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Patterns of Relative Diversity
Within Riparian Small
Mammal Communities, Platte
River Watershed, Colorado1
Thomas E. Olson2and Fritz 1. KnopP
Riparian communities in the western
states are mesic vegetative associations occurring along ephemeral, intermittent, and perennial streams.
Although relatively limited in area,
these communities contribute more
biotic diversity within a region than
upland vegetation communities
(Thomas et al. 1979).
Riparian communities have been
substantially affected by land-use
changes such as conversion to agriculture, grazing, and water management (Knopf et al. 1988).Further alterations in the western United States
have been caused by the widespread
naturalization of salt cedar (Tamarix
spp.) (Horton 1977) and Russianolive (Elaeagnus angust ifolia) (Olson
and Knopf 1986). Because of the biological significanceand potential for
perturbations caused by conflicting
land uses, riparian communities have
been the focus of numerous technical
conferences during the past 10 years
(Knopf et al. 1988).
An earlier study of the pattern of
avian species diversity in riparian
and upland study areas within a watershed (Knopf 1985) showed that
'Paper presented at symposium, Managemenf of Amphibians, Reptiles, and
Small Mammals in North America. (Flagstaft AZ, July 7 9-2 7, 7988.)
Wildlife Biologisf, Dames & Moore, 175
Cremona Drive, Suite A-E, Golefa, California 93 1 17.
3Leader,Riparian Studies, U.S. Fish and
Wildlife Service, 13CO Blue Spruce Drive, Fort
Collins, Colorado 80524-2098.
Abstract.-Relative diversity within and between
small mammal assemblages of riparian and upland
vegetation was evaluated at 6 study areas across
an elevational gradient. In contrast to avian diversity
analyses conducted at the same sites. species richness, relative diversity. and faunal similarity of small
mammals were greater among upland rather than
riparian communities across the cline. Beta diversity
between riparian and upland small mammal communities is greater at higher elevations within the watershed. These higher elevation portions of watersheds must be emphasized in management strategies to conserve regional integrity of native small
mammal faunas.
SPR
.SC
LPC
oIR
OLR
0 SFPR
C
0
L
O
R
A
D
0
Figure 1 .-Location of study areas within the Pkltte River drainage, northern Colorado, 1981.
although more species of birds occur
in riparian vegetation, upland sites
contribute more to avifaunal diversity between habitats (beta diversity)
and within a region (gamma diversity). Those findings were attributed
to greater similarity among riparian
avifaunas across the altitudinal cline
due to the riparian vegetation providing a corridor for movement of
birds within a region. Beta diversity
between upland and riparian avian
assemblages was greatest at the upper and lower ends of a watershed,
and the study concluded that avifau-
nal conservation efforts should be
concentrated at those sites.
Implications of the earlier avian
study to conservation of small mammal assemblages are unclear. Numerous studies (Anderson et al. 1980;
Honeycutt et al. 1981; Kirkland 1981)
have examined small mammal distribution along environmental gradients, but with a focus on upland
rather than riparian species assemblages. The objectives of this study
were to evaluate diversity within,
and between, small mammal assemblages of riparian and upland vegeta-
Figure 2.-Study areas: South Platte River (SPR), 1200 m; Lone Pine Creek (LPC), l9Wm; Sheep
Creek (SC), 2341 m; Illinois River (IR), 2500m; Laramie River (LR), 2631 m; South Fork of the Cache
la Poudre River (SFPR), 2747 m.
tion across an elevational gradient.
We believed that the results would
indicate relative elevations within
watersheds at which small mammal
conservation efforts should be focused. Such efforts could include
policies of state and federal agencies
concerning type of land use within
portions of watersheds.
Study Areas
Six study areas ranged in elevation
from 1200 to 2747 m within the Platte
River drainage of northern Colorado
(fig. 1). With the exception of an alpine area, riparian communities
within each major life zone of upland
vegetation along the Front Range
were represented (fig. 2). Within each
upland, we located a riparian site
that contained a permanent stream.
Cattle grazing had not occurred on
any of the study areas for at least
three years prior to 1981.
