Summary

RecommendationsBLM Wild Horse

and Burro

Population

Viability Forum,

April 21, 1999

by Linda Coates-Markle,

Montana/Dakotas Wild Horse

and Burro Specialist,

Bureau of Land Management,

Montana State Office

The final Note in a series 

of 13, Session 4

RECOMMENDATION #1: 

BLM should carefully consider its mandate (The Wild Free-Roaming Horse

and Burro Act) with respect to longterm genetic viability of populations of

wild horses and burros.

EXISTING POLICY: BLM regulations and policy state that wild horses

and burros shall be managed as viable,

self-sustaining populations of healthy

animals in balance with other multiple

uses and the productive capacity of

their habitat (CFR 4700.0-6).

DEFINITION: Self-sustaining refers

to the process whereby established

populations are able to persist and

successfully produce viable offspring

which shall, in turn, produce viable

offspring, and so on over the long term.

The absolute size which a population

must attain to achieve a self sustaining

condition varies based on the demographic and sociological features of the

herd (and adjoining herds), and these

aspects should be evaluated on a case by

case basis. In many cases it is not necessary that populations be isolated genetic

units, but both naturally-occurring and

management-induced ingress and egress

activity can be considered, in order to

maintain sufficient genetic diversity

within these populations.

DISCUSSION: Reproductive capacity is, to a large degree, dictated by

the genetic fitness of a population.

Generally speaking, the higher the

level of genetic diversity, within the

herd, the greater its long-term reproductive capacity. Inbreeding, random

matings (genetic drift), and/or environmental catastrophes can all lead to

the loss of genetic diversity within the

population. In most herds, though,

genetic resources will tend to be lost

slowly over periods of many generations

(~10 years/generation), and there is

little imminent risk of inbreeding or

population extinction. Potential negative consequences of reduced diversity,

however, may include reduced foal production and survival, as well as reduced

adult fitness and noted physical deformities. Smaller, isolated populations

(<200 total census size) are particularly

vulnerable when the number of animals participating in breeding drops

below a minimum needed level. This

minimum level can be calculated and

is different for each population (see

subsequent recommendations).

RECOMMENDATION #2:

BLM should continue to use (and

improve upon) defensible scientific aerial and/or ground survey techniques in

census activities for all managed wild

horse and burro herds. In order to fully

evaluate genetic viability issues, populations which participate in a measurable

level of natural ingress or egress activity

and which are, in reality, a component

of larger metapopulations, should be

identified, and the genetic impact of

this activity should be estimated.

EXISTING POLICY: BLM regulations and policy state that HMAs

should be inventoried and monitored

for population size, animal distribution, herd health and condition and

habitat characteristics at least every 4

years (CFR 4710.2). As such, BLM is

required to provide reliable estimates of

population size and distribution within each herd management area on a

regular interval.

DEFINITION: Metapopulation refers

to two or more local breeding populations which are linked to one another by

dispersal activities of individual animals.

These populations may have unique

demographic features (birth and death

rates) but ultimately may share some

genetic material if interbreeding is

occurring between individuals. This

sharing of genetic material may act to

enhance genetic diversity within participating herds, and as such, these

populations should be evaluated as

one larger metapopulation.

DISCUSSION: A complete population census of each herd management

area is unrealistic, especially for the larger populations (>200 total census

size). However, population size can

and should be estimated using reliable

scientific techniques. These survey

techniques are under continual revision

and BLM continues to participate in

these research efforts. On a more critical

level, however, is the determination of

size of the many smaller populations

(<200 total census size) over which

BLM has responsibility. Available data

indicates that almost 70% of the managed herds have AMLs (appropriate

management levels) set at 150 animals

or less. In fact, almost 40% of the herds

in Nevada, Utah, Wyoming, Colorado,

and Arizona (71 out of 177 total

HMAs) are indicated to have population sizes of less than 50 animals. There

is a real possibility that some of these

populations will be unable to maintain

self-sustaining reproductive ability, over

the long term, unless there is a natural

or management induced influx of

genetic information from neighboring

herds. An exchange of only 2 to 3 breeding age animals (specifically females),

every 10 years, is often sufficient to

maintain genetic diversity within a given

herd. Estimates of existing genetic diversity can be calculated for each wild horse

and burro population (see subsequent

recommendations).

RECOMMENDATION #3:

BLM should establish baseline genetic

diversity information for each population over which it has management

responsibility.

