Use of Habitat by Mountain Beaver in the Sierra Nevada

J. Wildl. Manage. 53(3):1989
BROWSE IN SPRUCE-FIR FORESTS * Newton et al.
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DANHAUS, M. LOTTMAN, AND S. DUBELMAN.
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LAUTENSCHLAGER,
1985.
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LEWIS,C. E., B. F. SWINDEL, L. F. CONDE, AND J.
E. SMITH.1984. Forage yields improvedby site
preparationin pine flatwoodsof north Florida.
South.J. Appl. For. 8:181-185.
MCCAFFERY, K. R., L. D. MARTOGLIO, AND F. L.
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Herbicide treatment of
Received 14 June 1988.
Accepted 4 January 1989.
BEAVERIN THESIERRANEVADA
USE OF HABITATBY MOUNTAIN
PAULBEIER,Departmentof Forestryand Resource Management,Universityof California,Berkeley,CA 94720
Abstract: I used stepwise logistic regressionto identify factors importantfor habitat use by SierraNevada
mountainbeaver (Aplodontia rufa californica). Elevation, stream gradient, and abundanceof willow (Salix
spp.), alder (Alnus spp.), and fir (Abies concolor and A. magnifica) had the strongest positive associations
with habitat use. Mountainbeaver probably did not respond to these factors directly, but rather to a cool
thermal regime, adequate soil drainage, and abundant food supply. Comparisonof current with historical
beaver use indicated no marked change since 1935.
J. WILDL.
MANAGE.53(3):649-654
The mountain beaver is found in moist environments with moderate to dense vegetation
in the Pacific Northwest and the Sierra-Cascades
(Ingles 1965). The species is restricted to moist
habitats because it has a poor ability to concentrate urine and requires free surface water or
succulent vegetation on a daily basis (Nungesser
and Pfeiffer 1964, Schmidt-Nielsen and Pfeiffer
1970). Although mountain beaver find appropriate vegetation or water on open slopes and
ridgetops in the forests of western Oregon and
Washington (Voth 1968, Kinney 1971), they are
restricted to riparian areas in the relatively xeric
Sierra Nevada (Stephens 1906, Orr 1949). However, not all areas with succulent vegetation are
occupied, and no quantitative work has related
occurrence of mountain beaver to physical and
vegetative characteristics of their habitats.
My objectives were to develop a model to
identify the physical and vegetative variables
associated with habitat use by mountain beaver
in the Sierra Nevada, to validate the model on
a set of stream reaches not used in development
of the model, and to use historical records to
determine whether habitat use has changed since
1935.
J. S. Slaymaker assisted on most of the fieldwork. J. L. Patton and B. R. Stein provided
access to the collections and field notes of the
Museum of Vertebrate Zoology at the University
650
MOUNTAINBEAVERHABITATUSE * Beier
of California, Berkeley. H. C. Black, J. E. Borrecco, J. Evans, D. R. McCullough, S. H. Jenkins,
and 2 anonymous reviewers offered helpful criticisms of the paper. This research was supported
by the California Department of Fish and Game
and the California Agricultural Experiment Station Project 4326-MS.
STUDY AREA
The study was conducted on the Truckee River and its tributaries from the confluence with
Deer Creek (Nevada County, Calif.) downstream to Verdi (Washoe County, Nev.), a drainage of approximately 600 km2 with 172 km of
streams ranging from 1,485 to 2,750 m in elevation. Stream banks were characterized by deciduous riparian vegetation that included aspen
(Populus tremuloides), black cottonwood (P.
trichocarpa), willows, alder, dogwood (Cornus
spp.), bitter cherry (Prunus emarginata), lodgepole pine (Pinus contorta), and herbaceous
plants. The vegetation of the area was described
in detail by Beier and Barrett (1987).
