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Small Mammal Community Structure in Old Growth
and Logge~d. Riparian Habitat 1
Lee H. Simons 2
Abstract.--Species richness and evenness were measured
in small mammal communities from old growth and logged
riparian habitat. Six species occurred in both habitats,
while Clethrionomys occidentalis occupied only old growth.
Similarities in understory vegetation, and proximity of old
growth to the logged area, may promote similar communities
in each habitat.
Commercial logging clearly affects community
characteristics of small mammals (Gashwiler 1970;
Kirkland 1977). Small mammals eat significant
amounts of conifer seeds, insects, and mycorrhizal
fungi (Maser et al. 1978). This study compares
small mammal communities from adjacent old growth
and logged riparian habitats.
Overstory and understory vegetation were quantified at each site with the point-quarter method
and ten random points (Cox 1976). Plants were
identified with Munz (1973). Small mammal diversity
and equitability were calculated with the ShannonWiener function (Krebs 1985). Statistical tests
follow Sakal and Rohlf (1981).
METHODS
RESULTS
Two study sites were established along Squaw
Valley Creek on the southern slope of Mt. Shasta,
Siskiyou County, California. The upper site was
in old growth forest at an elevation of 1400 m,
while the logged site was 1.5 km downstream at an
elevation of 1260 m. The logged area was clearcut,
except for several "seed-producing" trees, in 1967;
large stumps indicated that old growth existed there
prior to logging. An abrupt transition between
habitats occurred 0.5 km upstream from the logged
study site.
One hundred and fifty-six small mammals,
representing 7 species, were captured (fig. 1).
Six species occurred in both habitats, while
Clethrionomys occidentalis, the red-backed vole,
occurred only in old growth. The overall small
mammal frequency distributions were weakly associated with habitat type (P ~ .05). If red-backed
tn
...
Q)
45
D
40
~
:I
a.
co
For each site, 26 trap stations were randomly
assigned to either creek bank at intervals of about
9 m (range= 5-15 m). At each trap station two
Sherman livetraps were set with rolled oats: the
first within 5 em of water, the second at least 30
em away from water. Each study site was trapped
for 36 continuous hours (1800 to 0600 h) on four
occasions in 1982: 17-19 July, 31 July-2 Aug.,
7-9 Aug., 30 Aug.-1 Sept., and 30 Aug.-1 Sept.,
10-12 Sept., 17-19 Sept., 24-26 Sept. for the old
growth and logged areas, respectively, Traps were
checked at least every 12 hours and captured mammals were permanently removed.
50
0
-...
Oldgrowth Area
Clearcut Area
35
30
0
25
Q)
.a
E
20
:I
15
-
10
z
cu
0
~
0
P.m.
z. p.
S. p.
S. t.
M.l.
C. o.
E. a.
Species
Figure 1.--Total number of captures of seven species
of small mammals in two habitat types. P.M. =
Peromyscus maniculatus {deer mouse), Z.P. =
Zapus princeps (western jumping mouse), S.P.
Sorex palustris (northern water shrew), S.T.
Sorex trowbridgei (trowbridge shrew), M.L. =
~tus longicaudus (longtail vole), C.O. =
Clethrionomys occidentalis {red-backed vole),
E.A. = Eutamias amoenus (yellow pine chipmunk).
Statistical results are given in the text.
1paper presented at the first North American
Riparian Conference (University of Arizona, Tucson,
April 16-18, 1985).
2Lee H. Simons is a graduate student in the
Department of Zoology, Arizona State University,
Tempe, Arizona. I thank Laurie Stuart Simons for
help in the field and with analysis, and John Clark
and College of the Siskiyous, Weed, California for
the loan of small mammal traps.
505
voles are excluded, the rema1n1ng frequency distributions are independent of habitat type (P > .5).
Small mammal diversity and equitability were 1.83
and 0.65 in the old growth site, and 1.76 and 0.68
in the logged site. NeithP.r mP.asure is significantly different between habitats (P > .5; F tests).
VEgetative characteristics for each habitat
are summarized in table 1. Density and average
basal area of overstory were drastically lowered
in the logged area despite a 15 year lapse since
logging. Erosion and poor reforestation management
may have hindered regrowth. Understory vegetation
also differed between habitats, but the contrasts
were less extreme. Alder was dominant in both
understories, while willow partially replaced dogwood in the logged area. Total cover from understory vegetation was similar between habitats
because increased basal area offset decreased
density in vegetation on the logged site. Increased
herbaceous growth in the logged habitat also
appeared to increase cover.