RESOURCE NOTES

NO. 35 DATE 08/01/00

Range

Wild Horse andBurro Program
EXISTING POLICY: BLM regulations and policy state that HMAs

should be inventoried and monitored

for population size, animal distribution, herd health and condition and

habitat characteristics at least every 4

years (CFR 4710.2). Furthermore, the

purpose of monitoring is to collect

data necessary to evaluate progress (or

lack thereof) in achieving the objectives

of management. Within the context

of wild horse and burro populations,

the ability to maintain the quality of

“reproductively self sustaining” is

required. This can primarily be

accomplished through evaluation and

the maintenance of an acceptable level

of genetic diversity within the population over the long term.

DEFINITION: Establishing baseline

genetic diversity, for a wild horse population, often refers to typing up to 29

genetic marker systems from a sample of

individual animals (~25 individuals or

up to 25% of the population) within a

specific herd. Traditionally, these marker

systems have included blood group and

biochemical systems, and have required

fresh blood samples. These systems

were originally developed for verifying

parentage or founder animals within a

herd. Analysis of genetic diversity, however, can also be done through the use

of DNA genetic marker systems, and

direct testing can utilize almost any bodily product including hair or even feces.

Only DNA marker analysis can be used

for burros, however, due to the very

limited variation in blood protein genes.

DISCUSSION: Most wild horse

herds, sampled to date, have shown

fairly high levels of genetic diversity. In

some cases, however, this diversity is

attributed to a large number of low

frequency and relatively rare genetic

material which is often easily lost

from the herd. Thus, it becomes

important to understand the genetic

makeup of individual herds. Baseline

data needed to establish current levels

of genetic diversity in populations is

relatively easy to gather. Individual

samples cost about $25 to process, and

if ~25-50 individuals are sufficient to

establish baseline information for herds

ranging in size from 100 to 200 animals, then the cost would be approximately $1250 for herds of this size. As

a result, a comparison of genetic viability levels in the tested population can

be made to existing information from

over 100 domestic and wild horse

populations representing different

herd sizes and demographic backgrounds.

RECOMMENDATION #4:

BLM should establish a realistic management goal for maintenance of

genetic diversity within all managed

populations. Previous wildlife conservation research, and current efforts

with wild horses, suggest management

should allow for a 90% probability of

maintaining at least 90% of the existing population diversity over the next

200 years. Existing diversity should be

sufficient to ensure a self-sustaining

(see earlier definition) reproductive

capacity within the herd.

EXISTING POLICY: BLM regulations and policy state that wild horses

and burros shall be managed as viable,

self-sustaining populations of healthy

animals in balance with other multiple

uses and the productive capacity of

their habitat (CFR 4700.0-6). By definition this requires BLM to manage to

allow established populations to successfully produce viable offspring

which shall, in turn, produce viable

offspring, and so on over the long

term. This suggests that management

monitor levels of genetic diversity

within the population in order to

mitigate the effects of genetic drift

and possible inbreeding and population-associated problems due to loss

of diversity.

DEFINITION: Genetic diversity,

within wild horse and burro populations, refers to the entire complement of

genetic material representative of all

individuals (or a sample of individuals)

from within the population. Some

populations may possess genetic uniformity to a certain “type” or breed of

horse, but management interests are

specific to maintaining a maximum

diversity of genetic material which

appears representative of each herd.

Promotion of diversity will minimize

the effects of genetic drift, or the random loss of genetic material due to

mating processes, and maximize genetic health of the herds.

DISCUSSION: Once baseline genetic

data has been established, the main

focus of genetic management, especially for the smaller populations (<200

total census size), becomes the attempt

to preserve as much of the existing

genetic diversity as possible. Establishing

a genetic conservation goal will require

re-testing of herd diversity on at least

a five-year cycle, with subsequent evaluations of the potential impact of

management decisions (including

the establishment and/or revision of

appropriate management levels) on

that diversity. Management may need

to evaluate ways to introduce genetic

material into a herd which appears

genetically deficient in order to be

self-sustaining over the long-term (see

subsequent recommendations).

Baseline genetic data can also be incorporated into PVA (population viability

analysis) models, which attempt to

predict the impact of management

decisions (as well as environmental catastrophes) on existing diversity levels.

Most models require reasonably accurate data in terms of age class foaling

and mortality rates, as well as individual genetic information. As such, the

means to collect accurate data necessary for a genetically-based PVA, for

most herds, is probably unavailable

at the present time.

RECOMMENDATION #5: 

BLM should, in its efforts to evaluate

the genetic diversity and self sustaining

nature of managed herds, estimate the

genetic effective population size (Ne) of

all populations, or metapopulations,

with a total census size of 200 animals

or less.

EXISTING POLICY: BLM regulations and policy state that wild horses

and burros shall be managed as viable,

self-sustaining populations of healthy

animals in balance with other multiple

uses and the productive capacity of their

habitat (CFR 4700.0-6). By definition this requires BLM to manage to

allow established populations to successfully produce viable offspring which

shall, in turn, produce viable offspring,

and so on over the long term. 

