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Habitat Associations of Small
Mammals in a Subalpine
Forest, Southeastern
Wyoming1
Martin G. Raphael2
Subalpine forests of spruce, fir, and
lodgepole pine cover about 5 million
ha, or 38% of forested land in the
central Rocky Mountain regionmore than any other forest type (Alexander 1974, USDA Forest Service
1980). Subalpine forest is harvested
heavily, accounting for over 90% of
total sawtimber volume in this region
(USDA Forest Service 1980). These
forests also are managed to produce
water, and timber harvest practices
have been developed that can substantially increase water yield (Troendle 1983, Swanson 1987). The Coon
Creek Water Yield Augmentation
Pilot Project (Bevenger and Troendle
1984,1987) is a large-scale demonstration of the feasibility and costs/
benefits of increasing water yield
through specially designed clearcuts.
To evaluate the response of wildlife
species to such harvests, studies
were initiated to describe the pretreatment structure and composition
of the vertebrate community (Raphael 1987b) and, ultimately, to compare responses of vertebrates on the
treated watershed and on the unharvested control.
The present study summarizes the
structure of the small mammal com'Paper presented at Symposium, Management of Amphibians, Reptiles, and
Small Mammals in North America (Ragstaff,
AZ,July 19-21, 1988).
2ResearchEcdogist, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station. Forestry Sciences laboratory, 222 South 22nd Street, laramie, Wyoming 82070.
Abstract.-Mammal capture rates were greatest
at sites with mature timber and other old-~rowth
attributes. Shrews (both dusky (Sorex moniico\us)
and masked (S.cinereus)) and southern red-backed
voles (Clethrionomys gapperi) were much more
abundant at sites dominated by spruce or fir
compared to drier sites dominated by lodgepole
pine. Deer mice (Peromyscus maniculafus), in
contrast, were most abundant on drier, pinedominated sites. The southern red-backed vole,
because of its high abundance and strong
association with mature forest, is a good ecological
indicator of late seral conditions for forest planning
purposes,
Figure 1.-Map of study area showing location of study area and distributionof trapping
stations.
munity, describes habitat associations of the dominant species during
the pretreatment phase of the longer
term project, evaluates the efficacy of
an old-growth scorecard to rate oldgrowth characteristics of stands, and
assesses designation of mammals as
ecological indicators of old-growth
conditions.
STUDY AREA
Studies were conducted within two
watersheds, the Upper East Fork of
the Encampment River (911 ha) and
Coon Creek (1,615 ha). These adjacent watersheds are part of the Sierra
Madre range of southern Wyoming,
located about 25 km south of the
town of Encampment (fig. 1). Elevations vary from 2,600 to 3,300 m.
Soils are 50-150 cm deep and are well
drained.
Mean annual precipitation is about
100 cm, 70% falling as snow that usually covers the site from late September through late June at depths of 2-4
m in winter. Forest cover is dominated by lodgepole pine (- 60%of
land area), and a mixture of Engelmann spruce and subalpine fir. Pole
stands with trees ~ 2 cm
3 d.b.h. occur
on 24% of the two watersheds, mature stands occur on 72%, and meadows or rock outcrops cover 4%.
est personnel. Also recorded was the
presence or absence of permanent
streams within 100 m of each sampling station.
Small Mammal Trapping
Red Squirrel Counts
METHODS
Vegetation Sampling
In each watershed, 90 sampling stations were established at 200-m intervals along N-S lines that were 400 m
apart (fig. 1). At each of the 180 stations, an observer measured basal
area of each tree species using a 1factor metric reloskop. Canopy cover
was estimated from the average of
four readings taken at cardinal directions with a spherical densiometer.
Slope was measured with a clinometer and aspect was measured with a
hand-held compass. All snags >20
cm d.b.h. and 1.8 m tall were
counted within a 0.04-ha circular plot
centered at the station; cover percentages of shrubs, forbs, grasses,
rocks, litter, and bare ground were
visually estimated over the same
0.04-ha plot. Hard (class 1,2) and soft
[class 3,4,5 (Maser et al. 197911logs
also were counted on each plot.
Height and d.b.h. of one representative tree were measured at each station with a clinometer and metric
d.b.h. tape.
All stands on each watershed
were assessed by personnel of the
Medicine Bow National Forest and
assigned an old-growth rating based
on canopy structure, d.b.h., tree
height, snag size and density, and log
size and density (appendix). Possible
scorecard values range from 0 (no
old-growth characteristics) to 60
(maximum).
Stand maps were used to associate
a sampling station with the oldgrowth scorecard value for the stand
in which the station was located.
