Dean E. Pearson,Yvette K. Ortega, Kevin S. McKelvey, and Leonard F. Ruggiero, USDA Forest Service, Rocky
Mountan Research Staton Forestry Sciences Laboratory. PO Box 8089. Missoua. Montana 5'3807
Small Mammal Communities and Habitat Selection in Northern Rocky
Mountain Bunchgrass: Implications for Exotic Plant Invasions
Abstract
Agriculture and development have dramaticall) reduced the range of natire bunchgrass hahivats in h e Narlhrrn Rocky Muunlains. and the iniarion of exotic plants threatens to greatly alter the remaining pristine prairie. Small mammals play many important rules in ecmvstem functiuns. but little is known about small rnilrnrniil cummunitv comousition and structure in native bunchcomposition and relative abundance were cunsistent among sites. with deer mice (Peromyscus nianiculatus) dominating, followed
by muntilnr w l r s (Microrui montanus). which were uncommon. and montane shrews (Sorex m m i c o i u r ) , which were rare. Deer
m i c e and monvanr voles eahihited complcmentar) hahilat separation. Deer mice tended lo selrcl open micro>iles and avoid sites
will? high pcrccntagcs of vqctatirc covcr Male and female dccr mice dcmonstratcd strong habitat separation at two sites. but the
hahilat wriablcs partitioned bcrivccn sexes differed hy site. Montane voles avoided open sites and selcctcd for concave microsites
where lhe v p l a l i w corn was rclativcly densc. This information providcs an imponant hasclinc Cor undemanding pre~aettlrmrnt small miimmal communities in lhr rapidly dwindling. natiie hunchgmia habitats of the Northern Rocky Mountilins.
Introduction
Palouse prairie and sagebrush-steppe habitats of
the Northwest have been dramatically altered by
grazing, agriculture, and development (Tisdale
1961, Mueggler and Stewart 1980, West 1996,
see Flather et 81. 1999 for review), and most ecologically intact remnants persist as islands critically threatened by the invasion of exotic plants
(Daubenmire 1942, Tyser and Key 1988, Lacey
1989, DeLoach 1991, Tyser 1992, Tyser and
Worley 1992, Sheley et al. 1998). Valley bunchgrass habitats, occurring at or below lower timberline in the Northern Rocky Mountains. superficially resemble the Palouse prairie grasslands
of the Columbia River Basin, but are unique plant
communities differing in species composition and
in the historical dominance of rough fescue
(Festucascnbrella) (Lynche 1955, Stickney 1961,
Mueggler and Stewart 1980). Additionally, valley bnnchgrass habitats were historically much
less extensive than Palouse prairie, and currently
only 10 to 20% of these habitats remain in western Montana (Flather et al. 1999).
Small mammals play important roles in ecosystem functions (see Pearson 1999a) and may
figure prominently in the ecology of exotic plant
invasions within xeric grasslands of the West. For
instance, small mammals create disturbed sites
where exotics can establish (Mielke 1977), disperse seeds of exotic plants (McMurray et al. 1997,
Pearson and Ortega in press), consume native and
non-native plants and their seeds (Pyke 1986).
and depredate biological control agents released
to control exotic plants (Pearson 1999b, Pearson
et al. 2000). Reciprocally, exotic plant invasion
of native grasslands can alter biomass and species composition of small mammal communities
(Larrison and Johnson 1973), thereby modifying
ecological roles that small mammals play in grassland systems. Although little work has been done
toexamineeffects of exotic plants on smallmamma1 communities (Lamson and Johnson 1973,
Ellis et al. 1997, Pearson et al. 2000), current research suggests that exotic plant invasions can
alter small mammal ecology in complex ways.
