent.
9.2.3 Establishing a new herd
Establishing and maintaining related, isolated or semi-isolated
herds (i.e., parental and one or more satellite herds) is critical
to long-term species conservation in that multiple herds act to
increase effective population size (N e ) and reduce the total loss
of genetic variation over time (Lande and Barrowclough 1987).
Furthermore, the maintenance of a unique genetic population
in several small herds reduces the probability of accidental
extinction, such as from a natural catastrophe by disease, and
increases the opportunity for local adaptation (Franklin 1980;
Lacy 1987). In theory, and under experimental conditions,
several small groups (e.g., N e about 50) may preserve more
genetic diversity than a single herd with as many individuals as
the smaller herds combined (Margan et al. 1998). Genetic drift
within each related herd can be countered by the occasional
movement of individuals between related herds (Mills and
Allendorf 1996). Therefore, several moderately sized herds
(i.e., more than 300 and fewer than 1,000 animals) of the
same genetic stock can, if managed properly, act as a large
metapopulation with an effective population size sufficient to
impede genetic erosion (Lacy 1987). In this section, we articulate
considerations for the establishment and maintenance of new
bison herds from existing resources.
1. Source
88 American Bison: Status Survey and Conservation Guidelines 2010
Priority should be given to establishing satellite herds from
extant conservation herds, within the respective original ranges
for wood and plains bison, especially for those herds with unique
genetic characteristics (Halbert 2003; Wilson and Strobeck
1999) and those which appear to be free of domestic cattle
introgression (Ward et al. 1999; Halbert 2003; Halbert et al.
2005b). Beyond this, establishment of herds of mixed ancestry
should be considered to maximise genetic diversity and the
potential for adaptive response.
Although bison are likely to be more readily available from herds
subjected to artificial selection and some level of domestication,
we strongly recommend acquiring bison from “wild” herds not
subjected to these influences.
2. Number of animals
Little specific information is available regarding appropriate
foundation populations sizes. In general, a few (4-10) individuals
should be sufficient to avoid very short-term inbreeding effects
(Senner 1980). However, the loss of variation in such a small
population will be substantial after the first few years (Nei et al.
1975) and additional bison should be imported over a period
of several years to increase genetic variation. If the goal is to
conserve or duplicate most of the genetic material in a source
herd, many more animals are required. Shury et al. (2006)
proposed a base of 200 “founder” animals to preserve most of
the genetic variability in “re-established” wood bison herds.
3. Sex ratio
The initial imported bison should consist of approximately 50% of
each sex, and the herd should be maintained with a balanced sex
ratio to reduce inbreeding and maximise effective population size.
4. Breeding strategy
If a small number of bison are used to found a herd, and
especially if additional bison are not brought into the new herd,
breeding strategies to maximise the transfer of genetic diversity
across generations should be considered (e.g., avoid excessive
breeding by one or a few males). Appropriate genetic tools are
available to accurately assign parentage in bison (Schnabel et
al. 2000; Wilson et al. 2002), and these may be used to assist in
captive breeding decisions by evaluating the breeding success
of individual bulls and relatedness among calves.
5. Age composition and behaviour
Bison are social animals and the importance of social structure
within a herd is critical to overall herd health and survival
(McHugh 1958). We recommend establishing a new herd with
both adult and sub-adult individuals to prevent disintegration
of social structure and behavioural anomalies (e.g., foraging
behaviour; Ralphs and Provenza 1999).