The South Platte River (SPR) study
area was on the South Platte Wildlife
Management Area, 2 km south of
Crook, Logan County (elevation 1200
m). This community was dominated
by sand sagebrush mixed-prairie.
Several species of grass and 1woody
species, sand sagebrush (Artemisia
filifolia), occurred on the upland
sandhills. Dominant riparian species
were plains cottonwood (Populus
sargentii), western snowberry (Symphoricarpos occidentalis), and willows
(Salix spp.).
The Lone Pine Creek (LPC) study
area was 11 km west of Livermore,
Larimer County, at 1909 m elevation.
This area of mountain shrub transition vegetation was dominated by
true mountain mahogany (Cercocarpus montanus), antelope bitterbrush
(Purshia tridentata), and gooseberry
(RiSes spp.) in the upland site. The
riparian site was dominated by
plains cottonwood, willows, and
common chokecherry (Prunus virginiam). Rocky Mountain junipers
(Juniperus scopulorum) were scattered
throughout both sites.
The Sheep Creek (SC) study areas
was 21 km north of Rustic, Larimer
County, at an elevation of 2341 m
(fig. 2). Ponderosa pine (Pinus ponderosa) forest, along with scattered big
sagebrush (Artemisia tridentata) dominated the upland site. Riparian vegetation was dominated by narrowleaf
cottonwood (Populus angustifolia),
willows, and alders (AInus spp.).
The Illinois River (IR) study area
contained sagebrush steppe vegetation and was within the Arapaho National Wildlife Refuge, 10 km south
of Walden, Jackson County (elevation 2500 m). Upland vegetation was
predominantly big sagebrush. The
riparian site included eight species of
shrub willows dominated by S. geyeriana (Cannon and Knopf 1984).
The Laramie River (LR) study area
was 6.5 km north of Chambers Lake,
Larimer County (elevation 2631 m).
Aspen (Populus tremuloides) dominated the upland site, along with
Douglas fir (Pseudotsuga menziesii)
and lodgepole pine (Pinus contorfa).
The riparian site was comprised of
shrub willows.
The highest study area (elevation
2747 m) was along the South Fork of
the Cache la Poudre River (SFPR), at
the Pingree Park Campus of Colorado State University, Larimer
County. Upland vegetation was composed of lodgepole pine, limber pine
(Pinus pexilis), Engelmann spruce
(Picea engelmannii), Douglas-fir, subalpine fir (Abies lasiocarpa), and a
sparse understory of aspen. The riparian site was exclusively shrub willows.
Methods
Small mammal trapping was conducted in 1981 to determine the relative abundances of small mammal
species at the 6 study areas. In each
study area, two 400-m survey lines
were established, including one riparian and one upland site. Riparian
survey lines were within riparian
vegetation and generally paralleled
the stream course. Upland survey
lines began 500 m from the stream
and were oriented perpendicular to
the direction of the stream.
Trap surveys were conducted between 30 July and 26 August 1981.
Survey lines included 20 trap stations
spaced 20 m apart. Each trap station
contained 1rat trap and 2 museum
special snap traps located within a
1.8-m radius of the measured point.
Three traps were used at each station
to minimize any bias in the data toward more aggressive species, such
as Peromyscus maniculatus. Traps
were baited with a mixture of
ground raisins, carrots, and chipped
beef, blended in a peanut butter base,
and set for 3 consecutive nights in
the riparian and upland sites of a
study area simultaneously. Traps
were checked in the morning and
evening during the 72 hours. Thus,
trap effort per study area was 360
trap-nights, including 180 trap-nights
each in the upland and riparian sites.
Total number of trap-nights for all
study areas was 2160.
Diversity indices were calculated
to compare species diversity within
(alpha) and between (beta) riparian
and upland sites across the altitudinal cline. Because preference for type
of index varies, we selected two each
of the most commonly used indices
to measure alpha (Simpson Index,
Shannon-Weiner Index) and beta (coefficient of community, percentage
similarity) diversity (Whittaker 1975:
95,118).