Interpreting the degree to which the effects of
exotic plant invasions impact grassland ecosystems is contingent upon understanding conditions
prior to invasion. However, a dearth of community-level studies of small mammals within valley bunchgrass habitats renders the composition
and structure of these communities uncertain. For
instance, although some studies suggest that Microtus are prominent species within many grassland habitats in western Montana (Koplin and
Hoffmann 1968. Stoecker 1972. Hodgson 1972,
Northwest Science. Vol. 75, No. 2, 2001
107
Douglas 19761, the relative impottance ofMi(.rotus
in native bunchgrass habitats remains unclear.
In this paper. we present data on species compohition. relative abundance, and habitat use of
small mammals in three pristine valley bunchgrass habitats of west-central Montana to establish baseline information on small mammal communities within these ecosystems.
Methods
Study Sites
We sampled small niammal communities at three
locations in valley bunchgrass habitats of westcentral Montana: Bandy Ranch. Sieben Ranch,
and Wildhorse 1sland.All sites fall in10 the rough
fescueildaho fescue (Frstuca idnhoen.ris) habitat
types (Mueggler and Stewart 1980), but dominant grasses varied at microsites among rough
fescue. Idaho fescue. and hluebunch wheatgrass
(Agropyron spiratum) as a function of moisture
and aspect. All study areas were 25 ha. The Bandy
Ranch site is located about 8 km northwest of
Ovando. MT at 1370 m elevation. Uplands at the
Bandy Ranch are dominated by bunchgrass and
big sage (Artrmisia rridentatu) habitats and interspersed by glacial potholes and their associated wetland vegetation. Trapping at this site was
restricted to upland hunchgrass habitats, which
included some sage and some concave microsites
with mesic vegetation. Sieben Ranch is about 8
kmeast of Lincoln. MT at 1400 m elevation. Sieben
Ranch is dominated by bunchgrasses, but also
contains patches of big sage. Wildhorse Island is
the largest island in FlatheadLake, Lake County,
MT, covering nearly 10 km' and as such appears
to function essentially as a mainland for small
m;immal communities. Previous trapping studies on islands in Flathead Lake suggest that many
grassland small mammals have become established
on the various islands (Plopper 1968) and are likely
all present on the substantially larger Wildhorse
Island. The trapping area on Wildhorse Island
ranged from 900 to 1030 m in elcvation.
effectively sampling the small mammal community (Pearson and Ruggiero in review). Transects
were spaced2100 m apart to ensure independence.
The effective trapping area was therefore equal
at all sites. Traps were baited with peanut butter
and whole oats and checked from 0700 to 1100
hrs for four days. Small mammals were tagged
with #I 005-1 monel ear tags (National Band and
Tag Company, Newport, Kentucky 41072-0430),
and species, age, weight, sex, and reproductive
condition were determined before release at the
trap station. Live Micratns montanus and Microtus penn~sjlvaizicusare not easily differentiated
(Hoffmann and Pattee 1968). Therefore, since all
lnorlalities (20% of the total capture) were identified as montane voles (Micratus ~nontanus),and
the habitats were gencrally too dry for M.
penn.sjlvnnicus (Koplin and Hoffmann 1968,
Hodgeson 1972), we assigned all Microtu,~captures to M. montaniis.
Small mammal habitat was assessed by visually estimating percent cover for several vegetation categories within a 5-m radius of each trap
stalion. We estimated (I) total bunchgrasses - primarily bluebunch wheatgrass, Idaho fescue, and
rough fescue, but also June grass (Koleriu crisfara),
needle and thread grass (Stil~acomara), and other
less abundant native bunch grasses: (2) total nonbunchgrasses - mostly Poa species: (3) shrubs lnostly big sage and rabbit bmsh (Chrssothnmnu.~
spp.); (4) native forbs - primarily arrowleaf
balsamroot (Balsumorhizu sayittata) and lupine
(Lupinus spp.); and (5) exotic plants - spotted
knapweed, leafy spurge (Euphorbia esula), Eurasian toadtlax species (Linaria dalrnatica and L.
vulguri.~),and cheatgrass (Bromus rectorum).