METHODS
Each stream was divided into sampling reaches (approx 700 m long). Length varied from 400
to 900 m to put the end points at topographic
or vegetative discontinuities and minimize heterogeneity within the sampling unit. For each
reach, 7 physical and 12 vegetation variables
(Table 1, plus aspect) were measured between
5 May and 20 August 1985. I measured bank
slope class, stream width, and stream depth class
near the midpoint of the reach, avoiding atypical locations such as waterfalls and beaver impoundments. Elevation, aspect, and gradient
were obtained from 1:24,000 topographic maps;
aspect was categorized into 1 of the 8 cardinal
directions, or as flat (i.e., slope <2%). I measured
bare soil, litter cover, riparian zone width, and
abundance of herbaceous plants by taking the
average of ocular estimates at 100-m intervals
within each reach. The other vegetation variables were obtained by counts, except when it
was obvious that the species abundance would
be scored "3" (i.e., >20 plants/100 m of stream
length). Mountain beaver presence was based
on occurrence of the burrows and haypiles characteristic of the species, in association with bevel-cut woody stems (10-14 mm in diam). Beaver (Castor canadensis) activity in each reach
was categorized as active colony; abandoned
colony; beaver foraging, but no past or present
J. Wildl. Manage. 53(3):1989
colony; or no sign of beaver. Two beaver-use
variables were used in analyses. One variable
was dichotomous(beaverpresentor absent),and
the second was polychotomous(4 categories of
beaver activity).
Most habitat variables were measured on a
scale with only 4 possiblestates,and other variables (aspect and beaver use) were categorical;
such variablescannot be normally distributed.
The continuousvariableswere not normallydistributed(Kolmogorov-Smirnovtests, P < 0.01);
gradients,for instance, had a strongly bimodal
distributionwith peaks at 0 and 10%,and were
skewed to the right. Box'sM-statistic(Nie 1983)
indicated that group variances were heterogeneous (P < 0.01), even after logarithmic or
square-root transformations.Stepwise logistic
regression (SLR) is a classification procedure
well-suitedfor use with polychotomousand discontinuous explanatory variables (Walker and
Duncan 1967, Cox 1970)and is appropriateeven
when the conditions of normality and homogeneous variancesare not met (Anderson1972,
Press and Wilson 1978). Therefore, SLR was
used to identify variablesassociatedwith habitat
use by mountain beaver.
Variables that differed (P < 0.05) between
used and unused reaches were candidates for
entry into the logistic function. At each step,
the variable with the lowest P-value was included until the significanceof the F-statisticof
each remainingvariablewas >0.05. I randomly
selected half the cases for use in model development; the other half were used for model
validation.Priorprobabilitieswere based on the
proportionof cases falling into each group. The
ratioof each coefficientto its standarderrorwas
used to standardizecoefficient size to facilitate
comparisonof variables included in the function.
I contrasted the frequency distributionsof
gradientsand elevations at sites used by mountain beaver in the Truckee River Basin in 1985
to the gradients and elevations at pre-1935
mountain beaver sites in the Sierra Nevada
(basedon publishedrecords,collections,and field
notes of the Museum of Vertebrate Zoology
[MVZ],Univ. California, Berkeley) to investigate change in habitat use over time.
RESULTS
Habitat Use
Mountainbeaver were present on 56 (24%)
of the 237 reaches surveyed. The reaches on
MOUNTAIN BEAVER HABITAT USE * Beier
J. Wildl. Manage. 53(3):1989
651
Table 1. Meanvalues for 18 habitatvariableson reaches used (n = 56) versus not used (n = 181) by mountainbeaverin the
TruckeeRiverBasin,Californiaand Nevada, 1985. Significantdifferencesbetween means are denotedby an asterisk(t-test, P
< 0.01). The last 3 columns show statistics for variablesincludedin the logistic function,which was developed using 118
randomlyselected reaches.
