American Bison - Buffalo Field Campaign

engineer, both responding to, and creating, heterogeneity. Bison
traditionally exploited broad- and fine-scale variation in forages,
for example, sometimes migrating long distances in response to
snowfall or drought.
Guidelines to help preserve desirable behavioural patterns are as
follows:
1) Allow bison to respond to differences and changes in the
distribution, quality, and quantity of forages by moving
within, and between, ecosystems;
2) Provide herd ranges that include a broad variety of habitats
so that bison can exploit short-term (seasonal) and long-
term (annual, multi-year) heterogeneity in forages from
patch to landscape scales;
3) Bison herds should have the ability to create and
respond to spatial variation in forage quality, quantity,
and distribution that is the result of underlying variation in
resources necessary for plant growth, to variation resulting
from herbivore foraging (by bison, prairie dogs, and other
species), and to variation resulting from environmental
disturbances such as fire and flood;
4) Balance the advantages of larger population size against a
need to avoid permanent habitat damage.
These guidelines suggest that bison should have access to very
large areas in which they can exploit natural heterogeneity in
forage abundance and quality. Fences and other impediments to
movement should be minimised.
9.3.3 Mating behaviour
Differential reproduction resulting from mate competition is an
important evolutionary process and, as such, it is crucial to
allow bison to express natural mating behaviours. The following
guidelines for population management support this goal:
1) The sex ratio of a population should be nearly equal, and in
no case should either sex constitute more than 60% of the
population;
2) A population should include about 50 mature and
reproductively active males for every 100 cows (Gates 1996,
unpublished data; Gates et al. 2005; Komers et al. 1992);
3) Allow interaction and fighting between bulls.
The ratio of mature males to cows will generally be lower than
the overall sex ratio because males (bulls) achieve sexual
maturity at a greater age than females (cows) and the mortality
rate of males is higher than for females.
9.3.4 Limiting factors and natural selection
Chapter 6 described factors that were historically responsible
for seasonal and periodic fluctuations in the size and distribution
94 American Bison: Status Survey and Conservation Guidelines 2010
of bison populations. These factors, and the population
segments they tend to affect, are consistent with contemporary
observations (Chapter 6; Gaillard et al. 1998).
General guidelines consistent with our understanding of
“normal” demographic processes are:
1) Natural mortality rates should be highest for calves and the
oldest age classes;
2) A “normal” range for calf survival is 40-90%, and calf
survival should vary with winter severity, predation
pressures, and forage availability;
3) Natural survival rates for prime-age adults will normally be
about 95%;
4) Under good conditions (e.g., low density, mild winter, good
forage production), pregnancy rates for three-year-old
cows will be 70% or greater;
5) Under good conditions, pregnancy rates for prime-age
cows (generally about 4-15 years old) will normally be 70-
90% and some two-year-old cows (probably less than 5%)
will produce calves;
6) Disease will generally lower reproductive performance.
9.4 Habitat and Biodiversity Management
Bison can, and usually will, significantly influence habitat and
biological diversity, and bison are generally regarded as a
foundation species and ecosystem engineers. This is especially
true for ecosystems where bison are relatively abundant and
range over large areas. Modern, small-horned bison have a long
history as an integral part of two major ecosystems: the North
American Great Plains (plains bison) and the sedge-meadow
ecosystems of northern Canada and Alaska (wood bison).
Bison can profoundly affect ecosystem trophic structures,
bio-geochemical cycling, species composition, and patterns
of species diversity. Some major types of ecological processes
that bison influence are summarised in Table 9.1, while a more
detailed review is provided in Chapter 6.
Below we list guidelines for bison management that will
help conserve biological diversity. Decisions on active bison
management require knowledge of productivity, stocking
rates, and movement patterns. Good sources of information
for management of confined or semi-confined bison herds in
western habitats are the USDA’s Natural Resource Conservation
Service (NRCS) and its Field Office Technical Guides (http://
www.nrcs.usda.gov/technical/efotg/). These documents provide
information on primary productivity, recommended stocking
rates, animal conversion units, and other information relevant
to range management. The NRCS guides, however, focus on
obtaining the maximum sustained yield of livestock. There is no
comparable resource for biologists managing northern bison. For