Additionally, physiographic and abiotic features
measured included topography of the microsite
(i.e.. concave = 1 or not = 0; as an indicator for
low, moist sites). percent cover of bare ground,
and percent cover of rocks = 10 cm dia. Cover
estimates were made by the same two observers
at all sites.
Analytca Methods
Field Methods
We lrapped small manlmals i n August 1998 using 25 Shem~anlive traps placed a1 10-m intervals along eight transects per site. Transects were
used because they generate morecaptures andmore
species than grids of equal size, thereby more
108
Pearmn, Onega. McKel\ey, and Ruggiero
We present frequency distributions of the number of individuals captured hy species for each
site. We used logistic regression to determine which
habitat variables best differentiated trap stations
that captured small mammals from those that did
not. Traps having multiple captures were included
only once in the analysis for each species. By
employing thc gcncralircd cstimating equations
capability in SAS GENMOD, we developed unsuuctured correlation models that relax the standard assumption of independence among observations within transects (SAS Institute 1990).
Counting trap stations only once and controlling
for spatial correlations i n the data reduce the
pseudoreplication that can occur in data sets due
to repeated captures of a subset of individuals
within apopulation since individuals are most often
recaptured in the same or adjacent traps. Equations produced by logistic regression serve as resource selection functions an; the coefficients senre
as selection coefficients (p)whose values indicate the relative importance of the predictor variables in the equation (Manly et al. 1991). Positive coefficients indicate selection for the resource
and negative coefficients indicate avoidance when
coefficients differ significantly from zero (Manly
et al. 1991).We chose likelihood ratio tests over
Wald tests to determine the significance of selection coefficients because of their greater reliability (Hauck and Donner 1977).
We used a model selection procedure (Manly
et al. 1993. Asthur et al. 1996, Ruggiero el al.
1998) to determine whether habitat selection for
each species differed among sites. thereby indicating whether site data should be pooled or analyzed separately. This process involves generating a null or no-selection modcl whcrcin the constant
is estimated and all other coefficients are set to
zero. The deviance value (-210ge[likelihoodl)from
the null model is then compared to the deviance
valuc from a pooled sclcc~iontnodcl wherein coefficients are estimated using pooled data from
all sites. A significant improvement i n the pooled
sclcction model over ihe null model indicates that
habitat selection has occurred. and model selection proceeds by estimating a selection function
for each site independently. If the summcd dcviance values from the site-specific selection models is significantly smaller than the deviance from
the pooled selection model, thih indicatcs that
habitat selection differs among sites. and site-specific selection functions should hc uscd to cxamine habitat selection. Because habitat segregation
between sexes has been reported for deer mice
inhabiting xeric grasslands and savannas (Bowers and Smith 1979, Morrih 1984). we cxtcndcd
the model selection procedure using multinomial
logistic regression with SAS CATMOD to test
whether categorizing capture sites by sex (i.e.,
only males captured, only females captured, males
and fcmales captured) resulted in better habitat
use models than those disregarding sex. This analysis effectively examines the degree to which habitat
use by males and females directly overlaps (100%.
overlap results in no habitat separation), testing
for differences in habitat variables among the three
potentially distinct groups of capture sites. Because CATMOD does nolgeneratc likclihood ratio
tests for coefficients. we present Wald tests for
this analysis. Correlation was not built into the
multinomial models for habitat selection by sex
because CATMOD does not offer this option for
multinomial regression models.
Results
Bunchgrass dominated the herbaceous cover at
all study sites ranging from approximately 40%
cover on Wildhorse Island to 65% on Bandy Ranch
(Figure 1). Sieben Ranch had the lowest cover of
native forbs, nonbunchgrasses, and shrubs and the
highest cover estimates for bare ground. Wildhorse
Island had the highest densities of nonhunchgrasses
and native forhs. Exotic plants were generally rare
to absent on all sites. Spotted knapweed. toadflax, and leafy spurge cover estimates averaged
<I% for all sites. Chcatgrass was the most abundant and widespread exotic with average estimates
4 % on Sieben Ranch, 1% on Bandy Ranch. and
4% on Wildhorse Island. The proportion of conc a w microsites was approximately 40% on both
Bandy Ranch and Wildhorse Island and 20% on
Sieben Ranch, which also exhihited less largescale topographic relief.
