Density Dependence

in Population

Dynamics of Feral

Horses

By: Stephen H. Jenkins, Ph.D.

Department of Biology/314

University of Nevada

The fourth in a series of 13,

Session 1

Background

Populations of wild horses on

rangelands in the western U.S.

have the capacity for rates of

increase as high as 20 to 25%

per year. For this reason, intensive and expensive management

is often necessary to reduce

populations periodically to

appropriate management levels. The effects of density on

population dynamics of wild

horses are not presently known

although, in other species, negative density dependence can

contribute to population stability. Negative density dependence

implies that birth rate decreases

or death rate increases as the

density of a population increases. Several mechanisms may

contribute to these processes,

including competition for food,

disease or parasitism, social factors, and possibly predation.

Demonstrating the existence

of density dependence in populations of wild horses doesn’t

preclude the need for active

management, but may influence optimal decisions about

management.

Discussion

Two general methods can be

used to test for density dependence in population dynamics:

experimental manipulations

and statistical analyses of density estimates for unmanipulated

populations. Choquenot (1991)

showed experimentally that

juvenile survival, and hence rate

of population growth, depended on density for feral donkeys

in Australia. However, this

experiment was unreplicated,

and rigorous experimental studies of the population dynamics

of large, wide-ranging mammals

such as wild horses and burros

are difficult in general.

Therefore, I used time-series

analysis of data for seven populations of wild horses in the

western U.S. to look for evidence of density dependence.

Data for two of these populations came from intensive

ground surveys of known individuals (Berger, 1986 and L.

Coates-Markle, personal communication). The remaining

data came from aerial censuses.

Despite the large amount of

effort expended by BLM in censussing populations of wild

horses, few available data sets

meet the assumptions necessary for rigorous statistical tests

of the hypothesis of density

dependence. These assumptions

are that the population should

be relatively closed to immigration and emigration, that there

should be a series of counts in

the same season of successive

years without intervening

removals, that counts should

not be gross underestimates

because of poor sighting conditions, and that there should not

be illegal gathers and removals

during the sampling period.

There were 5 to 21 years of

usable data for the seven populations analyzed. I found negative

relationships between growth

rate and population size for all

seven cases, although only one

was statistically significant at the

conventional 5% level. However,

I used a form of meta-analysis

(Rice, 1990) to calculate that

the overall probability of getting

the complete set of results purely

by chance was only 0.001. If

these seven populations are

representative of others in the

western U.S., this is strong

evidence of negative density

dependence in wild horses.

Conclusion

If the negative density dependence found in this study is

strong enough in comparison to

other factors that impact population dynamics, such as severe

winters or droughts that cause

density-independent mortality,

then unmanaged populations of

wild horses might eventually stabilize at their ecological carrying

capacities. However, obvious

degradation of habitat might

occur at these population levels;

i.e., appropriate management

levels for horses may be substantially lower than ecological

carrying capacities. Nevertheless,

this analysis of density dependence implies that appropriate

management levels should be

RESOURCE NOTES

NO. 26 DATE 07/20/00

Range

Wild Horse andBurro Program
carefully justified by sound scientific research, in order to forestall

the potential argument that

populations should simply be

allowed to stabilize on their own.

Negative density dependence

also implies that population

growth rate increases as density

decreases. This means that

small populations may be able

to recover more rapidly than

expected based on their performance at higher densities. BLM

should continue to monitor

populations with aerial censuses;

whenever possible, these censuses should be done in the same

season of two or more successive years without intervening

removals in order to establish

more clearly the details of density

dependence so that more accurate population models can be

developed. In addition, we need

to collect specific data on the

effects of density on survival and

reproduction. The improved

models resulting from these data

will contribute to better prediction of potential effects of

alternative management plans.

Sources

Berger, Joel. 1986. Wild

horses of the Great Basin:

social competition and

population size. Chicago:

University of Chicago

Press, 326 p.

Choquenot, D. 1991.

Density-dependent Growth,

Body Condition &

Demography in Feral

Donkeys: Testing the food

hypothesis. Ecology vol.

72(3): 805-813. 

Rice, 1990: Biometrics

46:303-308.

Contact

Stephen H. Jenkins, Ph.D.

Department of Biology/314

University of Nevada

Reno, NV 89557

phone 775-784-6078

fax 775-784-1302

e-mail: jenkins@med.unr’’’.edu

Range

Resource Notes are intended to be early

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Information in this Resource Note is based on the opinion and

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

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