The former two differ in the general relationship between output
value and species diversity. Shannon-Wiener Index (H') varies directly
with number of species trapped,
while the Simpson Index (C) varies
inversely. Coefficient of community
(CC) values are ratios of the number
of species common to both riparian
and upland sites to the total number
of species occurring in the two sites
combined. Those values are based
only on presence or absence and vary
directly with diversity. Although
percentage similarity values are
based on the differences in importance values between the two sites,
they also vary directly with diversity.
Results
A total of 471 small mammals of 22
species was trapped in all study areas in 1981 (table 1). Three species
(14%of all species captured) were
trapped in riparian sites only, 9 species (41%) were trapped in upland
sites only, and 10 species (47%)were
trapped in both. Nine species (41%)
were rare, being represented by 2 or
fewer captures.
Within-Habitat Comparisons
Species composition within riparian
sites differed among the study areas.
Deer mice (Peromyscus maniculatus),
voles (Microtus spp.), and jumping
mice (Zapus princeps) accounted for
182 of 189 (96%)total captures at the
3 lower study areas, although jump
ing mice did not occur at SPR. In
contrast, shrews (Sorex spp.) accounted for 69% of all captures at the
remaining, higher areas. Of 68 small
mammals trapped at the higher sites,
only 14 (20%)were either voles or
jumping mice. No deer mice were
trapped in riparian sites at elevations
higher than 2293 m.
Changes in species composition of
small mammals in upland sites were
not distinct. Deer mice were the most
frequently trapped of all species at
the 4 intermediate study areas. Overall, 112 of 214 (52%)small mammals
trapped in the uplands were deer
mice. The next 3 species in abundance (least chipmunk [Tamias minimusl, northern grasshopper mouse,
[Onychomys leucogaster] and prairie
vole [Microtus ochrogasterl) accounted
for only 67 of 214 (31%)total captures. Of these 4 species, only the
deer mouse was trapped at all 6 sites.
Species richness varied among riparian and upland sites. The number
of small mammal species trapped in
riparian sites was least at the lowest
elevation study area (SPR) and greatest at the second highest study area
(LR) (table 2). All other riparian sites
were intermediate in species richness
with no apparent altitudinal trend.
Values for Simpson's Index (C) (a
measure of the concentration of
dominance) and Shannon-Wiener
Index (H')(Whittaker 1975:95)
yielded similar results.
The highest diversity among riparian sites occurred at LR, which had
the lowest dominance. The SPR
study area, which had a high C
value, also contained very low species diversity.
The number of small mammal species trapped in upland sites was
comparatively high at 2 of 3 study
areas under 2500 m (LPC and SC)
and at the highest elevation study
area (SFPR) (table 2). Simpson's Index values varied from a high at IR
(2500 m elevation) to a low at SFPR
(2747 m). Shannon-Wiener values in
upland sites ranged from a low of
0.22 at IR to a high of 0.74 at SFPR.
A matrix of percentage similarity
values (Whittaker l975:ll8) revealed
a mean similarity of 0.29 + 0.06
among upland sites and 0.18 + 0.05
among riparian sites. These results
suggest that small mammal communities in upland sites were more similar across the cline than were those in
riparian sites. Overall, beta diversity
along the altitudinal gradient was
greater (less faunal mixing) in riparian sites.
Between-Habitat Comparisons
Species richness was substantially
higher in upland sites than in adjacent riparian sites at the lowest and
highest study areas (table 2). The values were similar at 3 study areas of
intermediate elevation. Only at LR
(the second-highest area) was species
richness higher in the riparian site.
At that study area, number of species
trapped in riparian was greater than
the upland even when captures of
Lepus americanus and Thomomys
talpoides were excluded.
Coefficient of community (CC)
values (Whittaker 1975:118) suggest
that small mammal communities in
riparian and adjacent upland sites
were relatively similar at lower elevations, and became more dissimilar
at 2500 m and higher (table 3). More
species (3) were common to both riparian and upland sites at the 3
lower study areas than at the higher
areas. Percentage similarity (PS) val-
ues indicate the same trend, with the
exception of the lowest study area.
The low value at that study area is
due primarily to the abundance of
Peromyscus maniculafus dominating
this calculation (table 1).