Habitat types were also assigned to
each station based on classifications
used by Medicine Bow National For-
stations per day and most counts
were completed before noon.
Three observers visited each sample
station twice each year (totaling six
visits/station/yr) from 13June to 25
July 1985,18 June to 23 July 1986,
and 15 June to 17 July 1987. At each
visit, the observer recorded all red
squirrels seen or heard within a 100m radius of the station center. All
counts were begun within 30 minutes
after sunrise; each observer visited 15
To sample shrews, six pitfall traps
were installed in a 2 x 3 grid (15-m
spacing) centered on each station.
Each pitfall trap was a 3-gal plastic
bucket buried flush with the ground
surface and covered by logs or bark.
To capture other small mammals,
two 50-cm Sherman livetraps were
placed within 2 m of each pitfall station.
Mammals were trapped during
late summer from 1985 to 1987 (20
' I
August through 26 September 1985,5
August to 11 September 1986, and 4
August to 10 September 1987).Observers checked traps once daily during each of three, 10-day sampling
sessions each year. Sampling sessions
were separated by four days, encompassing six weeks each year. All captured specimens were identified, toe
clipped, sex determined, aged,
weighed, and checked for reproductive status (currently breeding or
not).
Dead animals were assigned a
permanent catalog number. Shrews
were preserved in 70% ethanol and
all other species were frozen for later
identification.
Data Analysis
Total numbers of detections (red
squirrels) or first captures (all other
species) were calculated at each station over the 3 years. Thus, the total
numbers of captures represented the
results of 450 trapnights of effort at
each of the 180 stations (81,000 total
trapnights). Despite efforts to close
pitfall traps between sessions, some
mammals were captured before the
start of each 10-day session. These
specimens were retained, but numbers were not included in analyses.
To assess habitat associations of
the more abundant mammals, I performed a principal components
analyses (with varimax rotation) using the SPSS/PC+ program package
(Norusis 1988). Principal components
analysis derives linear combinations
of attributes (in this case vegetation
characteristics as listed in table 1).
All components with eigenvalues
>1.0 were retained for subsequent
analyses. The equations were then
"solved" for each station, resulting in
a set of scores that were interpreted
as habitat gradients. I identified these
gradients from those original habitat
variables most highly correlated with
the principal components scores. To
relate abundance of the more abundant mammals to habitat features at
each station, I performed multiple regressions of capture rates at each station (dependent variable) with the
habitat gradients or principal components scores (independent variables).
To summarize patterns of co-occurrence of the more common mammal species, I performed an averagelinkage-between-groups cluster
analysis [UPGMA (Norusis 1988)l
based upon Pearson correlations between abundances of all pairs of species among the 180 stations. Results
of the cluster analysis were displayed
using a dendrogram showing the
relative similarities of all species. The
similarity measure, for this display,
was rescaled to values ranging from
0 (no similarity) to 25 (maximum
similarity).
RESULTS AND DISCUSSION
Vegetation
Structure and composition of vegetation (table l) were typical of subalpine forest in the central Rocky
Mountains (Alexander 1974; Raphael
1987a, 198%). Vegetation characteristics have been shown to be similar
between the two watersheds (Raphael 198%); therefore, no distinction was made between the two watersheds for this study.
Principal components analysis resulted in the creation of six synthetic
habitat gradients that, together, contained 68% of the total variance from
the 19 original habitat variables (table
2). I used the variables that were
most highly correlated with values of
each gradient to interpret the biological meanings of the gradients (table
2).
Mammals
Over the 3 years of study and over
all sampling stations, observers c a p
tured 4,553 individuals of 17 small
mammal species and recorded 987
detections of red squirrels (table 3).
The most abundant species was the
southern red-backed vole, accounting for over 50% of all captures.
Other dominant species included
masked shrew (15%),deer mouse
(15%~)~
red squirrel, dusky shrew
(6%),and chipmunks (2 species, 6%).
Specific Habitat Associations
Masked Shrew
Masked shrews were more abundant
than other shrews and were captured
more frequently in mature lodgepole
and spruce/fir sites (table 3) with
higher cover of herbs and grasses;
they were less abundant on dry,
south-facing sites (table 4). Their
abundance at each station was modeled (R2= 0.42) by a regression that
included gradients 2,1,6, and 4 (in
order of their statistical significance)
(table 4). Other studies (Negus and
Findley 1959, Spencer and Pettus
1966, Brown 1967a, Armstrong 1977)
also report this species' preference
for moist sites. However, I did not
find a strong association with bogs,
as reported by Brown (1967a) and
Spencer and Pettus (1966).
