AN ABSTRACT OF THE THESIS OF
David
C.
Payer for the degree of Master of
Science
in
Fisheries and Wildlife presented on May 15, 1992.
Title:
Habitat Use and Population Characteristics of Bighorn
Sheep on Hart Mountain National Antelope Refuge, Oregon.
Abstract approved:
Redacted for Privacy
Bruce E. Cob
ntz
I studied habitat use, productivity, and sex and age
structure
of
californiana)
California
bighorn
sheep
(Ovis
canadensis
on Hart Mountain National Antelope Refuge,
The population
Oregon, from 7 April 1990 to 31 August 1991.
included >300 sheep and has provided a source of animals for
transplant and hunting.
I identified 2 ewe ranges and 1 ram
range on Hart Mountain (HM) ,
Ridge
and 1 range each on Poker Jim
Ram and ewe ranges overlapped,
(PJR).
segregated sexually during spring and summer.
but sheep
In 1990, herd
ranges were smaller in spring (5E = 31.2 and 7.63 km2 for rams
and ewes, respectively) than summer
for rams and ewes, respectively).
larger than ewe ranges.
= 71.3 and 11.45 km2
Ram ranges were always
Ewes on PJR migrated to a HM range in
June 1990 and July 1991, resulting in greater sheep density on
this range
(24.5
respectively)
bighorn/km2
and 34.7 bighorn/km2 in 1990 and 1991,
than the other HM ewe range
in 1990 and 1991,
respectively).
returned to PJR by November of both years.
(5.3
and 8.9
These ewes
The other sheep
herds expanded into contiguous areas in summer.
Fidelity to
specific ranges appeared high.
Water was a limited resource on PJR, and its availability
affected sheep distribution there.
I observed PJR rams at
water sources <7 km from escape terrain.
Water and escape
terrain were interspersed and not limited on HM.
Seasonal
changes
changing
in
ewe
distribution
on
HM
reflected
physiographic requirements associated with lambing.
Rams on
both ranges used habitats including western juniper (Juniperus
occidentalis); ewes seldom did.
There were 76.8-85.4 rams:100 ewes, and 45.3-52.2% of
rams had .3/4-curl horns.
Twenty-one rams aged 3-9 yr were
radio-collared and monitored 53 yr in this and a related
study.
Only 4 mortalities occurred, all from hunting.
Ram
harvest could be increased without affecting productivity.
Lamb:ewe ratios did not differ between summer 1990 and
1991 (i = 51.8 lambs:100 ewes and 53.3 lambs:100 ewes in 1990
and 1991, respectively, P = 0.76), and did not differ between
ranges in either year (P = 0.14 and 0.26 in 1990 and 1991,
respectively).
Only 50% of lambs alive in August 1990 were
still alive the following summer.
Maintaining high productivity in this population will
require continued removals of animals from all sex and age
classes.
Habitat
protection
indicated;
special
attention
and
should
availability and vegetative structure.
improvement
be
given
is
to
also
water
Habitat Use and Population Characteristics of Bighorn Sheep
on Hart Mountain National Antelope Refuge, Oregon
by
David C. Payer
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Completed May 15, 1992
Commencement June 1993
APPROVED:
Redacted for Privacy
Professor of Wildlife Ecol6y in charge of major
Redacted for Privacy
Chairman of the Department of Fisheries and Wildlife
Redacted for Privacy
...3Dean of Graduat
chool (I
Date thesis is presented
Typed by
David C. Payer
May 15. 1992
ACKNOWLEDGEMENTS
This study was funded by the Oregon Department of Fish
and Wildlife,
with assistance
from
the Oregon Hunter's
Association and the Order of the Antelope.
B. Coblentz for guidance and encouragement.
I am grateful to
I also thank B.
Reiswig, M. Smith, E. Couch, W. Pyle and K. Mallory of the
U. S. Fish and Wildlife Service, and L. Conn of the Oregon
Department of Fish and Wildlife, for their contributions.
R.
Owens assisted in the field, and M. McCracken provided advice
on statistical analyses.
of this manuscript.
W. D. Edge reviewed earlier drafts
TABLE OF CONTENTS
INTRODUCTION
1
STUDY AREA
5
METHODS
Locating Bighorn Sheep
Habitat Use
Vegetation Sampling
Herd Ranges and Home Ranges of
Radio-Collared Rams
Population Characteristics
Statistical Analyses
Habitat Use
Population Characteristics
8
8
8
RESULTS
Bighorn Sheep Habitats
Hart Mountain
Poker Jim Ridge
Habitat Use
Seasonal Distribution
Use of Habitat Types: Spring vs. Summer 1990
Use of Habitat Types: Summer 1990 and 1991
Use of Habitat Characteristics: Spring vs.
Summer 1990
Use of Habitat Characteristics:
Summer 1990
and 1991
Home Ranges of Radio-Collared Rams
Seasonal Herd Range Sizes
Population Characteristics
Group Size
Ram Age Class Structure and Survival of
Radio-Collared Rams
Lamb Production and Survival
Ewe and Yearling Ratios
Helicopter Surveys and Population Structure
Bighorn Sheep Density
DISCUSSION
Habitat Use
Seasonal Distribution and Use of Habitat
Types and Characteristics
Range Fidelity
Population Characteristics
Group Size
Adult Ram Mortality and Age Class
Distribution
Lamb Production and Survival
Comparisons of Helicopter and Ground Surveys
Management Implications
11
12
12
13
13
15
18
18
18
21
25
25
30
30
34
41
46
47
48
48
50
50
56
56
57
60
60
60
71
72
72
73
74
79
80
LITERATURE CITED
APPENDICES
Appendix A:
Appendix B:
85
Results of Vegetation Sampling
Home ranges of Radio-Collared Rams
94
94
102
LIST OF FIGURES
Figure
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Page
The study area: Hart Mountain National
Antelope Refuge.
7
Bighorn sheep survey routes on Hart Mountain
National Antelope Refuge.
9
Bighorn sheep ranges on Hart Mountain National
Antelope Refuge, spring 1990.
27
Bighorn sheep ranges on Hart Mountain National
Antelope Refuge, summer 1990.
28
Bighorn sheep ranges on Hart Mountain National
Antelope Refuge, summer 1991.
29
Percent use of habitat types by rams and ewes
on Hart Mountain, spring and summer 1990.
31
Percent use of habitat types by rams on Poker
Jim Ridge, spring and summer 1990.
32
Percent use of habitat types by rams and ewes
on Hart Mountain, summer 1990 and 1991.
33
Percent use of habitat types by rams on Poker
Jim Ridge, summer 1990 and 1991.
34
Relative frequency distributions and median
values of distance to escape terrain, distance
to water, slope and elevation, and proportional
use of aspect categories, rams groups on Hart
Mountain, spring and summer 1990.
36
Relative frequency distributions and median
values of distance to escape terrain, distance
to water, slope and elevation, and proportional
use of aspect categories, ewe groups on Hart
Mountain, spring and summer 1990.
37
Relative frequency distributions and median
values of distance to escape terrain, distance
to water, slope and elevation, rams groups on
Poker Jim Ridge, spring and summer 1990.
39
Figure
13.
14.
Page
Relative frequency distributions and median
values of distance to escape terrain, distance
to water, slope and elevation, and proportional
use of aspect categories, ram and ewe groups on
Hart Mountain, summer 1990 and 1991.
42
Observed locations of ram and ewe groups on
Hart Mountain during summer 1990 and 1991 in
relation to distance to escape terrain, distance
to water and slope.
43
15.
Relative frequency distributions and median values
of distance to escape terrain, distance to water,
slope and elevation, rams groups on Poker Jim
Ridge and Hart Mountain, summer 1990 and 1991.
45
16.
Mean bighorn sheep group sizes on Hart Mountain
National Antelope Refuge.
49
Ram age class structure on Hart Mountain and
Poker Jim Ridge, summer 1990 and 1991.
51
Bighorn sheep nursery areas on Hart Mountain
National Antelope Refuge.
54
17.
18.
19.
20.
21.
22.
23.
Estimated number of lambs:100 ewes on South
Hart Mountain, North Hart Mountain and Poker
Jim Ridge, May-August 1990 and June-August
1991.
55
Monthly precipitation at Hart Mountain
National Antelope Refuge headquarters, 19891991, and 50-yr averages.
62
Response surface generated by logistic regression
model for differences in distance to escape
terrain and water of rams groups on Poker Jim
Ridge between spring and summer 1990, and a
scatter plot of observed values.
66
Response surface generated by logistic regression
model for differences in distance to escape
terrain and water of rams groups on Hart Mountain
between spring and summer 1990, and a scatter
plot of observed values.
68
Response surface generated by logistic regression
model for differences in slope and elevation at
sites used by ewe groups on Hart Mountain between
spring and summer 1990, and a scatter plot of
observed values.
69
Figure
24.
25.
Response surface generated by logistic regression
model for differences in distance to escape
terrain and water between rams groups on Poker
Jim Ridge and Hart Mountain in summer 1990 and
1991, and a scatter plot of observed values.
Page
71
Estimated bighorn sheep population size and number
removed through trapping and hunting, Hart
Mountain National Antelope Refuge, 1954-1991.
75
LIST OF TABLES
Table
1.
2.
3.
4.
5.
6.
7.
8.
Page
Comparisons of use of habitat characteristics
by bighorn sheep on Hart Mountain National
Antelope Refuge modeled using logistic
regression, and potential main effects entered
into full models.
16
Characteristics of shrubby and herbaceous
vegetation in bighorn sheep habitats on Hart
Mountain, June 1991.
24
Characteristics of shrubby and herbaceous
vegetation in bighorn sheep habitats on Poker
Jim Ridge, June 1991.
24
Estimated coefficients from logistic regression
of probability of a ram group observation on
Hart Mountain in spring and summer 1990 occurring
in spring against distance to escape terrain (m)
and distance to water (m).
36
Estimated coefficients from logistic regression
of probability of a ewe group observation on
Hart Mountain in spring and summer 1990 occurring
in spring against distance to escape terrain (m),
distance to water (m), slope (%) and
elevation (m).
38
Estimated coefficients from logistic regression
of probability of a ram group observation on
Poker Jim Ridge in spring and summer 1990
occurring in spring against distance to escape
terrain (m), distance to water (m), slope (%)
and elevation (m).
40
Estimated coefficients from logistic regression
of probability of a ram or ewe group observation
on Hart Mountain in summer 1990 and 1991 being
a ram group against distance to escape terrain
(m), distance to water (m) and slope (%).
43
Estimated coefficients from logistic regression
of probability of a ram group observation on
Poker Jim Ridge or Hart Mountain in summer 1990
and 1991 occurring on Poker Jim Ridge against
distance to escape terrain (m) and distance to
water (m).
45
Table
9.
10.
11.
12.
13.
Page
Home range sizes of radio-collared rams on Hart
Mountain National Antelope Refuge, 1990 and 1991.
46
Seasonal bighorn sheep herd range sizes (km2 on
Hart Mountain National Antelope Refuge, 1990 and
)
1991.
47
Range affiliation, number of months monitored,
and age and status at last contact for radiocollared rams on Hart Mountain National
Antelope Refuge.
52
Population structure of adult rams, ewes and
yearlings on Hart Mountain National Antelope
Refuge, 1990 and 1991.
58
Bighorn sheep density on 1990 and 1991 summer
ranges, Hart Mountain National Antelope Refuge.
59
LIST OF APPENDIX TABLES
Table
Page
A.1. Canopy cover (%) of major grass, forb and shrub
species in bighorn sheep habitats on Hart
Mountain, June 1991.
95
A.2. Canopy cover (%) of major grass, forb and shrub
species in bighorn sheep habitats on Poker
Jim Ridge, June 1991.
99
HABITAT USE AND POPULATION CHARACTERISTICS OF BIGHORN SHEEP
ON HART MOUNTAIN NATIONAL ANTELOPE REFUGE, OREGON
INTRODUCTION
Bighorn sheep (Ovis canadensis) were historically common
throughout western North America, but epizootics introduced by
livestock, competition with livestock for forage, loss of
habitat to human development and overhunting contributed to
their decline in the late 1800's and early 1900's (Cowan 1940,
Buechner 1960, Forrester 1971, Hibler et al. 1972, Spraker and
Hibler 1977, Lange et al. 1980).
g_t
californiana),
the
California bighorn sheep (Q.....
subspecies
native
to
southeastern
Oregon, were extirpated in Oregon by 1915 (Seton 1929).
In 1954, Hart Mountain National Antelope Refuge (HMNAR)
received a transplant of 20 California bighorn sheep from
British Columbia
(Deming
1961).
The refuge population
expanded steadily, and now includes >300 animals.
It serves
as an important source of bighorn sheep for transplants to
ancestral ranges throughout Oregon and surrounding states, and
is valued for trophy ram hunting.
Since 1960, 477 sheep have
been transplanted from the refuge and 203 rams have been
harvested (U.S. Fish and Wildl. Serv.,
unpubl. data).
In
order to maintain the high productivity and quality of this
population,
it
is
important to evaluate habitat
use and
population characteristics.
Habitat requirements of bighorn sheep include adequate
forage, water, thermal protection, escape cover and areas for
2
specific activities such as bedding,
lambing and rutting
(Ferrier and Bradley 1970, Shannon et al. 1975, Wilson et al.
1980, Tilton and Willard 1982).
Proximity to escape terrain
and open areas providing good visibility with adequate forage
are
of
primary
importance
(Buechner
1960,
Geist
1971,
Risenhoover and Bailey 1985, Wakelyn 1987).
Rams and ewes typically segregate except during the
breeding season (McCann 1956, Welles and Welles 1961, Blood
1963, Woodgerd 1964, Woolf et al. 1970, Geist and Petocz 1977,
Shank 1982,
Gionfriddo and Krausman 1986,
Festa-Bianchet
1991), and their use of habitats can differ substantially.
Ewes are generally more selective, preferring rugged terrain
closer to escape cover (Blood 1963, Woolf et al. 1970, Geist
and Petocz 1977, Van Dyke 1978, Leslie and Douglas 1979,
Gionfriddo and Krausman 1986).
This is particularly true
during the lambing season, when the need for protection from
predators is greatest (Spencer 1943, Geist 1971, Schaller
1977, Van Dyke et al. 1983, Festa-Bianchet 1988a).
Use of vegetative associations by rams and ewes often
differs, although this may result more from differences in
physiography than dietary preference per se (Van Dyke et al.
1983, Krausman et al. 1989).
At times of great energy demand,
eg. late pregnancy, physiological requirements create a need
for high quality forage, and could be partially responsible
for differences in habitat use (Morgantini and Hudson 1981).
Productivity of bighorn sheep populations is related
3
directly to habitat quality and inversely to population
density (Geist 1971, Douglas and Leslie 1986, Wehausen et al.
1987, Krausman et al. 1989).
Habitat quality can vary greatly
between seasons and years in arid ecosystems.
It is largely
determined by precipitation regimens, which control forage and
water availability (Beatley 1974, Douglas and Leslie 1986,
Wehausen et al. 1987).
