Effects of

Contraception and

Removal Treatments

on Pryor Mountain

Wild Horse

Population

Demographics and

Genetics

By: John E. Gross

CSIRO Davies Laboratory

Queensland, Australia

The twelfth in a series of 13,

Session 4

Background

Most free-ranging horse populations in the United States

inhabit Bureau of Land

Management (BLM) lands

and the BLM is mandated to

manage these herds. Wild

horse populations have the

ability to rapidly increase in

size, and land managers are

thus faced with the need to

control the size of a rapidly

increasing population.

Because the management

objective for many horse

populations is small (<150

horses), there are also concerns about the long-term

genetic viability of these

small populations. Managers

are thus faced with conflicting

needs to minimize population

size to control habitat damage or forage use, and to

maximize population size to

preserve genetic variation.

To evaluate consequences of

population control strategies

that used contraception,

periodic removals, or a

combination of the two, I

developed a population viability model that simulated

population dynamics and

changes in genetic variation

and applied the model to

horses on the Pryor Mountain

Wild Horse Range. Simulated

management actions were

applied to horses that were

young, old, or a random

selection from the population.

Discussion

Computer simulations were

used to evaluate management

alternatives that included

changes in population objective (AML), contraceptive

treatment, and removals.

Management actions (contraception and/or removal)

focused on treatment/removal

of young horses, old horses,

or a random selection of

individuals. Population

responses included average

population size, variation in

population size, population

age structure, growth rate,

changes in genetic variation

(heterozygosity), and loss of

alleles.

Model results showed that

simulated control strategies

had striking differences in

terms of population structure

and persistence of genetic

diversity. When comparing

contraceptives to removal, it

was found that use of contraceptives can greatly reduce the

variation in yearly population

size, and average population

sizes remained much closer to

the objective. However, population growth rate is relatively

insensitive to low levels of

infertility, and there are sharp

contrasts between the dynamics of populations controlled

by harvest or contraception.

With contraception, populations can increase rapidly

if a high level of infertility

is not achieved, but population size declines slowly

because natural mortality is

typically low. In contrast,

removals permit managers

to rapidly reduce population size, modify sex ratio,

or adjust the age structure

of the population. For most

wild horse populations,

about 70% of all reproductively active females will

need to be maintained in

an infertile state to achieve

a stable population size.

For the conditions simulated,

the age of animal treated had a

huge impact on the rate of loss

of genetic variation. Control

RESOURCE NOTES

NO. 34 DATE 08/01/00

Range

Wild Horse andBurro Program
strategies that delayed the age

of first reproduction, such

as contraception or removal

of young animals, greatly

reduced loss of genetic variation. In contrast, strategies

that emphasized reproduction

by young animals (contraception or removal of old horses)

increased the rate of loss of

genetic diversity. To retain

genetic diversity, a particularly

poor suggestion is to permit a

horse to reproduce and then

rendering it infertile. These

effects result from changes in

generation time – rate of loss

of genetic variation is directly proportional to generation

time, and strategies that

increase generation time (e.g.,

delaying reproduction) reduce

the rate of loss of genetic

resources. For the sizes of

populations simulated

(AML of 90 to 180 horses),

the age of treatment had a

greater effect than an overall

change in AML (total population size).

Conclusion

Regardless of control strategy,

genetic variation is lost much

more slowly if young animals

are treated (e.g., removed or

rendered temporarily infertile).

The most practical control

program will likely involve

both contraceptives and periodic removals. Contraceptives

can reduce growth rate and

are likely to be cost-effective,

while removals permit managers to rapidly adjust sex

ratio, age structure, or overall

population size. If singleyear contraceptives are used

to maintain infertility, a

very intensive management

program will be necessary. In

small populations, changes in

the age of horses treated will

likely have more impact on

loss of genetic variation than

changes in the number of

horses in the population.

Contact

John E. Gross, PhD.

CSIRO Davies Laboratory

PMB Aitkenvale

Queensland, 4814 Australia

phone 61 7 4753 8509

fax 61 7 4753 8600

email:

John.Gross@tag.csiro.au

Range

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