Two thousand four hundred trap nights produced 410 captures of 273 individual small mammals representing three species. Species rankabundance was consistent among locations with
deer mice dominant. followed by montane voles,
which were relatively uncommon, and montane
shrews, which were rare (Figure 2). It should also
be noted that northern pocket gophers ( T l ~ o r n o n ~ ~ s
talpoider). as indicated by the presence of winter
cores and fresh mounds, and yellow-pine chipmunks (Tcrmius nmoerrrrs), as indicated by visual
observations of the animals in shrub patches, were
present on the sites. However. we did not directly
sample and therefore quantify the abundance of
either of these species.
Small Mamnals in Native Bunchgrass
109
Sieben Ranch
Bandy Ranch
Forb
Ground
Nonbunch
Rock
Shrub
Totbunch
Figure 1. hlcan percent cover rslimatcs for vegetatiun, rocks. and hare ground within a 5-m radius of each trap b u i o n .
The pooled selection model for deer mice
provided a significant improvement over the noselection model (x' = 100.03. df = 7, P < 0.001),
indicating deer mice exhibited habitat selection
(Table 1). Furthermore, aggregation of the sitespecific selection models resulted in a model that
was significantly improved over the pooled selection model (x' = 92.89, df = 15, P < 0.001).
indicating that deer mouse habitat selection dif,
~~~,
~-~~~
Ranch deer mice avoided areas with higher forb
(x' = 8.23. df = 1. P = 0.004) and bunchgrass
cover (x2= 13.54, df = 1. P < 0.001) and exhibited marginally significant selection for rocky areas
(x' = 3.57, df = 1, P = 0.059). A selection coefficient for shrubs at Sieben Ranch could not be
110
Pearson. Ortega. McKelvey, and Ruggiero
estimated because values were zero for one category At the Bandy Ranch deer mice avoided arras
with higher cover of bunchgrass (x2= 5.57. df = 1,
P = 0.018) and nonbunchgass (x2= 13.99. df = I,
P < 0.001) and selected xeas with more bare ground
(x2= 8.35. df = 1, P = 0.004). On Wildhorse Island
deer mice responded to all variables measured except topography. Mice on Wildhorse avoided most
vegetative cover such as native forbs (p= 9.30. df
=I, P = 0.002), nonbunchgrass (x2= 5.62, df = I, P
= 0.01 8), and shrubs (x2= 4.75, df I, P = 0.029)
in favor of areas with higher percent cover of bare
ground (x' = 26.21. df = I , P < 0.001) and rock (x2
= 14.90. df = I . P c 0.001). However, deer mice on
Wildhorse selected for bunchgrass cover (f = 6.92,
df = I , P = 0.009), whereas they avoided it at other
sites.
-
I
Deer Mouse
l m
Montane Vole
Bandy Ranch
Siehen Ranch
Wildhorse Island
Figure 2. Species composition and relative abundance of small mammals capturcd in three location\ within valley hunchgas*
habitats of a,rst-cmtral Mvluntana.
TABLE 1. Selection cuefficients for deer mice in pristine valley hunchgrass hahitats of W C S I ~ C h~ l ~u n, t~i i ~based on logirric
regression analysis. Parenthese, indicate the number of stations capturing mice. A iclccdon coefficient was not
calculaud Tor ahruba at Sirhrn Rmch due to singularity of the variable.