Discussion
To date, studies of small mammal
distribution along environmental
gradients (Anderson et al. 1980;
Armstrong et al. 1973; Honeycutt et
al. 1981; Grkland 1981) have been
conducted in upland sites. Knopf
(1985) compared distribution of
breeding birds in riparian and adjacent upland sites within the 6 areas
used in this study. The focus of this
study was to analyze patterns of
small mammal faunal similarity
within and between riparian and adjacent upland sites in the same watershed. Such patterns, although based
on relatively small sample sizes, may
indicate elevations along the gradient
at which management should be emphasized to conserve regional diversity.
A pronounced change in species
composition occurred within riparian
sites at 2500 m elevation. The study
areas below that elevation, representing foothills and plains, were dominated by deer mice and voles. At
2500 m and above, dominance
shifted vrimarilv to shrews. The
means for PS values comparing the 3
lower study areas (0.31 + 0.10) and 3
higher study areas (0.43 2 0.02) were
both considerably higher than the
mean for all study areas (0.18 + 0.05).
Faunal similarity changed as riparian
sites shifted from cottonwood-willow to willow shrub systems. This
shift in small mammal community
composition could have reflected a
shift from xeric site willows (S.
amygdaloides, S. exigua) to mesic site
willows as described in Cannon and
Knopf (1984). Other factors may have
influenced composition of small
mammal communities. Among those
suggested in previous research are
A
d
soil type, nutrient availability, and
vegetation structure (Huntley and
Inouye 1984, Moulton et al. 1981).
Others have found specific microhabitat components to be important (cf. M'Closkey 1981, Szaro and
Belfit 1987).
Dominance by deer mice was particularly obvious at the lowest site,
SPR, where 65 of 67 captures were of
this species. The remaining 2 small
mammals trapped were western harvest mice (Reithrodontomys megalotis).
These findings were supported by an
earlier study of total small mammal
richness conducted in the same study
area. During the 1982 and 1983 field
seasons of that study, 98.3% of all
small mammals captured in 25,800
trap-nights were deer mice and western harvest mice (Bennett 1984).
High numbers of deer mice
trapped could indicate behavioral
differences (deer mice being more
aggressive), rather than a dominance
in absolute numbers. We believe,
however, that the number trapped
reflected higher relative abundances
of Peromyscus maniculatus for several
reasons. First, although this species
was the most frequently trapped species, it dominated only 4 of 12 total
sites, and was infrequent to absent at
7 sites (table 1). Total captures in 180
trap-nights at each of those 4 sites
(riparian at SPR, upland at IR, both
sites at LPC), ranged from 18 to 68.
That is, deer mice captures accounted for no more than 38 percent
of all available traps at any site.
Moreover, in the riparian site at SPR
(where deer mice were most commonly caught), the percentages of all
captures that were deer mice were
similar for this study (97%)and that
of Bennett (1984) (95%).
Dominance by ecological generalists at the lowest site, SPR, likely is
explained by periodic catastrophic
events, specifically flooding. In contrast to periodic severe flooding observed in floodplains of the western
Great Plains, riparian systems at
higher elevations are not subject to
severe overbank flooding. During a
study of riparian avifauna at SPR,
annual spring flooding varied tremendously (Knopf and Sedgwick
1988).Maximum mean daily flow in
1982 was 44 m3/sec, compared to 405
m3/sec in 1983, when all of the riparian zone, as well as portions of adjacent upland habitat were flooded. No
overbank flooding occurred in 1982.
Habitats of small mammals in lower
riparian systems are periodically
subjected to total inundation for variable amounts of time. Those habitats
appear to be too unstable to assure
prolonged survival by species populations, and are recolonized by individuals from the uplands following
each perturbation.
Changes in small mammal communities among upland sites were
less pronounced. Faunal similarity
was greatest at the intermediate sites,
especially LPC (1909 m), SC (2293 m)
and IR (2500 m). The mean of PS values comparing those sites was 0.57 +
0.12, compared to the overall mean
of 0.29 + 0.06. Deer mice were a
dominant species at all sites but SPR
(sand sagebrush mixed-prairie) and
LR (aspen). The distribution of other
species appeared to be influenced by
changes in upland vegetation types
along the altitudinal gradient. For
example, northern grasshopper mice
were relatively abundant at the lowest site, which contained grassland
areas. Boreal redback voles (Clethrionomys gapped were similarly abundant at the highest site in spruce-fir.
