Microhabitat and Dietary Partitioning in Three Species of Shrews at Yellow Bay Montana

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Graduate Student Theses, Dissertations, & Professional Papers Graduate School

1990

Microhabitat and dietary partitioning in three species of shrews at Yellow Bay Montana

Karen E. McCracken The University of Montana

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MICROHABITAT AND DIETARY PARTITIONING IN THREE SPECIES OF

SHREWS AT YELLOW BAY, MONTANA

Karen E . McCracken

B.A., University of New Hampshire, 1984

Presented in Partial Fulfillment of the Requirements

for the degree of Master of Arts

University of Montana

1990

Approved by

ChairmjBT^ Board of Examiners

Dean, Graduate Schoo

. 5 . Date

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

McCracken, Karen E., M.A. July 1990 Zoology

Microhabitat and Dietary Partitioning In Three Species of Shrews at Yellow Bay, Montana

Director; Kerry R. Foresman

I analyzed habitat use patterns and food habitats of three species of shrews (Sorex vagrans. Sorex cinereus. and Sorex montlcolus) In a grand fir (Abies grandis)/ queencup beadllly (Cllntonla unlflora) habitat In northwestern Montana. Distribution of the three species differed significantly across meslc and xerlc sites, with Sorex vagrans captures being most numerous on the meslc site, Sorex cinereus captures most numerous on the xerlc site, and Sorex montlcolus. at Is lower eleva- tlonal range, was Infrequently captured on both plots. Thirty-five live-trapped Sorex vaerans and Sorex clnereus were marked with phos phorescent dust and released at their site of capture. Mapping of Individual's movements revealed no detectable differences between species, but the most conspicuous habitat component used by both was the space between fallen trees (decomposition classes 1-3) as runways. Mlcrohabltat structure used by the three species was Investigated using multivariate analysis of 15 traps1te characteristics. There were significant differences between Sorex vagrans and Sorex clnereus on five variables, and between Sorex vagrans and Sorex montlcolus on one variable. These results suggest that Sorex vagrans and Sorex clnereus partition the space within the habitat at both large and small scales. Habitat segregation may be due to asymmetric competition between the two species. Involving Interference competition, exploitative competi tion, or both.

Summer foods of the three species (20 Sorex vagrans. 10 Sorex clnereus. and 4 Sorex montlcolus) were examined using stomach contents. Major foods. In order by percent frequency, were Sorex vagrans: ants, true flies, beetles, and adult lepldopterans; Sorex clnereus: ants, adult lepldopterans, beetles, and spiders; Sorex montlcolus: ants, spiders, true flies, and millipedes. No significant differences In prey Items consumed was observed among the three species. These data Indicate that, at the taxonomic level of Order, there Is a high degree of overlap In foods among the three species. However, they could be separating foods on the basis of prey size, foraging method, or at a lower taxonom ic level.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

First I thank my advisor. Dr. Kerry Foresman, for introducing

me to shrews and guiding me through the development of this project.

His enthusiasm and advice have been invaluable to me repeatedly

throughout the past year and a half.

Next, I thank my committee members, Drs. Richard Hutto and James

Habeck, for reviewing and offering comments on my original thesis

proposal and earlier thesis drafts. This project also benefited from

discussion with Andy Sheldon, to whom I extend my thanks, and with

Don Loftsgaarden, who spent many hours discussing multivariate statistics

with me. Colin Henderson also gave generous advice on the stomach

contents analysis.

I thank both the American Museum of Natural History’s Teddy Roose

velt Fund and also the University of Montana for providing partial

funding for this project.

Lastly, I must thank my parents, Cynthia and Gerald Dunn, for pro

viding moral and financial support for my education through the years.

My deepest thanks however are reserved for my husband Michael, whose

companionship and support I cannot imagine having done without.

iii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ABSTRACT...... ii

ACKNOWLEDGEMENTS...... iii

LIST OF TABLES...... v

LIST OF FIGURES...... vi

CHAPTER

I. Introduction...... 1

II. Methods and Materials...... 6 Study Area and Grids...... 6 Trapping Procedure...... 8 Microhabitat Variables...... 9 Diet, Sex-Ratio, and Age-Class Analysis...... 10

III. Results...... 13 Capture Records and Spatial Overlap...... 13 Sex Ratios and Demography...... 17 Movements of Livetrapped Shrews...... 17 Microhabitat Use by Species...... 22 Diet Analysis...... 25

IV. Discussion...... 27 Habitat Usage by Species...... 27 Microhabitat Use by Species...... 28 Diet Analysis...... 31

LITERATURE CITED...... 33

APPENDIX...... 37

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

Table

1. Numbers of captures and individuals observed for three species of shrews...... 14

2. Overlaps in capture sites among three species of shrews...... 15

3. Individual shrews observed in each plot type...... 16

4. Number of captures broken down by sex for three species of shrews trapped at Yellow Bay, Montana...... 18

5. Distribution of captures between two age classes for three species of shrews trapped at Yellow Bay, Montana...... 19

6. Estimates of species means (x ± standard deviations (s)) for each of the 15 habitat variables surrounding trapsites...... 23

7. Standardized discriminant function coefficients and linear correlations(r) between each variable and the discriminant function, for microhabitat variables used to distinguish between vagrans and cinereus capture sites...... 24

8. Frequency of prey items identified in 20 microscope fields from the stomachs of Sorex vagrans, Sorex cinereus, and Sorex monticolus from northwestern Montana...... 26

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

Figure

1. Map of the study site...... 7

2. Movements of two livetrapped S. cinereus...... 20

3. Movements of two livetrapped vagrans...... 21

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION

Field studies of North American shrews have been limited primarily

to distributional descriptions and habitat preferences (Clothier, 1955;

Long, 1972; Brown, 1967; Wrigley et al., 1979; Buckner, 1966; Haveman,

1973), but it is not known what small- scale habitat features may be

important in the coexistence of closely related, competing shrew species.