Population density exerts its effect
on productivity through resource depletion (Geist 1971, Festa-
Bianchet
and
1988a)
diseases
such
as
the
lungworm
(Protostronqvlus spp. ) -pneumonia complex (Stelfox 1971, Hibler
et al. 1982, Festa-Bianchet 1988b).
Low productivity may also
result from human disturbance (Dunaway 1971 cited in Hicks and
Elder 1979, Etchberger et al. 1989, Stockwell et al. 1991) and
depletion of rams through overhunting (Nichols 1978, Heimer
and Watson 1986, Heimer 1988).
HMNAR supports up to 3 distinct subpopulations of ewes
and 2 of rams, the ranges of which differ substantially in
regards to physiographic and vegetative structure
1978, Cottam 1985).
(Kornet
Previous studies of bighorn sheep on
HMNAR focused on productivity and habitat use of the ewe-lamb
portion of the population (Kornet 1978, Cottam 1985), and
little
has
been
published
about
rams
on
the
refuge.
Furthermore, current characteristics such as population age
and
sex
structure,
range
sizes,
sheep
densities,
lamb
production and recruitment have not been intensively studied.
The purpose of this study was to provide information useful
4
for managing the entire HMNAR bighorn population, and develop
recommendations.
resources
My objectives were to:
important
to
bighorn
sheep,
(1)
Identify
thereby providing
direction for efforts to protect or improve bighorn habitat;
(2)
document differences
in
habitat use and population
characteristics between ewes and rams, and between ranges and
seasons, in order to more fully appreciate and manage for
differing needs; (3) determine home ranges, range fidelity and
age structure of rams to aid the development of harvest
strategies; and (4) document lamb production and survival, and
identify limiting factors.
5
STUDY AREA
HMNAR is located in Lake County, Oregon, 104 km northeast
of Lakeview.
The refuge is administered by the U.S. Fish and
Wildlife Service (USFWS), and includes approximately 1,120 km2
within the Great Basin Desert.
The most prominent feature of HMNAR is a 48.3 km-long
fault block mountain.
The mountain is oriented north-south;
the west side drops abruptly from a maximum elevation of 2,445
m on Warner Peak to the Warner Valley at 1,370 m.
several
canyons,
precipitous
cliff
bands,
It features
talus
slides,
sagebrush (Artemisia tridentata)-bunchgrass slopes and stands
of western juniper
(Juniperus occidentalis)
mountainmahogany (Cercocarpus ledifolius).
and curlleaf
The top of the
mountain is a rolling to flat sagebrush-bunchgrass plateau,
varying
in width from 0.5-3.0
The plateau slopes
km.
gradually down to the Catlow Valley (elevation 1,525 m) on the
east side.
The eastern slopes include areas of sagebrush-
bunchgrass,
small canyons and stands of western juniper.
Vegetation
is
generally
typical
of
the
shrub-steppe
communities described by Franklin and Dyrness (1973).
The
northern
headquarters)
portion
of
HMNAR
(north
of
is known as Poker Jim Ridge (PJR),
southern portion as Hart Mountain (HM).
into North Hart Mountain
(NHM),
refuge
and the
I further divided HM
that area north of Hart
Canyon, and South Hart Mountain (SHM), that area south of and
6
including Hart Canyon (Fig. 1).
Average annual precipitation at refuge headquarters from
1939-1991 was 29.0 cm (range 13.7-48.3 cm)
data).
(USFWS, unpubl.
The majority of precipitation occurs as winter and
spring snows (Deming 1961).
Nineteen-ninety was a relatively
dry year (19.0 cm), whereas 1991 was relatively wet (38.7 cm).
May 1991 was the third wettest month in 50 years with 11.2 cm
of precipitation.
The
south
end
of
HMNAR
tends
precipitation than the north (Deming 1961).
to
receive
more
Furthermore, the
west face of HM has numerous perennial springs, whereas there
are none on PJR.
There are a few standing water sources on
PJR, but these go dry during the summer in some years.
There
are also 4 water catchment devices on PJR; 2 were in place
prior to initiation of this study, and 2 were constructed in
August 1990.
Current land use on HMNAR is predominantly recreational,
emphasizing consumptive and nonconsumptive wildlife values.
Cattle were grazed on the refuge in 1990, but no livestock
grazing occurred in 1991.
7
Fig. 1.
The study area:
Hart Mountain National Antelope Refuge.
8
METHODS
Locating Bighorn Sheep
I observed bighorn sheep from 7 April-31 August 1990, 7
June-31 August 1991, and 1 week in mid-November 1990 and 1991.
Travel was primarily on foot, with frequent overnight stays in
the more remote areas of the range.
I travelled 6 non-
overlapping routes covering the entire range of bighorn sheep
on HMNAR,
each requiring
a
full
day to
complete.
An
additional route was added in July-August 1990 to accommodate
the expanded range of PJR rams during those months (Fig. 2).
Bighorn sheep were located and observed with the aid of
7X35 binoculars and a 22X spotting scope.
Radio telemetry
assisted in locating and identifying <18 rams radio-collared
by the Oregon Department of Fish and Wildlife (ODFW).
I used
the technique of homing (Mech 1983) with a Telonics TR-2
receiver and Yagi directional antenna (Telonics, Inc., Mesa,
Ariz.).
Sheep group locations were plotted on U.S. Geological
Survey (USGS) topographic maps (1:24,000), and recorded using
the Universal Transverse Mercator (UTM)
system (Grubb and
Eakle 1988).
Habitat Use
I studied habitat use by bighorn sheep during spring and
summer 1990 and summer 1991.
The differentiation between
spring and summer in both years was based on plant phenology
9
Fig. 2.
Bighorn sheep survey routes on Hart Mountain National Antelope
Refuge.
Routes: 1-3 = Poker Jim Ridge, 3 added in summer 1990 only, 4-6
= Hart Mountain, 7 = base of cliffs, Hart Mountain and Poker Jim Ridge.
10
and sheep movements during the 1990 field season.
spring
included the period of peak forb abundance and lambing, and
summer commenced when forbs receeded and the majority of ewes
with lambs left lambing areas and formed large ewe-lamb bands.
On this basis, 7 April-9 June was designated as spring, and 10
June-31 August as summer.
For each sheep group observation, I used USGS topographic
maps to determine the following habitat characteristics:
Elevation,
slope
(%),
aspect (north,
east,
south or west
corresponding to 315°-44°, 45°-134°, 135°-224° and 225°-314°,
respectively), distance to water (= (x2 + y2)05, where x =
horizontal distance and y = vertical distance), and distance
to escape terrain (similarly calculated).
Escape terrain was
defined as "steep rocky terrain on which mountain sheep would
safely outdistance or outmaneuver predators" (Gionfriddo and
Krausman 1986).
I surveyed and mapped escape terrain and
water sources in bighorn sheep ranges during September 1990.
Potential water sources were identified through examination of
infra-red aerial photographs and visual scanning of the west
face of the mountain, and were then visited on foot.
I never
observed, nor was there evidence of, bighorn sheep using the
water catchment devices on PJR, so these were not included as
water sources when measuring distance of groups to water.
I recorded habitat type for each sheep group location.
Habitat types within sheep ranges were differentiated based on
vegetative and physiographic features thought to be important
11
to bighorn sheep, including dominant grass and shrub species,
visibility,
presence or absence of trees,
steepness,
and
proximity to escape terrain and water (Buechner 1960, Geist
Shannon et al.
1971,
Wakelyn 1987).
(1973)
1975, Reisenhoover and Bailey 1985,
Plant names followed Hitchcock and Cronquist
and Garrison et al.
according
to
dominant
(1976).
vegetative
Habitats were named
and/or
physiographic
features.
Vegetation Sampling
Mean herbaceous and shrub species canopy cover, shrub
height and shrub density were estimated for each habitat type
by methods similar to those of Poulton and Tisdale (1961).
I
established three 50-m line transects in representative stands
of vegetation within each habitat type during June 1991, at
peak forb abundance.
Percent herbaceous cover within 20 x 50
cm plots placed every 5 m along each transect was visually
estimated (Daubenmire 1959).
The number of shrubs rooted in
a 1-m belt adjacent to each transect was counted to estimate
shrub density.
Mean shrub height was calculated by measuring
the individual shrub nearest each 5-m point on each transect.
Percent canopy cover of shrubs was determined by the line
interception method (Canfield 1941).
In those habitats supporting tree growth,
I estimated
tree density by the nearest neighbor method (Cottam and Curtis
1956).
Fifty trees/habitat were selected at random, and the
12
distance to the nearest tree was measured.
Density was
calculated using the formula (2d)-2, where d was the mean
distance between nearest neighbors.
Herd Ranges and Home Ranges of Radio-Collared Rams
I used the minimum convex polygon (MCP) method (Southwood
1966) and the computer program McPAAL (Stuwe and Blohowiak
1985)
to characterize bighorn sheep herd ranges.
locations constituted individual fixes.
Group
Seasonal ranges for
spring 1990 and summer 1990 and 1991 were mapped and their
sizes determined.
I defined home range of radio-collared rams as the total
area an animal was observed to occupy over the course of the
study
(Leslie and Douglas 1979).
Home range sizes were
determined like herd ranges with collared ram locations
constituting fixes.
Home range determined by the MCP method
is negatively biased by small sample size (Beckoff and Mech
1984), so I only included rams with .17 locations.
Home range
of 6 rams was mapped.
Population Characteristics
When a bighorn sheep group was located, I attempted to
count the number of sheep and determine the age class and sex
of each individual.
Age and sex classification followed the
system of Geist
(1971),
yearlings,
I,
class
II,
wherein rams were classified as
III,
or
IV,
ewes as adults or
13
yearlings, and young of the year as lambs.
I designated ram
groups as those consisting solely of adult rams (class I-IV),
ewe groups as those consisting of adult ewes with or without
lambs or yearlings of either sex, and mixed groups as those
including both adult rams and adult ewes.
Helicopter surveys conducted by ODFW in June 1990 and
1991 provided population size estimates.
I
coupled the
results of these surveys with the sex and age class ratios
determined during ground surveys to estimate absolute numbers
of animals in each sex and age class.
Helicopter survey data
was also used in conjunction with herd range size estimates to
determine bighorn sheep density
ranges.
on summer
sheep/km2)
(no.
Density estimates for the NHM ewe range included the
migratory PJR ewe subpopulation.
Statistical Analyses
Statistical analyses were performed using the computer
packages
SAS
(SAS
Institute,
Inc.
1990;
descriptive
statistics, Student's t- and Wilcoxon rank-sum tests, analysis
of
variance
[ANOVA]
and
regression
model
performance
assessment), Statgraphics (STSC, Inc. 1991; chi-square tests)
and
GLIM
(Baker
and
Nelder
1985;
interactive
logistic
regression modeling).
Habitat Use
I
defined
bighorn
sheep
habitat
type
use
as
the
14
proportion of group observations occurring in each habitat
type, and stratified it by range, sex and season.
Data from
the NHM and SHM ewe ranges was combined to simplify comparison
between
sexes
on
HM.
I
used
the
chi-square
test
of
homogeneity (Mendenhall 1971:299) to test null hypotheses of
no difference in habitat type use between sexes and seasons
(2-way contingency tables).
I
If Ho was rejected at a = 0.05,
constructed 95% confidence intervals
Bonferroni approach
(Neu et al.
differences existed.
1974)
(CI's)
using the
to determine where
Ewe use of PJR during summer 1990 and
1991 was too limited to analyze statistically, and HM and PJR
ranges were not compared because available habitat
types
differed.
I used logistic regression (Neter et al.
1989:581-616,
Meyers 1990:317-332) to model differences in use of specific
habitat characteristics between sexes, ranges and seasons.
Potential
explanatory variables were distance
to
escape
terrain, distance to water, slope, elevation and aspect, and
sex, range or season was the response variable (Table 1).
Response variables were binary, and one outcome was designated
the "successful" case.
effects,
quadratic
I constructed full models with main
terms
and
2-way
interactions,
then
eliminated variables in a step-wise fashion if P > 0.05.
Aspect was not considered a potential main effect on PJR
because slopes were generally mild and aspect varied little
over the area.
Elevation was not considered when comparing HM
15
and PJR because the range of values observed on HM was not
available on PJR.
Performance of minimal models was assessed by examination
of
the
-2
Log Likelihood
for
the contribution
of
the
explanatory variables. This was a likelihood ratio chi-square
statistic for testing Ho: 81 = .... = Bk = 0, where the B's
were regression coefficients for the explanatory variables
(SAS Institute, Inc. 1990:1088-1089).
sensitivity
(percent
observed
of
I also determined the
event
responses
with
predicted probability of event > 0.50), specificity (similar
statistic
for
no
classification rate
event
of
responses)
and
overall
the models
(SAS
Institute,
correct
Inc.
1990:1091-1092).
Response surfaces were generated for models with 2 main
effects.
Main effects were plotted against the predicted
probability of the "successful" case (Murtaugh 1988).
Population Characteristics
The hypotheses of no difference in mean group size
between seasons, ranges and sexes were tested using Student's
t-test
(Devore and Peck 1986:370-374)
when
2
means were
compared, and a single factor ANOVA (Devore and Peck 1986:558-
564) when comparing >2.
I
used the chi-square test
of homogeneity and the
Bonferroni method (Neu et al. 1974) to compare ram age class
structure on HM to that on PJR during summer 1990 and 1991.
16
Table 1. Comparisons of use of habitat characterisitcs by bighorn sheep
on Hart Mountain National Antelope Refuge modeled using logistic
regression, and potential main effects entered into full models.
Comparison upon which model was
baseda
Potential main effects
Ewes, HM (NHM and SHM pooled),
spring vs. summer, 1990
Distance to escape terrain
Distance to water
Slope
Elevation
Aspect
Rams, HM, spring vs. summer, 1990
Distance to escape terrain
Distance to water
Slope
Elevation
Aspect
Rams, PJR, spring vs. summer, 1990
Distance to escape terrain
Distance to water
Slope
Elevation
Rams, HM vs. ewes, HM (NHM and SHM
pooled), summer 1990 and 1991
(summer data pooled)
Distance to escape terrain
Distance to water
Slope
Elevation
Aspect
Rams, PJR vs. rams, HM, summer
1990 and 1991 (summer data pooled)
Distance to escape terrain
Distance to water
Slope
aHM = Hart Mountain, NHM = North Hart Mountain, SHM = South Hart Mountain,
PJR = Poker Jim Ridge.
A
similar
approach
was
taken
to
compare
age
class
distributions from the field study to those from helicopter
surveys.
Yearling ratios were expressed as observed number of
yearlings:100 ewes, and were stratified by year and range.
Data from the NHM and PJR ranges was combined because the PJR
ewes, lambs and yearlings spent much of the summer on NHM, and
the range fidelity of these animals was not determined.
I
used the chi-square test of homogeneity to test the following
17
null hypotheses:
(1)
Refuge-wide yearling ratios did not
differ between 1990 and 1991; and (2) yearling ratio on SHM
did not differ from that on NHM/PJR in 1990 or 1991.