Sclccrion cocfficicnts
Modcl
Forh
Ground
Null
0.00
0.00
Poolcd (199)"
0.011*
0.028**
-0.134'"0.003
Siehen (48)'
Bandy ( X I *
0.016
0.041**
Wildhorsc(126)**
0.019** 0.036**
Sire deviance totals
Null vi, pooled (sslscrion)
Pooled (ielectionl Vs site elru rut ion)
*
**
Nunbunch
0.00
0.003
-0.010
-0.013**
0.013*
Ruck
0.00
0.05hx*
0.145
0.019
0.117**
Shrub
Bunch
TODO Deviance
df
P
0.00
-0.015"
0.015
-0.146*
indicates rigniticant diffcrcncc a1 a = 0.05
mdicates rigniiicant difference at n = 0 0 l
Sex-based habitat selection models for deer
mice weresignificantforSiebenRanch(~'= 17.33.
df = 6, P = 0.008) and Wildhorse Island (x' =
24.09. df = 7, P < 0.001) and marginally signifi-
cant for Bandy Ranch (x' = 13.12, d l = 7. P =
0.069). Females at Sieben Ranch used sites with
more bare ground (x'= 4.85, df = I , P = 0.027)
and more total bunchgrass cover (x' = 7.75, df =
Small Mammals in Native Bunchgrass
111
1, P = 0.005) than males. At Wildhorse Island,
females wed sites that were less concave (x'=
6.17. df = I, P = 0.013) and had less shrub cover
than male5 (x2= 88.3, df = I , P = 0.004). and
females selected microsites with lower shrub cover
4.15, df = 1, P = 0.042) than stations where
both males and females were captured. At Bandy
Ranch, habitat partitioning between sexes was not
significant for any individual comparisons ( P >
0.140). Chi-square goodness of fit tests assuming equal sex ratios indicated that scx ratios for
Wildhorse Island were male biased (1.40 males
to females; x'= 6.61, df = 1, P = 0.010), hut the
sex ratio for Bandy Ranch was not biased (1.59
males to females;
2.27, df = 1, P = 0.132).
nor was the sex ratio for Sieben Ranch (0.88 males
to females;
0.38. df = I, P = 0.540).
The pooled seleclion model for montane voles
provided a significant improvement over the noselection model (x2 = 18.00. df = 7. P = 0.012).
However. the site-specific model provided little
improvement over the pooled model (x' = 22.94.
df = 16, P = 0.1 15), indicating either that habitat
selection by montane voles did not differ among
sites. or that we lacked sufficient power to detect
such differences due to low captures of this species. Overall. montane voles selected concave
micrositcs (x' = 6.30, df = I, P = 0.01 2) and avoided
areas with high percentages of bare ground
=
7.18, df = I , P = 0.007). but did not exhibit selection with regard lo othcr variables measured.
(X:=
x2=
x'=
(x'
Discussion
Community Cornpostion
The small mam~nalcommunity we identified for
valley bunchgrass habitats in west-central Montana was remarkably consistent with regard to
species composition and relative abundance among
the three distinct locations [rapped. Dcer mice
dominated each native bunchgrass site. Montane
voles were consistently captured. hut relatively
uncommon. Montane shrews were rare at two
locations, and none were captured on Wildhorse
Island. Valley bunchgrass habitats of west-cenlral Montana appear to contain fewer species than
small mammal communities described for neighboring grasslands and shrub-stcppe of eastern
Washington. eastern Oregon, southern Idaho. and
the westcrn Great Plains of eastern Montana
(Lanison and Johnson 1973, Rogers and Hedlund
1980. Gano and Rickard 1982, Grant et al. 1982.
112
Pcarson, Ortega, McKelvep. and Ruggiero
MacCracken et al. 1985, Groves and Steenhof
1988, Koehler and Andcraon 1991 . Elliot et al.