Even with the recent passage of legislation such as the National

Forest Management Act of 1976, whereby the monitoring of all vertebrate

species became law, most management emphasis has been placed on game

species, or threatened and endangered species (Szaro, 1988). Small

mammals, including shrews, are a component of the fauna in most North

American terrestrial habitats, and their importance has generally been

overlooked (Gibbons, 1988). For example, small mammals may be impor

tant indicators of habitat quality and health due to their relatively

sedentary characteristics, and they serve as the primary prey base for

more conspicuous animals (particularly birds of prey, in the case of

shrews).

Closely related, sympatric species commonly subdivide available

food resources in one of three ways, by differing in (1) What they eat,

(2) Where they forage, or (3) When they are active (Fianka, 1975).

Slight differences in any of these may allow coexistence of species in

a community, presumably by reducing interspecific competition for lim

ited resources (MacArthur, 1958; Levin, 1970; Hawes, 1977). Since

shrews have very high metabolic rates which necessitate virtually con

stant food search interspersed with brief periods of inactivity (Rust,

1978; Buckner, 1964), significant temporal separation as a means of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reducing competition seems unlikely. Therefore, foraging microhabitats

and diet represent reasonable starting points for a study of ecological

separation among three species of coexisting shrews.

Shrews are small, inconspicuous, and widely distributed insectivores.

Previous studies have correlated species diversity and abundance with

in a community with moisture levels (Buckner, 1966; Getz, 1961), avail

ability of different foraging microhabitats (Dickman, 1988), and in

vertebrate biomass (Rolling, 1959; Haveman, 1973). In addition, many

studies have described vegetation types within which particular species

have been found, but coexistence of species within single habitats has

not been examined yet. The following three species occur commonly

throughout western Montana, and their extensive overlap in habitat use

makes them an interesting group for the study of within-habitat separa

tion.

The masked shrew (Sorex cinereus), which has the largest range of

any North American shrew (Junge and Hoffmann, 1981), has been found in

many types of habitats, suggesting that it is a habitat generalist.

Brown (1967) found S, cinereus represented in ail but one habitat type

(short-grass prairie) sampled in southern Wyoming, but the species was

most abundant in moist bog localities. These results are generally

consistent with those found by Wrigley et al. (1979) in Manitoba,

where S^. cinereus was also present on all habitats studied but one -

jack pine - lichen woodland on sand esker. Sorex cinereus was consis

tently present in high numbers in grass - sedge marsh and willow - alder

fen, both hydric habitats.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The vagrant shrew (Sorex vagrans) is found primarily in the

northern and central Rocky Mountains west of the continental divide,

its range extending westward to the Pacific coast as far south as Cal

ifornia at elevations below 1600 meters (Hennings and Hoffmann, 1977).

They are also reported to exist in a variety of habitats - forest,

meadow, and riparian - but are ordinarily considered mesic (Junge and

Hoffmann, 1981). In Montana, the most productive habitats for vagrans

were moist areas near water, either under overhanging roots along small

streams, in damp meadows, or in patches of Equisetum (Clothier, 1955).

Brown (1967) found S. vagrans consistently in sympatry with Sorex

cinereus, but higher population densities of S. vagrans were in mesic

communities associated with bogs (aspen, lodgepole pine, spruce - fir,

alpine tundra).

The range of the montane shrew (Sorex monticolus) extends from

the mountains of Alaska to northern Mexico (Junge and Hoffmann, 1981),

most usually in high altitude spruce - fir forest and alpine tundra

(Hennings and Hoffmann, 1977). However, Hennings and Hoffmann (1977)

also report that Sorex monticolus is.sometimes.found as low as 1000

meters in mid - altitude forests of Douglas - fir, lodgepole pine,

western larch, and grand fir, where they may occur in sympatry with

both Sorex vagrans and Sorex cinereus. Little is known about specific

habitat preferences of Sorex monticolus, but Wrigley et al. (1979)

collected 17 specimens of monticolus in Manitoba, all within 100

meters of watercourses. The habitat types in which they were found

were grass - sedge marsh, willow - alder fen, mesic alder - willow

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shrubs, aspen forest with understory of alder, and aspen - spruce for

est with alder.