DEFINITION: The genetic effective

population size (Ne) is a measure of

the total number of mares and stallions

Range
which contribute genetically, through

successful breeding, to the next generation. Although no standard goal for Ne

currently exists for wild horse and

burro herds, a goal of Ne=50, which

comes from domestic breeding

guidelines, can be conservatively

applied. Populations, where Ne is

calculated to be less than 50, may

experience higher rates of loss of

genetic diversity than would be considered acceptable under recommended

management goals (see earlier recommendation).

DISCUSSION: Limited research into

wild horse herds (Pryor Mountain Wild

Horse Range and Assateague Island

National Seashore populations) has

demonstrated that the “Ne”, for a herd

under a natural age structure, is about

30-35% of the total census population

size. In other words, a total population

size of about 150 animals might support only a minimum (Ne=50) genetic

effective population size. Ne, however,

is difficult to calculate for wild horses,

since the calculation is complicated by

a number of issues. The harem structure of the population, for example,

greatly limits male participation in

breeding, creating an uneven ratio of

breeding sexes which reduces Ne and

contributes to a high variation in individual reproductive success. Extreme

fluctuations in population size, due to

the effects of removals, can also act to

reduce the value of Ne. Ne is also

highly influenced by the sex ratio and

age class structure of a population. A

sex ratio which favors males and results

in larger numbers of smaller sized

harems, within the herd, will act to

increase Ne (and male participation in

breeding) to a point. A population

with an age structure involving high

numbers of young animals (<5 years of

age) will have a lower value of Ne than a

similar sized population with a larger

component of older breeding-age animals (>5 years of age). Also, there is no

single, uniformly accepted method to

calculate Ne. However, researchers have

used and applied several formulas to

certain wild horse herds and have

found this comparative approach to

provide the best estimates. Generally,

the best possible data on population

sex ratios and age structures, coupled

with reasonable estimates of foaling

and mortality rates, will enable managers to evaluate the genetic health of

most herds.

RECOMMENDATION #6: 

BLM should evaluate viable management alternatives for conserving or

enhancing genetic diversity within

populations (or metapopulations)

having a known limited level of diversity, a total census size of less than 200

animals and/or an estimated genetic

effective population size (Ne) of less

than 50.

EXISTING POLICY: BLM regulations and policy state that wild horses

and burros shall be managed as viable,

self-sustaining populations of healthy

animals in balance with other multiple

uses and the productive capacity of

their habitat (CFR 4700.0-6). By definition this requires BLM to manage to

allow established populations to successfully produce viable offspring

which shall, in turn, produce viable

offspring, and so on over the long

term. This suggests that management

monitor levels of genetic diversity within the population in order to mitigate

the effects of genetic drift and possible

inbreeding.

DEFINITION: Viable management

alternatives for conserving genetic diversity within managed wild horse and

burro herds may take several forms.

Some options to be considered might

include: altering population age structure (through removals) to promote

higher numbers of reproductively-successful animals; altering breeding sex

ratios (through removals) to encourage a

more even participation of breeding

males and females; increasing generation

intervals (and reducing the rate of loss of

genetic material) by removing (or contracepting) younger versus older mares;

and/or introducing breeding animals

(specifically females) periodically from

other genetically similar herds to help in

conservation efforts. In this last scenario,

only one or two breeding animals per

generation (~10 years) would need to be

introduced in order to maintain the

genetic resources in small populations of

less than 200 animals.

DISCUSSION: Simply increasing the

total herd size by adding additional animals (adjusting the management AML

upward) is not the only viable technique

for enhancing the genetic effective population size (Ne) of a wild horse and

burro population. With sound

knowledge of existing herd demographic information, management

alternatives for specific populations

can be evaluated through research

modeling efforts. As such, management also has the option of adjusting

certain aspects of herd structure in

order to promote genetic conservation. Major options for consideration

were presented in the above definition. It should also be noted that any

adjoining herds, which are naturally participating in an exchange of animals and

genetic material through interbreeding,

are probably self-maintaining their

genetic diversity and management

should consider both supporting and

estimating this type of activity.

RECOMMENDATION #7:

BLM should continue to evaluate

incidences of club foot and parrot

mouth, and other such physical deficiencies, within individuals of wild

horse and burro populations, on a

case by case basis. Currently, there is

no solid evidence that these physical

conditions are purely geneticallybased and that they may contribute

to a long-term loss of genetic health

in the herd.