1997). Although some species that could occur
in these habitats as uncommon or rare community member5 such as meadow voles (M.
p~rms~1vnnicu.s).
jumping mice (Za~ius
pf-inceps).
vagrant shrews (S. vugrarrs), and cinereus shrews
(S. cinereus) may have gone uncaptured, most
species described for adjacent grasslands such as
the prairie vole (M. oclvoguster), sagebrush vole
(Lemi.rcus currutus), grasshopper mouse (Onocom?.s la~ogu.ster),plains pocket mouse (Per-og~
narhus prrrvus), olive-backed pocket mouse ( P
faiciarus), western harvest mouse (Reithrodontomys megaloris), and kangaroo rat (Dipodon~yc
ordii), do not occur within western Montana or
are extremely rare (only a few records) (Hoffmann
and Pattie 1968). One possible explanation for
the lower species richness of valley bunchgrass
habitats is that the presence of valley glaciers and
historic Lake Missoula (Ah and Hyndman 1986)
extirpated more spccialized grassland species from
the region. Forested mountain ranges surrounding these grasslands may further serve to prevent
recolonization.
Conclusions regarding species composition and
relative abundance based on a single year's data
require some consideration of annual variability
given the multiannual fluctuations exhibited by
many small mammal species (Krebs 1996). Examination of the literature strongly suggests that
deer mouse domination of xeric grasslands is the
norm within much of the Northwest (Granter al.
1982. MacCracken et al. 1985, Pyke 1986. Groves
and Steenhof 1988. Elliot et al. 1997). However,
deer mice can periodically decline to hecome less
abundant than montane voles, especially if vole
populations increaseconcurrently with thcdecline.
Montane voles becameessentially codominant with
deer mice in the second year of Pyke's (1986)
study in Washington. and montane voles clearly
outnumbered other small mammals at Grant et
a l ' s (1982) ungrazed site in the Bridgcr Mountains of Montana. Although shrews fluctuate and
can be common in moist habitats (Spencer and
Pettus 1966). shrews are not reported as common
within dry habitats of the Northwest (Negus and
Findley 1959, Rickard 1960, Clark 1973. Pearson
1999a). Our results considered in the context of
small mammal studies within other grasslands of
the Northwest suggest that deer mice generally
dominate valley bunchgrass habitats. with voles
being uncommon and shrews rare. However.
montane voles may periodically dominate these
communities during population highs. Montane
voles may also play a more prominent role in hlgher
elevation, more mesic bunchgrass hahitats (e.g.,
Grant et al. 1982).
Habtat Selection
Deer mice are habitat generalists at the macrohabitat scale. as indicated by their viability within
a wide range of habitats (Handley 1999). However, their ability toefficiently exploit a wide range
of resources can rcnder them quite speciali~edin
their selection of microsites within habitats that
differ i n resource availability (e.g., Pearson 1994).
In native valley bunchgrass habitats of west-central Montana, we found that deer mouse selection of microhabitats varied by site. This differcntial sclcction for rcsourccs huch as bunchgrass
(selected for at Wildhorse Island, but avoided at
Bandy and Siehen Ranches) probably retlects 1)
the fact that the distribution of these resources
varies by site and 2) the tict that deer mice are
responding to a complex of resources, many of
urhich were not measured. For example, as bunchgrass distributions change so may the abundance
and therefore relative value of associated resources
huch as sccdh and insects that dccr micc arc foraging on. However, at all sites, deer mice tended
to avoid heavy vegetative cover in favor of more
open and in some cases, rockier hahitats. Elliott
et al. (1997) sin~ilarlyreponed that dccr mice
selected microhabitats with more hare ground and
less grass cover in southeastern Wyoming grasslands. That deer mice increase in response to grazing (Lamison and Johnson 1973, Grant et al. 1982,
Rosenstock 1996) may also be evidence of selection for more open vegetation, as fra7ing decreases the total vegetative biomass on a sire (Grant
ct al. 1982).