With habitat use being similar among species of shrews, studies

have begun to address aspects of food selection. Because of their ex

ceptionally high metabolic rates, shrews have been used extensively,

primarily in laboratory setting, as models for testing various optimal

foraging theories. Pierce (1987) found that common shrews (Sorex

araneus) searched more efficiently than predicted on the basis of a

random model, and they were able to increase searching efficiency by

remembering where they had searched. Barnard et al. (1985) used S.

araneus as a model for 'risk sensitive foraging,' and found that pref

erence for a constant versus risky reward was influenced by variance

in the risky reward choice. Dickman (1988) found that six different -

sized insectivores showed no tendency in a laboratory setting to spec

ialize on different - sized prey when provided with a choice, but max

imized net energy intake by preferentially feeding on large prey.

However, he suggested with complementary field experiments that body

size differences may influence prey &i2i%-^Tsul'e(ccicrn indiTcctly by allow

ing larger species to exclude smaller species from more productive

habitats (i.e., those harboring larger prey).

Comparative studies on the diets of sympatric shrews have revealed

much overlap in prey items eaten, suggesting that different species

respond in a similar fashion to the available prey. Whitaker and French

(1984) found considerable overlap between the diets of cinereus and

hoyi in New Brunswick, with insect larvae the most predominant

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. component, and similar smaller amounts of spiders and Coleoptera (beetles)

consumed. Ryan (1986) also found a high degree of dietary overlap

between these two species, with Hymenoptera, Arachnida, Lepidoptera

larvae, and Coleoptera accounting for the majority of stomach material

identified. However, a few studies have indicated that in assemblages

of coexisting shrew species, the larger species consume lumbricids

(earthworms) while the smaller species do not (Butterfield et al., 1981;

Pernetta, 1976), while Yalden (1981) suggested that prey size is rela

tively more important to shrews than taxonomic category. The role of

competitive interactions as a proximate factor in determining diets of

sympatric shrews is unknown, but some have suggested that perhaps prey

is normally so abundant that competition between organisms is limited

to unpredictable periods of severe environmental stress ("ecological

crunches") (Wiens, 1977). The possibility exists that coexisting shrew

species will respond opportunistically and in a similar fashion to any

prey encountered, and that any differences observed are simlpy due to

sample variation.

So, microhabitat differences amsng tbs .thsecskrew species are

unclear. Several possibilities exist;

(1) Microhabitat selection reflects broader habitat differences,

(2) Microhabitats differ otherwise, or

(3) Food differences exist that permit coexistence among the three

species.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. METHODS AND MATERIALS

Study Area and Grids

My research was conducted at the University of Montana Biological

Station at Yellow Bay, on the east shore of Flathead Lake, from July

5 to November 11, 1989. The actual study sites were located in a grand

fir (Abies grandis) forest which extends north from the Biological

Station's entrance road and east from the lakeshore to Highway 35, and

in the small section of forest extending south from the entrance road

to the Yellow Bay State Recreation Area (Figure 1). Common plant species

were western larch (Larix occidentalis), paper birch (Betula papyrifera),

ponderosa pine (Pinus ponderosa), Douglas - fir (Pseudotsuga mensiesii),

Engelmann spruce (Picea engelmannii) , and mountain maple (Acer glabrum)

(Cloonan, 1986). A similar area in close proximity and north of the

entrance road has been the site of small mammal censuses since the late

1940's.

Initially, two live - trapping grids were laid out over the study

area, one grid in a mesic region of forest located to the south of the

entrance road (henceforth referred to as the "wet" plot), the other in

a xeric region of forest to the north of the Station road, situated

on its upper edge near the property line (referred to as the "dry"

plot). The wet plot had a small stream flowing through the center of

the grid and is approximately 890 m in elevation, while the dry plot

was situated over 150 m from the closest standing water (Flathead

Lake), and is approximately 902 m in elevation. The canopy of the wet

plot is dominated by grand fir, while a greater proportion of Douglas -

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A f l A » ^ A Kallspell / % ■-^Study Areal o. / MONTANA I

îigfork^ A<^

'1 1 1 !

É?v I A

Orchard

Dry 2» Plot

Flathead Lake

10 km Yellow Bay

132.4 ffl

5 0 0 ft

Figure 1. Map of the study area (adapted from Cloonan, 1986)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fir is present in the canopy of the dry plot. Neither areas have ever

been logged. Each grid consisted of 144 trapping stations arranged

in a 12 X 12 fashion with stations 5 m apart. Each station consisted

of a no. 10 can placed in the ground as a pitfall trap, which has

been shown to be more accurate than snaptraps in estimating the true

numbers of shrews in a given locality, especially the smallest species

(Brown, 1976; Macleod and Lethiecq, 1963). Traps were modified after

each trapping session by the addition of a shrew escape ramp - a 20 -

cm long, 5 - cm wide device constructed of a mesh wire. The ramp allow

ed a shrew to "escape" once captured in an attempt to reduce trap mor

tality and possible effects on movement elicited by the interruption

of behavior during most livetrapping experiments.

Trapping Procedure

In order to examine and mark captured shrews, traps were set out

initially without escape ramps. Station of capture was recorded, and

each individual identified, sexed, weighed, and toe - clipped. Indiv

iduals were then individually marked with a different color of phos

phorescent dust (Radiant Color) by placing -t'kc-- ividual in a small

bag with a small portion of the dust and gently shaking (Lemen and

Freeman, 1985). This technique has been shown to be comparable to

radiotracking for measuring home - range size and position in small

mammals (Jike et al., 1988), but is less expensive and easier to use.