DISCUSSION
The strong similarities in small mammal community structure between old growth and logged
habitats was not anticipated. Small mammal communities change extensively in response to logging
(Gashwiler 1970; Ramirez and Hornocker 1981); and
the sparse overstory in the logged area contrasted
sharply with the dense old growth forest. But each
Table !.--Summary of vegetative characteristics of
overstory and understory in (A) old growth,
and (B) logged habitat. Probabilities of
differences by chance: NS = (P > .1),
* = (0.1 > P > .OS), ** = (.OS> P > .01),
*** = (P < .001) 1 •
SPECIES
OVERS TORY
Abies
-----concolor
Pinus
---rambertiana
DENSITY
(stems/ha)
B
A
AVE. BASAL
AREA (cm2)
A
B
RELATIVE
FREgUENCY
A B
44
17
2000
1000
27
30
22
3
9900
4600
19
30
70
13
8600
1600
27
26
13
2900
2000
1663
Pseudotsu~a
menziesii
Calocedrus
decurrens
Total/Mean
UNDERSTORY
Abies
-----concolor
Alnus
~nuifolia
ChrysoleEsis
sem;eervirens
Salix
s;eecies
Ceanothus
velutinus
Cornus
~tallii
Total/Mean
39
174
L;6 5815
***
***
27 35
100 100
NS
1133
88
13
29
21
6
3220
2133
35
60
37
53
477
234
5
7
16
12
0
155
0
12
0
11
179
155
8
323
5
6
954
5963
157
2922
--rr
**
8
6
~
*
21 12
100 100
***
lT-tests for density and basal area;Tests of
independence for relative frequency.
506
study site was close to the other habitat (0.51.0 km) which insured that mammal species were
available for colonization. Also, cover from understory vegetation was similar in both areas. These
data suggest that understory cover supersedes overstory in providing habitat for most of the small
mammals in this study.
Red-backed voles are a notable exception.
Clearcuts generally support these voles only when
logs or other debris are present (Tevis 1956).
Data presented here indicate that vegetative cover
alone is not sufficient to retain these voles.
Apparently debris provides some additional'resource
other than cover. Red-backed voles feed primarily
on mycorrhizal fungi and are important dispersers
of mycorrhizal spores to sapling roots (Maser et al.
1978). Because mycorrhizae enhance growth rates of
trees in coniferous forests, retention of voles
(as dispersers of mycorrhizae) may assist in stand
regeneration (Maser et al. 1978). Since retention
of voles requires that some slash be left on clear
cut sites (Tevis 1956), this practice is suggested
to accelerate recovery of clear cut areas. Further
research in local management areas should test the
interpretation that understory vegetation and some
debris are sufficient to maintain high small mammal diversity in riparian clear cut zones.
LITERATURE CITED
Cox, G. W. 1976. Laboratory manual of general
ecology. 232 p. Wm. C. Brown Company Pub.,
Dubuque, Iowa.
Gashwiler, J. S. 1970. Plant and mammal changes on
a clearcut in West-Central Oregon. Ecology 51
(6): 1018-1026.
Kirkland, G. L., Jr. 1977. Responses of small mammals to the clearcutting of northern Appalachian forests. J. of Mamm. 58(2):600-609.
Krebs, C. J. 1985. Ecology: The experimental analysis
of distribution and abundance. Third edition.
Harper and Row, Pub., New York.
Maser, c., J. M. Trappe, and R. A. Nussbaum. 1978.
Fungal-small mammal interrelationships with
emphasis on Oregon coniferous forests. Ecology
59(4):799-809.
Munz, P. A. 1973. A California flora with supplement.(in collaboration with D. D. Keck) University of California Press, Berkeley.
Ramirez, P., Jr. and M. Hornocker. 1981. Small mammal populations in different-age clearcuts in
Northwestern Montana. J. of Mamm. 62(2):400-403.
Sakal, R. R. and F. J. Rohlf. 1981. Biometry: The
principles and practice of statistics in biological research. Second edition. W. H. Freeman
and Company, San Francisco.
Tevis, L., Jr. 1956. Responses of small mammal
populations to logging of Douglas-fir. J. of
Mamm. 37(1):189-196.