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American Bison - Buffalo Field Campaign

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Table 9.1 Ecosystem processes that bison can strongly influence. See<br />

Hobbs (1996); Knapp et al. (1999); Larter and Allaire (2007); and Truett<br />

et al. (2001).<br />

Process Description<br />

Create patches Grazing can produce a dynamic<br />

mosaic of vegetation patches that<br />

differ in seral stage and that differ due<br />

to variations in grazing intensity<br />

Enhance nutrient<br />

cycling rates<br />

Enhance habitat<br />

quality<br />

3) Preserve genetic integrity and health. Maintain bison<br />

lineages and carefully evaluate all movements of bison<br />

between populations. Consider potential genetic<br />

consequences of all management actions, especially for<br />

small herds.<br />

<strong>Bison</strong> grazing can enhance nutrient<br />

turnover and change dominant system<br />

mode from detritus-decomposition to<br />

consumption-defecation<br />

<strong>Bison</strong> grazing can increase habitat<br />

suitability for prairie dogs, pronghorn,<br />

and other species<br />

Modify fire regimes <strong>Bison</strong> consume fine fuels and create<br />

trails and trampled areas that reduce<br />

fire intensity and extent, and modify<br />

the effect of fire on vegetation<br />

heterogeneity<br />

Create disturbances Trampling and wallows create<br />

seedbeds for some species; localised<br />

tree stands that are not tightly clumped<br />

are susceptible to major damage by<br />

rubbing, horning, and thrashing of<br />

bison.<br />

Stimulate primary<br />

production<br />

<strong>Bison</strong> grazing removes senescent<br />

material from the sward and increases<br />

light penetration, nutrient availability,<br />

and growth<br />

Disperse plant seeds <strong>Bison</strong> transport seeds in leg fur and<br />

gut, and may enhance establishment<br />

(of native and exotic plants) via<br />

consumption, seed coat digestion, and<br />

defection in nutrient-rich media.<br />

Maintain floral<br />

diversity<br />

Support carnivores<br />

and scavengers<br />

<strong>Bison</strong> grazing can result in greater<br />

grass and forb species diversity<br />

<strong>Bison</strong> are prey to some large<br />

carnivores, and bison carcasses can<br />

contribute to supporting scavengers.<br />

4) Routine assessment is central to science-based<br />

conservation of bison. Routine monitoring and evaluation<br />

of demographic processes, herd composition, habitat, and<br />

associated ecological processes are central to evaluating<br />

herd health and management efficacy. Assessments are<br />

necessary to anticipate or respond to conservation needs<br />

and sound data is the basis for informed management.<br />

The scientific basis and rational of principles for conserving<br />

bison is provided in the more detailed guidelines in this chapter<br />

86 <strong>American</strong> <strong>Bison</strong>: Status Survey and Conservation Guidelines 2010<br />

and other chapters that review bison ecology, genetics, and<br />

ecological restoration.<br />

9.2 Guidelines for Population and<br />

Genetic Management<br />

The general goals for population and genetic management are to<br />

achieve and sustain a population with a healthy level of genetic<br />

variation and a sex and age composition typical of viable wild<br />

bison populations. Management actions needed to achieve<br />

these goals will vary with the size, history, and circumstances<br />

of each particular population. In this section, we articulate<br />

more specific management objectives, summarise background<br />

information relevant to our recommendations (see also Chapter<br />

6), and provide both general and specific guidelines.<br />

In bison, loss of genetic variation is a concern primarily when the<br />

number of actively breeding animals or the founding population<br />

size is small. Our best estimates are that bison populations<br />

can generally be considered “not small” (for genetic purposes)<br />

when they exceed about 1,000 animals, the population has<br />

approximately equal numbers of bulls and cows, and the size<br />

of the population is stable. For the purposes of this report,<br />

the genetic objective is to attain a 90% probability of retaining<br />

90% of selectively neutral genetic variation for 200 years. This<br />

objective is less stringent than some published objectives,<br />

and thus our estimates for sustainable population sizes are<br />

smaller than those that result from estimates based on more<br />

conservative criteria (Reed et al. 2003; Soule et al. 1986). In all<br />

populations, the rate of loss of genetic diversity is directly related<br />

to how rapidly individuals in a population replace themselves<br />

(generation time) and to the size of the breeding population.<br />

Most guidelines for genetic management in this document can<br />

be understood in the context of just these two factors.<br />

Most populations are not uniform, but have genetic variation<br />

related to the spatial substructure of the population (Manel et<br />

al. 2003). Demographic and genetic substructure occurs at<br />

a large geographical scale due to traditional use of particular<br />

parts of a range (e.g., breeding range fidelity, seasonal ranges,<br />

calving areas) by segments of a population (e.g., bison in YNP;<br />

Christianson et al. 2005; Gardipee 2007; Gogan et al. 2005;<br />

Halbert 2003; Olexa and Gogan 2007). Within herds, bison are<br />

thought to form family groups (i.e., matrilineal groups, mother<br />

cows with their preparturient daughters) and these family<br />

groups constitute fine-scale population structuring. These types<br />

of population structure are important because they increase<br />

the likelihood that animal removals without plans to explicitly<br />

accommodate substructures of cows could disproportionately<br />

impact a particular segment of the population and result in<br />

a greater loss of genetic diversity than necessary. Removal<br />

strategies should be designed to accommodate the potential<br />

spatial structure of herds, and institute procedures that ensure

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