Individuals were then released at their sites of capture. Escape ramps

were then placed in traps for the next one to two days and movements

of individuals were monitored by following the phosphorescent trails

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. left by the animals, employing ultraviolet light at night. For anal

ysis of the marked individual's movement, 30 - cm wire flags were placed

every 10 cm along trails. The next morning, the paths were quantified

by determining the angle of turn every 10 cm, and a map was created

for the movement of each marked individual (Lemen and Freeman, 1985).

At the end of the livetrapping period, traps were left open in

order to catch individuals marked during the livetrapping period, and

to catch other individuals for use in the diet analyses. Traps were

checked either weekly or biweekly through the beginning of November.

Microhabitat Variables

At the height of the herbaceous growth for the season, I measured

15 vegetation variables at each trapping station (see Appendix). Spec

ific variables were those suspected of influencing the microhabitat

within which an individual was found (Dueser and Shugart, 1978; Belk

et al., 1988). Sampling units centered on trap sites were (1) a 1.0 -

m diameter ring centered on the trapsite (for herbaceous and woody stem

counts), and (2) a 5 - m - radius circular plot, divided into four 90 -

degree quarters for all other variables (Dueser and Shugart, 1978). I

used statistical methods similar to those outlined by Dueser and Shugart

(1978), including compiling number of captures, number of individuals

observed, and number of capture sites for each species in each plot

(including multiple captures for individuals). I also calculated per

cents of captures at trapsites at which no other species were encount

ered, and species abundances and distributions between the wet and dry

plots. With this information, each pair of species was tested for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. association with a two - way contingency analysis of the frequencies

of their presence and absence using a Chi - square test.

For the microhabitat variables, the mean and standard deviation

of each variable for each species was calculated and tested for skew

ness and kurtosis. Since none of the variables violated the assump

tions of multivariate distributions with equal variances, a one - way

multivariate analysis of variance (MANOVA) was performed to distinguish

variables for which capture sites for a particular species were signif

icantly different from any other species. I used Discriminant Function

Analysis to assess the relative importance of the 15 habitat variables

to the discrimination among shrew species.

Diet, Sex-Ratio, and Age-Class Analyses

In order to determine dietary separation among the three species,

stomach analyses were performed on any individuals who died, on indiv

iduals taken at the end of the trapping period, and on additional

specimens that were taken throughout the study from plots that were

close in proximity and similar in overall habitat structure to the

trapping plots. The shrews were kept frozen until the end of the

trapping period and then dissected in order to determine sex and repro

ductive condition, and to remove and store stomachs and intestinal

tracts in 10% formalin. At the same time, skulls were examines to

classify individuals into age classes on the basis of tooth wear. An

individual was classified as "juvenile" if the tooth cusps were sharp

and the tooth pigmentation relatively unworn. An individual was class

ified as "adult" if the teeth and pigmentation exhibited a high degree

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of wear. (It may be worth mentioning here that "juvenile" in this

case does not necessarily connote "nonbreeding," but designates an in

dividual born in the current year. An "adult" then constitutes an in

dividual that was born the previous year, and overwintered.

Samples of invertebrates were also taken randomly throughout the

trapping season in order to serve as reference for microscopic iden

tification of stomach fragments. At the end of each sampling period,

invertebrates that had fallen into the pitfalls were placed in vials

containing a 10% formalin solution and saved for later reference (Kirk

land et al., 1988).

Reference invertebrates were identified to the lowest taxonomic

level at which all prey items could be identified (Order), and reference

slides for each invertebrate taxon were made by separating body parts

(head capsules, legs, wings, etc.) and mounting them on slides.

I placed the excised stomach of an individual shrew in a petri

dish and cut it open from the esophagus to the pylorus. The food par

ticles were then teased out with a pair of forceps and the inside of the

stomach was flushed with 10% formalin to remove all particles. The

material from the stomach was then gently washed over a -10 - mm screen

to insure mixing of the prey fragments and to remove any particles too

small to be identified. Five permanent slides were then made with the

material from each stomach. Material from the intestine was not con

sidered in the analysis since it was generally unrecognizable. For

each stomach sample, 20 random fields were examined at lOOx to identify

prey items consumed (Holecheck and Varra, 1981). Identification was

based on comparison with reference slides using histological features

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. such as cell shapes, presence or absence of hairs or settae, and color.

Percent frequency of occurrence (the percent of microscope fields in

which a prey item was found for each species) was compiled for statis

tical analysis. I used a one - way ANOVA to test whether the shrew

species consumed various prey with equal frequency.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RESULTS

Capture Records and Spatial Overlap

Of the 159 captures, 57% were Sorex vagrans, 35% were Sorex cin

ereus, and 8% were Sorex monticolus (Table 1). The majority of cap

tures for each species was at trapsites at which no other species was

captured. For each species, the percentage of such sites was 72%

(Sorex vagrans), 60% (Sorex cinereus), and 57% (Sorex monticolus)

(Table 2). I captured both Sorex vagrans and Sorex cinereus at 14

trapsites, Sorex vagrans and Sorex monticolus at 4 trapsites, Sorex

cinereus and Sorex monticolus at 2 trapsites, and all three species at

1 site. Despite the overlaps, no two species showed a significant asso- 2 ciation (each X > 3.84, df=l, p?.05). Excluding overlaps, the 159

captures were at 97 different trap sites - an average of 1.6 captures

per site.