I also
tested the hypotheses that male:female yearling ratios did not
differ from parity in 1990 and 1991 using the chi-square
goodness-of-fit test (Devore and Peck 1986:637-640).
Lamb production was expressed
as observed number of
lambs:100 ewes, and was stratified by range, season and month.
I defined mean lamb production as observed number of lambs:100
ewes over the course of a summer.
ranges was again combined.
Data from the NHM and PJR
I used the chi-square test of
homogeneity to test the following null hypotheses concerning
lamb production and survival:
(1)
Mean refuge-wide lamb
production did not differ between 1990 and 1991; (2) mean lamb
production did not differ between SHM and NHM/PJR in 1990 or
1991; (3) refuge-wide lamb:ewe ratios did not differ between
months in May-June 1990 or June-August 1991; and (4) refuge-
wide lamb:ewe ratio in August 1990 did not differ from the
yearling:ewe ratio in summer 1991.
I similarly compared
lamb:ewe and yearling:ewe ratios derived from ground surveys
to those from helicopter censuses in June 1990 and 1991.
18
RESULTS
Bighorn Sheep Habitats
Twelve habitat types,
6
each on PJR and HM, were
identified and characterized within the herd ranges of bighorn
sheep on HMNAR (Tables 2-3, Appendix A).
Hart Mountain
Low Sagebrush Plateau.
This habitat covered the entire
upper plateau of NHM, and most of the upper plateau of SHM
within bighorn sheep range.
sagebrush
(Artemisia
The shrub layer consisted of low
arbuscula)
(Chrysothamnus vicidiflorus).
and
green
rabbitbrush
Shrub height was the lowest of
all HM habitats, but shrub density was the greatest.
grass species were Idaho fescue
(Festuca idahoensis)
Sandberg's bluegrass (Poa secunda).
Major
and
Common forbs included
spreading phlox (Phlox diffusa), lupine (Lupinus spp.) and
granite gilia (Leptodactylon pungens).
Mountain
Big
Sagebrush Plateau.
The mountain big
sagebrush (Artemisia tridentata var. vasevana) plateau habitat
occurred in a few restricted areas within bighorn sheep range
on the upper plateau of SHM.
Shrub height was greater than
that in the low sagebrush plateau habitat, but density was
less.
The dominant shrub species was mountain big sagebrush,
but green rabbitbrush was also common.
Herbaceous species
19
included Idaho fescue,
bottlebrush squirreltail
hystrix),
Sandberg's
bluegrass,
(Actropyron
spicatum),
granite
bluebunch
gilia,
(Sitanion
wheatgrass
lupine
and
wooly
groundsel (Senecio canus).
Cliff/Talus-Shrub.
Precipitous cliffs dissected by
horizontal benches and steep talus slides made up a large
portion of the upper west face of HM.
was devoid of vegetation.
Much of this habitat
Vegetation did occur on cliff
benches and in islands within the talus slides, and these
areas supported the greatest variety and canopy cover of shrub
species within HM bighorn sheep ranges.
in
vegetated
areas
were
mountain
Common shrub species
big
sagebrush,
bush
rockspirea (Holodiscus dumosus), green rabbitbrush, mountain
snowberry
(Symphoriocarpus
(Chrysothamnus nauseosus)
Herbaceous species were
fescue,
cheatgrass
oreophilus),
gray
and wax currant
less common,
brome
(Bromus
and
rabbitbrush
(Ribes cer ui).
included Idaho
tectorum)
and
Pacific
Monardella (Monardella odoratissima).
Mountain Big Sagebrush-Bunchgrass.
supported
large
expanses
of
The west face of HM
shrub-bunchgrass
habitat,
generally on moderate slopes below the cliff/talus areas.
shrub layer was dominated by mountain big sagebrush.
The
Green
rabbitbrush, mountain snowberry and Wyoming big sagebrush
(Artemisia tridentata var. wyomingensis)
were also present.
20
Grasses
included
Idaho
fescue,
bluebunch
wheatgrass,
bottlebrush squirreltail and Sandberg's bluegrass.
The most
common forbs were lupine, littleflower collinsia (Collinsia
parviflora) and spreading phlox.
Riparian.
west
face
Perennial springs and creeks occurred on the
of
HM,
riparian zones.
the
HM
and surrounding areas formed distinct
The riparian habitat was the most limited of
habitats,
but
supported
the
greatest
cover
of
herbaceous species. Giant wildrye (Elymus cinereus) dominated
the grass layer.
Cheatgrass brome, Kentucky bluegrass (Poa
oratensis) and western wheatgrass
also common.
(Agropyron smithii) were
Typical forbs included stinging nettle (Urtica
dioica), common monkeyflower (Mimulus quttatus), California
falsehellabore
(Veratrum
californicum),
bedstraw
spp.), cushion eriogonum (Eriogonum ovifolium)
solomonplume (Smilacina stellata).
(Galium
and starry
Mean shrub height was the
greatest of all HM habitats, but shrubs were not common.
The
dominant shrub species was Wood's rose (Rosa woodsii). Rushes
(Juncus spp.)
areas.
and sedges (Carex spp.)
were found in some
Low elevation riparian areas included
stands of
quaking aspen (Populus tremuloides) and ponderosa pine (Pinus
Ponderosa), but these areas were not used by bighorn sheep and
were not considered part of the HM bighorn sheep habitats.
21
Juniper/Mountainmahogany-Mountain Big Sagebrush.
Mixed
stands of western juniper and curlleaf mountainmahogany were
found on the west face of HM, particularly SHM.
These were
relatively open stands, averaging 35.9 trees/ha.
herbaceous species cover was sparse.
Shrub and
The dominant shrub
species was mountain big sagebrush, and bush rockspirea and
mountain snowberry were also present.
Grasses occurring in
this habitat included Thurber needlegrass (Stipa thurberiana),
bluebunch wheatgrass,
Idaho
(Oryzolosis jymenoides).
fescue
and
Indian ricegrass
Forbs were rare, the most common
being lupine.
In some areas, western juniper did not occur and curlleaf
mountainmahogany was very dense, averaging 1166.4 trees/ha.
The understory was completely devoid of any herbaceous or
shrubby growth.
These areas received no sheep use and were
not considered part of the HM bighorn sheep habitats.
Poker Jim Ridge
Low Sagebrush Plateau.
This habitat occupied the upper
plateau of PJR above an elevation of 1830 m.
was the dominant shrub.
Major grass species were Sandberg's
bluegrass and bottlebrush squirreltail.
bighead clover (Trifolium macrocephalum),
spp.),
sandwort
stenophyllus),
(Arenaria
hollyleaf
Low sagebrush
spp.),
clover
Forbs
included
lomatium (Lomatium
goldenweed
(Trifolium
(Haplopamus
aymnocarpum),
littleflower collinsia and locoweed (Astragalus obscurus).
22
Low Sagebrush-Bunchgrass.
The low sagebrush bunchgrass
habitat was found on the upper plateau of PJR below an
elevation of 1830 m, and on the gentle east-facing slopes
above the Catlow Valley.
shrub.
Low sagebrush was again the dominant
Grass canopy cover was nearly twice that of the low
sagebrush
plateau;
grass
species
included
bluebunch
wheatgrass, Sandberg's bluegrass and bottlebrush squirreltail.
Forbs were relatively abundant, including spreading phlox,
tapertip hawksbeard (Crepis acuminata), goldenweed, bighead
clover, littleflower collinsia, pink microsteris (Microsteris
aracilis) and lupine.
Cliff-Shrub.
The west face of PJR dropped precipitously
from the upper plateau to the Warner Valley.
Much of the face
was bare rock, but vegetation occurred on narrow horizontal
benches and in some gullies.
In vegetated areas, shrubs were
common but grasses and forbs were sparse.
Primary shrub
species were mountain big sagebrush, bush rockspirea, low
sagebrush and mountain snowberry.
Cusick bluegrass
( aa
cusickii), bottlebrush squirreltail and bluebunch wheatgrass
were the most common grasses.
Forb species included Pacific
monardella, granite gilia and littleflower collinsia.
Juniper-Low Sagebrush. Large areas of the eastern slopes
of PJR supported open stands of western juniper with a low
sagebrush - bunchgrass understory.
Juniper density averaged 9.7
23
trees/ha.
Bluebunch wheatgrass was the dominant grass; other
common grasses were bottlebrush squirreltail,
Sandberg's
bluegrass and prairie junegrass (Koelaria Dvramidata).
species
included bighead clover,
Forb
littleflower collinsia,
tapertip hawksbeard and longleaf phlox (Phlox longifolia).
Juniper-Mountain Big Sagebrush. Denser stands of western
juniper with an understory of taller shrubs and few herbaceous
species occurred in association with small canyons and rocky
breaks on the east side of PJR.
habitat was 90.8 trees/ha.
Mean juniper density in this
Mountain big sagebrush was the
dominant shrub, but wax currant, Wyoming big sagebrush and
mountain snowberry were also common.
The sparse herbaceous
layer included bottlebrush squirreltail, bluebunch wheatgrass,
Thurber
needlegrass,
arrowleaf
balsamroot
(Balsamorhiza
sagittata), lupine and littleflower collinsia.
Wvomina Bia Saaebrush-Bunchgrass.
This habitat occurred
on the floor of Catlow Valley between the lower east side of
PJR and Rock Creek, and at all elevations on the northern end
of PJR.
It had the lowest vegetation cover of any PJR
habitat.
The shrub layer was dominated by Wyoming big
sagebrush, and green rabbitbrush was occasionally encountered.
Herbaceous species included Thurber needlegrass,
bluebunch
wheatgrass, Sandberg's bluegrass and cheatgrass brome.
24
Table 2.
Characteristics of shrubby and herbaceous vegetation in bighorn
sheep habitats on Hart Mountain, June 1991.
Shrubs
Grasses
Forbs
Total
Canopy
Cover
Total
Canopy
Cover
Total
Canopy
Cover
Density
Mean
Height
(%)
(no./m4)
(cm)
(%)
(%)
Low sagebrush
plateau
23.0
5.6
8.8
16.3
14.2
Mountain big
sagebrush plateau
32.8
3.8
28.2
16.1
15.2
Cliff/talus-shrub°
39.0
2.6
42.8
8.5
6.7
Mountain big
sagebrushbunchgrass
23.6
1.8
33.6
16.7
13.1
2.1
0.3
50.2
56.4
42.9
23.7
1.2
45.3
10.9
7.0
Habitat
Riparian
Juniper/mountain
mahogany-mountain
big sagebrush
°Results reflect characteristics within islands of vegetation.
Table 3. Characteristics of shrubby and herbaceous vegetation in bighorn
sheep habitats on Poker Jim Ridge, June 1991.
Shrubs
Grasses
Forbs
Total
Canopy
Cover
Total
Canopy
Cover
Total
Canopy
Cover
Density
Mean
Height
(%)
(no./m4-)
(cm)
(%)
(%)
Low sagebrush
plateau
17.8
1.9
16.8
8.1
16.8
Low sagebrushbunchgrass
20.7
1.4
23.3
15.0
18.3
Cliff-shrub°
32.1
1.8
27.2
6.2
8.4
Juniper-low
sagebrush
17.6
1.4
23.4
25.5
15.6
Juniper-mountain
big sagebrush
25.9
0.7
82.0
7.3
12.1
Wyoming big
sagebrushbunchgrass
14.9
0.8
50.1
11.9
Habitat
°Results reflect characteristics within islands of vegetation.
4.6
25
Habitat Use
Analysis of habitat use by bighorn sheep on HMNAR was
based on observations of 896 groups collected 7 April-31
August 1990 and 10 June-31 August 1991.
Adult bighorn sheep
segregated sexually during spring and summer, and occupied 5
distinct ranges.
Ram and ewe ranges varied seasonally and
overlapped considerably
(Figs.
3-5).
Numbers
of
group
observations per range stratified by sex in spring 1990,
summer 1990 and summer 1991, respectively, were:
(1) Rams,
HM--49, 88 and 124;
(3) ewes,
(2) rams, PJR--61, 94 and 95;
SHM--42, 72 and 74;
(4)
ewes, NHM--14, 57 and 61; and (5)
ewes, PJR--31, 8 and 26.
Seasonal Distribution
Rams on PJR were observed most frequently on the upper
plateau during spring and early summer.
In mid-summer they
expanded their range to include the gentle east slopes and
Catlow Valley in the vicinity of Rock Creek and Flook Knoll.
Rock Creek from the Morgan drift fence to Flook Knoll was an
important summer water source.
The creek went dry in mid-
August 1990 and rams shifted to the south end of PJR in the
vicinity of Petroglyph Lake, which provided a late summer
water source.
This did not occur in 1991.
Juniper stands on
the east side of PJR were used for thermal cover.
The HM ram range included the entire length of the
mountain from 1.0 km south of the refuge access road to the
26
south end of the upper plateau.
Spring and early summer use
was generally restricted to the upper plateau within 0.5 km of
the rim and the upper half of the west face, but expanded
during late summer to include the lower half of the west face
down to Warner Valley.
Numerous springs and creeks on and
below the cliffs provided water sources.
Thermal cover was
provided by vegetation and overhanging rock shelves.
The PJR ewe range included the upper plateau within 1.0
km of the rim and the steep west face.
There were no
perennial water sources and few opportunites for thermal
cover.
1991,
This area was abandoned by mid-June 1990, and mid-July
and
I noted coincident increases in ewe,
yearling numbers on NHM.
lamb and
The migration corridor appeared to
follow the cliffs and cross the refuge access road at 1,5241,737 m elevation.
The NHM ewe range extended north of Juniper Canyon to the
refuge access road, and the SHM range extended from Hart
Canyon to the south end of the upper plateau.
In spring and
early summer, ewe groups were observed primarily on cliffs and
broken terrain on the upper half of the west face, and on the
upper plateau within 0.5 km of the rim.
In late summer, the
ranges expanded to include lower slopes on the west face.
In
1990, the NHM range also included the upper plateau and the
east side of the mountain above Willow Creek.
During November of both years, I observed mixed groups in
ewe ranges on SHM, NHM and PJR.
Two radio-collared rams from
27
Fig. 3.
Bighorn sheep ranges on Hart Mountain National Antelope Refuge,
spring 1990.
Determined by minimum convex polygon method (Southwood
1966). Ranges: NHM = North Hart Mountain, SHM = South Hart Mountain, PJR
= Poker Jim Ridge.
28
Fig. 4.
Bighorn sheep ranges on Hart Mountain National Antelope Refuge,
summer 1990.
Determined by minimum convex polygon method (Southwood
1966).
Ranges: NHM = North Hart Mountain, SHM = South Hart Mountain, PJR
= Poker Jim Ridge.
29
Fig. 5.
Bighorn sheep ranges on Hart Mountain National Antelope Refuge,
summer 1991.
Determined by minimum convex polygon method (Southwood
1966). Ranges: NHM = North Hart Mountain, SHM = South Hart Mountain, PJR
= Poker Jim Ridge.
30
PJR moved to the north end of NHM in November 1990,
subsequently returned to PJR 2 days later.
and
This was the only
documented movement of radio-collared rams between ranges.
