Viability of Feral

Horse Populations

on Atlantic

Coastal Barrier

Islands:

Implications for

Management

By: H. Brian Underwood

USGS Patuxent Wildlife

Research Center, College of

Environmental Science &

Forestry, Syracuse, NY.

The eleventh in a series of 13,

Session 4

Background

Feral and free-ranging horses

have a long and illustrious

legacy common to many

Atlantic coastal barrier

islands. I analyzed the population dynamics of the horses

of Assateague Island National

Seashore (ASIS), which

encompasses the northern 28

km of the island. Here the

horses live a wild existence

and are managed as a living

cultural resource. Using

population reconstruction

methods and computer

simulation of feral horse

life-history, I developed a

demographic population viability analysis (PVA) for this

population for the expressed

purpose of assessing the

impacts of fertility control,

disease epidemics, and catastrophic storm events on population persistence. Partial

pedigrees were constructed to

examine lineage-specific

effects on population demography and genetic zygosity.

Discussion 

The reconstructed population varied from a low of 35

horses in 1975 to a high of

171 in 1988, and grew at an

average annual rate of 15%,

which by western experience

is sub-par, but matches earlier

estimates of >10%. Two-hundred forty-seven observations

of survival histories (58 uncensored) of horses demonstrated

that reconstructed March population size added significant

prognostic value to the prediction of hazards for ASIS

horses. There were no statistically detectable differences in

survival between the sexes

and no dramatic differences

in the age at first foaling over

time. Analysis of the interbirth interval revealed a

strong population size influence, however. There was a

pronounced change in the

early-age hazards as population

size increased, an observation

unique among wild horse

population studies. Adult survival changed very little and

remained quite high over a

wide range in population

sizes. A meaningful population decline induced through

fertility control could be

approached at efficacies of

$80% sustained over a period

of 10 years. This translates

into treating about 60 mares

each year. Interestingly, just 3

of 17 progenitor mare matrilines were responsible for

most of the foals born into

the population over the 18

years examined in this study.

Conclusion

The levels of adult horse

mortality due to storms, diseases and accidents, which

are impossible to predict, had

the most profound effect on

population growth and viability. The simulation also

suggested that management

directed at specific matrilines

exerts a disproportionate

effect on rate of increase,

population viability and

genetic zygosity. As a precursor to management, PVA

forces one to consolidate the

existing knowledge base,

explore management alternatives, and identify weaknesses

in the data that might compromise biological inferences

drawn from the exercise.

While a prudent first step,

PVA is not suited for every

purpose. Managers should

RESOURCE NOTES

NO. 33 DATE 08/01/00

Range

Wild Horse andBurro Program
Range

Resource Notes are intended to be early

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consult with a population

biologist who (1), has firsthand experience with the various approaches and, (2) can

facilitate and incorporate

knowledge from the field.

Contact 

Dr. H. Brian Underwood

Patuxent Wildlife Research

Center, Cooperative Park

Studies Unit, 350 Illick Hall

State University of New York

College of Environment

Science & Forestry

Syracuse, NY 13210-2778

phone (315)470-6820

fax (315)470-6934

e-mail: hbunderw@syr.edu