Mountain beaver
Variable
Present
13.6
1.9
5.2
Stream gradient (%)
Bank slope classt'
Bare soil (%)
Logistic function
Absent
*
5.0
1.6
6.0
17.3
4.4
*
Stream depth class'
1.6
*
2.6
Elevation (m)
Riparian zone width
2,137
19.4
*
1,823
26.9
Stream width
(m)
Litter cover (%)
Aspen,'
Cottonwood,'
Alder,
Willow'
Dogwood'
Bitter cherry"'
Lodgepole pine'
Yellow pine,'
Fir,'
Herbaceous plants,
Constant
(m)
4.5
1.1
0.3
2.1
2.4
1.3
0.8
0.9
0.6
1.6
2.3
*
*
*
*
*
*
3.2
0.5
0.9
1.7
2.0
1.0
1.2
0.9
1.1
0.9
2.3
Ratioa
Tolerance
0.244
3.28
0.63
0.0069
3.67
0.70
0.885
1.396
2.39
2.78
0.86
0.95
1.395
2.24
0.59
Coefficient
-25.76
-4.09
a
Ratio of coefficient to its SE.
b Slope class: =1 1-30, 2 = 31-40, 3 = 41-60, 4 = >60%.
c Depth class: 1 1-23, 2 = 24-46, 3 = 47-76, 4 = >76 cm.
(1 Abundance index: 0 = absent, 1 = 1-5, 2 = 6-20, 3 = >20 trees or shrubs in riparian zone/100 m of stream length.
e Abundance index: 0 = absent, 1 = rare, 2 = moderate, 3 = abundant.
which mountain beaver were present differed
significantly (P < 0.01) from the unused reaches
with respect to 10 of 18 non-categorical habitat
variables (Table 1). Of the categorical variables,
there was significantly less beaver activity on
reaches used versus not used by mountain beaver (x2 = 23.6, 3 df, P < 0.01), but aspect was
not associated with mountain beaver presence
(x2 = 5.9, 8 df, P > 0.40). On average, reaches
with mountain beaver had steeper gradients,
narrower and shallower streams, were higher in
elevation, had a greater abundance of alder,
willow, fir, and aspen, and a lesser abundance
of cottonwood and yellow pine (Pinus ponderosa, P. jeffreyi, P. washoensis, and hybrids) than
unused reaches.
Gradients and elevations differed most strikingly between used and unused reaches (Fig.
1). The logistic regression model, using 118 randomly selected reaches, included these 2 variables and abundances of alder, willow, and fir
(Table 1). When applied to the 119 cases not
used to develop the model, the logistic function
correctly classified 65% of the used reaches and
97% of the unused reaches.
Although 60% of 237 reaches showed sign of
past or present use by beaver, only 16 (29%) of
56 reaches used by mountain beaver also had
sign of beaver use and none of these 56 reaches
contained active beaver colonies. However, SLR
did not select either of the 2 beaver-use variables, and the forced addition of a beaver-use
variable into the function did not improve the
classification.
HistoricalPatterns of HabitatUse
The collections and field notes at the University of California's MVZ provided 29 mountain
beaver sites in the Sierra Nevada between 1912
and 1935 for which elevation and gradient could
be determined. These records indicate no change
in habitat use by mountain beaver. Only 4 of
the 29 sites were < 1,700 m, whereas 13 were
>2,300 m in elevation. No site had a stream
gradient <5% and only 3 sites had gradients
between 5 and 9%, whereas 13 had gradients
>19%. Collectors C. L. Camp and T. I. Storer
provided 12 of the 29 records, and searched
extensively for mountain beaver in the Yosemite
region during 1915-19. Their field notes suggest
that mountain beaver were not widely distributed at that time.
I
.......
MOUNTAINBEAVERHABITATUSE * Beier
652
A
J. Wildl. Manage. 53(3):1989
the ground for >4.5 hours/day (Ingles 1959,
Voth 1968), and high ambient temperatures
56 USEDREACHES
probably exclude the species from low eleva181 UNUSED
REACHES
50tions in the Sierra Nevada.