Dccrnmusc dcction forharc ground in hunchgrass habitats, which are inherently low in vegetative cover, may result from predator influences
on deer mice or deer mice respondins to their
own prey-base. For example, use of open habitat
could indicate that predation risk from weasels.
which favor heavy ground cover, cxcccds that from
raptors, which prefer to hunt where ground cover
is minimal (e.g.. Korpiniaki et al. 1996).Alternativcly, dccr micc. which arc primarily inxctivorous in westem passlands (Sieget al. 1986. Pearson
et al. 2000), may be morc effective at preying on
particular insects in open habitats. or their prefel~edpreymay attain higher densities within such
habitats. Deer mice also tended to select rockier
areas, though this relationship was only marginally significant at Sieben Ranch where rocks were
relatively rare. Rocks may provide escape cover
that the predominantly herbaceous vegetation docs
not. However, most rocks wcrc relatively scattered, partially buried, and small (approximately
I to 3 dm). Possibly, rocks provide cover for secure burrow sites as deer mouse burrow cntranccs
were sometimes observedin association with rocks.
We observed habitat partitioning between male
and female deer mice at Sieben Ranch and
Wildhorse Island. Variables separating male and
female deer mice differed by site, with females
at Sieben Ranch using microsites having more
bare ground and more bunchgrass c o w than males.
and females at Wildhorse Island using microsites
with less shrub cover than malcs. Females at
Wildhorse Island also used concave sites less often than males. Since deer mice, as a species,
avoided bunchgrass at Sieben Ranch and shrubs
at Wildhorse Island, thc xxual separation on these
variable axes would appear to result from a further partitioning of these resources hetween the
sexes. At Wildhorse Island, the species-level response to topography may have hccn masked by
habitat partitioning between the sexes, as males
exhibited no avoidance of concave sites. whereas
topography values at female trap stations and stations where both malcs and females were captured indicated relatively strong avoidance. At
Sichen Ranch, a similar phenomenon was observed: males exhibited no selection for bare ground
while females differed significantly from the males
in their seleclion for bare ground. In general. females appeared to be more selective of microhabitats than were males. At Wildhorse Island thc
skcwcd scx ratio could have facilitated this differential habitat selection between sexes by allowing the proportionally fewer females to bemore
selective of microhabitat space. However, one
might expect that the more abundant male> would
overwhelm such an effect by encompassing all
female microsites as well as additional hahitats.
Moreover, sex ratios did not differ from random
elscwhcrc, and so could not be invoked to explain
differential habitat selection at Sieben Ranch.
Small Mammals in Native Bunchgrass
113
Determining which sex triumphs when a resource is partitioned within a spccics is contingent upon correctly assessing the resource gradient. Bowers and Smith (1979) produced a
defensible argument for female deer mice outcompeting males by showing that females dominated microsites having higher measured soil
moisture in habitats where water was clearly limited. Seagle (1985) and Belk et al. (1988) contend that structural features such as logs and woody
vegetation are the preferred resources in some
habitat5 and have shown that females are more
closely associated with these resources than are
males. They therefore argue that females effectively dominate the preferred resources i n these
systems. This argument is reasonable, but untested.
Woody strucLurc provides one important vector
along a complex of resource gradients. but unless it is the limiting resource. and the addcd hcnefits of cover do not compromise other resource
nceds (i.e.. the collective resource gradient is linear and correlated with structure), it is unclcar
whether the tradeoff, in other resource vectors
associated with increasing cover will lead to increased fitness. Similarly, without a defensible
measure of resource quality. we do not attempt
to assess which sex dominated the preferred resources within the bunchgrass community that we
studied. However, based on our results. we conclude that I ) resources were sufficiently limited
to warrant intraspecific partitioning within the
bunchgrass community (c.g., Bowers and Smith
1970). 2) resources were sufficiently variable to
allow for their partitioning among conspccifics
(see Seagle 1 9 8 3 , and 3) intraspecific resource
selection pressures differed among sites.
