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MANAGING VEGETATION FOR PRONGHORNS
IN THE GREAT BASIN
Jim Yoakum
ABSTRACT
Forage
Pronghorn densities and distribution in Great
Basin rangelands are directly related to the
quantity and quality of vegetation. Wild fires
and prescibed burning often enhance pronghorn
habitats by favorably changing the vegetation.
Properly designed vegetation manipulation
projects can also improve pronghorn habitats.
Pronghorns benefit when tall, thick stands of
shrubs are thinned to produce a low, open
shrubland with an abundance of grasses and
forbs. Rangeland seedings should maintain 5 to
10 percent shrubs. Mixture seedings are
preferred over monocultures. The planting of
forbs, such as dryland alfalfa, in rangeland
seedings provides preferred forage species.
Specific case histories are reviewed to
describe relationships between pronghorns and
rangeland improvement projects on Great Basin
shrub steppes.
An adult pronghorn needs approximately 2 pounds
(.90 kg) of air-dry forage per day (Severson
and others 1968). This forage must be from
plant species that are palatable to pronghorns
in order to provide nutrition for survival and
reproduction (Stoszek and others 1978).
There have been numerous food habit studies of
pronghorns in the Great Basin (Mason 1952 for
Oregon; Yoakum 1958 for California, Idaho,
Nevada, and Oregon; Beale and Smith 1970 for
Utah; and Hansen 1982 for Nevada.) These
reports substantiate that pronghorns are
opportunistic herbivores selecting the most
palatable and succulent forage throughout the
year. Pronghorns forage on at least 10 species
of grasses, 70 species of forbs, and 20 species
of shrubs in the Great Basin, using about 7
percent grasses, 22 percent forbs, and 71
percent shrubs on a year-long basis (Yoakum
1958).
INTRODUCTION
Unfortunately, few food habit studies have
related pronghorn diets to forage available.
Such comparisons are needed to analyze
plant preferences and competition with
other herbivores, and for managers to allocate
vegetation among the herbivores on a rangeland.
Recently, Hansen (1982) completed a thorough,
year-long study in northwestern Nevada in·a low
sagebrush community. He found the pronghorns
consuming nearly equal proportions of forbs (46
percent) and shrubs (45 percent), and 5 percent
grasses. This was the highest use of forbs on
a year-long basis reported in the Great Basin.
This high use of forbs was comparable to diet
studies for the grassland biome, e.g., 54
percent in Colorado (Hoover and others 1959);
70 percent in New Mexico (Russell 1964); 76
percent in Kansas (Hlavachick 1968); and 68
percent in Texas (Roebuck 1982). Roebuck's
statement "Use of shrubs occurred only when
forbs were not available ••• "documents
pronghorn preference for forbs.
Vegetation is one of the most important habitat
components controlling pronghorn (Antilocapra
americana) distribution and abundance. The
quantity and·quality of different plant
communities affect pronghorn densities
(Sundstrom and others 1973; Yoakum 1972).
Therefore, the production and survival of
pronghorns is directly related to how
vegetation is maintained or manipulated.
This paper reviews the relationship of
vegetation to pronghorns regarding: (1) forage
and cover habitat requirements; (2) management
objectives for forage classes, composition, and
structuTe; and (3) recommended practices for
maintaining and enhancing vegetation.
PRONGHORN REQUIREMENTS FOR VEGETATION
The pronghorn's requirements for vegetation have
been identified for sagebrush-grassland steppes
of the Great Basin (Yoakum 1974). Vegetation
characteristics including ground cover,
composition, species variety, succulence, and
structure were described. It was pointed out
that too much or too little of any one of these
vegetation characteristics could limit pronghorn
densities. Pronghorn forage needs and plant
structure requirements are specific and
critical.
Cover
Vegetation characteristics of pronghorn fawn
bedsites were documented in Idaho (Autenrieth
1976) and Montana (Pyrah 1974). Both studies
identified tall sagebrush habitats as important
bedsites for fawns. However, Beale and Smith
(1973) in Utah, Barrett (1978) in Alberta,
Bodie (1979) in Idaho, and McNay and O'Gara
(1982) in Nevada did not reach this same
conclusion. Beale and Smith (1973) and Bodie
(1979) found high predation on fawns in tall
shrublands.
Jim Yoakum is a Vildlife Biologist with the USDI
Bureau of Land Management, at the University of
Nevada, Reno
189
The habitat requirements of pronghorn fawns
require further study. Tall shrubs may provide
important protection cover for fawns in some
biomes, but not in others. For example, shrubs
often comprise less than 5 percent of
vegetative cover in the grasslands yet
grasslands support the highest pronghorn
densities in North America (Yoakum 1968).