It is not obvious why steep gradients would
be preferred by mountain beaver. Gradients
30were not highly correlated with the other explanatory variables (r2 = 0.26 with elevation,
pl
00<
less with other variables). Gradient may be imKXK
C>
10OCK
OCK
portant because it promotes water drainage, thus
00<
CO)
w
nrr7iF%
Ix
preventing burrows from flooding. This interLI
0-4
5-9
10-14 15-19
pretation is supported by observations on 3
0
reaches used by mountain beaver in which there
GRADIENT
(%)
was a break in slope from steep (> 10%) to gentle
B
(<5%) gradient, with no noticeable change in
56 USEDREACHES
LL
bank slope, or other variables. In
vegetation,
181 UNUSED
REACHES
0
such cases, mountain beaver were found only
50in the steeper part of the reach.
Crouch (1964) found greater numbers of
mountain beaver on west and northwest facing
30slopes, an observation consistent with the importance of cool temperatures, and with the fact
that moisture-bearing storms come from these
directions. Although this trend was not evident
10in the Truckee River Basin, only 19 of the 237
LXXX
A
AAA
reaches had southeast to southwest aspects. This
< 1,700 1,700-2,000 2,000-2,300 > 2,300
may have been too small a sample to detect
ELEVATION
aversion
to these warmer slopes.
(M)
variables were also important, alVegetation
1.
Relative
distributions
for
Fig.
frequency
gradients(A)and
elevations(B) on reaches used versus not used by mountain though they contributed less to the logistic funcbeaver in the Truckee River Basin, Californiaand Nevada,
tion. Willow, alder, and fir are important foods
1985.
(P. Beier, unpubl. data) and also provide thermal and escape cover. In Washington and Oregon, some sites used by mountain beaver had
DISCUSSION
very little alder and willow, but were abundant
HabitatUse
in ferns, which formed the bulk of the animal's
Elevation was the most important factor re- diet (Voth 1968, Allen 1969). These data suggest
lated to habitat use by mountain beaver. Ste- that although mountain beaver require an adphens (1906) also described mountain beaver equate supply of succulent vegetation, including
habitat in the Sierra Nevada as high elevation
evergreen winter food, the species composition
hillsides, but in western Oregon, the species is of the vegetation can be variable. I suggest that
abundant at sea level (Voth 1968, Johnson 1971, habitat use by mountain beaver involves strict
Kinney 1971). In the Sierra Nevada, high elerequirements for an appropriate thermal revation was probably preferred because it is as- gime and adequate soil drainage, and somewhat
sociated with lower mean temperatures. The more flexible requirements for food.
ability of a fossorial mammal to tolerate warm
Although mountain beaver and beaver did
temperatures decreases as body mass increases not co-occur in the Truckee River Basin, beaver
(McNab 1979). This relationship may play a role variables did not improve the classification funcin the distribution of mountain beaver, 1 of the tions. Thus, the lack of spatial overlap between
largest fossorial mammals. In laboratory exper- the 2 species in the Sierra Nevada is probably
iments, mountain beaver were unable to sustain due to different habitat requirements rather than
activity at temperatures >28 C (Johnson 1971, to displacement. This conclusion is supported
Kinney 1971). Mountain beaver forage above by observations of the 2 species coexisting at
*i*
I
,.A.
J. Wildl. Manage. 53(3):1989
many sites in Washington and Oregon (J. E.
Borrecco, U.S. For Serv. and J. Evans, U.S. Fish
Wildl. Serv., pers. commun.).
Historical Patterns of Habitat Use
In their pre-1935 descriptions of mountain
beaver habitat in the Sierra Nevada, Camp
(1918) and Grinnell and Storer (1924) did not
comment on gradient or elevation. However,
Stephens (1906:94) found mountain beaver in
the Sierra Nevada "in canyons and on mountain
sides where suitable springs occur, usually at
considerable altitude."
Historical records indicate that the mountain
beaver's association with high elevations and
steep gradients is not the result of human activities excluding the animal from low elevation,
low gradient sites since 1935. The field notes of
C. L. Camp and T. I. Storer also suggest that
suitable habitat has been scarce in the Sierra
Nevada. T. I. Storer (1919 field notes:829) commented on their rarity: "It would seem that the
hold which these animals have in the scheme of
existence is exceedingly slight, being so especially fitted for a particular kind of life and a
particular sort of habitat." Because I could find
no pre-1906 references to mountain beaver in
the Sierra Nevada, I could not detect any shifts
in habitat use by mountain beaver prior to the
1906-35 era.