Fcw studies have examined microhabitat selection of montane voles in the Northern Rocky
Mountains. In our balky bunchgrass habitats.
montane voles strongly avoided trap stations in
open areas wilh more barc ground and selected
concave microsites where total herbaceous cover
was high. The higher herbaceous cover at concave microsiles appcarcd lo rcsult from higher
soil moisture. Hodgson (1972) found that montane voles were negativelycorrelated with shrubs.
with the cxception of big sage, and positively
correlated with soil moisture and higher graminoid
cover in southwestern Montana. Similarly, Belk
el al. (1988) ohscrvcd that montane volcs selected
habitats with higher herbaceous cover and lower
shruh cover within a mixture of habitats in Utah:
114
Pearwn. Ortega. McKelley, and Ruggiero
and Randall and Johnson (1979) showed that
montane voles favored bunchgrass habitat over
hhrubby habitats in Washington, but overflowed
into shrubby habitats when populations increased.
Collectively, these studies indicate that montane
voles avoid open habitats in favor of microsites
with higher vegetatix covcr and higher soil moisture. Montane voles may also avoid shrubs, with
the possible exccption of big sage. but since we
attempted to exclude sage from our sampling and
other shrubs were rarc on the study sites, we could
not effectively assess these relationships.
Deer mice and montane volcs appear to occupy complemmtary niches in valley bunchgrass
habitats of west-central Montana. Whereas deer
mice generally sclected for dryer, more open habitats with relatively little vegetative cover, montane voles avoided open areas in favor of moister
concave micrositcs with higher herbaceous cover.
Such complementary habitat selection could be
construed as habitat partitioning resulting from
interspecific competition. as has been argued for
deer mice and other Microtus species in grasslands of British Columbia (Redfield el al. 1977)
and Colorado (Abramsky et al. 1979). However.
resource competition for food seems unlikely as
deer mice are primarily insectivorous and gnmivorous in this rcgion (Sieg et al. 1986, Pearson et
al. 20001, and montane voles are herbivorous
(Zimmernian 1965. Lindroth and Batzli 1984).
InLcrfcrence competition is similarly reduced by
dihparatc activity patterns that help separate nocturnal dccr mice (Falls 1968) from the largely
diurnal montane voles (Drabek 1994). Furthermore, although voles avoid open habilats u,here
deer mice abound, deer micc arc smaller than
Miuotus (by 30% on avcragc in this study) and
thus unlikely to exclude voles through interference competition. Although decr micc avoid the
heavier herbaceous cover, thcy arc ncither excluded
from it nor from the concave rnicrosites where
voles abound. Complementary habitat selection
between deer mice and montane voles in vallcy
bunchgrass habitats may arise from noncompctitive coexistence as has been shown for dccr mice
and southern red-backed voles (Clefhriorzontys
gnpperi) in Virginia (Wolff and Dueser 1986).
Determining the causal mechanism for habitat
separation betwccn dccr mice and montane voles
within vallcy hunchgrass habitats would require
reciprocal removal studies.
Conclusions
Exotic plants have transformed vast rcgions o l
western grasslands and will continue to impact
the remainder of these systems with potentially
complex ecological consequences. However, it is
difficult to oredict the ~otentialinloacts that exotic plants may have on small mammal communities hecausc little is known about small mammal ecology in intermountain grasslands. Our
rcsults emphasize the habitat specificity of montane voles and the habitat flexibility of deer mice
in thc lowelevation intermouutain grasslands and
leadus to hypothesize that generalist spccies such
as deer mice may he favored over habitat and dietary specialists such as voles and shrews given
invasions of such exotics as spotted knapweed,
Icafy spurge, and Eurasian toadtlax species. Bccause small mammals play important speciesspecific roles in ecosystem functions. changcs
- in
3mall mammal \pecics composition will likely
affect prcdator communities and the ecological
roles that small mammals play with regard to
herhiwry. sccdandinsect predation, seed dispersal,
etc. This paper presents important baseline in-
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Recerved I 1 Jrtnuan 2000
Acc?pled 25 October 2000
Small Mammals in NaLive Bunchgrass
117