Use of Habitat Types:
Hart Mountain.
Spring vs. Summer 1990
Use of HM habitat types did not differ
between spring and summer 1990 for rams (X2 = 0.74, 4 df, E =
0.95), but did for ewes (X2 = 12.15, 4 df, P = 0.016)
6).
(Fig.
Use of the riparian habitat was too infrequent to
consider it separately, so it was included in the surrounding
habitat (generally mountain big sagebrush-bunchgrass).
Ewes
used the cliff/talus-shrub habitat more frequently, and the
mountain big sagebrush-bunchgrass habitat less frequently, in
spring than summer.
poker Jim Ridge.
Use of habitat types by rams on PJR
differed between spring and summer 1990 (X2 = 34.4, 5 df, k <
0.001) (Fig. 7).
The low sagebrush plateau habitat was used
more frequently in spring, and the juniper-low sagebrush and
Wyoming big sagebrush-bunchgrass habitats were used more
frequently in summer.
Use of Habitat Types:
Summer 1990 and 1991
Hart Mountain. Proportional use of habitat types in both
summers differed between rams and ewes (1990: X2 = 35.0, 4 df,
P < 0.001; 1991: X2 = 44.3, 4 df, P < 0.001) (Fig. 8).
Rams
31.
80
Spring
70-
, Summer
Rams
6050-
40302010-
0
0
4-)
U
3
4
5
4
5
80
Spring
a)
04
2
1
0
70-
VA
Summer
Ewes
6050
403020
10
0
1
2
3
Habitat Type
Fig. 6.
Percent use of habitat types by rams and ewes on Hart Mountain,
spring and summer 1990.
Habitat types:
1 = low sagebrush plateau, 2 =
mountain big sagebrush plateau, 3 = cliff/talus-shrub, 4 = mountain big
sagebrush-bunchgrass, 5 = juniper/mountainmahogany-mountain big sagebrush.
* = Different use between spring and summer, 95% simultaneous confidence
intervals (Neu et al. 1974).
32
m
M
50-
4.)
a)
40-
U
w
30-
)34
1
2
3
4
5
6
Habitat Type
Fig. 7.
Percent use of habitat types by rams on Poker Jim Ridge, spring
and summer 1990.
Habitat types:
1 = low sagebrush plateau, 2 = low
sagebrush-bunchgrass, 3 = cliff-shrub, 4 = juniper-low sagebrush, 5 =
juniper-mountain big sagebrush, 6 = Wyoming big sagebrush-bunchgrass. *
= Different use between spring and summer, 95% simultaneous confidence
intervals (Neu et al. 1974).
used the cliff/talus-shrub habitat less, and the mountain big
sagebrush-bunchgrass and juniper/mountainmahogany-mountain big
sagebrush habitats more, than ewes in both summers.
The
riparian habitat was again not considered separately.
Use of habitat types did not differ between summers for
ewes on HM (X2 = 1.10, 4 df, P = 0.89), but did for rams (X2
= 11.99, 4 df, P = 0.017), although none of the individual
differences were significant at a = 0.05.
33
1
Rams
2
f/ A
3
4
5
4
5
Ewes
Summer 1991
*
1
2
3
Habitat Type
Percent use of habitat types by rams and ewes on Hart Mountain,
summer 1990 and 1991.
Habitat types: 1 = low sagebrush-plateau, 2 =
mountain big sagebrush-plateau, 3 = cliff/talus-shrub, 4 = mountain big
Fig. 8.
sagebrush-bunchgrass, 5 = juniper/mountainmahogany-mountain big sagebrush.
*
= Different use between summer 1990 and 1991,
95% simultaneous
confidence intervals (Neu et al. 1974).
34
U
30
4-)
0
0
U
14
0
20-
2
1
3
4
5
6
Habitat Type
Percent use of habitat types by rams on Poker Jim Ridge, summer
1 = low sagebrush plateau, 2 = low
1990 and 1991.
Habitat types:
sagebrush-bunchgrass, 3 = cliff-shrub, 4 = juniper-low sagebrush, 5 =
Fig. 9.
juniper-mountain big sagebrush, 6 = Wyoming big sagebrush-bunchgrass. * =
Different use between summer 1990 and 1991, 95% simultaneous confidence
intervals (Neu et al. 1974).
Poker Jim Ridge.
Use of habitat types differed between
summers for rams on PJR (X2 = 30.2, 5 df, P < 0.001) (Fig. 9).
The low sagebrush plateau habitat was used less,
and the
juniper-low sagebrush habitat more, in 1990 than 1991.
Use of Habitat Characteristics:
Rams. HM.
Spring vs. Summer 1990
Median distance of HM ram groups to escape
terrain and water was less in spring than summer 1990 (Z =
-2.06, P = 0.040; and Z = -2.53, P = 0.011, respectively)
35
(Fig. 10).
Significant differences were not found in median
slope (Z = 1.07, P = 0.29) and elevation (Z = -0.0067, P =
0.99), or proportional use of aspect categories (X2 = 4.72, 3
df, P = 0.19) (Fig. 10).
The fitted logistic regression model included the main
effects distance to escape terrain and water only (Table 4).
The contribution of explanatory variables to the fitting of
the model was significant
(X2
= 12.24,
2
df,
P = 0.002).
Sensitivity, specificity and overall correct classification
rate were 30.6%, 84.1% and 65.0%, respectively.
Ewes. HM.
Median distance of HM ewe groups to escape
terrain was less in spring than summer 1990 (Z = -3.74, P <
0.001), and median distance to water, slope and elevation were
greater (Z = 1.99, P = 0.047; Z = 3.12, P = 0.002; and Z =
2.40, P = 0.017, respectively).
Proportional use of aspect
categories did not differ (X2 = 4.56, 3 df, P = 0.21)
(Fig.
11).
The fitted logistic regression model included 4 main
effects (distance to escape terrain, distance to water, slope,
elevation) and 2 interaction terms (distance to escape terrain
x slope, distance to water x slope)
(model
1,
Table 5).
Explanatory variables contributed significantly to the fitting
of the model (X2 = 30.26,
6 df, P < 0.001).
Sensitivity,
specificity and overall correct classification rate were 9.4%,
89.1% and 65.9%, respectively.
36
v
0.040
E
0.011
0.13
29
sprtng
M001an
Spriny
101.0 m
121.2 m
mocia40
0.19
0.13
3 0.09
0.03
0.01
0.07
0.11
Summer
Median
Summer
155.8 m
391.5 m
Medlar)
0.17
0.21
0
8
5
2
0
14
11
C
0.29
8
5
2
14
11
DlOtAned to Meter (m a 100)
Distance to [scope Terrain (m x 1001
0.29
0.15
Spr%ng
median
Spring
Spring
Median
43.0
1996.0 m
0.4
0.1
0.19
0.2
'70.05
0.09
0.01
0.05
0.11
0.1
0.4
Summer
Medlar)
Summer
36.0 %
0.21
0.15
0
20
40
60
80
100
120
Sumter
2004.0 m
Medlar)
0.6t14
16
18
20
22
24
26
S
tlematlon (m x 100)
Slope (4)
M
Aspect
Fig. 10.
Relative frequency distributions and median values of distance
to escape terrain, distance to water,
slope and elevation,
and
proportional use of aspect categories, ram groups on Hart Mountain, spring
and summer 1990.
P values given for tests of difference between spring
and summer distributions (see text).
Table 4.
Estimated coefficients from logistic regression of probability
of a ram group observation on Hart Mountain in spring and summer 1990
occurring in spring against distance to escape terrain (m) and distance to
water (m).
Variable
Intercept
Estimated Coefficient
0.6422
Standard Error
0.4031
Distance to escape
terrain
-0.001821
0.0007819
Distance to water
-0.002088
0.0008378
37
p < o.00l
p
0,047
0.8 (.
Spring
Median - 0 m
0.4
0
A
A
A
0.4
Summer
Median
62.2 m
0.8
O
2
5
8
11
O
14
8
E - 0.002
E
0.22
Spring
Median
20
16
12
Distance to mater (m a 100)
Distant. to Escape Terrain (a s 100)
0.25
0.017
spring
median
67.0
0.21
0.56
Spring
2210.0 at
0.15
0.36
0.05
0.16
0.05
0.04
0.1.2
0.02
0.08
0.24
0.15
Summer
Median
Summer
Median
53.0 %
0.15
2126 m
Summer
0.44
0.25
O
20
SO
80
110
Slope (6)
140
170
13
15
17
19
21
23
25
Elevation lm a 1001
Fig. 11. Relative frequency distributions and median values of distance
to escape terrain,
distance to water,
slope and elevation,
and
proportional use of aspect categories, ewe groups on Hart Mountain, spring
and summer 1990.
P values given for tests of difference between spring
and summer, and arrows indicate influential outliers (see text).
I suspected the complexity and poor sensitivity of the
above model was due to the inclusion of 5 summer observations
with outlying values for distance to escape terrain (Fig. 11,
arrows).
These observations were the same ones comprising the
5 lowest elevations.
Their exclusion resulted in a simpler
model including only the main effects slope and elevation
38
Estimated coefficients from logistic regression of probability
of a ewe group observation on Hart Mountain in spring and summer 1990
occurring in spring against distance to escape terrain (m), distance to
water (m), slope (%) and elevation (m).
Table 5.
Modela
Variable
Estimated Coefficient
Standard Error
1
Intercept
-12.09
4.146
Distance to escape
terrain
-0.006762
0.003492
Distance to water
0.001186
0.0008994
Slope
0.02749
0.01262
Elevation
0.004598
0.001846
(Distance to escape
terrain) x (slope)
0.0001029
0.00005646
(Distance to water) x
-2.161 x 10'5
0.00001445
-11.54
3.177
(slope)
Intercept
2
Slope
0.02062
0.005672
Elevation
0.004437
0.001397
aModel 1 includes all observations, model 2 derived from data set with
some outliers omitted (see text).
(model 2, Table 5), which contributed significantly to model
fit (X2 = 21.60, 2 df, P < 0.001).
Sensitivity was still low,
but both sensitivity and specificity were improved (11.3% and
90.3%, respectively). Overall correct classification rate was
66.7%.
Rams. PJR.
Median distance to escape terrain and slope
were less in spring than summer 1990 for ram groups on PJR (Z
= -2.71, P = 0.007; and Z = -3.31, P = 0.002, respectively),
and median distance to water and elevation were greater (Z =
4.58, P < 0.001; and Z = 4.37, P < 0.001)
The
fitted
(Fig. 12).
logistic regression model
included main
effects and quadratic terms for distance to escape terrain and
39
e 4 0.001
0.007
0.1S
Spring
median
Spring
modian
845.9 m
4454.7 m
0
0.05
0.05
0.1
0.2
Summer
Median
Summer
Median
1594.7
3207.2 m
0.15
.3r
80
60
40
20
0
20
0
40
0.002
80
60
Distance to *****
Distance to Recaps Terrain (s. a 100)
. 100)
< 0.001
0.25
0.6
0.4
Spring
Median
1881.5
Summer
Median
1794.0
0.15
0.2
0.05
0.00
0=
0.15
0.4
Summer
Median
8.0
0.25
0.610
10
30
50
70
90
IS
Slope (%)
16
17
18
19
20
Slevatlon (5 0 100)
Fig. 12. Relative frequency distributions and median values of distance
to escape terrain, distance to water, slope and elevation, ram groups on
Poker Jim Ridge, spring and summer 1990.
P values given for tests of
difference between spring and summer, and arrows indicate influential
outliers (see text).
water, slope and elevation, and a significant distance to
escape terrain x distance to water interaction (model 1, Table
6).
The contribution of the variables to the fitting of the
model was
significant
(X2
=
54.26,
5
df,
P
<
0.001).
Sensitivity, specificity and overall correct classification
rate were 73.8%, 76.6% and 75.5%, respectively.
40
Table 6.
Estimated coefficients from logistic regression of probability
of a ram group observation on Poker Jim Ridge in spring and summer 1990
occurring in spring against distance to escape terrain (m), distance to
water (m), slope (%) and elevation (m).
Modela
Variable
1
Intercept
7.838
9.856
Distance to escape
terrain
0.01035
0.003453
Estimated Coefficient
(Distance to escape
terrain)4
Distance to water
0.006964
5.455 x 10-7
0.002648
(Distance to water)2
-4.457 x 10-7
2.301 x 10-7
(Distance to escape
terrain) x (distance
to water)
-1.829 x 10.6
5.706 x 10-7
Slope
2
-1.611 x 10-6
Standard Error
0.1779
0.1682
(Slope)2
-0.01370
0.007877
Elevation
-0.01622
0.006774
Intercept
-14.33
5.129
Distance to escape
terrain
(Distance to escape
terrain)4
Distance to water
0.009151
-1.475 x 10-6
0.004378
0.003100
5.145 x 10-7
0.001774
(Distance to water)2
-2.741 x 10-7
1.623 x 10-7
(Distance to escape
terrain) x (Distance
to water)
-1.521 x 10.6
4.997 x 10-7
aModel 1 includes all observations, model 2 derived from data set with
some outliers omitted (see text).
Although the above model fit the data reasonably well,
it's complexity limited its usefulness.
I sought a simpler
model by excluding those summer observations with outlying
slope values (Fig. 12, arrows).
These observations were of
groups on the steep west face of PJR, and 8 of 10 were small
groups, <3 rams.
These groups may have been responding to
41
some stimulus that caused them to seek escape terrain, such as
my approach (although this was not observed).
Exclusion of 7
observations with slope >50% did not simplify the model much,
but exclusion of 10 observations with slope >30% resulted in
a model with main effects and quadratic terms for distance to
escape terrain and water, and a significant distance to escape
terrain x distance to water interaction
The addition
of
elevation
to
(model 2, Table 6).
this model resulted
in
a
significant decrease in model deviance (X2 approximation =
6.48,
1 df, P = 0.011), but elevation was highly correlated
with both distance to escape terrain and water
(Pearson
correlation coefficient = -0.820 and 0.829, respectively).
The standard errors of the coefficients of the other variables
decreased substantially when elevation was dropped, further
emphasizing the collinearity.
Sensitivity, specificity and
overall correct classification rate of the final model were
72.1%, 71.4% and 71.7%, respectively.
Use of Habitat Characteristics:
Rams vs. ewes, HM.
Summer 1990 and 1991
Median distance of HM ram groups to
escape terrain was greater than that of ewe groups in summer
1990 and 1991 (Z. = 8.83, P < 0.001), and median distance to
water, slope and elevation were less (Z = -6.70, P < 0.001; Z
= -4.34, P < 0.001; and Z = -5.81, P < 0.001, respectively).
Proportional
use
of
aspect
categories
significantly (X2 = 20.62, 3 df, P < 0.001)
also
differed
(Fig. 13).