Neither species was trapped elsewhere. Honeycutt et al. (1981) also
reported that the distribution of
some species along an altitudinal
gradient in Utah was strongly influenced by type of vegetation. We
(Knopf and Olson 1984) have noticed
regional differences in small mammal communities in riparian zones of
similar woody communities but different herbaceous composition that
can be attributed to variations in site
dryness.
Beta diversity was low (high CC
values) at elevations of less than 2500
m (SPR, LPC, and SC), indicating
that small mammal communities in
riparian and adjacent upland sites
were quite similar. At 2500 m (IR),
the CC value declined to 0 (no species common to both sites), then remained low at the higher study areas
that contained aspen and spruce-fir
uplands. With the exception of an
extremely low value at SPR (caused
by the overwhelming dominance of
deer mice in the riparian site), PS values followed the same pattern. Thus,
within the Platte River watershed,
beta diversity between riparian and
upland small mammal communities
is much greater at the upper end of
the altitudinal cline.
These results differ from the
avifaunal studies of Knopf (1985)
who found beta diversity between
riparian and upland sites to be greatest at the higher and lower ends of
the watershed, and upland/riparian
assemblages to be similar at intermediate study areas. Also in contrast to
Knopf's (1985) findings were greater
relative diversity in, and faunal similarity among, upland communities.
In support of the avian study conclusions, however, riparian sites at the
higher elevations contributed substantially to small mammal beta and
gamma (regional) diversity.
Implications to Conservation
Historically, management of riparian
zones has occurred primarily on areas at lower elevations. Management
that is concentrated in a limited number of habitats or at selected elevations may result in higher local (alpha) diversity at the expense of beta
and gamma (regional) diversity
(Samson and Knopf 1982). Despite
different beta diversity patterns, our
findings support the conclusion by
Knopf (1985) that greater emphasis
needs to be placed upon conservation of riparian communities at
higher elevations regionally.
Knopf et al. (1988) have recommended that agencies develop guidelines for regionwide rather than local
management of riparian systems. Respective agencies should realize that
small mammal communities at
higher elevations contribute more to
regional diversity than those at lower
elevations. In order to conserve regional integrity in native small mammal faunas, land uses allowed in,
and adjacent to, high elevation riparian zones should be critiqued as carefully as those in lowland floodplains.
For example, livestock grazing can
affectstructure of small mammal associations by reducing understory
vegetation (Moulton et al. 1981).
Grazing and other activities that potentially reduce understory vegetation in higher elevation riparian
zones can seriously affect abundances of certain species such as
shrews that are not present at lower
sites. The consequences to regional
diversity of small mammals would
be greater than livestock grazing at
lower elevations because our findings suggest that: (1) higher elevation
(above 2500 m) sites contribute more
to regional diversity of small mammals; and (2) small mammal comrnunities in some lower elevation riparian zones are composed mostly of
species populations of ecological generalists that are regulated by catastrophic, natural perturbations.
Acknowledgments
We thank Richard W. Cannon, John
F. Ellis. and Elizabeth A. Ernst for
field and analytical assistance. Ron
Desilet assisted in locating field sites.
Eugene C. Patten of Arapaho National Wildlife Refuge and Marvin
Gardner of the South Platte Wildlife
Management Area granted access to
the 1R and SPR sites, respectively.
The U.S. Forest Service and Colorado
State University allowed us to work
within their holdings. This research
is a product of Cooperative Agreement 2463-4 between the Colorado
Division of Wildlife and U.S. Fish
and Wildlife Service. This manuscript was improved by the com-
ments of Steven J. Bissell, Michael A.
Bogan, Lawrence E. Hunt, Me1
Schamberger, Robert C. Szaro, Don
E. Wilson, and an anonymous
reviewer.
Literature Cited
Anderson, Douglas C., James A.
MacMahon, and Michael L. Wolfe.
1980. Herbivorous mammals
along a montane sere: community
structure and energetics. Journal
of Mammalogy 61:500-519.
Armstrong, David M., Benjamin H.