The distribution of captures between the wet and the dry plots 2 differed significantly among the three species (X =22.1, df=2, p<.000)

(Table 3). Sixty-six shrews were caught on the wet plot during the

trapping period for a trapping success rate of 1.2%, and a total of

80 shrews were caught on the dry plot for a trapping success rate of

1.3%. Sorex monticolus was observed infrequently on both plots (n=13).

Of these 13 individuals, 4 (31%) were distributed on the wet plot and

9 (69%) on the dry plot. Sorex vagrans was the most common species

caught on the wet plot (62% versus 38% for the dry plot), while Sorex

cinereus was more numerous on the dry plot (78% versus 22% for the wet

plot).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1. Numbers of captures and individuals observed for three species of shrews.

Captures Individuals Observed

Sorex vagrans 90 82

Sorex cinereus 56 51

Sorex monticolus 13 13

Totals 159 146

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2. Overlaps in capture sites among three species of shrews.

S. vagrans S. cinereus S. monticolus Totals

S. vagrans 47 14 4 65

S. cinereus 14 24 2 40

S. monticolus 4 2 8 14

Totals 65 40 14 119

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3. Individual shrews observed in each plot type.

Sorex vagrans Sorex cinereus Sorex monticolus Totals

Wet plot 51 (62%) 11 (22%) 4 (31%) 66

Dry plot 31 (38%) 40 (78%) 9 (69%) 80

Totals 82 (100%) 51 (100%) 13 (100%)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sex Ratios and Demography

The proportion of male captures did not differ significantly from

the proportion of female captures among the three species (Table 4)

(X^“3.97, df=2, p>. 138). Of the S. cinereus individuals trapped, 54%

were males, 46% females. Of the S. vagrans caught, 49% were males, 51%

females. For monticolus, 23% were males, 77% females.

The distribution of age classes differed among the three species

(Table 5) (X^=31.45, df=2, p<.000). For monticolus, 70% of those

individuals trapped were juveniles. For S. vagrans, the majority (96%)

of individuals caught were also juveniles. For cinereus, however,

the percentage of juveniles and adults caught were similar (57% and

43%, respectively).

Movements of Livetrapped Shrews

Thirty - five individual shrews (29 Sorex vagrans and 6 Sorex

cinereus) were color marked using the phosphorescent dust technique.

Two individuals were tracked twice within an interval of several weeks;

all others were tracked only once, since they were not livetrapped

again. Because so few individuals were recaptured, home range analysis

was not possible. No obvious differences were apparent between move

ments of each species (see Figures 2 and 3 for representative movement

patterns). Both species used the space beneath fallen trees (decompo

sition classes 1-3) extensively as cover (33/35 shrews). Fourteen

shrews also used thickets or brushy cover and two shrews used fern

(herbaceous) cover. The edges of logs, bases of trees, and stream banks

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4. Number of captures broken down by sex for three species of shrews trapped at Yellow Bay, Montana.

Species Male Female Totals

Sorex cinereus 27 23 50

Sorex vagrans 37 39 76

Sorex monticolus 3 10 13

Totals 67 72 139

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 5. Distribution of captures between two age classes for three species of shrews trapped at Yellow bay, Montana.

Species Juveniles Adults Totals

Sorex cinereus 29 22 51

Sorex vagrans 78 3 81

Sorex monticolus 9 4 13

Totals 116 29 145

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■ D O Q. C g Q. tuna 6h smAirlimb : ôVëMyiMgwmTT jumps on trunk , / ■ D CD

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8-3-89 A. Individual # 10 B. Individual # 20

Figure 2. Movements of 2 Livetrapped Sorex cinereus (1.5" = 5 meters) ■DCD O Q. C g Q.

■D 41 56 CD

WC/) oar 119 o' 3 capture site 0 3 InldhrMual «5 twigs on water surface CD 8 (metked simultaneously) ci' 3" 40 1 3 CD 99 118 "n c stream 3. 3" CD Nî ■DCD rotting tree stump O Q. C a O 3 ■D 3 9 dead snag O icr^ CD Q. 100 117

■D CD disappears beneath log-nest? C/) C/) fern cover 38 logs

capture site A. Individual #1 7-20-89 101 116 B. Individual #4

Figure 3. Movements of 2 Livetrapped Sorex vagrans (1.5" = 5 meters) were also commonly followed (28 shrews). In three instances, two

shrews captured in the same can (presumed littermates), and each mark

ed with a different colored dust, were followed beneath the same logs

used as runways. Also, the distribution of dust suggested that shrews

commonly investigated any hole/nest site they encountered.

Microhabitat Use By Species

Five of the vegetation variables surrounding the trap sites dif

fered significantly (p<. 05) among the three species (Table 6). Sorex

vagrans capture sites differed from those of Sorex cinereus capture

sites on five variables, including woody stem density, fallen log abun

dance, fallen log density, fallen log distance, and distance to water.

Between vagrans and Sorex monticolus only one habitat difference was

noted, that of fallen log density. No significant differences were

observed between cinereus and S. monticolus capture sites.