MANAGEMENTIMPLICATIONS
In the Sierra Nevada, mountain beaver occur
in small patches of high elevation, steep, moist
habitat, often separated from other populations
by distance and topography. The historical record indicates that this is a long-standing pattern, because the Sierra Nevada apparently offer
only marginal habitat conditions.
Multivariate wildlife habitat models can be
used for 2 different purposes, namely to identify
factors important in habitat use, and to predict
species occurrence (Shugart 1981). With respect
to the first goal, the SLR model suggests that a
cool thermal regime and adequate soil drainage
are the most important variables governing habitat use by mountain beaver in the Sierra Nevada, and that an adequate supply of succulent
food, including some evergreen foliage, is also
necessary. Therefore, management activities
such as road building, livestock grazing, and
herbicide applications will also influence suita-
MOUNTAINBEAVERHABITATUSE * Beier
653
bility of the habitat for mountain beaver, by
altering soil drainage and forage composition.
As a predictor of mountain beaver occurrence, the SLR model correctly identified 97%
of the reaches not used by mountain beaver,
and 65% of the used reaches. From a management perspective, the latter rate is more critical,
because the failure to identify used habitats is
a more serious error than incorrectly predicting
the presence of a population (Rice et al. 1981).
Because the model may frequently fail to alert
the manager to potential impact on the species,
it should not be used to predict mountain beaver
occurrence in the Sierra Nevada.
LITERATURECITED
ALLEN,L. 0. 1969. Preferential food habits of
Aplodontia rufa, M.Ed. Thesis, Central Washington State Coll., Ellensburg.55pp.
ANDERSON,J. A. 1972. Separate sample logistic discrimination. Biometrika 59:19-35.
BEIER,P., ANDR. H. BARRETT.1987. Beaver habitat
use and impact in the Truckee Basin, California.
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CAMP, C. L. 1918. Excavations of burrows of the
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517-536.
Cox, D. R. 1970. The analysis of binary data. Methuen and Co. Ltd., London, U.K. 142pp.
CROUCH, G. L. 1964. Forage production and utilization in relation to deer browsing of Douglasfir seedlings in the Tillamook burn, Oregon. Ph.D.
Thesis, Oregon State Univ., Corvallis. 162pp.
GRINNELL, J., AND T. I. STORER. 1924. Animal life
in the Yosemite. Univ. California Press, Berkeley.
752pp.
INGLES,L. G. 1959. A quantitative study of mountain beaver activity. Am. Midl. Nat. 61:419-423.
1965. Mammals of the Pacific states. Stanford Univ. Press, Stanford, Calif. 506pp.
JOHNSON, S. R. 1971. Thermal regulation, microclimate, and distribution of the mountain beaver,
Aplodontia rufa pacifica Merriam. Ph.D. Thesis,
Oregon State Univ., Corvallis. 164pp.
KINNEY, J. L. 1971. Environmental physiology of
a primitive rodent. Ph.D. Thesis, Univ. Oregon,
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McNAB, B. K. 1979. The influence of body size on
the energetics and distribution of fossorial and
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NIE, N. H., editor. 1983. SPSSX user's guide.
McGraw-Hill,New York,N.Y. 806pp.
1965.
Water balance and maximum concentrating capacity in the primitive rodent, Aplodontia rufa.
Comp. Biochem. Physiol. 14:289-297.
ORR, R. T. 1949. Mammals of Lake Tahoe. Calif.
Acad. Sci., San Francisco. 127pp.
PRESS, S. J., AND S. WILSON. 1978. Choosing beNUNGESSER, W. C., AND E. W. PFEIFFER.
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tween logistic regressionand discriminantanalysis. J. Am. Stat. Assoc. 73:699-707.