Dusky Shrew
Dusky shrews were captured in
greater numbers in more moist, mature spruce/fir sites (table 3). They
were most strongly associated with
dense herbaceous cover and (to a
lesser degree) with old-growth attibutes. Unlike the masked shrew, their
abundance was positively and significantly correlated with gradient 3
(moist, streamside sites; tables 2,4).
Like masked shrews, they were less
abundant on southerly, steeper sites.
The regression model explained 41%
of variance in abundance (table 4).
Brown (1967a) captured this shrew in
a greater variety of habitats and in
drier sites than the masked shrew.
Negus and Findley (1959) also reported use of a greater variety of
habitats; Spencer and Pettus (1966)
found dusky shrews in association
with marshy habitats.
The association of this shrew with
old-growth conditions has not, to my
knowledge, been previously reported.
Least Chipmunk
The abundance of least chipmunk
was significantly and negatively correlated with gradient 4 (bareground)
and positively correlated with gradient 6 (southerly exposure). Although
the regression was statistically significant, it explained only 5% of variance in abundance (table 4); thus, the
regression model was not statistically
meaningful.
None theless, the assoda lions suggested by the model, particularly the
preference for open, drier slopes, are
in accordance with results of other
studies (e.g., Telleen 1978, Clark and
Stromberg 1987).
Uinta Chipmunk
Uinta chipmunks were most abundant on rocky slopes (gradient 51, as
also reported by Clark and
Stromberg (1987).They were relatively more abundant in younger
stands (gradient 2). The regression
model explained 17%of the variation
in abundance of this species (table 4).
Compared with the least chipmunk,
this species is reported to be more
restricted to subalpine forest habitats
(Negus and Findley 1959).Telleen
(1978) found an association with
closed canopy, open understory
habitats.
Red Squirrel
Red squirrel abundance was somewhat greater on dry, gently sloping
sites (gradients 3,5), but only 16%of
variation in abundance was explained by the regression model.
These squirrels were abundant
throughout the study area, which
seemed to be comprised of excellent
red squirrel habitat; Therefore, variation in vegetation among sites was
probably minor in relation to the po-
tential variation that would distinguish suitable from unsuitable habitat. Clark and Stromberg (1987) describe red squirrels as widespread
throughout coniferous forest habitats
of Wyoming.
Deer Mouse
Deer mice were associated with
streamside sites having lower basal
area of subalpine fir (gradient 3). Although widespread on the study
area, they tended to be more abundant on open, lodgepole-dominated
sites and meadows than on spruce/
fir sites. The regression model explained 15%of the variance in deer
mouse abundance (table 4). Contrary,
to these results, other studies (Brown
1967b, Campbell and Clark 1980,
Ramirez and Hornocker 1981) reported associations of deer mice with
xeric sites away from streams. The
species is known to be abundant on
cutover sites (Ramirez and Hornocker 1981, Scrivner and Smith
1984, tolerant of a wide range of ecological conditions (Clark and
Stromberg 19871, and omnivorous
(Clark 1975).
Southern Red-Backed Vole
This vole, the most abundant species
on the study area, was most abundant in mature spruce/fir stands
(table 3). Its abundance was also
greater in stands that had more herb
and grass cover (gradient 11, on
northerly slopes (gradient 61, and on
sites with greater basal area of subalpine fir and greater log cover (gradient 3). Its abundance was modeled
well by the regression, which accounted for 46% of variation in redbacked vole abundance among sites
(table 4).
The association of red-backed
voles and mature spruce/fir forest is
well documented (Ramirez and Hornocker 1981, Allen 1983, Scrivner and
Smith 1984).This association may be
due, at least in part, to the greater
cover of logs and other woody debris
that provides protection during critical periods of freezing and thawing
(Merritt 1976,1985; Merritt and Merritt 1978, Sleeper 1979)and supports
fungi used as food (Williams 1955,
Clark and Stromberg 1987, Wywialowski and Smith 1988).
Western Jumping Mouse
Jumping mice were most abundant
in spruce/fir and mature lodgepole
habitats (table 3). As reported in
other studies (Negus and Findley
1959, Brown 1967b, Clark 1971,
Scrivner and Smith 1984, these mice
were associated with dense herbaceous or grassy vegetation (gradient
1)along moist streamsides (gradient
3) in more mature stands (gradient 2)
(table 4). The regression model accounted for 20% of variation in abundance across all stations. These mice
feed primarily on grass seeds and
fungi (Jones et al. 1978, Vaughan and
Weil1980), which may account for
their close association with grassy
streamside habitats.