Discriminant function analysis confirmed that of the habitat var

iables measured, the most important ones which distinguished between

vagrans and S. cinereus capture sites were distance to water (r=.67),

fallen log density (r=-.64), fallen log abundance (r=.55), fallen log

distance (r=.45), and woody stem density (r=.31) (Table 7). (Sorex

monticolus was not included in the analysis due to small sample size.

Based on the discriminant function obtained, a reclassification of the

capture sites of the two species yielded 78% of the cases correctly

classified.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 6. Estimates of species means (x ± standard deviations (s)) for each of the 15 habitat variables surrounding trapsites.

S. cinereus S. vagrans S. monticolus

(n=50) (n==80) (n=12)

Variable^

GTS .255 ± .122 .253 ± .101 .204 ± .072

DTD 3.541 ± .808 3.677 ± .767 3.440 ± .900

UTS .071 ± .026 .076 ± .023 .083 ± .025

UTD 3.261 ±1.033 3.208 ± .969 3.155 ± .911

WSD * 9,020 ±5.568 6.013 ±7.320 8.500 ±6.695

HSD 5.900 ±5.898 6.300 ±7.985 5.000 ±4.369

HS# 1.440 ±1.003 1.725 ±1.340 1.333 ± .651

TSS .583 ± .484 .496 ± .485 .503 ± .509

TSDEN .154 ± .193 .193 ± .206 .177 ± .199

TSDIS 7.154 ±3.348 6.389 ±3.601 6.669 ±3.503

FLA ** 2.699 ±1.813 4.395 ±2.352 2.973 ±2.328

FLS .151 ± .139 .192 ± .124 .122 ± .086

FLDEN **,*** 1.102 ± .661 1.915 ±1.014 1.292 ±1.093

FLDIS ** 2.685 ±1.441 1.829 ±1.255 2.169 ±1.138

DTW ** 83.196 i:34.594 44.396 ±43.954 77.017 ±41.642

* denotes significant difference between vagrans ans cinereus at p<. 05

** denotes significant difference between vagrans and cinereus at p<.005

*** denotes difference between S. vagrans and monticolus (p<.005)

1 See appendix for key. 23

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 7. Standardized discriminant function coefficients and linear correlations (r) between each variable and the discriminant function, for microhabitat variables used to distinguish between S, vagrans and S. cinereus capture sites.

Standardized Correlation(r) Discriminant with the Function Discriminant Variable^ Coefficient Function

OTS .28714 .01215

OTD -.21552 -.12269

UTS -.12481 .03752

UTD .10638 .03752

WSD -.25606 -.31493

HSD .00375 .03869

HS# .12994 .16239

TSS .71468 .12532

TSDEN .00172 -.13434

TSDIS .63682 .15319

FLA .21666 .55079

FLS -.42381 -.22366

FLDEN .47273 -.63745

FLDIS .48025 .45209

DTW .81970 .67039

1 See appendix for key.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Diet analysis

The most important foods for Sorex vagrans were ants (frequency

of occurrence = 38.8%), Dipterans (18.2%), beetles (10.2%), and adult

Lepidopterans (8.0%) (Table 8). For Sorex cinereus, two types of prey

comprised 74% of items eaten - ants (38.0%) and adult Lepidopterans

(36.0%). Also eaten in significant amounts were spiders (17.0%) and

beetles (17.0%). The greatest frequency of ants eaten was recorded

for Sorex monticolus (47.5%). Spiders (18.8%), Dipterans (15.0%0, and

millipedes (10.0%) followed ants as major foods of Sorex monticolus.

There were no significant differences in proportionate distribution

of diet items among the three species of shrews (one - way ANOVA,

F£1.41, p>.05).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 8. Frequency of prey items identified in 20 microscope fields from the stomachs of Sorex vagrans, Sorex cinereus, and Sorex monticolus from northwestern Montana.

S . vagrans S . cinereus s . monticolus (n=20) (n=10) (n=4)

Prey Type

Formica 38.8 38.0 47.5

Diptera 18.2 2.0 15.0

Coleoptera 10.2 17.0 —

Lepidoptera 8.0 36.0 — — (Adult) Gastropoda 7.2 1.6 5.0 (Slug) Arachnida 5.2 17.0 18.8

Chilopoda 4.8 -- --

Collembola 4.4 3.0 5.0

Oligochaete 2.6 —— ——

Diplopoda 2.0 2.0 10.0

Hemiptera 1.2 --: ----

Orthoptera 0.8 1.0 1.3

Opiliones ---- 1.0 --

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DISCUSSION

Habitat Usage By Species

Distribution of the three species differed significantly across

mesic and xeric sites within the same general habitat type. Sorex

monticolus, at its lower elevational range, was rare on both plots.

