American Bison - Buffalo Field Campaign

Selection for diversity in one system, such as blood group
proteins, or biased selection for maintaining specific rare genetic
characteristics could lead to reduced diversity in other parts
of the genome (Chambers 1998; Hedrick et al. 1986). Biased
selection for maintaining rare alleles is especially questionable
if it is not known what the rare allele does, or if it is detrimental
(i.e., it may be rare because it is being expunged from the
bison genome through natural selection). Variation throughout
the genome, rather than the maintenance of one specific rare
allele, conveys evolutionary flexibility to a species (Chambers
1998; Vrijenhoek and Leberg 1991). Therefore, it is crucial for a
genetic management plan to consider all available measures for
managing genetic diversity in the policies and procedures for
breeding and culling decisions.
An assessment of overall genetic diversity should examine
at least 25-30 loci distributed across the nuclear genome
(Chambers 1998; Nei 1987). While genetic diversity for some
herds has been assessed (Baccus et al. 1983; Berger and
Cunningham 1994; Knudsen and Allendorf 1987; Wilson and
Strobeck 1999), these studies did not include a sufficient
number of loci and comparisons between studies are not
possible due to differences in marker systems (allozymes vs.
microsatellites). Other studies have included larger numbers of
loci and populations; however, several conservation herds have
not been fully examined (e.g., some U.S., Canadian and Mexican
state, federal and private bison herds; Halbert 2003; Halbert and
Derr 2008). Clearly it is important to create a more complete
assessment of bison genetic diversity to allow for more informed
management decisions.
In general, maintaining genetic diversity of American bison
requires an understanding of herd population dynamics to
assess the probability of long-term persistence of that diversity.
Most bison populations are composed of fewer than 1,000
individuals, and it is possible for a relatively small number of
dominant males to be responsible for a high percent of the
mating in a given year (Berger and Cunningham 1994; Wilson
et al. 2002; Wilson et al. 2005; Halbert et al. 2004). This, in
turn, can reduce genetic diversity over time, especially in
the absence of natural migration and exchange of genetic
diversity among populations (Berger and Cunningham 1994).
The potential for disproportionate reproductive contributions
emphasises the importance of maintaining large herds with
large effective population sizes, that given proper management,
will prevent loss of genetic diversity (Frankham 1995; Franklin
1980). Assessment of genetic uncertainty, based on Ne, founder
effects, genetic drift, and inbreeding, is a required component
of a population viability analysis (PVA) (Gilpin and Soulé 1986;
Shaffer 1987).
4.2 Hybridisation
Hybridisation involves the interbreeding of individuals from
genetically distinct groups, which can represent different
species, subspecies, or geographic variants (Rhymer and
Simberloff 1996). Some authors argue that hybridisation is a
potentially creative evolutionary force, which generates novel
combinations of genes that can help species adapt to habitat
change, although such hybrids often experience reduced fitness
(Anderson and Stebbins 1954; Lewontin and Birch 1966; Hewitt
1989). Hybridisation through artificial manipulation or relocation
of animals, however, can compromise genetic integrity through
genetic swamping of one genome over another and disruption
of locally adapted gene complexes (Avise 1994). It can also
produce offspring that are devalued by the conservation and
legal communities (O’Brien and Mayr 1991; Chapter 7). The
genetic legacy of introducing plains bison into a wood bison
population in northern Canada, and crossbreeding bison and
cattle, have made hybridisation a controversial topic in bison
conservation.
4.2.1 Plains bison x wood bison
Based on their geographic distribution and morphology, plains
bison and wood bison were historically distinct entities (Chapter
3). It can be argued that the introduction of plains bison into
range occupied by wood bison was a “negligible tragedy” (Geist
1996), because some consider the two groups to be ecotypes
(Geist 1991). Others maintain that the interbreeding of these
two types should have been avoided to preserve geographic
and environmental variation (van Zyll de Jong et al. 1995). The
introduction of either subspecies into the original range of the
other could, in theory, erode the genetic basis of adaptation
to local environmental conditions (Lande 1999). Therefore,
hybridisation between plains and wood bison should be
considered detrimental to maintaining the genetic integrity and
distinctiveness of these two geographic and morphologically
distinct forms.
While historically there may have been natural hybridisation
events between the subspecies in areas of range overlap, the
current hybridisation issue is the consequence of an ill-advised
and irreversible decision made nearly 85 years ago. In 1925,
the Canadian government implemented a plan to move more
than 6,000 plains bison from the overcrowded Wainwright
National Park to Wood Buffalo National Park (WBNP). Biological
societies from U.S. and Canada strenuously challenged this
action, as interbreeding would eliminate the wood bison form,
resulting hybrids might not be as fit for the environment, and
diseases such as bovine tuberculosis (BTB) would spread to
formerly healthy animals (Howell 1925; Harper 1925; Lothian
1981; Saunders 1925). Proponents of the plan countered the
criticism by questioning the subspecies designations, arguing
American Bison: Status Survey and Conservation Guidelines 2010 21