42
r<
e . 0.001
0.5
0.001
0.2
Name
Rams
Median
225.9
Median
136.4 en
Sees
Median
540.7 m
0.3
0.1
0
! 0.1
0.1
0.3
[yea
Median
38.9
0.5
0.2
0
2
5
8
1
Is
0
Dltanc to Recaps Terrain ( 4 100)
6
2
10
IS
14
Distance to WAtOr fin
71
22
1001
2 < 0.001
0.10
0 < 0.001
0.2
Ram
Rao.
nedln
33.5
Median
1995.0
twee
Median
2123.0
0.1
0.1
ri 0.00
0
0.00
0.1
[we
Median
SO
110
Slope 10)
53.0
140
170
13
15
17
19
21
25
23
Elevation ( a 100)
Fig. 13. Relative frequency distributions and median values of distance
to escape terrain, distance to water,
slope and elevation,
and
proportional use of aspect categories, ram and ewe groups on Hart
Mountain, summer 1990 and 1991. P values given for tests of difference
between rams and ewes (see text).
Logistic regression analysis indicated significant main
effects and quadratic terms for distance to escape terrain,
distance to water and slope, and a distance to escape terrain
x distance to water interaction (Table 7).
Contribution of
explanatory variables to the
fitting
the model
significant
df,
(X2
=
170.09,
7
P
of
<
0.001).
was
Model
sensitivity, specificity and overall correct classification
43
Table 7. Estimated coefficients from logistic regression of probability
of a ram or ewe group observation on Hart Mountain in summer 1990 and 1991
being a ram group against distance to escape terrain (m), distance to
water (m) and slope (%).
Variable
Estimated Coefficient
Standard Error
Intercept
-0.6510
0.5588
0.001605
0.004349
Distance to escape terrain
(Distance to escape terrain)2
1.456 x 10-6
-3.791 x 10-6
Distance to water
0.001172
0.001697
-4.308 x 10-6
1.168 x 10.6
(Distance to escape terrain)
x (distance to water)
5.198 x 10-6
1.797 x 10.6
Slope
0.02161
0.01412
(Distance to water)2
0.0001402
-0.0003161
(Slope)2
Ewes
Rama
150
120
tt 90
a.
0
60
24
4
20 e
24 c
20
30
16 "el,
12
16
...P.,:P.
12
8
4 act
0f
6
to
(M X
Fig. 14.
12
Pe per
100)
0
15
rain
s:,''
:.
:1'
0 6"\
4,
'''
4'
0
8 .5.1.
4 tc,
3
Distan,
6
-0 to z
9
22
0
4".
(41 .112,7:7 Te,railn5
Observed locations of ram and ewe groups on Hart Mountain during
summer 1990 and 1991 in relation to distance to escape terrain, distance
to water and slope.
Arrow indicates potential influential outlier (see
text).
44
rate were 69.8%, 71.6% and 70.8%, respectively.
1 examined
three dimensional plots of the data for trends and possible
influential outliers (Fig. 14); one ewe group observation was
suspect (Fig. 14, arrow), but its exclusion did not change the
variables included in the fitted model, and changed their
estimated coefficients only slightly.
Rams. PJR vs. HM.
Median distance to escape terrain and
water was greater for ram groups on PJR than HM in summer 1990
and 1991
(Z = 8.79, P < 0.001; and ZE = 15.52, P < 0.001,
respectively), and median slope and elevation were less (Z =
-11.04, P < 0.001; and Z = -11.78, P < 0.001, respectively)
(Fig. 15).
The fitted logistic regression model included the main
effects distance to escape terrain and water only (Table 8).
Explanatory variables contributed significantly to the fitting
of the model (X2 = 515.00, 2 df, P < 0.001).
Sensitivity,
specificity and overall correct classification
97.4%, 99.5% and 98.5%, respectively.
rate were
45
e < 0.001
f < 0.001
U.61
0.6
P78
P.18
Medlan
madLan - 3772.5 m
782.1 m
0
2
0.2
i 0.2
0.2
O.
.
HO
104
median
225.9 m
0.6
Median
336.4 m
0.6
1
1
20
0
40
80
60
0
Distance to Serape Terrain (m x 100)
80
60
40
20
Distance to Water (a x 1001
c 0.001
0.4
0.32
median
1804.0 m
0.22
0.2
0.12
0.02
0
0.08
0.2
0.18
MM
Median
xM
33.5 %
0.
1995.0 m
0.28
0
20
50
80
Slope (6)
110
140
14
16
18
20
22
24
26
Elevation I. a 100)
Fig. 15. Relative frequency distributions and median values of distance
to escape terrain, distance to water, slope and elevation, ram groups on
Poker Jim Ridge and Hart Mountain, summer 1990 and 1991. P values given
for tests of difference between Poker Jim Ridge and Hart Mountain (see
text).
Table 8. Estimated coefficients from logistic regression of probability
of a ram group observation on Poker Jim Ridge or Hart Mountain in summer
1990 and 1991 occurring on Poker Jim Ridge against distance to escape
terrain (m) and distance to water (m).
Variable
Estimated Coefficient
Standard Error
Intercept
-8.741
1.597
Distance to escape terrain
0.004294
0.001435
Distance to water
0.006089
0.001426
46
Home Ranges of Radio-Collared Rams
Mean home range size of radio-collared rams was 15.5 (n
= 3, SE = 1.9) and 25.2 (n = 11, SE = 2.8) km2 on HM and PJR,
respectively (Table 9).
= -1.73, P = 0.11).
The difference was not significant (t
The home range of 3 rams from each range
was mapped (Appendix B).
Table 9. Home range sizes of radio-collared rams on
Hart Mountain National Antelope Refuge, 1990 and 1991.
Range
Hart Mountain
Poker Jim Ridge
Ram
Number
na
0640
18
11.8
0690
29
16.7
1265
18
18.1
1240
17
14.3
1390
21
13.6
1510
28
26.8
1520
30
25.2
1530
26
28.9
1600
32
41.1
1620
18
20.5
1640
24
39.9
0620
30
29.5
0630
24
17.4
0660
18
20.2
Home Ransot
Size (km
aN = number of observations used to calculate home
range size.
bMinimum convex polygon method (Southwood 1966).
47
Seasonal Herd Range Sizes
Summer ranges were larger than corresponding spring
ranges, and ram ranges were larger than ewe ranges (Table 10,
Figs.
3-5).
corresponding
All
1990
1991 ranges
ranges were
summer
(Table
10).
larger
than
Poker Jim Ridge
received limited use by ewes in both summers, and range sizes
were not determined.
Table 10. Seasonal bighorn sheep herd range sizes (km2) on Hart Mountain
National Antelope Refuge, 1990 and 1991.
Group
Spring Herd Range
Summer Herd Range
Year
Type°
Rangeb
Sizec (n)d
Sizec
1990
Ram
HM
38.0 (49)
68.1 (88)
PJR
24.4 (61)
74.5 (94)
SHM
13.8 (42)
16.7 (72)
NHM
1.4 (14)
6.2 (57)
PJR
7.7 (31)
Ewe
1991
Ram
Ewe
HM
**
47.8 (124)
PJR
**
29.6 (95)
SHM
**
12.9 (74)
NHM
**
3.4 (61)
aRam groups included adult rams a2-yr old.
Ewe groups included adult
ewes, yearlings and lambs.
bHM = Hart Mountain, PJR = Poker Jim Ridge, SHM = South Hart Mountain, NHM
= North Hart Mountain.
cRange size determined by minimum convex polygon method (Southwood 1966).
dN = Number of sheep group observations used for calculation of herd range
size.
*Ewes emigrated from PJR in summer 1990.
**No data available for spring 1991.
48
Population Characteristics
Group Size
Mean group size of all bighorn sheep on HM and PJR was
9.52 (SE = 0.44) and 8.23 (SE = 0.50), respectively, based on
observations of 7,576 sheep in 835 groups.
I stratified group
size by sex, range and season (Fig. 16).
Mean ram group size was larger on PJR than HM in spring
1990 (t = 4.07, P < 0.001) and summer 1991 (t = 3.29, P =
0.001), but did not differ in summer 1990
0.50).
(t = 0.68, P =
Mean ewe group size did not differ between ranges in
any season, although too few ewe groups were observed on PJR
in summer 1990 (n = 7) to be included in the analysis (spring
1990:
F = 0.21, P = 0.81; summer 1990:
t = -1.13, P = 0.26;
summer 1991: F = 1.62, P = 0.20).
I pooled ewe group size data from NHM and SHM and
compared it to ram group sizes on HM by season.
Mean ram
group size was consistently smaller than ewe group size on HM
(spring 1990: t = -3.25, 2 = 0.002; summer 1990: t = -5.61, 2
< 0.001; summer 1991: t = -6.54, P < 0.001), but they did not
differ on PJR (spring 1990: t = -0.32, P = 0.75; summer 1991:
t = -1.95, P = 0.06).
For a given range and sex, there was some variation in
trends in group size by season.
Mean group sizes of ewes on
NHM and SHM, and rams on HM did not differ between spring and
summer 1990 (ewes, NHM: t = -1.30, P = 0.20; ewes, SHM: t =
-0.69, P = 0.49; rams, HM: t = 1.33, P = 0.19), but rams on
49
PJR formed larger groups in spring (t = 6.45, P < 0.001).
Mean group size of rams on PJR and ewes on NHM and SHM did not
differ between summer 1990 and summer 1991 (rams, PJR: t =
-1.68, P = 0.09; ewes, NHM: t = 1.54, P = 0.13; ewes, SHM: t
= -1.11, P = 0.27), but ram groups on HM were larger in summer
1990 (rams, HM: t = 2.50, P = 0.13).
20
RHM
18-
RPJR
ESHM
ENHM %V4 EPJR
16IMO.
IMM
CMS
IMMO
IMM
IOU
MOM
14-
IOU.
12-
NUM.
ISOM
IMM
IOW.
ISOM
MO.
IMMO
Inn.
10-
MOM
MOO
8-
MOM
MOO
MM.
N
MOM
EMU
MOO
MOM
MM.
0.
ONO
IMMO
IMMO
IMO.
s
MOM
IMMO
IOU
6-
IMMO
I.M.
IMMO
ISOM
MOM
IMMO
4-
IMM
IMO.
IMMO
IONE
IMM
IMMO
IMMO
MOM
IMMO
IMO.
20
Spring 1990
Summer 1990
MUM
MOO
MOO
MM.
MOM
ME
UM.
MM.
ONO
MOM
EOM
ROM
MOM
MM.
MM.
MOM
MOM
E
Summer 1991
Season
Fig.
16.
Mean bighorn sheep group sizes on Hart Mountain National
Antelope Refuge.
Vertical lines represent standard errors.
Group types:
RHM = rams, Hart Mountain, RPJR = rams, Poker Jim Ridge, ESHM = ewes,
South Hart Mountain, ENHM = ewes, North Hart Mountain, EPJR = ewes, Poker
Jim Ridge.
50
Ram Age Class Structure and Survival of Radio-Collared Rams
Ram age class structure differed between PJR and HM in
summer 1990 and 1991 (1990: X2 = 36.15, 3 df, P < 0.001; 1991:
X2 = 18.33, 3 df, P < 0.001) (Fig. 17).
The HM subpopulation
included a larger proportion of class I rams and a smaller
proportion
of
subpopulation.
class
rams
III
in
1990
than
the
PJR
In 1991, HM included a larger proportion of
class II rams and a smaller proportion of class IV rams than
PJR.
Median age of radio-collared rams was 5 yr (range 3-9 yr)
when collared, and 8 yr (range 6-12 yr) at last contact.
Four
known mortalities occurred from February 1989 to September
1991, all the result of hunting (Table 11).
Lamb Production and Survival
Each of the 3 ewe ranges included an active nursery
during the 1990 and 1991 lambing seasons (Fig. 18).
These
areas were within the cliff/talus-shrub habitat on NHM and
SHM, and the cliff-shrub habitat on PJR.
I first saw newborn
lambs 20 April 1990 on SHM, and subsequently observed them in
the other nursery areas.
Few neonates were observed after 1
June.
Ewes
with
newborn
lambs
remained
isolated
from
conspecifics and within nursery areas for 2-3 weeks postpartum.
In early May, I observed small groups of adult ewes
with lambs on the cliffs.
These groups became increasingly
51
50
Hart Mountain
% / //A Poker Jim Ridge
Summer 1990
40-
30-
20-
10-
0
U
I
a)
a)
II
III
IV
50
124
Hart Mountain
Poker Jim Ridge
Summer 1991
40-
30-
20-
*
10-
0
I
II
III
IV
Age Class
Fig. 17.
Ram age class structure on Hart Mountain and Poker Jim Ridge,
summer 1990 and 1991.
Age classification follows Geist (1971).
* =
Different proportion between Hart Mountain and Poker Jim Ridge, 95%
simultaneous confidence intervals (Neu et al. 1974).
52
Table 11.
Range affiliation, number of months monitored, and age and
status at last contact for radio-collared rams on Hart Mountain National
Antelope Refuge.
Range
Affiliation
Hart
Mountain
Poker Jim
Ridge
Elapsed Time
Between First
and Last Contact
Age at Last
Contact
(Months)
(Years)
0640
29
11
Alive
0690
30
8
Alive
1610
9
8
Hunter-killed
16108
9
6
Alive
1265
30
10
Alive
1240
30
9
Alive
1320
31
8
Hunter-killed
1390
30
7
Alive
1420
29
7
Alive
1500
19
7
Alive
1510
30
6
Alive
1520
30
8
Alive
1530
30
10
Alive
1600
30
11
Alive
1620
29
12
Alive
1640
30
11
Alive
1670
31
9
Hunter-killed
0610
31
6
Hunter-killed
0620
30
8
Alive
0630
30
12
Alive
0660
29
8
Alive
Ram
Number
Status at Last
Contact
°Same radio-collar placed on second ram after first killed.
53
mobile, and by late May larger groups including ewes, lambs
and yearlings formed on the cliffs and in adjoining plateauand
sagebrush-bunchgrass habitats.
Ewes and
initially
lambs
remained in close proximity within these groups, but lambs
became increasingly independent, and by mid-June it was common
to see nursery bands consisting of several lambs with only a
few ewes.
I also observed groups of ewes with full udders but
no lambs by their side.
Mean number of lambs:100 ewes during summer 1990 was
51.8, and did not differ from 1991 (53.3; X2 = 0.092, 1 df, E
=
0.76,
n
=
respectively).
not
differ
1242
and
799
sheep
in
1990
and
1991,
Mean number of lambs:100 ewes on NHM/PJR did
from
SHM
respectively; X2 = 2.15,
in
summer
1990
(56.1
and
47.0,
1 df, E = 0.14) or 1991 (48.5 and
57.5, respectively, X2 = 1.27, 1 df, P = 0.26).
I estimated lamb:ewe ratios by month for each of the 3
ewe ranges in May-August 1990 and June-August 1991,
then
combined these figures to yield refuge-wide monthly lamb:ewe
ratios (Fig. 19).
The latter did not differ between months in
1990 (X2 = 4.39, 3 df, P = 0.22) or 1991 (X2 = 3.16, 2 df, E
= 0.21).