The larger Sorex vagrans (mean weight = 4.75 g) was more common on

the mesic site, while the smaller Sorex cinereus (mean weight = 3.43 g)

was more common on the xeric site. As the most distributionally wide

spread shrew in North America, Sorex cinereus can exist in a wide

range of habitats, which suggests that its scarcity on the mesic plot

is probably not due to a marked habitat preference for more xeric con

ditions. Hanski and Kaikusalo (1989) report that a key pattern in the

distribution of Finnish shrews is a greater abundance of the larger

species in more productive (i.e. moister) habitats. Dickman (1988)

also concludes that body size differences may confer an asymmetric

advantage to the larger species in a community of insectivores. Hanski

and Kaikusalo propose that the larger species competitively excludes

the smaller species from the most productive habitat patches, but the

smaller species finds a refuge in less productive habitat patches,

where they may be relatively successful due to their decreased energy

requirements. They call this their "interference competition - habitat

patchiness" hypothesis. Possible tests of this hypothesis include

measuring shrew species compositions at the same site before and after

clear - cutting. Clear - cutting should reduce the habitat "patchiness"

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of a given site thus making coexistence of many different - sized

species more difficult (Hanski and Kaikusalo, 1989). Also, this hy

pothesis would predict that removal of the larger species should result

in the smaller species shifting into micrhabitats previously occupied

by the larger (Saarikko, 1989).

This idea that differences in body size can facilitate the coex

istence of closely related species is widespread. The tendency for

larger organisms to be competitively superior to smaller ones (termed

"asymmetric" or "hierarchical" competition) has been demonstrated for

plants (Keddy, 1989). The primary mechanism for plants appears to be

exploitative competition - the taller plant intercepts more sunlight

while simultaneously shading the shorter plant, thereby inhibiting its

growth. Although asymmetrical competition has been argued to be the

rule rather than the exception in animal communities (Wilson, 1975;

Persson, 1985), a single mechanism has not been recognized - expoita-

tive competition, interference competition, or both have been impli

cated, although in the majority of studies some type of interference

competition was involved. Conversely, smaller animals in a community

have lower food requirements, and may be superior to larger ones with

respect to exploitative competition (Persson, 1989). This phenomenon

has been viewed as a preceding condition for the evolution of interfer

ence competition.

Microhabitat Use By Species

Obtaining multiple captures of several individual shrews using

the phosphorescent dust technique was relatively unsuccessful. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lack of recaptures in pitfalls may be due to avoidance of concentrated

odors from previous captures (Szaro et al., 1988), or that shrews

"learned" to recognize and avoid pitfall edges. Of the individuals

trapped successfully, the "following" behavior noted by other authors

was evident in both Sorex cinereus and Sorex vagrans, as they commonly

traversed the edges of stream banks, logs, and tree bases.

The space under fallen trees used as corridors or runways was the

most obvious component of cover used by both Sorex cinereus and Sorex

vagrans. The reasons for this use are probably complex, but may in

clude predator avoidance, food resource distribution, and interspeci

fic competition (Belk et al., 1988).

Multivariate analysis of the microhabitat variables measured at

trapsites revealed differences between Sorex cinereus and Sorex vagrans

capture sites with respect to five variables. Sorex vagrans were cap

tured at sites significantly closer to the nearest water, and at great

er fallen log densities and abundances than Sorex cinereus, while Sorex

cinereus capture sites exhibited greater woody stem densities and dis

tances to fallen logs than Sorex vagrans captur sites. Again, compet

itive interference or patch mosaic segregation by species may play

some role in determining the microhabitat usage patterns of the two

species where they live sympatrically, but more investigation is war

ranted. Hawes (1976) proposes that odor may function as a possible

reproductive isolating mechanism in two sympatric populations of Sorex

monticolus and Sorex vagrans in British Columbia, and it is possible

that such a pheromone may also function in territoriality by eliciting

avoidance behaviors.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Between Sorex vagrans and Sorex monticolus capture sites, only

one microhabitat difference was noted, that of fallen log density.

Since no differences were observed between Sorex cinereus and Sorex

monticolus capture sites, it is probable that a small sample size in

Sorex monticolus captures contributed to these results. Also, the

ecologically similar Sorex vagrans and Sorex monticolus have only re

cently been considered separate species, and they exhibit much overlap

in morphological features, especially in northwestern Montana (Hennings

and Hoffmann, 1977). The possibility that they may interbreed and

produce hybrids also confounds attempts to distinguish patterns in

microhabitat usage between the two species.

These data suggest that Sorex vagrans and Sorex cinereus partition

the space within the grand fir habitat at both large and smaller scales.

For birds in particular, the "choice" of where to settle and/or forage

has been viewed as a hierarchical decision - making process (Tinbergen,

1981; Hutto, 1985). It is possible that small mammals also exhibit

such an organization of stepwise decisions, beginning at the time of

dispersal with the decision of where to settle at locally. The micro

distribution of organisms has generally been thought to be a function

of subtle variation in food availability.

Not considered in this investigation were potential differences

in microhabitat use by each age class and sex within a species, or

possible seasonal shifts in microhabitat use. Belk et al. (1988) showed

that females of four species of ground - dwelling rodents occupied

more "structured" habitat than males of the same species (relatively

more fallen logs, trees, and shrubs), and that the majority of species

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. studied exhibited seasonal shifts in micrhabitat use from close associ

ation with fallen logs and brush in June to areas of more herbaceous

growth in July and August. Van Horne (1982) found that adult deer mice

used habitat with more cover than juvenile habitat, and hypothesized

that intraspecific competition was responsible for niche displacement

of juveniles. Since the age - class distribution among species in this

investigation is asymmetric (overwintered Sorex cinereus represented

almost 50% of all Sorex cinereus individuals captured compared to only

4% of Sorex vagrans individuals captured had overwintered), this rep

resents a possible bias in the data.

