American Bison - Buffalo Field Campaign

animals are proportionately removed from different population
segments. This could potentially be accomplished by removing
animals from different parts of the range.
A variety of factors can lead to increased rates of genetic
diversity loss. After accounting for population size, the most
important factors are likely to be non-random mating (i.e., a few
bulls are responsible for siring most calves), skewed sex ratios,
and large variation in population size.
9.2.1 Guidelines that apply to
most conservation herds
Very few conservation herds will persist without the need for
some form of population control. Many guidelines in this
chapter were included with the specific intent to support
development of informed population management plans. Many
of the following guidelines apply to most conservation herds,
and are likely to be included in comprehensive management
plans for conservation herds:
1) Maintain a sex ratio with neither sex constituting more
than 60% of the population. Ideally, the adult sex ratio will
be slightly female biased (e.g., 55 cows per 100 animals),
reflecting observations that mortality rates of males tend
to be slightly greater than those for females. Avoiding a
high ratio of females to males helps ensure participation in
mating and transfer of genetic diversity by a larger number
of bulls. In large populations, mating competition will likely
be sufficient when there are 20 or more mature bulls (six
years old and older) per 100 cows. Maintaining mating
behaviour, as noted above, calls for a more equal sex ratio.
2) Avoid removing a significant proportion of the population.
For populations subjected to population control actions,
culling should be on a yearly, or every other year, schedule,
rather than periodically at longer intervals. We cannot
offer a definitive definition of ‘significant’, as the effects
of population fluctuations will be greater as population
size diminishes and varies with other circumstances. As a
general guideline, we suggest limiting removals of animals
to less than 30% of the population;
3) Avoid disproportionate removal of matrilineal female
groups (mother cows and their preparturient daughters).
More specifically, attempt to retain the older cows
matrilineal groups;
4) Remove animals from all spatial segments of the
population;
5) Emulate natural mortality patterns—higher mortality/
removal rates for juveniles and old age classes (more than
15 years);
6) In small populations, consider actions that reduce variation
in the breeding success among individuals. This could be
accomplished by reducing the opportunities for continued
breeding by highly successful bulls.
7) Avoid human selection for market traits such as docility,
carcass composition, body shape, or productivity, as such
interventions contradict natural selection and conservation
of genetic variability;
8) Routine supplemental feeding to increase productivity, or
to support a population size that exceeds range carrying
capacity, is discouraged for conservation herds;
9) Where practical, the full suite of natural limiting factors
should be allowed to influence populations, including
winter deprivation and predation. This will result in variable
rates of reproduction and survival.
The need for active genetic management will vary with herd
size, genetic composition, and management goals. In general,
genetically diverse herds with more than 1,000 animals are
unlikely to require active management to retain most of their
genetic diversity for the next 200 years (Gross et al. 2006).
Hedrick (2009) suggests a herd size of 2,000-3,000 to avoid
inbreeding depression. In very small herds (fewer than about
250 animals), long-term genetic health will require occasional
supplementation with genetic material from other herds. The
exact number of animals needed to supplement a particular herd
will vary with the genetic composition of the source and target
herds, but a supplement of four to five breeding animals per
decade should be sufficient for long-term herd genetic health
(Wang 2004). In addition to the guidelines below, managers
should follow the IUCN guidelines for translocation of wild
animals between established herds, being especially careful
about genetic purity (i.e., cattle genes and geographically
appropriate sources of stock) and diseases (http://www.kew.
org/conservation/RSGguidelines.html).
Active management to retain genetic variation (other than
translocations) may be most important for intermediate-sized
populations with about 250-750 animals because this is the
size range where active management may prevent or greatly
reduce the need for translocating animals to ensure long-term
the genetic health of a herd (Gross et al. 2006). For conservation
herds, the overall objective is to retain allelic diversity, which
is the best indicator of the genetic resources available to the
population. By contrast, genetic heterozygosity may be a better
short-term indicator of the mating structure of the herd. In
addition to the guidelines provided above, removal of young
animals, prior to their first breeding, can significantly enhance
the retention of genetic diversity (Gross et al. 2006). Removal
of young animals to preserve genetic diversity may seem
counterintuitive. Genetic material is lost only when animals in a
population are replaced. Removal of young animals increases
the length of the generation (replacement) interval, and this
thereby prolongs the retention of genetic material.
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