The refuge-wide estimate of lambs:100 ewes in August 1990
(51.3, n = 531 ewes and lambs) differed from the corresponding
estimate of yearlings:100 ewes in summer 1991 (25.7,
ewes and yearlings, X2 = 27.2,
n
= 655
1 df, P < 0.001), inferring
loss of lambs between September 1990 and June 1991.
54
Fig. 18. Bighorn sheep nursery areas on Hart Mountain National Antelope
Refuge. Nurseries: SHM = South Hart Mountain, NHM = North Hart Mountain,
PJR = Poker Jim Ridge.
55
100
908070-
6050-
403020100
July
June
May
August
100
908070-
6050-
403020100
,
June
,
July
,
August
Month
Fig. 19. Estimated number of lambs:100 ewes on South Hart Mountain, North
Hart Mountain and Poker Jim Ridge, May-August 1990 and June-August 1991.
Ranges: SHM = South Hart Mountain, NHM = North Hart Mountain, PJR = Poker
Jim Ridge, OVERALL = data from all ranges combined.
56
Ewe and Yearling Ratios
There
was
no
difference
in
observed
refuge-wide
yearling:ewe ratios between 1990 and 1991 (22.6, n = 1333 and
25.7, n = 655 yearlings:100 ewes, respectively; X2 = 1.14, 1
df, P = 0.29), nor was there a difference between yearling:ewe
ratios on SHM and NHM/PJR in 1990 (25.4 and 20.3 yearlings:100
ewes, respectively, X2 = 2.42, 1 df, P = 0.12) or 1991 (22.1
and 29.8, respectively, X2 = 2.35, 1 df, P = 0.13).
The overall ratio of male to female yearlings did not
differ from parity in 1990 (1.02:1, n = 246 yearlings, X2 =
0.016, 1 df, E = 0.90) or 1991 (1.06:1, n = 134 yearlings, X2
= 0.12, 1 df, P = 0.73).
Helicopter Surveys and Population Structure
Helicopter surveys by ODFW in June 1990 and 1991 provided
minimum estimates of the number of rams, ewes and yearlings by
range.
I multiplied these estimates by the proportions of
animals in each sex and age class determined by ground surveys
to yield estimates of absolute numbers of sheep by range, sex
and age class (Table 12).
were combined.
The NHM and PJR ewe subpopulations
Estimated total population, excluding lambs,
was 274 and 281 in 1990 and 1991, respectively.
Overall
estimates of rams:100 ewes were 76.8 and 85.4 in 1990 and
1991, respectively.
Ewes and yearlings were not differentiated in helicopter
surveys, but rams were assigned to age classes.
The results
57
presented in Table 12 were based on my field data for ram
class proportions.
The ODFW ram class data did not differ
significantly in most cases (PJR 1990: X2 = 5.82, 3 df, P =
0.12; HM 1990: X2 = 5.80, 3 df, P = 0.12; PJR 1991:Y = 3.00,
3 df, P = 0.39), but did for HM in 1991 (X2 = 22.31, 3 df, P
< 0.001).
In the latter case, the proportion of class I rams
was greater and the proportion of class IV rams less in the
field data than the helicopter survey data.
There was no difference between lamb:ewe and yearling
ratios from helicopter surveys and June ground surveys in 1990
(X2 = 0.77,
1 df, E = 0.38) or 1991 (X2 = 3.0,
1 df, E =
0.083).
Dighorn Sheep Density
Estimated densities of bighorn sheep on summer ranges
varied by year, sex and range (Table 13).
Ram density was
consistently less than ewe density, and sheep density was
greater in 1991 than 1990 on corresponding ranges.
Ewe,
yearling and lamb density on NHM was approximately 4x greater
than SHM.
58
Table 12.
Population structure of adult rams, ewes and yearlings on Hart
Mountain National Antelope Refuge, 1990 and 1991.
Rams
Age Classc
Group
Ewes
Ydd
Y9e
SHM
53
5
6
NHM/PJR
85
9
10
SHM
64
7
7
NHM/PJR
66
10
10
Year
Type°
Rangeb
1990
Ram
HM
PJR
Ewe
1991
Ram
Ewe
I
II
III
IV
Total
17
21
15
7
60
7
13
19
7
46
HM
10
20
18
6
54
PJR
10
16
25
12
63
°Ram groups included adult rams a2-yr old. Ewe groups included adult ewes
apd yearlings.
°HM = Hart Mountain, PJR = Poker Jim Ridge, SHM = South Hart Mountain,
NHM/PJR = North Hart Mountain and Poker Jim Ridge combined (see text).
CClassification follows Geist (1971).
dYd = yearling males.
eY9 = yearling females.
59
Table 13.
Bighorn sheep density on 1990 and 1991 summer ranges, Hart
Mountain National Antelope Refuge.
Group
Total No.
Bighornc
No. Bighorn/
HM
60
0.9
PJR
46
0.6
SHM
89
5.3
NHM
152
24.5
HM
54
1.1
PJR
63
2.1
SHM
115
8.9
NHM
118
34.7
Year
Type'
Rangeb
1990
Ram
Ewe
1991
Ram
Ewe
km"
°Ram groups included adult rams z2-yr old.
Ewe groups included adult
ewes, yearlings and lambs.
bHM = Hart Mountain, PJR = Poker Jim Ridge, SHM = South Hart Mountain, NHM
= North Hart Mountain.
cTotal numbers of sheep inhabiting ranges based on Oregon Department of
Fish and Wildlife helicopter census for ewes and yearlings, and ground
census results for lambs.
Sheep numbers on the NHM ewe range include
agimals from PJR (see text).
°Calculated as total number of sheep divided by size of range (see Table
10).
60
DISCUSSION
Habitat Use
Seasonal
Distribution
and
Use
Habitat
of
Types
and
Characteristics
Seasonal migrations of bighorn sheep occur in response to
changes
in
resource
availability
and
physiological
behavioral states (Geist 1971, Shannon et al. 1975).
and
Benefits
of migrations are expressed in terms of growth, survival and
reproduction
(Bergerud
1974),
and
costs
include
energy
expenditure, vulnerability to predation and time spent in
suboptimal habitat (Nelson and Mech 1981).
Bighorn sheep may
have several seasonal ranges, and migrations between them tend
to be traditional (Geist 1971).
Of the 5 subpopulations of bighorn sheep I identified on
HMNAR, only PJR ewes used a distinct migration corridor to
move between ranges.
The lack of perennial water sources
within acceptable habitat for ewes on PJR was probably the
primary stimulus for the migration.
Many of these animals
were lactating and thermal cover was scarce, both of which
increased
water
Succulent
forage
requirements
(Turner
and ephemeral water
and
Weaver
1980).
from precipitation
presumably satisfied the requirements of these ewes during
spring.
Migration to NHM occurred in June 1990 and July 1991,
coincident with the progression of plant phenology from an
early-season condition of adundant forbs and new grass and
61
shrub growth, to a late-season condition of poorer quality
vegetation and the beginning of the hot, dry days of summer.
The delay of migration in 1991 was probably a response to
greater precipitation in spring and early summer of that year
(Fig. 20), resulting in prolonged availability of succulent
forage and ephemeral water sources.
In years of above average
precipitation, when water was available on PJR, some ewes
remained on this range throughout the summer (Kornet 1978,
Cottam 1985).
The other subpopulations of bighorn sheep
on HMNAR
gradually expanded into contiguous areas from spring to summer
1990.
No distinct migrations like that of PJR ewes were
observed; the cut-off dates between seasons therefore provided
useful, albeit somewhat arbitrary, descriptors of seasonal
changes
in
range
size
and
use
of
habitat
types
and
physiographic features.
Herd
range
size
is
a
function
of
home ranges
of
individuals within a herd and the degree of overlap between
those ranges.
Bighorn sheep are gregarious (Blood 1963,
Simmons 1980), and thus home ranges within a herd overlap
considerably (Leslie and Douglas 1979).
This occurred in this
study, as demonstrated by the home range plots of selected
radio-collared rams on HM and PJR (Appendix B).
Thus,
I
believe observed increases in herd range size between spring
and summer 1990 reflected increases in home range size, rather
than dispersal of individuals or small groups.
62
12
1989
1990 % / //A 1991
50-yr. mean
10-
E
U
F
D
Fig. 20. Monthly precipitation at Hart Mountain National Antelope Refuge
headquarters, 1989-1991, and 50-yr averages.
Habitat quality,
determined by the availability and
juxtaposition of forage, water and escape terrain, is a major
determinant of home range size in bighorn sheep (Krausman et
al. 1989).
On HMNAR, escape terrain was obviously constant,
but forage and water availability generally decreased over
time in spring and summer.
Range sizes increased during the
period,
the
consistent
with
hypothesis
that
resource
limitations cause bighorn sheep to range over larger areas to
satisfy physiological needs (Leslie and Douglas 1979, Krausman
et al. 1989).
The preference of ewes with young lambs for
areas close to escape terrain (Blood 1963, Woolf et al. 1970,
63
Geist and Petocz 1977, Leslie and Douglas 1979, Gionfriddo and
Krausman 1986) was another factor acting to keep ewe ranges
smaller in spring than summer; much of the ewe population was
restricted to nursery areas and adjacent precipitous terrain
until late May 1990.
Finally, the addition of PJR ewes to the
NHM ewe range probably caused expansion of the NHM range in
summer 1990.
The regression models developed for differential use of
specific habitat characteristics identified variables that
explained the majority of variation between between seasons,
sexes and ranges.
No causality was inferred from the models
themselves, but knowledge of physiological and behavioral
requirements
of
bighorn
sheep
and
the
availability
of
resources on HMNAR allowed speculation regarding reasons for
observed differences.
The models were
explanatory variables chosen for inclusion,
other biotic and abiotic factors
(eg.
by the
limited
and obviously
visibility,
compositon and quality) affected sheep distribution.
forage
However,
slope, elevation, apect and proximity to escape terrain and
water are known to be important determinants of habitat use
(Shannon et al. 1975, McQuivey 1978, Tilton and Willard 1982,
Van Dyke et al. 1983, Reisenhoover and Bailey 1985, Gionfriddo
and Krausman 1986, Krausman and Leopold 1986, Fairbanks et al.
1987,
Wakelyn 1987),
and bighorn sheep are opportunistic
foragers, adapting their diet to available plant communities
(Browning and Monson 1980, Keating et al.
1985, Miller and
64
Gaud 1989).
Furthermore,
I defined habitat types based on
physiographic and vegetative features, and differences in
their use between seasons and sexes provided additional
insight into differential use of habitat.
Observed movements of PJR rams between spring and summer
1990 were probably largely the result of changing water
requirements associated with changes in forage succulence and
availability of ephemeral water sources.
In mid-summer, ram
activity shifted from the upper plateau to the east slopes and
Catlow Valley in the vicinity of Rock Creek.
In late summer,
when the creek went dry, a shift towards Petroglyph Lake
occurred.
The lake was much less remote, being within sight
of the refuge access road and receiving some human use, but
was closer to escape terrain.
The former may have been
contributed to its being a "second choice" water source, and
the latter may have offset effects of human disturbance.
Logistic regression identified distance to water and
escape terrain as significant explanatory variables (model 2,
Table 6).
The response surface generated by this model for
the probability of a group observation at a given distance
from water and escape terrain being a spring observation (Fig.
21) demonstrated the movement of rams away from the upper
plateau (close to escape terrain, far from water) towards Rock
Creek (far from escape terrain, close to water) and Petroglyph
Lake (close to escape terrain and water) in summer.
Note that
the probability of a given observation described by the
65
response
surface
being
a
1 - P(spring) (Fig. 21).
summer
observation was
simply
The model is only valid within the
observed range of main effects (scatterplot, Fig. 21), and
inferences could not be made over the entire response surface.
The PJR ram range included 4 artificial water catchment
devices (guzzlers), but I did not observe use of any of these.
Two were constructed in August 1991,
and were similar to
guzzlers proven successful on Nevada bighorn sheep ranges (L.
Conn, ODFW, pers. commun.).
If they are accepted by PJR rams,
the rams will not have to move as far for water, and seasonal
distributions and habitat relationships may change from those
reported here.
Differences in use of habitat types by PJR rams between
spring and summer 1990 (Fig. 7) also reflected the trend of
dispersal from the upper plateau (low sagebrush plateau) in
early summer and utilization of Rock Creek, which was within
the Wyoming big sagebrush-bunchgrass habitat.
There was also
a significant increase in use of the juniper-low sagebrush
habitat in summer; it provided thermal cover, had the greatest
grass canopy cover of any PJR habitat (Table 3), and may have
held succulent forage longer due to favorable microclimates
created
by
shade
relatively low
(R.
of
juniper
trees.
= 9.7 trees/ha)
Tree density was
and did not compromise
visibility greatly. Use of the juniper-mountain big sagebrush
habitat did not differ between seasons, although it and the
juniper-low sagebrush habitat were interspersed.
Tree density
66
Summer
Spring
80
0
0
- 60
1.0
0.8
s
4
0'
x 40
0.6
0
C
0.4
a
80
0.2
4 a
20
60 4'
40
0
20
piata nce
20
40
0 Esca
(m
0
-,'0 4,0
60
Pe Terrai
100)
0
20
40
60
80
Distance to Escape Terrain (m x 100)
Fig. 21.
Response surface generated by logistic regression model for
differences in distance to escape terrain and water of ram groups on Poker
Jim Ridge between spring and summer 1990, and a scatter plot of observed
values.
P(spring) = predicted probability of a group observation at a
site described by the response surface occurring in spring.
here was much greater (R = 90.8 trees/ha) and grass canopy
less
(Table
3),
making it a less suitable bighorn sheep
habitat (Shannon et al. 1975, Tilton and Willard 1982, Wakelyn
1987) .
In contrast to PJR, shifts in distribution of rams on HM
between spring and summer 1990 were subtle.
types did not differ (Fig.
Use of habitat
6), and there were only slight
changes in distance to escape terrain and water (Fig. 10).
Logistic regression analysis indicated these were significant
explanatory variables (Table 4), but sensitivity and correct
classification rate were low (30.6% and 65.0%, respectively).
The response surface generated by the model demonstrated that
67
distance to escape terrain and water was predicted to vary
little over the observed range of values (Fig. 22).
These
variables were poor predictors of seasonal shifts on HM
because,
unlike
PJR,
water
and
escape
terrain
interspersed throughout the range and were not
factors.
were
limiting
Other variables (eg. reduced forage availability)
were probably responsible for the observed range expansion in
summer.
I believe that observed shifts in HM ewe distribution
between spring and summer 1990 were largely associated with
lambing and lamb rearing.
In spring, ewes tended to use
steeper terrain closer to escape cover (Fig. 11), consistent
with typical physiography of
lambing areas
(Geist
1971,
Schaller 1977, Van Dyke et al. 1983, Gionfriddo and Krausman
1986, Festa-Bianchet 1988a).
of
elevations
with
a
They also used a narrower range
greater
median
value
(Fig.
reflecting the locations of the cliff bands on HM.
11),
There was
a shift in summer use to areas closer to water, which was
generally available at and below the base of the cliffs.
The
response surface generated by the logistic regression model
(Fig. 23, from model 2, Table 5) demonstrated the relative
preference of ewes for steeper terrain in the spring.
