Genetic Effective

Population Size in

the Pryor Mountain

Wild Horse Herd:

Implications for

conservation genetics and viability

goals in wild horses

By: Francis J. Singer and

Linda Zeigenfuss

Biological Resources Division

of US Geological Survey,

Natural Resources Ecology Lab,

Colorado State University

The seventh in a series of 13,

Session 2

Background

Genetics are typically presumed

to be the least important component of minimum viable

population predictions (and the

population viability analysis

process), and catastrophe is the

most important. Catastrophe

can be guarded against with

large populations of longer predicted persistence times, but

also with better management of

any given population. Consider

the concepts of food-limited

ecological carrying capacity

(hereafter ECC) and economic

carrying capacity. The tarpan

and Przewalski’s wild horses of

Europe and Asia might have

been limited by predation by a

combination of wolves, brown

bears and one or more large

cats, but predation (mostly by

mountain lions) is significant

in only a very small number of

wild horse herds in the US west.

Most herds grow at phenomenal

rates, for ungulates, of 16-22%

per year. We observe that most

wild horse herds are managed

close to economic carrying capacity (which is typically 50-65% of

ECC in numbers) and, at this

lowered population level, animals

are in better body condition,

survival is higher (there is less

starvation or dehydration),

recruitment is higher, there is

less conflict with other vertebrates and soil and vegetation

resources, population fluctuations

are less, and there is less risk of a

resource-limited catastrophe.

Furthermore, while genetics is

not a consideration in many

free-ranging vertebrates, genetic

conservation will become a serious consideration over future

decades in wild horse management since so many of the herds

are now isolated and small. In

the Intermountain West region,

61% of all wild horse populations numbered less than 100

and 41% numbered less than

50 animals. Herds managed at

these low numbers for decades

might become inbred.

Discussion

Evidence from the Pryor

Mountain wild horse herd

supports the hypothesis that

long-term management of

wild horse numbers below the

unmanaged maximum, has

resulted in improved wild horse

conditions, apparently improved

range conditions, and a lower

probability of a large starvation

losses. In the 1990’s, foal rates

were higher, 0.608, for the

youngest breeding mares (3-5

year-olds), the most sensitive

group to better conditions, than

in the 1970-86 period, 0.392

(P=0.048). Population growth

rates did not differ statistically

between the periods (P=0.528),

but numerical growth rates were

higher (lambda =1.176) in the

1990’s than in the 1970-86 period (lambda =1.119) and the

coefficient of variation on growth

rates was less (3.21 vs. 17.09) in

the 1990’s. Genetic effective

population size (commonly

referred to as Ne) is defined as

the number of breeding individuals (both male and female) that

contribute to the next generation. Ne is a useful number

since it can be used to calculate

the loss of genetic variation

through genetic drift and/or

inbreeding from one generation

to the next with the formula

1/4Ne. But Ne is a difficult

number to calculate for wild

horses, since the calculation is

complicated by overlapping

generations, a harem structure

greatly limiting male participation in breeding (an uneven ratio

of breeding sexes reduces Ne),

high variance in reproductive

success of both sexes, population

fluctuations due to removals, and

by a typical failure to breed until

the age of 3 years for mares and 7

years for stallions. No single, universally acceptable formula exists

to deal with these complexities.

We studied the population and

mating dynamics of the Pyror

RESOURCE NOTES

NO. 29 DATE 07/26/00

Range

Wild Horse andBurro Program
Mountain wild horse herd,

1992-98, in collaboration with

a local volunteer (Rev. Floyd

Schwieger) and Linda CoatesMarkle of the Bureau of Land

Management, Billings Resources

Area. We found the 7 estimators

for Ne averaged 27% of the census population size (N) until

1994. Birth sex ratio and management scenarios that resulted in

more males on the range and

smaller average harem size since

1994 (which increased male participation in breeding) increased

Ne to 36% of census N.

The complexities of the loss of

genetic resources were also investigated through a contract from

USGS to John Gross, Colorado

State University, who used a

individually-based, stochastic

simulation model. Management

actions that were simulated

included population sizes that

varied from 130 to 1000,

removals that were random or

focused on young (0-8 years) or

older (12-25 years) animals, and

contraceptive applications that

focused on young, or older

horses. Simulations showed that

populations managed with a target size of fewer than 500 horses

were at some risk of losing more

than 90% of selective neutral

genetic variation over a long

period of 200 years. Populations

of more than 500 horses were at

low risk. Removals or contraceptions that focused on young

animals that delayed the average

age of reproduction (and

increased generation length) better maintained genetic diversity.

We are currently investigating

the complexities of the simple

calculations of Ne through

detailed stallion parentage

studies using DNA (to test

how many foals are sired by

harem stallions) and to test if

high genetic heterozygosity is

maintained through selection

(i.e. is positively correlated to

high reproductive success). This

work is in collaboration with Gus

Cothran of the Equine Studies

Lab, University of Kentucky

through a contract to USGS.

Conclusion

No standard goal for Ne or for

loss of genetic resources currently exists for wild horse herds. If

a goal of Ne=50 was applied,

the goal for maintenance of

domestic livestock production

and thus probably an absolute

minimum for a population in

the wild, census N would need

to be in excess of 139-185 wild

horses, the excess to account for

3-5 removals per wild horse

generation. Management could

greatly alter this relationship by:

(a) altering breeding sex ratios to

increase Ne through removals,

(b) increasing generation length

through removal scenarios (which

reduces the rate of loss of genetic resources, or (c) introducing

breeding animals periodically

from other genetically similar

herds to maintain genetic

resources. Only one to two

breeding animals per generation

(= about every 10 years in wild

horses) would maintain the

genetic resources in small populations of about 100 animals,

thus obviating the need for larger

populations in all cases. We stress

that there is little imminent risk

of inbreeding since most wild

horse herds sampled have large

amounts of genetic heterozygosity, genetic resources are lost

slowly over periods of many

generations, and wild horses are

long-lived with long generation

interval.

Contact

Francis Singer, PhD.

Biological Resources Division,

USGS, Midcontinent Ecological

Science Center, USGS, and

Natural Resource Ecology Lab

Colorado State University

4512 McMurray Avenue

Fort Collins, Colorado 

80523-1499

phone 970-491-7056

fax 970-491-1965

email: francis@nrel.colostate.edu.

Range

Resource Notes are intended to be early

announcements of technical and informational topics for

Bureau of Land Management personnel and some of their customers.

Information in this Resource Note is based on the opinion and

experience of the author and has not been peer-reviewed. Conclusions

and opinions expressed herein do not

necessarily represent those of BLM. Use of trade names does not

imply U.S. Government endorsement of commercial products.

If you have received a copy of or found out about Resource

Notes in an indirect way and would like to be included in future

mailings, please send the following:

NAME, TITLE, MAILING ADDRESS and a list of the two or three

subject areas that you are most interested in or that most directly

relate to your job. Send this information to Phil Dittberner, BLM,

RS-140, P.O. Box 25047, Denver, CO. 80225-0047 or

phil_dittberner@blm.gov or FAX 303-236-3508.

If you would like to prepare a Resource Note for

distribution, or you have an idea and author in mind for a good

Resource Note, please contact Phil Dittberner at

303-236-1833, FAX 303-236-3508 or phil_dittberner@blm.gov with

the topic and the name of writer, including an address, phone number, and e-mail address.

Thank you for your interest in Resource NoteS.