American Bison - Buffalo Field Campaign

season: Wolfe and Kimball (1989) reported an increase in the
percentage of calves from 10.2% in late May to 12.2% in late
July (i.e., count too early and you may miss some).
Similarly, segregation of age and sex classes may influence
estimates of population composition. Meagher (1973) reported
that calves formed 20% of mixed age and mixed sex bison
herds, but 11% of the total Yellowstone bison population.
Other biases are also possible. Carbyn et al. (1998) reported
an unweighted average of 36 calves per 100 adult females
for bison in Delta Area of WBNP for 1989-1996 (Table 6.3),
while others reported between 20 and 30 calves per 100 adult
females for the same area and during the same time period
(Bradley and Wilmshurst 2005). Similarly, Carbyn et al. (1998)
reported an average of 20 yearlings per 100 adult females for
this time period, while others reported more than 10 yearlings
per 100 adult females for only one of those years (Bradley and
Wilmshurst 2005). Thus, composition estimates need to be
interpreted with considerable caution and would benefit by
inclusions of confidence intervals.
Few data sets permit evaluation of reproductive success and
survival of young in relation to population densities (Table 6.4).
The higher ratios of calves and yearlings per 100 adult females
in the Mink Lake area of WBNP compared to MBS (Table 6.4)
reflect differences between increasing and declining populations
(Larter et al. 2000). Lower calf and yearling to adult female ratios
were linked to a period of population decline at WBNP (Bradley
and Wilmshurst 2005). Reynolds et al. (2003) reported density
dependent fecundity in bison at EINP.
Information on the age structure of free-ranging bison
populations not subjected to regular culling is limited. Wood
bison at the MBS were assigned to age and sex classes in July
1993: calves and yearlings were not assigned to sex classes,
all females two or more years old were assigned to a single
category, and males more than two years old were assigned to
one of four age categories following Komers et al. (1992). Here,
the population age structure is presented with an assumption
of an equal sex ratio in calves and yearlings (Figure 6.2).
Irrespective of the sex, the relatively low numbers of calves and
yearlings suggest a low recruitment rate (Figure 6.2).
6.3.2 Reproduction
The age of first reproduction is sensitive to nutritional condition
and, therefore, highly variable. The proportion of females calving
as two-year-olds (conceiving as one-year-olds) ranges between
4-12% (Table 6.5). However, female bison typically enter oestrus
as two-year-olds, and give birth to their first calf at three years
(Table 6.5). Mature females in some populations reproduce
each year (Rutberg 1984; Shaw and Carter 1989; Wolff 1998),
although in other populations mature females may not breed
in some years (Fuller 1962; Green 1990; Halloran 1968; Soper
1941; Van Vuren and Bray 1986; Wolfe et al. 1999). This is
particularly true of females breeding as two- to four-year-olds
(Green 1990). Fuller (1962) noted that for wood bison in the Hays
Camp area of WBNP, 21% of the females more than three years
old at the time of parturition were lactating, but non-pregnant,
while the same was true for 9% of the females in the Lake Claire
area of the park. This proportion may vary within the same
population at different densities of bison and other ungulate
species relative to forage conditions (Halloran 1968; Shaw and
Carter 1989). The young born to females following a year of not
breeding were larger and more fecund than the young of females
who bred the previous year (Green and Rothstein 1991). Females
continue to breed until more than 16 years of age (Green 1990).
Bison are typically monoparous, with twinning reported only
occasionally (Reynolds et al. 2003).
Male bison maintained on supplemental feed are physiologically
capable of breeding as early as 16 months of age (Helbig et al.
2007), and those not receiving diet supplements may breed at
two to three years old (Maher and Byers 1987). However, males
generally do not breed until they are five or six years old and
large enough to compete with older and more experienced bulls
(Fuller 1960; Komers et al. 1994; Meagher 1973; Rothstein and
Griswold 1991).
The age of first successful reproduction may be modified
by disease in bison of the Jackson, Yellowstone and GWBE
populations. More than 90% of the first pregnancies were
lost in brucellosis infected captive female bison (Davis et al.
1990; 1991). In free-ranging bison, the impact of brucellosis
on the age of first successful reproduction will vary with
the proportion of first time breeders in the population, the
proportion of those breeders infected with brucellosis, and
the severity of the infection (Bradley and Wilmshurst 2005).
Diseases may also modify reproductive performance of older
females. At WBNP, both tuberculosis (BTB) and brucellosis
may impact the reproductive success of females of all age
classes within select population segments (Joly and Messier
2004; 2005). In two population segments of wood bison at
WBNP, infection with brucellosis or BTB alone did not impact
pregnancy status, but infection with both diseases reduced the
probability of pregnancy by 30% (Joly and Messier 2005). In a
third population segment, infection with BTB alone reduced the
probability of pregnancy by 75% (Joly and Messier 2005).
6.3.3 Mortality factors and survival
Proximate causes of mortality in contemporary wood bison
herds include wolf predation and the exotic diseases brucellosis
and BTB (Fuller 1962; Calef 1984; Carbyn et al. 1993; Joly and
Messier 2001, 2004; 2005; Wilson et al. 1995 in Bradley and
Wilmshurst 2005). In addition, some wood bison succumb to
irregular outbreaks of anthrax (Bacillus anthracis) (Gates et al.
American Bison: Status Survey and Conservation Guidelines 2010 49