The low
sensitivity and correct classification rate of the model
(11.3% and 66.7%, respectively) may have resulted from the
fact that elevation was really
a poor predictor,
being
correlated with slope (not apparent from the correlation
68
Summer
Spring
15
0
12
0
0.8
c
g 0.6
S
0.
6
0.4
-.7
* I e
..°
15
0.2
3
12
°:
64.
e
:p
9
0
o' Y.°
AP cs
6
e
$1,
,
e
e
I
...
cos
2
3
4
,
,o eic
fm
0
6
Distance
10
12
ap. Derr ain
I00) p.
*4'
%.0
O
0
2
4
6
8
lo
12
Distance to Escape Terrain (m x 100)
Fig. 22.
Response surface generated by logistic regression model for
differences in distance to escape terrain and water of ram groups on Hart
Mountain between spring and summer 1990, and a scatterplot of observed
values.
P(spring) = predicted probability of a group observation at a
site described by the response surface occurring in spring.
coefficient because gentle slopes occurred both below and
above
the
cliffs)
and
biotic
factors,
such
as
forage
availability and vegetative structure (Shannon et al. 1975).
Observed differences in habitat type use (Fig.
6) were
consistent with the hypothesis that changes were associated
with lambing.
Ewes made greater use of the cliff/talus-shrub
habitat (where nursery areas were located) in spring and the
mountain big sagebrush-bunchgrass habitat in summer.
The
latter occurred below the cliffs, had greater grass and forb
cover (Table 2), and included most of the water sources.
Use of habitat types did not differ between summer 1990
and 1991 for rams and ewes on HM (Fig. 8), but did for rams on
69
Summer
Spring
;*
'44
a
.
.
0
;
.0
,
0
rn
S.
0
30
60
90
Slope (5)
120
150
Fig. 23.
Response surface generated by logistic regression model for
differences in slope and elevation at sites used by ewe groups on Hart
Mountain between spring and summer 1990, and a scatter plot of observed
values.
P(spring) = predicted probability of a group observation at a
site described by the response surface occurring in spring.
PJR (Fig. 9).
Precipitation in March-June 1991 was 387% that
of 1990 (Fig. 20), resulting in considerably better forage
conditions and, at least on PJR, greater water availability in
summer 1991.
The smaller herd ranges in 1991 reflected
increased availability of resources.
did not differ because
Use of HM habitat types
habitats were
interspersed,
and
resource availability was probably greater there than on PJR.
Bighorn sheep on PJR appeared more affected by drought;
in
1990 (a drought year relative to 1991), ewes migrated 1 month
70
earlier and rams used the low sagebrush plateau less, and the
juniper-low sagebrush habitat more, than in 1991.
Rock Creek
did not go dry in 1991 and rams were never observed near
Petroglyph Lake, apparently preferring the more remote water
source.
Differences in summer habitat use between HM ewes and
rams reflected the relative preference of ewes for rugged
terrain close to escape cover.
Ewes used the precipitous
cliff/talus-shrub habitat more, and the gentler mountain big
sagebrush-bunchgrass and juniper/mountainmahogany-mountain big
sagebrush habitats less, than rams in both summers (Fig. 8).
The restricted visibility in the latter habitat may also have
contributed to its relative avoidance by ewes (Shannon et al.
1975, Tilton and Willard 1982).
The logistic regression model
(Table 7) was complex and a response surface could not be
generated
because
it
included
>2
dimensional plots of the data (Fig.
main
effects,
but
3-
14) clearly showed the
relative preference of ewes for steeper slopes closer to
escape terrain.
Differences in habitat use between PJR and HM rams on
summer ranges were marked,
and reflected differences
in
resource availability and juxtaposition between the ranges.
Rams on PJR sacrificed proximity to escape terrain for water,
and probably also for higher quality forage and thermal cover.
The logistic regression model (Table 8) fit the data well, and
generated a response surface that clearly demonstrated how HM
71
RN
PJR
80
60
e:
0.8
OS
8
ao
0.6
;I
3.
.
j
eft.
eB
0.4
.2
.
.
80 ,
0.2
.,
!V
60 44-
20 -. .
:
.
...
4.
%
40 dj7 e
AO N
0
0
40
bletenc0 to Es
a
"Pa
200)
60
Ter
80
rain
0 fi
' '.
.4,
*
20 et +
20
.
1
.
1
1
0
20
40
60
80
Distance to Escape Terrain (m x 100)
Response surface generated by logistic regression model for
Fig. 24.
differences in distance to escape terrain and water between ram groups on
Poker Jim Ridge and Hart Mountain in summer 1990 and 1991, and a scatter
plot of observed values.
P(PJR)
= predicted probability of a group
observation at a site described by the response surface being a Poker Jim
Ridge group.
rams remained close to escape terrain and water, whereas PJR
rams used sites on a gradient between these (widely separated)
resources (Fig. 24).
Range Fidelity
Several studies of bighorn sheep have documented a high
degree of fidelity to specific ranges (Geist 1970; FestaBianchet 1986a,b; Goodson and Stevens 1988), and this was
corroborated here.
I observed only 2 instances of movement by
radio-collared rams between PJR and HM.
Both were probably
temporary forays; they were PJR rams observed on the northern-
72
most end of HM during the 1990 rut, and they returned to PJR
within 2 days.
Ewes were never observed crossing between
ranges except during mid-summer and fall migrations between
PJR and NHM, and I believe they were also quite traditional in
use of ranges.
Population Characteristics
Group Size
Bighorn sheep group sizes vary in response to "the needs
and opportunities of the moment" (Wilson 1975).
Sheep may
form smaller groups when resources are limiting, thus avoiding
resource depletion (Leslie and Douglas 1979, Berger 1979b,
Simmons
Group
1980).
size may
also
be
a
function
of
vegetative and physiographic structure; the formation of large
groups enhances predator surveillance, allowing sheep to use
habitats
further
from escape
terrain
or with decreased
visibility (Reisenhoover and Bailey 1985, Warrick and Krausman
1987, Krausman et al. 1989).
Group sizes observed in this study (Fig. 16) were similar
to those reported previously for California bighorn sheep on
HMNAR (Kornet 1978, Cottam 1985) and elsewhere (Blood 1963,
Hansen 1982).
Ram group size was larger on PJR than HM in
spring 1990 and summer 1991.
PJR rams were generally observed
further from escape terrain and used low visibility habitats
(ie. those including juniper) more frequently than HM rams.
Larger group sizes on PJR were probably a response to the
73
lesser security of these rams' habitat.
There was no difference between HM and PJR ram group size
in summer 1990, although PJR ram groups were more variable.
Drought conditions (Fig.
20) caused forage and water to be
more limited in summer 1990 than the other seasons of this
study, and as previously explained, probably affected PJR more
than HM.
Smaller and more variable ram groups on PJR probably
reflected a compromise in response to the degree of security
and the limited resource base.
On HM, ewe groups were larger than ram groups in all
seasons, consistent with the findings of Leslie and Douglas
(1979)
and Geist
for bighorn sheep elsewhere.
I
included lambs and yearlings when counting individuals,
and
(1971)
this inflated ewe group size.
The presence of lambs probably
also favored larger groups by increasing the need for predator
surveillance.
Adult Ram Mortality and Age Class Distribution
Twenty-one rams aged 3-9 years were radio-collared and
monitored for up to 31 months in this study, and no cases of
natural mortality were observed (Table 11).
This suggests
that adult rams experienced low natural mortality rates, and
hunting was probably the most important mortality factor.
The
majority of the 203 rams harvested since 1965 have been class
III or IV (Van Dyke 1990), which generally corresponds to
yr old (Geist 1971).
6-
74
Class III and IV rams comprised 45.3% and 52.2% of all
rams observed in 1990 and 1991, respectively (Fig. 17).
The
PJR ram subpopulation included a slightly greater proportion
of older rams than that on HM (Fig. 17), probably due to
greater hunting pressure on HM through the 1990 season (L.
Conn, ODFW, pers. commun.).
Regulations were changed in 1991
to create equal pressure on both ranges, and ram age class
distributions may equalize.
I estimated the overall ram:ewe ratio to be 76.8:100 and
85.4:100 in 1990 and 1991, respectively (Table 12).
greater
than
the
65-70
rams:100
ewes
expected
This is
in
an
unexploited bighorn sheep population (Leslie and Douglas 1979,
Wehausen et al. 1987) because annual removals of large numbers
of ewes, lambs and yearlings for transplant (Fig. 25) offset
relatively conservative ram harvests.
Lamb Production and Survival
I observed no difference in overall lamb production and
survival between 1990 and 1991.
The ability of a ewe to
conceive, carry a lamb to term, and successfully nurse it is
related to her condition.
Condition is a function of resource
availability (Douglas and Leslie 1986, Krausman et al. 1989),
which is dependent on rainfall (Beatley 1974, Douglas and
Leslie 1986, Wehausen et al. 1987).
Timing and amount of
precipitation in 1989, 1990 and early 1991 were similar (Fig.
20), so comparable lamb production was expected. Rainfall was
75
500
450400350Cl)
300
w
0
N
na)
250 -
200-
a 150100-
1954
1964
1969
1974
1979
1984
1994
Year
Estimated bighorn sheep population size and number removed
25.
through trapping and hunting, Hart Mountain National Antelope Refuge,
U.S. Fish and Wildl. Serv., Lakeview, Or.
1954-1991. Source:
Fig.
greater during spring and early summer 1991 than 1990 (Fig
20), but this had no apparent effect on lamb survival.
An inverse relationship between bighorn sheep density and
lamb recruitment has been suggested (Geist 1971, Douglas and
Leslie 1986, Wehausen et al. 1987).
Increased density can
cause forage depletion, resulting in smaller, inherently less
viable lambs (Geist 1971), or delays in conception related to
poor condition of the ewe when entering the rut
Bianchet 1988a).
(Festa-
A ewe's ability to produce milk is related
to forage availability, and thus declines through the summer
76
This puts late-born lambs at an energenic
(Berger 1979a).
disadvantage for over-winter survival.
The size of the HMNAR bighorn sheep population has
remained stable during the
last decade,
largely due to
substantial annual removals of ewes, lambs and yearlings for
transplant and a ram harvest (Fig. 25).
These removals have
contributed
the
the
to
productivity
of
population
by
preventing the attainment of ecological carrying capacity
(Bartmann et al.
1992).
If removals through trapping and
harvest declined, productivity would decline as population
size approached carrying capacity.
I observed no difference in lamb production and survival
between NHM and SHM in either year, despite the fact that
during summer,
sheep density on NHM was approximately 4x
greater than SHM
(Table
16).
The effect of
differing
densities on the 2 ranges was probably overshadowed by that of
precipitation.
Wehausen et al.
(1987) noted that in arid
ecosystems, fluctuations in annual forage production related
to precipitation patterns generally exert a much greater
influence on nutrient availability than population density.
Furthermore, my density figures reflected density on summer
ranges, when PJR ewes had migrated to NHM.
During much of the
year, including late gestation and early lactation when ewe
energy requirements were greatest (Maynard et al. 1979:476),
the NHM and PJR ewe subpopulations were on separate ranges.
Given sustained drought conditions, I believe the densities I
77
observed could become an additive factor limiting lambing
success on NHM, however.
The incidence of parasitic diseases such as lungworm
increases with increasing sheep density (Hibler et al. 1982).
Lungworms have been implicated as a predisposing factor in
highly fatal pneumonia epizootics involving bighorn sheep
(Marsh 1938, Buechner 1960, Forrester 1971,
Uhazy et al. 1973, Onderka and Wishart 1984).
Stelfox 1971,
Mortality is
usually related to a combination of lungworm, bacterial (eg.
Pasteurella spp., Corynebacterium spp.) and possibly viral
(eg. Parainfluenza-3) infection (Forrester 1971).
In utero
infection is possible (Hibler et al. 1972, Kistner and Wyse
1979, Schmidt et al. 1979, Foreyt et al. 1983), predisposing
lambs to potentially fatal bacterial and viral pneumonias
(Hibler
1981).
Lungworm
is
suspected
to
be
limiting
recruitment in some populations of California bighorn sheep in
Oregon (M. J. Willis, ODFW, pers. commun.), and was implicated
as a cause of lamb mortality on NHM and PJR in 1982 and 1983
(Cottam 1985).
Fecal samples collected on HMNAR between 16
July-27 August 1990 contained 0-205 larvae/gm feces
21.1,
n
= 162, SE = 2.6)
(k =
(D. C. Payer, unpubl. data), which
was considered light infection of no pathogenic significance
(W.
J.
Foreyt,
Wa.
State Univ.,
pers.
commun.).
Larval
shedding varies with season and physiological status, however,
and is probably not a reliable indicator of herd health
(Festa-Bianchet
1990).
Lungworm
was
present
in
this
78
population, and is therefore a potential limiting factor.
Cottam (1985) reported lamb:ewe ratios of 90:100, 39:100
and 52:100 on SHM, NHM and PJR, respectively, in July 1985.
His SHM lamb:ewe ratio was 1.5-2x those I observed in July
1990
and
1991
(Fig
Three
19).
factors may have been
responsible:
(1) The SHM range was first colonized in the
early 1980's
(Cottam 1985),
so sheep density and forage
competition were low at the time of Cottam's study;
density
effects were
augmented
(2)
by greater than average
precipitation in fall and winter 1982 (28.3 cm), resulting in
favorable forage conditions in 1983; and (3) lungworm may have
been a significant cause of lamb mortality on NHM and PJR in
1983, but SHM was spared because of low environmental burdens
of infective lungworm larvae (Cottam 1985).
Predation limits lamb survival in some bighorn sheep
populations.
Losses to predators are related directly to
predator density and inversely to availability of escape
terrain (Hass 1989). Adequate escape terrain was available in
all 3 ewe ranges on HMNAR.
Potential predators of bighorn
lambs included coyotes (Canis latrans), bobcats (Felis rufus),
golden eagles (Aquila chrysaetos) and perhaps mountain lions
(Felis concolor), but predator density was not assessed.
I
occasionally observed interactions between coyotes and ewes
with lambs, but coyotes were always either chased off or sheep
sought escape
cover.
I
did not observe
any cases
of
predation, and believe that predation did not significantly
79
affect lamb survival during spring and summer.
Observed lamb:ewe ratios did not differ by month in
spring and summer 1990 or summer 1991 (Fig.
19), although
significant loss of lambs is expected within the first few
months of life (Geist 1971, Hass 1989, Krausman et al. 1989).
The majority of
lambing occurred
in April-May 1990
and
presumably also in 1991, and it is unlikely that late births
completely offset the expected decline in lamb:ewe ratio.
There may have been little early lamb mortality, or it may
have escaped detection due to a potential bias in my data
towards small lamb:ewe ratios in May-June when ewes with young
lambs were secluded and difficult to count (Geist 1971).
