American Bison - Buffalo Field Campaign

een estimated to be between 0.16 and 0.42 (Berger and
Cunningham 1994; Shull and Tipton, 1987; Wilson and Zittlau,
2004), although Shull and Tipton (1987) suggested that the ratio
could be as low as 0.09 in some managed populations.
It is possible that American bison experienced reductions in
overall genetic diversity due to the population bottleneck of the
late 1800s; however, this effect may not have been as great as
once expected. McClenaghan, Jr. et al. (1990) found that plains
bison have greater genetic variability than several other mammals
that experienced severe demographic bottlenecks. Furthermore,
Wilson and Strobeck (1999), Halbert (2003) and Halbert and
Derr (2008) found levels of DNA microsatellite variability in bison
populations to be similar to other North American ungulates.
Some authors speculate that prior to the bottleneck, the
American bison, with the possible exception of the wood bison,
expressed surprising homogeneity despite its extensive range
(Roe 1970; Seton 1910). Plains bison ranged over large areas.
This suggests that extensive animal movements, and thereby
gene flow, may have existed among populations (Berger and
Cunningham 1994; Wilson and Strobeck 1999). Similar to other
large mammals, bison are expected to be less genetically diverse
than small mammals (Sage and Wolff 1986). Despite founder
effects and low gene flow, which increase genetic distance
values, recent studies demonstrate that the genetic distances
between existing bison herds are lower than expected, indicating
that existing isolated populations are likely derived from one large
gene pool (Wilson and Strobeck 1999). Furthermore, foundation
herds for contemporary bison originated from across the species’
range, suggesting that much of the pre-existing diversity was
likely retained (Halbert 2003). Analysis of ancient DNA may
provide an opportunity for assessing pre-bottleneck genetic
diversity for comparative purposes (Amos 1999; Cannon 2001;
Chambers 1998). Unfortunately, it is not possible to recover the
genetic material lost as a result of the bottleneck underscoring
the importance of maintaining existing genetic diversity while
minimising any future genetic erosion.
Inbreeding, or the mating of related individuals, can lead to the
expression of deleterious alleles, decreased heterozygosity,
lower fecundity, and developmental defects (Allendorf and
Leary 1986; Berger and Cunningham 1994; Lande 1999; Meffe
and Carroll 1994). Inbreeding is difficult to assess and does not
always have measurable deleterious consequences (Berger and
Cunningham 1994; Meffe and Carroll 1994); however, it remains
a potential cause of reduced diversity in bison. To decrease
the effects of inbreeding, some bison herds were founded or
augmented with animals from different regions (Wilson 2001).
Over time, the translocation of animals among herds may have
reduced the impacts of inbreeding and founder effects, which
are most severe in isolated, small populations with low levels
of genetic diversity. While few bison herds have truly exhibited
signs thought to be the result of inbreeding depression, such
20 American Bison: Status Survey and Conservation Guidelines 2010
as high rates of physical abnormalities, reduced growth rates,
and reduced fertility, inbreeding depression has been linked to
low levels of calf recruitment and high levels of calf mortality
in a plains bison herd (Halbert et al. 2004; 2005), and has been
suggested to affect male reproductive success in another
population (Berger and Cunningham 1994).
Although existing bison populations may be derived from a
largely homogeneous gene pool, recent studies using DNA
microsatellites reveal that several plains bison herds are
genetically distinguishable (Halbert and Derr 2008; Wilson and
Strobeck 1999). This raises the issue of whether conservation
herds should be managed as a large metapopulation, with
translocation of bison among herds to maintain local diversity, or
as closed herds to preserve emerging localized differentiation.
Some populations may be adapting to non-native habitats
or changing conditions in the natural environment, and
would, therefore, benefit from localized differentiation. Other
populations may be adapting to, or inadvertently selected
for, unnatural conditions, and would benefit from periodic
augmentation (Wilson et al. 2002b). A precautionary approach
may be to diversify conservation efforts by transferring randomly
selected animals among some herds, to maximise intra-
population genetic diversity, while maintaining other herds as
closed populations with the possibility of the establishment of
satellite populations to increase overall effective population sizes
(Halbert and Derr 2008). Managers should carefully consider
the implementation of metapopulation management plans as a
tool to preserve genetic diversity due to historical differences in
morphology, behaviour, physiology, and disease status (Lande
1999; Ryder and Fleischer 1996; Wilson et al. 2002b) and to limit
the spread of domestic cattle genes between bison populations
(Halbert et al. 2005a; 2006).
Genetic analysis could be used to monitor genetic diversity
by building an inventory of diversity held within conservation
herds. There are several measures of genetic diversity including
heterozygosity, alleles per locus, and proportion of polymorphic
loci (Amos 1999; Templeton 1994; Wilson et al. 2002b). While
early work on bison genetics involved blood groups (Stormont
1982; Stormont et al. 1961), some authors suggest that such
studies are inappropriate for assessing genetic diversity
because selection for blood group type may be high, violating
the assumption of selective neutrality (Berger and Cunningham
1994; Knudsen and Allendorf 1987; Yamazaki and Maruyama
1974). More recent studies have used allozymes (Knudsen and
Allendorf 1987; McClenaghan et al. 1990), mitochondrial DNA
(MtDNA) (Polziehn et al. 1996), nuclear DNA restriction fragment
length polymorphisms (Bork et al. 1991), and DNA microsatellites
(Wilson and Strobeck 1999) to assess diversity. Investigation of
individual genomic regions can reflect overall diversity, allowing
for data from various techniques to be combined to provide an
accurate representation of genetic diversity (Chambers 1998).