EXISTING POLICY: BLM regulations and policy state that HMAs

should be inventoried and monitored

for population size, animal distribution, herd health and condition and

habitat characteristics at least every 4

years (CFR 4710.2). Furthermore, the

purpose of monitoring is to collect

data necessary to evaluate progress (or

lack thereof) in achieving the objectives of management.

DEFINITION: Physical deficiencies

may be encountered at different rates for

different wild horse and burro herds.

Conformational deformity and/or misalignment is often expressed in the legs,

feet and mouth but may be apparent

in other structural areas as well.

Range
Despite the existence of a specific deficiency, however, an individual animal

may otherwise be healthy, bear acceptable condition and be fit enough to

contribute socially and genetically to

the herd. If an individual animal is

successful in these merits, there seems

little reason to remove it simply on the

grounds of physical imperfection by

human standards.

DISCUSSION: These types of physical deficiencies are thought, by

researchers, to potentially be both

genetically and environmentally (poor

forage base during fetal development)

induced. As such, efforts to remove

individual animals bearing this condition from herds may or may not result

in a significant loss of expression of

that trait from the herd. Success in this

area will be related to the source of the

genotype and whether it results from

inbreeding, founder effect, and/or

genetic drift. However, since multiple

genes are probably responsible for the

expression of these traits, it is likely

that the genetic predisposition will

remain in the herd despite the fact

that minimal expression is observed.

Eventually over time, then, the trait

may continue to reappear. Future

research may illuminate different theories regarding this situation. In the

meantime, the impact of humaninduced selection, over factors of natural

selection, should be evaluated carefully

and with due consideration as to the

possible long-term impacts on individual herds. In other words, if the animal

is otherwise healthy, maintaining a status within the social structure of the

herd, and contributing to the gene pool

through successful breeding, it might be

left on the range. However, if a population excess has been determined, and

an individual animal is young and has

yet to contribute to the gene pool, it

may be a candidate for removal and

adoption or sanctuary-placement.

Likewise, if the animal is older, less

healthy, and has withdrawn from the

herd, it may also be a candidate for

removal with sanctuary placement.

RECOMMENDATION #8:

BLM should continue to manage wild

horse and burro herds, beneath the

level which is scientifically referred to

as the ecological carrying capacity (EEC)

of the population. This is the level at

which science has determined that density-dependent population regulatory

mechanisms would take effect within

the herd. Most herds are currently

managed close to their “economic

carrying capacity” which is approximately 50-65% of EEC. At this level

of management, health of both the

horse herd and range ecosystem are

prioritized.

EXISTING POLICY: BLM regulations and policy state that wild horses

and burros shall be managed as viable,

self-sustaining populations of healthy

animals in balance with other multiple

uses and the productive capacity of

their habitat (CFR 4700.0-6). Thus

appropriate management levels (AMLs)

are established which provide for a level

of use by wild horses and burros which

results in a thriving natural ecological

balance and avoids deterioration of

the range. Furthermore, proper management requires that wild horses and

burros be in good health and reproducing at a rate that sustains the population

and that population control methods be

considered before the herd size causes

damage to the rangeland.

DEFINITION: Ecological carrying

capacity (EEC) of a population, is a

scientific term which refers to the level

at which density-dependent population

regulatory mechanisms would take

effect within specific herds. At this

level, however, the herds would show

obvious signs of ill fitness including

poor individual animal condition,

low birth rates, and high mortality

rates in all age classes due to disease

and/or increased vulnerability to

predation. In addition, supporting

range conditions would be noticeably

deteriorated, with much of the available habitat showing symptoms of

irreparable over-grazing.

DISCUSSION: Populations of wild

horses on western rangelands have the

capacity for rates of increase as high as

20-25% per year. Recent research has

shown that unmanaged populations

of wild horses and/or burros might

eventually stabilize (due to densitydependent regulatory mechanisms)

at very high numbers, near what is

known as their food limited ecological

carrying capacity. At these levels, however, the herds would show obvious

signs of ill-fitness including poor individual animal condition, low birth

rates, and high mortality rates in all

age classes due to disease and/or

increased vulnerability to predation. In

addition, supporting range conditions

would be noticeably deteriorated, with

much of the available habitat showing

symptoms of irreparable over-grazing.

Most wild herds are currently managed close to economic carrying

capacity which allows the herds to be

healthy with strong foal production

and high individual survival rates. This

approach should be continued, as it

benefits the populations and also

allows for the maintenance of healthy

and in-balance rangeland systems.

Contact

Linda Coates-Markle

Program Specialist

Montana State Office, MT-010

Billings, Montana

phone (406) 896-5223

fax (406) 896-5281

email lcmarkle@mt.blm.gov

Range

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