I observed a significant difference between lambs:100
ewes in August 1990 (51.3) and yearlings:100 ewes in summer
1991 (25.7).
from HMNAR
lambs:ewes
Although 49 ewes and lambs were transplanted
in January
(22.5:100)
1991,
the
approximated
ratio
of
transplanted
the yearling:ewe ratio
observed in summer 1991 (M. J. Willis, ODFW, pers. commun.).
My estimates thus indicated approximately 50% lamb mortality
between September 1990-June 1991.
I observed equal numbers of
male and female yearlings in 1991.
Assuming an equal sex
ratio at birth (Woodgerd 1964, Geist 1971), there was no
differential mortality of male and female lambs.
Comparisons of Helicopter and Ground Surveys
Helicopters have been used extensively to census big game
80
populations (Thompson and Baker 1981, Bleich 1983).
This
study provided an unusual opportunity to compare the results
of helicopter censuses to those of repeated ground surveys in
regards to age and sex ratios.
Agreement was good; observed
lamb:ewe and yearling ratios and age class distribution of
rams, with the exception of PJR rams in 1991, did not differ
significantly between the two census methods.
The major
shortcoming of the helicopter censuses was that ewes and
yearlings were not differentiated, making it impossible to
estimate lamb recruitment.
Management Implications
My data suggested that water was a limiting resource on
PJR, and its availability affected the distribution of bighorn
sheep there.
Petroglyph Lake and the section of Rock Creek
from the Morgan drift fence to Flook Knoll were critical
summer water sources for rams,
and these areas should be
managed for sheep use by avoiding conflicts with domestic
livestock
and
human
disturbance.
Petroglyph
Lake
is
especially susceptible to the latter; in times of intensive
ram use, especially late summer of drought years, it should be
closed to human access.
The lack of a perennial water source in the ewe range on
PJR was probably the primary stimulus for the ewes' summer
migration to NHM.
The migration corridor followed the cliffs
and crossed the refuge access road at 1,524-1,737 m elevation.
81
Migration corridors are often not considered when managing
bighorn sheep habitat, resulting in adverse effects on the
populations involved (Wakelyn 1987, Risenhoover et al. 1988).
This corridor should be considered critical bighorn sheep
habitat on HMNAR.
This study did not fully address vegetative structure and
its effects on bighorn sheep distribution.
The vegetative
characteristics I measured in sheep habitats were biased by
non-random transect site selection and small sample size.
Vegetation succession resulting in low-visibility habitats is
a major cause of bighorn sheep habitat loss, and prescribed
burning is an effective means of reversing this trend (Hobbs
and Spowart 1984, Wakelyn 1987, Etchberger et al. 1989).
I
recommend further study to more fully characterize vegetative
structure of bighorn sheep habitats, followed by a program of
prescribed burning designed to maintain or improve habitat
visibility.
Based on visibility and observed use,
the
habitats that would probably benefit most from burning include
mountain big sagebrush plateau and mountain big sagebrushbunchgrass on HM, Wyoming big sagebrush-bunchgrass on PJR, and
juniper habitats on both ranges.
Annual removals of sheep from all sex and age classes
through trapping and hunting have contributed significantly to
the high productivity of this population by preventing the
attainment of
ecological carrying capacity.
If
removal
programs are discontinued, population quality will eventually
82
decline.
Other populations of California bighorn sheep in
Oregon, particularly those experiencing limited or no growth,
would also experience enhanced productivity following removal
of some animals of all sex and age classes.
Combined with a
program of habitat protection and improvement, the result
would be increased population size.
The ram harvest on HMNAR, particularly that on PJR, could
be increased without compromising productivity.
adding
another
season
monitoring the results.
the
current
3/4
curl
for
6
rams
on
PJR,
I recommend
and
closely
An any-ram hunt would be simpler than
rule,
and
would
be
biologically
justifiable (Murphy et al. 1990).
Capture efforts need to be carefully planned to obtain
maximum benefits.
Bighorn sheep on HMNAR appeared to have a
high fidelity to their ranges; depopulation of a range could
cause loss of traditional movement patterns (Risenhoover et
al. 1989), adversely affecting efficiency and productivity.
Seasonal distribution and migrations should be considered when
planning capture efforts to achieve specific population goals.
For example, >30 ewes, lambs and yearlings were removed from
NHM in January 1991 (M. J. Willis, ODFW, pers. commun.), yet
sheep density was greater on this range in summer 1991 than
1990 (Table 13).
This occurred because PJR ewes migrated to
NHM in both years, and range size was smaller in 1991 (Table
10).
To alleviate summer over-crowding on NHM and not
critically depopulate the resident subpopulation, sheep should
83
be removed from PJR.
Migration of PJR ewes to NHM increased sheep density on
the latter range during the driest months of the year.
could
limit
productivity
of
ewe
both
This
subpopulations,
It could
especially under conditions of sustained drought.
also increase potential for disease transmission.
If
a
perennial water source existed on PJR, ewes would probably not
migrate, or would do so in reduced numbers.
The guzzler
within ewe range on PJR was not used, and construction of a
guzzler acceptable to ewes is indicated.
Further research is
necessary to develop an appropriate design.
Managing this population for maximal productivity will
require continued survey efforts.
Surveys should be performed
(1) Mid-June, to assess
3 times/yr in each range as follows:
lamb
production
(lambs:100 ewes)
and
recruitment
(yearlings:100 ewes); (2) early October, to assess early (pre-
winter) lamb survival (lambs:100 ewes); and (3) late March, to
assess total population size, and age and sex structure.
The
suggested times and goals for these surveys will provide the
greatest amount of information with the least disturbance to
sheep during critical periods such as lambing or rutting.
The major flaw with surveys as they are now performed is
that no estimates of recruitment are possible.
The only
practical way to conduct these surveys is by helicopter, but
if yearlings and ewes cannot be differentiated, sampling the
population
on
foot
to
estimate
yearling:ewe
ratios
is
84
indicated.
The
accuracy
of
aerial
counts
could
be
concurrently assessed by doubling sampling subpopulations with
ground counts.
85
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APPENDICES
94
APPENDIX A
Results of Vegetation Sampling
Table A.1.
Canopy cover (%) of major grass, forb and shrub species in bighorn sheep habitats on Hart
Mountain, June 1991.
Grasses
Forbs
Canopy
Cover
Shrubs
Canopy
Cover
Canopy
Cover
Habitat
Major Species
(%)
Major Species
(%)
Major Species
(%)
Low sagebrush
plateau
Idaho fescue
(Festuca
idahoensis)
9.0
Spreading phlox
(Phlox diffusa)
4.3
Low sagebrush
(Artemisia
arbuscula)
19.9
Sandberg's
bluegrass (Poa
5.7
Lupine
(Lupinus spp.)
1.7
Green rabbitbrush
(Chrvsothamnus
viscidiflorus)
2.3
Granite gilia
(Leptodactvlon
pungens)
1.6
Hollyleaf clover
(Trifolium
pvmnocarpum)
1.0
secunda)
Mountain big
sagebrush plateau
Idaho fescue
11.2
Granite gilia
4.7
Mountain big
sagebrush
(Artemisia
tridentata var.
vasevana)
27.7
Bottlebrush
squirreltail
(Sitanion hvstrix)
1.8
Lupine
4.5
Green rabbitbrush
3.5
Sandberg's
bluegrass
1.7
Wooly groundsel
(Senecio canus)
1.3
Bluebunch
wheatgrass
(Apropvron
spicatum)
1.3
Canopy cover (%) of major grass, forb and shrub species in bighorn sheep habitats on Hart
Table A.1.
Mountain, June 1991 (cont.).
Shrubs
Forbs
Grasses
Canopy
Cover
Canopy
Cover
Canopy
Cover
Habitat
Major Species
(%)
Major Species
(%)
Major Species
(%)
Cliff/talus-shrub
Idaho fescue
2.7
Pacific
monardella
(Monardella
odoratissima)
2.0
Mountain big
sagebrush
14.7
Cheatgrass brome
(Bromus tectorum)
2.1
Varileaf phacelia
(Phacelia
heterophvlla)
1.2
Bush rockspirea
(Holodiscus
dumosus)
7.8
Sandberg's
bluegrass
1.7
Green rabbitbrush
6.2
Mountain
snowberry
(Svmphoriocarpus
oreophilus)
4.0
Gray rabbitbrush
(C. nauseosus)
3.2
Wax currant
(Ribes cereum)
3.1
Mountain big
sagebrushbunchgrass
Idaho fescue
9.4
Lupine
3.7
Mountain big
sagebrush
17.2
Bluebunch
wheatgrass
2.6
Littleflower
collinsia
(Collinsia
parviflora)
2.2
Green rabbitbrush
2.7
Table A.1.
Canopy cover (%) of major grass, forb and shrub species in bighorn sheep habitats on Hart
Mountain, June 1991 (cont.).
Grasses
Forbs
Canopy
Cover
Shrubs
Canopy
Cover
Canopy
Cover
Habitat
Major Species
(%)
Major Species
(%)
Major Species
(%)
Mountain big
sagebrushbunchgrass (cont.)
Bottlebrush
squirreltail
2.6
Spreading phlox
1.2
Mountain
snowberry
1.6
Sandberg's
bluegrass
1.3
Wyoming big
sagebrush (A.
tridentata var.
wvomingensis)
1.1
Giant wildrye
(Elvmus cinereus)
36.7
Wood's rose (Rosa
1.5
Kentucky bluegrass
(P. pratensis)
Riparian
Stinging nettle
(Urtica dioica)
12.6
8.1
Common
monkeyflower
(Mimulus
outtatus)
7.8
Sedge
(Carex spp.)
5.4
California
falsehellabore
(Veratrum
californicum)
7.5
Cheatgrass brome
3.3
Bedstraw
(Galium spp.)
3.4
Rush
(Juncus spp.)
2.0
Cushion eriogonum
3.2
woodsii)
(Erioctonum
ovifolium)
Western wheatgrass
(A. smithii)
1.0
Starry
solomonplume
(Smilacina
stellata)
2.1
Canopy cover (%) of major grass, forb and shrub species in bighorn
Table A.1.
Mountain, June 1991 (cont.).
sheep habitats on Hart
Shrubs
Forbs
Grasses
Canopy
Cover
Canopy
Cover
Canopy
Cover
Habitat
Major Species
(%)
Major Species
(%)
Major Species
(%)
Juniper/
mountainmahoganymountain big
sagebrush
Thurber needlegrass
(Stipa thurberiana)
3.4
Lupine
3.2
Mountain big
sagebrush
13.4
Bluebunch
wheatgrass
3.0
Bush rockspirea
8.4
Idaho fescue
1.7
Mountain
snowberry
1.9
Indian ricegrass
(Oryzopsis
hvmenoides)
1.8
Canopy cover (%) of major grass,
Table A.2.
Ridge, June 1991.
forb and shrub species in bighorn sheep habitats on Poker Jim
Shrubs
Forbs
Grasses
Canopy
Cover
Canopy
Cover
Canopy
Cover
Habitat
Major Species
(%)
Major Species
(%)
Major Species
(%)
Low sagebrush
plateau
Sandberg's bluegrass
(Poa secunda)
4.5
Bighead clover
(Trifolium
macrocephalum)
4.0
Low sagebrush
(Artemisia
arbuscula)
17.8
Bottlebrush
squirreltail
(Sitanion hvstrix)
2.9
Lomatium
(Lomatium spp.)
2.9
Sandwort
(Arenaria spp.)
2.3
Goldenweed
(Haplopappus
stenophvllus)
1.6
Hollyleaf clover
(T. qvmnocarpum)
1.4
Littleflower
collinsia
(Collinsia
parviflora)
1.0
Locoweed
1.0
Low sagebrush
20.7
(Astracialus
obscurus)
Low sagebrushbunchgrass
8.0
Spreading phlox
(Phlox diffusa)
3.6
Sandberg's bluegrass
3.7
Tapertip
hawksbeard (Crepis
acuminata)
2.2
Bottlebrush
squirreltail
1.7
Goldenweed
2.2
Bluebunch wheatgrass
(Aciropvron spicatum)
Canopy cover (%) of major grass, forb and shrub species in bighorn sheep habitats on Poker Jim
Table A.2.
Ridge, June 1991 (cont.).
Major Species
(%)
Low sagebrushbunchgrass
Canopy
Cover
Canopy
Cover
Canopy
Cover
Habitat
Shrubs
Forbs
Grasses
Major Species
(%)
Bighead clover
2.0
Littleflower
collinsia
1.8
Pink microsteris
(Microsteris
Gracilis)
1.7
Lupine (Lupinus
1.1
Major Species
(%)
(cont.)
sPP)
Cliff-shrub
Cusick bluegrass (P.
cusickii)
2.1
Pacific monardella
(Monardella
odoratissima)
3.2
Mountain big
sagebrush
(Artemisia
tridentata var.
vasevana)
12.4
Bottlebrush
squirreltail
1.5
Granite gilia
(Leptodactylon
2.5
Bush rockspirea
(Holodiscus
dumosus)
6.9
1.1
Mountain snowberry
(Svmphoriocarpus
oreophilus)
6.4
Low sagebrush
5.1
Low sagebrush
17.6
pungens)
Bluebunch wheatgrass
Juniper-low
sagebrush
1.0
Littleflower
collinsia
Bluebunch wheatgrass
13.5
Bighead clover
3.9
Bottlebrush
squirreltail
7.3
Littleflower
collinsia
3.3
1-..
00
Table A.2.
Canopy cover (%) of major grass, forb and shrub species in bighorn sheep habitats on Poker Jim
Ridge, June 1991 (cont.).
Grasses
Shrubs
Forbs
Canopy
Cover
Canopy
Cover
Canopy
Cover
Habitat
Major Species
(%)
Major Species
(%)
Juniper-low
sagebrush
Sandberg's bluegrass
2.3
Tapertip
hawksbeard
3.2
Prairie junegrass
(Koelaria
pvramidata)
1.3
Longleaf phlox
(Phlox lonpifolia)
1.0
Thurber needlegrass
(Stipa thurberiana)
1.8
Arrowleaf
balsamroot
(Balsamorhiza
Major Species
(%)
5.3
Mountain big
sagebrush
15.8
2.8
Wax current (Ribes
5.3
(cont.)
Juniper-mountain
big sagebrush
sactittata)
Bluebunch wheatgrass
1.3
Lupine
cereum)
Wyoming big
sagebrushbunchgrass
Bottlebrush
squirreltail
1.0
Littleflower
collinsia
1.6
Wyoming big
sagebrush (A.
tridentata var.
wyomingensis)
4.7
Bottlebrush
squirreltail
5.2
Lineleaf fleabane
0.80
Wyoming big
sagebrush
13.1
(Ericieron
linearis)
Bluebunch wheatgrass
2.8
Sandberg's bluegrass
2.0
Cheatgrass brome
(Bromus tectorum)
1.8
Green rabbitbrush
(Chrvsothamnus
viscidiflorus)
102
APPENDIX B
Home Ranges of Radio-Collared Rams
103
Fig. B.1.
Home ranges of selected radio-collared rams on Hart Mountain
National Antelope Refuge. Ranges labeled with ram identification number.
104
Fig. B.1.
(Cont.)
105
Fig. B.1.
(Cont.)