RICE, J., R. D. OHMART, AND B. ANDERSON. 1981.
Bird community use of riparianhabitats. Pages
186-196 in D. E. Capen, ed. The use of multivariatestatisticsin studiesof wildlife habitat.U.S.
For. Serv. Gen. Tech. Rep. RM-87.
SCHMIDT-NIELSEN, B., AND E. W. PFEIFFER. 1970.
Urea and urinary concentrating ability in the
mountainbeaver, Aplodontia rufa. Am. J. Physiol. 218:1370-1375.
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variate methods and their application to studies
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ed. The use of multivariatestatisticsin studiesof
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wildlife habitat. U.S. For. Serv. Gen. Tech. Rep.
RM-87.
F. 1906. Californiamammals.WestCoast
STEPHENS,
Publ. Co., San Diego, Calif. 351pp.
VOTH,E. H. 1968. Food habitsof the Pacificmountain beaver, Aplodontia rufa pacifica Merriam.
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263pp.
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Received 2 October 1987.
Accepted 4 January 1989.
PREDATION
ON UNIONIDCLAMSBY MUSKRATS
SIZE-SELECTIVE
LAURIEE. CONVEY,Departmentof Zoology,Universityof Alberta,Edmonton,AB T6G2E9, Canada
JOHNMARKHANSON,Departmentof Zoology,Universityof Alberta,Edmonton,AB T6G2E9, Canada
WILLIAM
C. MACKAY,
Departmentof Zoology, Universityof Alberta,Edmonton,AB T6G2E9, Canada
Abstract: We collected shells of the northern floater (Anodonta grandis simpsoniana) from muskrat (Ondatra zibethicus) middens on the shore of a small lake in the boreal forest zone of Alberta. Muskratsate a
mean of 228 ? 23.6 (SE) clams/day or 1.4 ? 0.15 kg/day (soft body mass measuredas wet wt) from 22 July
to 1 September 1986. The overall length and age distributionsof clams eaten (median length = 64.3 mm,
median age = 7.5 yr) were larger and older (P < 0.001) than a random sample of clams from the lake
(median length = 49.1 mm, median age = 6.2 yr). Muskratsmay have a significant effect on the size and
age distributionsof clams in the lake.
MANAGE.53(3):654-657
J. WILDL.
The muskrat is a herbivore that occasionally
eats animal matter (Enders 1932, Smith 1938,
Bellrose 1950). Most studies of muskrat diet have
reported only the plant species in their diet (Butler 1940, Takos 1947, Danell 1978) because most
studies of muskrat diets are conducted in marshes where plant matter is abundant relative to
suitable animal prey. A few studies noted the
consumption of animal prey (e.g., crayfish, fish,
insects, snails, young birds, other muskrats, frogs,
turtles, salt- and freshwater mussels) (Errington
1941, Bellrose 1950, Triplet 1983). In addition,
muskrats have been accused of destroying mussel beds (Headlee 1906, Van Cleave 1940, Joy
1985) based on the observation of shells discarded on feeding platforms. However, prey
selection and consumption rates of clams by
muskrats have not been documented. The goals
of our study were to determine the size and age
distributions of clams consumed by muskrats, to
test for spatial and temporal variation in their
size and age distributions, and to determine
whether the size and age distributions of clams
eaten differed from the size and age distributions of clams in the lake.
We thank S. A. Boutin for reviewing this paper. C. Podemski provided extensive field assistance. LEC was supported by a Natural Sciences
and Engineering Research Council of Canada
(NSERC) Summer Undergraduate Award. JMH
was supported by an NSERC Postdoctoral Fellowship. This study was done at the Meanook
Biological Research Station and was funded by
a Boreal Alberta Research grant to JMH and an
NSERC operating grant to WCM.
METHODS
We collected intact valves of unionid clams
from muskrat middens along the shore of Narrow Lake (54?34'N, 113?37'W), a small (1.14
km2), deep (x depth = 14.2 m), moderately productive (x total phosphorus = 12.9 mg/m2) lake