General Relationships
Moisture and stand maturity were
two habitat features that separated
patterns of abundance of the various
species. This is illustrated most effectively through the cluster analysis
based on interspecific correlations of
relative abundance (fig. 2). The dendrogram shows two groups: one
comprised of the two shrews, two
voles, and jumping mouse; and one
comprised of the red squirrel, two
chipmunks, and deer mouse. The former group is associated with more
moist, old-growth conditions (table
4). The latter group is associated with
drier, less mature conditions.
The association of species with
old-growth conditions is of special
interest because of concern over
identifying species that are ecological
indicators of old-growth (USDA Forest Service 1985; Nordyke and
Buskirk, these proceedings). The
Medicine Bow National Forest, the
site of this study, lists the southern
red-backed vole as an ecological indicator representing late successional
stages in conifer forests. Because the
forest uses the old-growth scorecard
to rate old-growth conditions,
whether or not red-backed vole
abundance is related to old-growth
index values is of interest. Raphael
(1987b)confirmed such a trend based
on analyses of the first 2 years of the
present study.
The trend is even more pronounced when all 3 years are included in the analyses (fig. 3). Southern red-backed voles are increasingly
abundant as old-growth scorecard
index values increase. Similar trends
are evident for masked and dusky
shrews (fig. 3).
CONCLUSIONS
The small mammal community, as
sampled in this study, was similar in
composition to that described in
other studies in subalpine forests of
the Rocky Mountain region (cf. Ra-
phael1987a). The southern redbacked vole was the most abundant
species and can be considered the
species most representative of mature spruce/fir forest stands. Stand
age and moisture conditions were
the two most important generalized
gradients that were predictors of
summer abundance of the various
species. The southern red-backed
vole was confirmed as a suitable ecological indicator of old-growth forest;
but, two other species, the masked
shrew and the dusky shrew, are
good candidates as well.
thank personnel of the Wyoming
Game and Fish Department, the
Medicine Bow National Forest, especially the Hayden Ranger District, for
their cooperation; Carron Meaney of
the Denver Museum of Natural History and Steven Buskirk of the University of Wyoming for technical assistance and use of museum facilities;
and Gregory D. Hayward, Graham
W. Smith, Mark R. Stromberg, and
Richard H. Yahner for comments on
the manuscript.
LITERATURE CITED
ACKNOWLEDGMENTS
I am indebted to Christopher Canaday, Anita Kang, Jeffery Waters,
Gary Rosenberg, Scott Stoleson, Sandra Spon, Sandra Pletschet, Steven
Larson, Lindsay Hall, Thomas
Batchelor, Daniel Maltese, and Lisa
Smith for their help in the field. I also
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SOMO
MIMO
SOCI
CLGA
ZAPR
TAH U
TAM l
PEMA
TAU M
RELATIVE SIMILARITY
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6,
OLD-GROWTH SCORECARD VALUE
Figure 3.-Mean abundance of selected small mammal species in relation to old-growth
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Appendix.-Old growth habitat scorecard (Anonymous 1985) used to rate stands in the Rocky Mountain Region. Point values from 1 to 5 are assigned to each category A-L Values are summed over all rows and the grand total is used as the index value.
Point value
A. Overston/
3 or more species
3 or more species
Spruce and/or Fir <5096
2 species
Spruce and/or Fir >50%
2 species
Spruce and/or Fir 4 0 %
1 species
100%
3 or more species
Spruce and/or Fir ~ 5 0 %
2 species
Spruce and/or Fir 930%
2 species
Spruce and/or Fir ~5096
1 species
100%
3 or more species
3 or more species
Spruce and/or Fir >50%
Spruce and/or Fir 4 0 %
2 species
Spruce and/or Fir >50%
2 species
Spruce and/or Fir <50%
1 species
100%
Spruce and/or Fir >50%
B. Midstory
3 or more species
Spruce and/or Fir >50%
C. Understory
D. Total Canopy Cover
70%+
E. Overstory, Canopy Cover
50-30%
70-50%or 30-10%
100-70%or 10-1%
70-40%or 20-10%
100-70%or 10- 1%
15'-13'
12'-10'
8'-6'
5'-3'
F. Midstory Canopy Cover
40-20%
G. Overstory Ave. DBH (Live)
16' +
H. Midstory Ave. DBH (Live)
9'+
I. Standing Snags Ave. DBH (Record only those snags above 6' in height.)
16'+
15'-13'
12'-10'
J. Standing Snags #/Acre (Record only those snags above 6' in height and 7' DBH.)
6+
-
K. Dead, Down Logs Ave. DBH
16'+
6-4
-
15"-13'
L. Dead. Down Logs #/Acre (Record only those above 7" DBH.)
12 +
12-6
Column
Totals
3-1
12'-10'
6-2