American Bison - Buffalo Field Campaign

Figure 7.5 Area classes of ranges available for existing bison conservation herds.
7.5 Mate Competition
The sex and age structure of small populations are sometimes
manipulated to reduce the risk of escapes, remove aggressive
animals, compensate for unequal mating by males, alter
fecundity, or to increase the rate of population increase. A
common practice for both small conservation herds and
commercial herds is to remove males before they become
morphologically and behaviourally mature (six to seven years
old and older), when they may become dangerous to people or
other animals and property (e.g., fences). Furthermore, the sex
ratio may be manipulated to maintain only sufficient young males
to ensure fecundity (e.g. 10 males: 100 females). In contrast, in
non-manipulated wild herds the mature male: female ratio can
exceed 50:100 (Gates et al. 1995) and mate competition among
males is assured.
The bison is a polygynous species in which mature males (six
or seven years old) compete vigorously for mating opportunities
(Komers et al. 1992). In the absence of mature males, juvenile
and subadult males are capable of breeding successfully, but
60 American Bison: Status Survey and Conservation Guidelines 2010
there is little competition among
them for mating opportunities
(Komers et al. 1994a,b). We
considered that the presence of
two or more mature males indicates
the potential for mate competition.
Sixteen percent of plains bison
conservation herds did not contain
mature males. In contrast, two or
more mature males were maintained
in all wood bison conservation
herds, thus providing opportunity for
mate competition.
7.6 Presence of Wolves
Key species, such as bison, have
a disproportionate influence
on the patterns of occurrence, distribution, and density of
other species. Where present, bison influence the structure,
composition, and stability of plant (Campbell et al. 1994; Knapp
et al. 1999) and animal communities (Bogan 1997; Roe 1970;
Truett et al. 2001). Grazers like bison also enhance mineral
availability and nutrient cycling through faeces and urine
deposition, and carcass decomposition (Augustine and Frank
2001; Towne 2000; Wallis DeVries et al. 1998). The presence of
wolves, the only effective predator of bison (aside from humans),
is an indicator that the maximum number of interactions is
possible between bison and other species in an ecosystem. If
wolves are present we assumed that all other natural limiting
factors would likely be present in the ecosystem. Wolves are
associated with only 10% of plains bison conservation herds (6
of 62) in contrast to 82% of wood bison herds (9 of 11).
7.7 Presence of Reportable Diseases
Although diseases may limit bison population growth and
productivity they are unlikely to cause extirpation. However,
the presence of diseases reportable
under federal or state/provincial
statutes may lead to management
interventions that impact
conservation (Chapter 5). The
type of intervention varies with the
disease and jurisdiction (Chapter 5).
For example, captive conservation
herds that test positive for BTB
or brucellosis would normally be
depopulated, while less serious
interventions (such as the use of
Figure 7.6 Number of bison conservation
herds in four size classes.