American Bison - Buffalo Field Campaign

6.3.4 Population growth rates
The rate of increase of a population is influenced by sex ratio
and age structure, forage and habitat availability and quality,
immigration and emigration combined with reproductive and
mortality rates. The highest rates of increase occur in captive
bison herds, in the absence of predators, where the sex ratio is
skewed towards reproductive age females, some supplemental
feeding occurs, and most, or all, of the population is rounded
up annually and “surplus” bison removed. The Tallgrass Prairie
Preserve (Oklahoma) population attained a rate of increase
of about 50% under such conditions (R. Hamilton, personal
communication).
The maximum exponential rate of increase (r m ) is the rate at
which a population with a stable age structure will grow when
resources are not limiting (Caughley 1977). The observed
exponential rate of population growth over time (ˆr ) may
approximate r m for populations introduced into areas where
resources are abundant (Caughley 1977). The observed rate
of growth may be expected to deviate from r m over time as
a population increases, and per capita resources become
limiting. The length of time for a population to double in size
may be calculated as (natural log (ln) of 2)/ ˆr (Johnson 1994).
Plains bison re-introduced to the NBR in 1909 were permitted
to increase without management intervention for 14 years
(Roelle 1977 in Fredin 1984). The observed exponential rate
of growth of the population in this period, with a starting
population of 37, was ˆr = 0.2053 (Figure 6.3). The population
grew at a rate of 20.5% each year, with a doubling time of 3.4
years, or, given the birth-pulse characteristic of bison, it would
realistically double every four years. The northern Yellowstone
plains bison herd was intensively managed in the early 20th
century, with supplemental feed provided in
winter (Meagher 1973). Numbers increased from
21 in 1902 to 239 in 1915, after which bison
were removed from the herd (Meagher 1973).
The observed exponential rate of increase for the
northern Yellowstone herd for this 14-year period
was ˆr = 0.1787. The population doubling time at
this rate of increase was four years.
Plains bison, allowed to become free ranging
in the Jackson Valley, Wyoming in 1969,
experienced minimal management intervention
until 1980, when these animals began utilising
supplemental forage intended for elk at the
NER (USFWS-NPS 2007). Limited numbers of
plains bison were killed by agency personnel
or licensed hunters between 1980 and 2002
(USFWS-NPS 2007). Plains bison numbers have
been estimated annually by staff of GTNP; S.
Cain; personal communication). The observed
exponential population growth rate for the 33-year
period from 1969 to 2002 was 0.129 (Figure 6.4).
The observed exponential rate of increase for the
Jackson herd for the 14-year period from 1980 to
1993, with a starting population size of 37, was ˆr
= 0.1197. At these rates of increase, a population
would double every six years.
Figure 6.3 Growth of the National Bison Range plains
bison population between 1909 and 1922 (14 years)
starting with 37 bison (upper panel), and the northern
Yellowstone National Park population between 1902 and
1915 (14 years) starting with 21 bison (lower panel).
American Bison: Status Survey and Conservation Guidelines 2010 53