Therefore, it should be protected and encouraged
as a component of the natural vegetative
community in pronghorn habitats.
Too much or too little of any habitat component
can be benefical or detrimental to wildlife
(Dasmann 1964). This is especially true for
shrubs on pronghorn habitat in the Great Basin.
A shortage of shrubs can reduce survival during
winters when snows cover most vegetation and
only shrubs protrude to provide forage, and
perhaps when there are too few for adequate
cover for fawns. Too many shrubs, on the other
hand, impede rapid mobility from predators and
compete for moisture and soil nutrients needed
to produce other preferred forage species. A
plant community containing 5 to lO·shrub species
covering 5 to 30 percent of the ground provides
optimum vegetation on pronghorn habitat.
MANAGEMENT PROCEDURES
Because vegetation is used daily by pronghorns,
and is one of the most important habitat
components, there is need to document the
objectives and practices of managing vegetation
for the pronghorn's welfare. I have previously
identified four management objectives (Yoakum
1980):
1.
2.
3.
4.
Initially inventory vegetation quantity
and quality and follow-up periodically
with monitoring studies.
Compare the quantity and quality of
vegetation provided by a site with
pronghorn habitat requirements.
When the vegetation on a site provides
high quality pronghorn habitat, then
by design, maintain these quality
conditions.
Improve habitats which have
deteriorated, or which lack pronghorn
habitat requirements.
Grasses
Although pronghorns usually do not feed heavily
on grasses, shortgrass prairies east of the
Rocky Mountains are noted for maintaining some
of the highest pronghorn densities in North
America (Yoakum 1972; Sundstrom and others
1973). These grasslands provide good forage
(primarily forbs) and protection from predators
(predators are easily seen; vegetation does not
impede quick escape).
Grasses are important during winters as they are
high in energy. Even though pronghorns do not
consume large quantities, they do eat grasses
each month of the year. Their preference
appears to be for finer textured species such
as the Poas compared to the rougher textured
Agropyr~ Cheatgrass (Bromus tectorum) is
readily eaten and was the only graminae found
in a year-long food habit study in Oregon
(Yoakum 1958). Rangelands having 20-50 percent
grass ground cover were rated as preferred
habitats in southeastern Oregon (Yoakum 1980).
Forbs
Forbs are preferred year-round forage; their
presence in the plant community is a habitat
requirement. Managers allotting forage to
different herbivores should avoid or alleviate
severe competition for forbs which pronghorns
need to subsist. Managers manipulating
vegetation can encourage growth of native
forbs. Livestock grazing systems can be
designed recognizing that forb seeds generally
mature later than grass seeds. Where
rangelands are reseeded, use mixtures of seeds
that include several species of forbs.
Compared to 20 years ago, many forb seeds are
readily available at reasonable prices.
Composition
Suitable pronghorn habitats in the Great Basin
support mixtures of grasses, forbs, and shrubs.
When available, each forage class is eaten in
all seasons of the year. The proportion of each
forage class present is a major criterion
determining a habitat's degree of suitability
for pronghorn occupancy (Kindschy and others
1982). In general, rangelands with about
one-third each of grass, forbs, and shrubs are
desirable. Total vegetative ground cover should
averaqe about 50 percent, and produce 500 to
1,000 lbs of forage per acre (563 to 1 125)
kg/ha).
Shrubs
Shrubs are an extremely important component of
pronghorn habitats, particularily in the Great
Basin. The availability of shrubs for forage
during severe winters has been directly linked
to pronghorn survival (Bayless 1969; Barrett
1982). Sagebrush (Artemisia sp.), bitterbrush
(Purshia tridentate), and rabbitbrush
(Chrysothamnus spp.) are especially important
to pronghorns in the Great Basin. These plants
should not be eliminated from pronghorn
rangeland. Illegal spraying of sagebrush on
public lands in Wyoming resulted in an
administrative law decision requiring the
appellant to replant sagebrush for wildlife
(Diamond Ring Ranch, IBLA 73-48, August 17,
1973). Rabbitbrush is considered an
undesirable forage plant for livestock; however,
it is a highly preferred forage for pronghorns.
Structure
Plant structure is directly related to pronghorn
occupancy of rangelands (Yoakum 1972). The
height of vegetation appears to be a major
factor. Plant communities with heights greater
than 24 inches (60 em) are less frequently
occupied. This behavior may have been selected
for over eons of time, since (1) predators can
hide in high, thick vegetation, and (2) the
pronghorn's ability to escape rapidly is impeded.