Diet Analysis

There were no significant differences among diets of sympatric

Sorex vagrans, Sorex cinereus, and Sorex monticolus. Similarity of

diet does not necessarily reflect a lack of competition, however, since

insect genera within an Order, or even species within a genus, can

differ considerably in microhabitat, plant species, or portion of a

plant that they occupy (Martin, 1987). Since prey items were only

identified to the level of Order, these types of differences would be

obscured. Even though the three species took similar amounts of

particular insect Orders, they may have eaten different species or

genera and collected them through different foraging methods or micro-

sites. More study needs to be undertaken on foraging behaviors of

North American Sorex in the field. Prey size has also been suggested

as being relatively more important than taxa, especially with different

sizes of shrews (Yalden, 1981), but was not considered in this inves-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. tigation. Dickman (1988) observed that the larger members of pairs

of sympatric insectivores ate larger prey than the smaller, by par

tially excluding the smaller species from more productive microhabitats.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LITERATURE CITED

Barnard, C.J., C.A.J. Brown, A.I. Houston, and J.M. McNamara. (1985). Risk - sensitive foraging in common shrews: an interruption model and the effects of mean and variance in reward rate. Behav. Ecol. Sociobiol. 18:139-146.

Belk, M.C., H.D. Smith, and J. Lawson. (1988). Use and partitioning of montane habitat by small mammals. J. Mamm. 69(4): 688-695.

Brown, L.N. (1967). Ecological distribution of six species of shrews and comparison of sampling methods in the central Rocky mountains. J. Mamm. 48(4):617-623.

Buckner, C.H. (1964). Metabolism, food capacity, and feeding behavior in four species of shrews. Ca. J. Zool. 42:259-279.

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Butterfield, J., Coulson, J.C., and S. Wanless. (1981). Studies on the distribution, food, breeding biology, and relative abundance of the pygmy and common shrews (Sorex minutus and Sorex araneus) in upland areas of northern England.

Cloonan, C.L. (1986). Successional pathways of a grand fir (Abies grandis) forest based on thirty - three years of evidence. Unpub lished masters thesis.

Clothier, R.R. (1955). Contribution to the life history of Sorex vagrans in Montana. J. Mamm. 36(2):214-220.

Cottam, G., and J.T. Curtis. (1956). The use of distance measures in phytosociological sampling. Ecology. 37(3):451-460,

Dickman, C.R. (1988). Body size, prey size, and community structure in insectivorous mammals. Ecology. 69(3):569-580.

Dueser, R.D., and H.H. Shugart, Jr. (1978). Microhabitats in a forest - floor small mammal fauna. Ecology. 59(1):89-98.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hanski, I. and A. Kaikusalo (1989). Distribution and habitat selection of shrews in Finland. Ann. Zool. Fennici. 26:339-348.

Haveman, J.R. (1973). A study of population densities, habitats, and foods of four sympatric species of shrews. Unpublished masters thesis.

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Hawes, M.L. (1977). Home range, territoriality, and ecological separ ation in symapatric shrews, Sorex vagrans and Sorex obscurus. J. Mamm. 58(3): 354-367.

Hennings, D. and R.S. Hoffmann. (1977). A review of the taxonomy of the Sorex vagrans species complex from western North America, 68:1-35.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX: Descriptions and sampling methods for microhabitat variables measured.

VARIABLE METHOD

1) Overstory tree size 1) Average diameter (meters) of nearest (OTS) overstory tree, in quarters (Cottam and Curtis, 1956).

2) Overstory tree 2) Average distance (meters) from trap to dispersion (OTD) nearest overstory tree, in quarters (Cottam and Curtis, 1956).

3) Understory tree size 3) Same as OTS, for understory trees. (UTS)

4) Understory tree 4) Same as OTD, for understory trees dispersion (UTD)

5) Woody stem density 5) Live woody stem count at ground level (WSD) within a 1.0m diameter circle centered on trap (Dueser and Shugart, 1978).

6) Herbaceous stem density 6) Same as WSD, for herbaceous stems. (HSD)

7) Number of herbaceous 7) Herbaceous species count within a 1.0 species (HS#) m diameter circle centered on trap (Dueser and Shugart, 1978).

8) Tree stump density 8) Average number of tree stumps greater (TSDEN) than or equal to 7.50 cm in diameter, in quarters around trap (Dueser and Shugart, 1978).

9) Tree stump size 9) Same as OTS, for tree stumps. (TSS)

10) Tree stump dispersion 10) Same as OTD, for tree stumps. (TSDIS)

11) Fallen log density 11) Average number of fallen logs greater (FLDEN) than or equal to 7.50 cm in diameter, per quarter (Dueser and Shugart, 1978)

12) Fallen log size (FLS) 12) Same as OTS, for fallen logs.

13) Fallen log dispersion 13) Same as OTD, for fallen logs. (FLDIS)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14) Fallen log abundance (FLA) 14) Average total length of fallen logs, per quarter (Dueser and Shugart, 1978).

15) Distance to water (DTW) 15) Distance (meters) from trap to nearest standing/running water.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.