Banta, and Edward J. Pokropus.
1973. Altitudinal distribution of
small mammals along a cross-sectional transect through the Arkansas River Watershed, Colorado.
Southwestern Naturalist 17:315326.
Bennett, Lisa. 1984. The initial effects
of cattle introduction on a riparian
small mammal community, northeastern Colorado. M.S. Thesis,
Colorado State University, Fort
Collins.
Cannon, Richard W. and Fritz L.
Knopf. 1984. Species composition
of a willow community relative to
seasonal grazing histories in Colorado. Southwestern Naturalist
29:234-237.
Honeycutt, Rodney L., Michael P.
Moulton, Jerry R. Roppe, and
Lynn Fifield. 1981. The influence
of topography and vegetation on
the distribution of small mammals
in southwestern Utah. Southwestern Naturalist 26:295-300.
Horton, Jerome S. 1977. The development and perpetuation of the permanent tamarix type in the phreatophyte zone of the Southwest. p.
124-127. In Importance, preservation and management of riparian
habitat: a symposium. USDA Forest Service General Technical Report RM-43. Rocky Mountain Forest and Range Experiment Station,
Fort Collins, Colo.
Huntley, Nancy, and Richard S.
Inouye. 1987. Small mammal
populations of an old-field
chronosequence: successional patterns and associations with vegetation. Journal of Mammalogy
68:739-745..
Kirkland, Gordon L. 1981. The zoogeography of the mammals of the
Uinta Mountains regions. Southwestern Naturalist 26:325-339.
Knopf, Fritz L. 1985. Significance of
riparian vegetation to breeding
birds across an altitudinal cline. p.
105-111.In Riparian ecosystems
and their management: reconciling
conflicting uses. First North
American Riparian Conference.
USDA Forest Service General
Technical Report RM-120,523 p.
Rocky Mountain Forest and Range
Experiment Station, Fort Collins,
Colo.
Knopf, Fritz L., R. Roy Johnson, Terre11 Rich, Fred B. Samson, and
Robert C. Szaro. 1988. Conservation of riparian ecosystems in the
United States. Wilson Bulletin
100:278-284.
Knopf, Fritz L., and Thomas E.
Olson. 1984. Naturalization of
Russian-olive: implications to
Rocky Mountain wildlife. Wildlife
Society Bulletin 12:289-298.
Knopf, Fritz L., and James A.
Sedgwick. 1988. Latent population
responses of summer birds to a
catastrophic climatological event.
Condor 892369-873.
M'Closkey, Robert T.1981. Microhabitat use in coexisting desert
rodents-the role of population
density. Oecologia 50:310-315.
Moulton, Michael P., Jerry R. Choate,
and Steven J. Bissell. 1987. Small
mammals on revegetated agricultural land in eastern Colorado.
Prairie Naturalist 13:99-104.
Moulton, Michael P., Jerry R. Choate,
Steven J. Bissell, and Robert A.
Nicholson. 1981. Associations of
small mammals on the central
high plains of eastern Colorado.
Southwestern Naturalist 26:53-57.
Olson, Thomas E., and Fritz L.
Knopf. 1986. Naturalization of
Russian-olive in the western
United States. Western Journal of
Applied Forestry 1:65-69.
Samson, Fred B., and Fritz L. Knopf.
1982. In search of a diversity ethic
for wildlife management. Transactions of the North American Wildlife and Natural Resources Conference 47:421-431.
Szaro, Robert C., and Scott C. Belfit.
1987. Small mammal use of a desert riparian island and its adjacent
scrub habitat. USDA Forest Service Research Note RM-473,5 p.
Rocky Mountain Forest and Range
Experiment Station, Fort Collins,
Colo.
Thomas, Jack Ward, Chris Maser,
and Jon E. Rodiek. 1979. Wildlife
habitats in managed rangelandsthe Great Basin in southeastern
Oregon. Riparian zones. USDA
Forest Service General Technical
Report PNW-80, 18 p. Pacific
Northwest Forest and Range Experiment Station, Portland, Ore.
Whittaker, Robert H. 1975. Communities and ecosystems. MacMillan
Co., New York, 385 p.