190
MANAGEMENT PRACTICES
Enhance Habitats in Low Quality Condition
Management of pronghor~ habitat should be
based upon a careful comparison of plant
inventory and trend studies with pronghorn
habitat requirements. This comparison will
indicate whether to maintain vegetation in
present condition or attempt to make range
conditions more favorable to pronghorns. Plant
community inventory and monitoring procedures
were discussed in a previous paper (Yoakum
1982); greater emphasis will now be given to
discussing the maintenance and enhancement of
habitats.
If a habitat is in good conditionlit is producing
its natural potential of pronghorns; therefore,
manipulation of vegetation cannot be justified
as a means to improve conditions for pronghorns.
Only on those sites which provide inadequate
vegetative conditions but which provide the right
combination of other habitat factors can it be
justified to manipulate vegetation for
pronghorns. Pronghorns thrive on rangelands in
a subclimax vegetative condition (Kindschy and
others 1982). Such conditions can be the result
of wildfires caused by lightning, grazing by
herbivores, or vegetation manipulation.
Vegetative communities in the Great Basin contain
a variety of grasses, forbs, and shrubs. Range
improvement projects that provide similar mixed
forage classes are best suited to pronghorn
requirements.
Maintain Existing Quality Habitats
A cardinal rule of wildlife habitat management
is that when an environment exists in good
condition achieving its ecological site
potential, then maintain that site in that good
condition. The site will support the variety
of wildlife species that has adapted over
centuries to that ecosystem (Shelford 1963;
Thomas 1979).
Structure Manipulation
Extensive areas of dominant (more than 30 percent
plant composition), high (exceeding 24 inches [60
em]) shrublands are low-density rangelands for
pronghorns compared to similar sites with fewer
shrubs and more grasses and forbs. These
shrublands can be treated to make the vegetal
structure more favorable for pronghorns.
Following this ecological principle will not meet
all management objectives, such as producing
maximum numbers of pronghorns. For example, some
desert-shrub communities in the Great Basin have
60 percent or more shrubs. If this is the
site's natural potential, it will not support
a large pronghorn population, because the site
has a low carrying capacity for pronghorn.
Management should not expect the site to produce
more pronghorns.
Shrub control has been a major practice on
western rangelands during the past 4 decades.
Manipulating sagebrush with large brushland plows
was one method used extensively. Sometimes up
to 95 percent of the sagebrush (Vallentine 1971)
was removed. However, the practice often kills
other plants, especially forbs and perennial
bunchgrasses used by wildlife. Chaining, another
mechanical shrub control method, is accomplished
by dragging a heavy anchor chain in a U-shape
behind two crawler tractors traveling in a
parallel direction. Chaining does not kill as
many shrubs as plowing and is less damaging to
native grasses and forbs.
When the pr~sent vegetation provides all of the
biological requirements of pronghorns, then
maintenance of that site is of utmost importance
to maintain those pronghorns. Carrying capacity
for pronghorns varies from rangeland to
rangeland; always depending on the mix of
pronghorn habitat requirements that a site
provides. The carrying capacity of a site can
change through natural plant succession as a
result of a wildfire. It is because of these
changes that the habitat manager must monitor
sites periodically.
Fire is common on rangelands and is one of
nature's primary ways of developing and
maintaining grasslands (Sauer 1950). Burning is
the oldest known practice used by man to
manipulate vegetation on grazing lands
(Vallentine 1971). Accidental burns can be more
deleterious than beneficial to rangeland
resources; however, prescribed burning can be
beneficial and economical as a habitat management
technique. Prescribed burning is systematically
planning the firing of lands when weather and
vegetation favor a particular method of burning
that can be expected to maximize benefits.
Pronghorns are products of their environment.
If rangelands have the right combination of
habitat factors, then the areas have the
potential to produce maximum numbers of
pronghorns. However, if a rangeland lacks just
one factor, or if a factor is low in quantity
or quality, then that site is limited in its
ability to produce maximum numbers of
pronghorns. This concept of carrying capacity
is well documented for various species of big
game (Russo 1964; Dasmann 1971; Caughley 1979)
but it appears to be not so well understood for
the pronghorn, especially on rangelands that are
managed for multiple use. I repeat this basic
principle of habitat management for emphasis:
Recognize habitats in good ecological condition,
and then by objective, maintain them.
Management of such sites will provide not only
for pronghorns, but also will provide natural
environments for the security of other wildlife
species endemic to the areas.
Artificial Seedings
If preferred plant species are scarce, pronghorn
habitats can be seeded artificially. Scarcity
of favored plants can result from repeated wild
fires destroying endemic sagebrush-grassland
types (Leopold 1966), and also when mining
operations strip off the natural vegetation.
Under such circumstances on public lands, the
Surface Mining Act of 1977 requires
rehabilitation of the site to its original
vegetative conditions, including the replanting
191
in changing the vegetative structure from a
dominant, high-shrub community to one of
low-growing grasses and forbs with sparse stands
of sagebrush. In addition, over this 40-year
period the ranchers improved the habitat from
a low-quality vegetative community to one highly
favorable to pronghorns. Vegetation trend
transects completed in 1982 disclosed that the
plant composition was 57 percent grass, 47
percent dryland alfalfa, and 2 percent shrubs
on treated rangelands. During this 40 year
period, pronghorns first ventured into the
valley for short periods and then moved back to
their historic ranges. As additional acreages
were manipulated and winters remained mild, the
pronghorns became established in the valley
year-long. The population expanded to over 600
animals within a 20-year period and supported
one of the highest doe:fawn ratios in Oregon
(Torland 1980).
of sagebrush.
Past artificial seeding programs in the western
United States can~b~ classified into three
categories:
1.
2.
3.
Single species seedings.
Simple mixture seedings.
Complex mixture seedings.
The effects of these seedings on pronghorns
warrant discussion.
Single Species Seedings
Crested wheatgrass (Agropyron cristatum) has
been used most frequently in single species
seedings in the Great Basin. Other species of
grasses have been planted, but none so
extensively as crested wheatgrass.
The effect of crested wheatgrass on pronghorn
populations has been not well studied to date.
Reeher (1969) conducted a 2 year study on
seedings, noting that pronghorns used plowed and
seeded projects more than sprayed and seeded
sites. Spalinger (reported in Yoakum 1980)
conducted an analysis of pronghorn fecal samples
collected on a crested wheatgrass seeding from
Malheur County, Oregon. The samples were
obtained during late winter of 1977. Although
crested wheatgrass was the dominant plant in the
site (frequency of 52 percent), only an
estimated 2 percent was found in the pronghorn
fecal samples.
Complex Mixture Seedings
Complex mixture seedings contain a number of
different plant species (Plummer and others
1968; Yoakum and others 1980). These mixtures
vary, but for wildlife habitat restoration,
Plummer and others (1968) recommended mixtures
to have a minimum of 6 species each of grasses,
forbs, and shrubs. Plummer and his coworkers
have investigated seedings for the past 30 years
on over 24,000 acres of successfully treated
rangelands. Their findings are the culmination
of intensive research and tried and tested field
results. Their publication, "Restoring Big Game
Ranges in Utah," is a classic and a valuable
guide for range and habitat managers concerned
with restoring western rangelands, It cannot
be stressed too strongly that all vegetation
improvement projects should be planned in
conformance with the basic principles and
practices for successful range restoration
advocated by Plummer and others (1968). These
procedures have wide application on pronghorn
rangelands throughout the west.
Simple Mixture Seedings
When 2 to 5 spehies are planted concurrently,
the practice is referred to as a simple mixture
seeding. Often this has been the planting of
1 or more grasses and 1 or 2 forbs. Such
seedings have been well used by pronghorns,
especially when dryland alfalfa (Medicago spp.)
was used and 10 percent or more of the native
shrubs were retained.
Complex mixture seedings serve pronghorns best
because they re-establish a mixed plant
community of grass, forbs, and shrubs,
approximating natural conditions much more than
monotypic cultures. They meet the vegetation
requirements of pronghorns and many other
wildlife species.
Dryland alfalfa has been one of the most
successful forbs seeded on pronghorn rangelands
in southeastern Oregon (Kindschy and others
1982). In excess of 56,000 acres were planted
in 36 separate seedings. Dryland alfalfa was
aerially seeded over plowed sagebrush rangelands
which had been drilled with crested wheatgrass.
Recent analysis of 20 of these seedings
disclosed that dryland alfalfa maintained 10
percent composition over a 10-year period,
increasing the forb composition from 2 percent
to 7 percent in seeded areas.
ACKNOWLEDGEMENTS
Appreciation is extended to certified wildlife
biologists Phil Davis, Don Klebenow, and
Marshall White for reviewing the manuscript.
Another seeded grass-forb project proved
beneficial to pronghorn in Bear Valley, Oregon.
Forty years ago, pronghorns were not in the
valley although herds occupied adjacent areas.
Bear Valley was predominantly private rangelands
used for grazing domestic livestock. The
vegetation was changed through manipulation
practices. The objective was to decrease the
abundant, tall, unpalatable (to cattle)
sagebrush and to plant crested wheatgrass and
dryland alfalfa for livestock. This resulted
192
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-:,;U.S. GOVERNMENT PRINTING OFFICE: 1983-776-032/1042 REGION NO.8