Regional Patterns of Small Mammal Abundance

and Community Composition in Oregon and

Washington

Authors

KEITH B. AUBRY is a research wildlife biologist and

MARK J. CRITES1

 is a wildlife biologist, USDA Forest

Service, Pacific Northwest Research Station, Olympia,

Washington 98502; STEPHEN D. WEST is an associate

professor, College of Forest Resources, University of

Washington, Seattle, Washington 98195.

Abstract

Pitfall traps were used by a team of investigators to study

forest-floor small mammal communities in 144 differentaged Douglas-fir forest stands in three physiographic provinces in the Pacific Northwest: the Oregon Coast Range,

Oregon Cascade Range, and Southern Washington Cascade

Range. All three small mammal assemblages were numerically dominated by a few species. The Trowbridge’s shrew

was the most abundant species in all three provinces and

accounted for 47 percent of 8661 small mammals captured.

Most remaining captures were of only one or two other

species: in the Coast Range, the western red-backed vole

and Pacific shrew; in the Oregon Cascades, the western

red-backed vole; and in the southern Washington Cascades,

the southern red-backed vole and montane shrew. Other

species each accounted for less than 6 percent of total

captures in all three provinces.

Ordination analysis of small mammal communities, both

among and within provinces, revealed few ecologically

interpretable patterns or differences among forest ageclasses. Most variation was attributable to differences in

species composition resulting from zoogeographic barriers.

The shrew-mole and red tree vole were closely associated

with old-growth forests in all provinces where they occurred.

These species should be given high priority in the development of management strategies for small mammals in Pacific

Northwest forests.

Introduction

The fate of old-growth Douglas-fir forests in the Pacific

Northwest has become one of the most controversial resource management issues of this century (see Norse 1990,

Norse and others 1986, Wilcove 1988). Research on the composition, structure, and function of these forests, however,

has only recently been undertaken (see Franklin and others

1981, Gutierrez and Carey 1985, Meehan and others 1984,

Raphael 1984, Raphael and Barrett 1984, Spies and others

1988).

1

 Present address: wildlife biologist, USDA Forest Service, Pacific Northwest

Region, Portland, Oregon 97208.

285
Before these studies, lists of small mammal species believed

to be associated with old-growth forests were based primarily

on natural history observations and speculation (for example,

Franklin and others 1981, Lumen and Neitro 1980, Meslow

and others 1981). The comprehensive program of research

described in this volume was undertaken to provide quantitative information on wildlife habitat relationships in Douglasfir forests, and to provide a scientific basis for the management of old-growth forests (Carey and Spies, this volume;

Ruggiero and Carey 1984).

The primary objectives of the small mammal studies conducted in the Oregon Coast Range (Corn and Bury, this

volume a), Oregon Cascade Range (Gilbert and Allwine, this

volume a), and southern Washington Cascade Range (West,

this volume) were to identify small mammal species that are

associated with old-growth Douglas-fir forests within each

province, and to investigate potential correlations between

small mammal abundances and vegetative or structural

features of unmanaged Douglas-fir forests. The purpose of

this paper is to combine these three data sets and compare

and contrast observed habitat association patterns among

physiographic provinces to evaluate the extent to which

observed patterns vary on a regional scale. Data from studies

conducted in the Northern California province were not

included because the ecological characteristics of stands

occurring there were considered to be too different from

those in Oregon and Washington for the data to be combined

into a single data set (T. Spies and J. Franklin, pers. comm.).

By combining the data in a single analysis, results from each

province can be compared quantitatively with those from

other provinces. In addition, larger sample sizes resulting

from the pooling of data increase the power of statistical

analyses, especially for relatively rare species. Knowledge of

regional variation in the habitat relationships of small mammal species associated with old-growth forests will enable

wildlife managers to set priorities for management activities

within different areas of the Pacific Northwest, and will

provide additional insights into the habitat relationships of

small mammals in unmanaged Douglas-fir forests. The

specific objectives of this paper are to:

l Describe the forest-floor small mammal communities

occurring in unmanaged Douglas-fir forests in three

physiographic provinces in Oregon and Washington; and

l Compare patterns of community composition, species

abundances, and habitat associations among provinces.

Methods

Study Areas

Forest-floor small mammals were studied in 144 stands in

Oregon and Washington in 1984 and 1985 by a team of

investigators. Study sites were located in three physiographic

provinces (Franklin and Dymess 1973): 45 in the Oregon

Coast Range, 54 in the Oregon Cascade Range, and 45 in the

southern Washington Cascade Range. Data were collected in

all stands in both years, with the exception of the Oregon

Cascades province in which only 15 stands in the central

Cascade Range were sampled in 1985.

Homogeneous forest stands of at least 20 ha with Douglas-fir

as the dominant tree species were selected for study. None of

the sites had been logged; all had resulted from natural regeneration after catastrophic wildfires. All stands are within the

Western Hemlock Zone and lower elevations of the Pacific

Silver Fir Zone (Franklin and Dyrness 1973).

Stands were classified into three broad age-classes: young

(35-79 yrs), mature (80-195 yrs), and old-growth (200-730

yrs). Ages were based on growth-ring counts, either by

increment coring or examination of cut stumps in nearby

stands (Spies and others 1988). Old-growth stands typically

contained high proportions of Douglas-fir and western hemlock and, in wet sites, western redcedar. Mature and young

stands were dominated by Douglas-fir. In all age-classes,

other species such as red alder, vine maple, bigleaf maple,

Pacific silver fir, and western hemlock occurred in lesser

amounts. See Spies and Franklin (this volume) and Spies

(this volume) for more detailed descriptions of the vegetative

and structural characteristics of the study stands.

Within each province, stands of comparable moisture condition were placed into an age gradient as a means of assessing variation in small mammal abundances resulting primarily

from differences in stand age-class. The age gradient consisted of 127 stands, including 43 from the Coast Range, 48

from the Oregon Cascades, and 36 from the southern Washington Cascades. Although the range of ages sampled was

similar among provinces, old-growth stands in the southern

Washington Cascades contained nine stands that were 25 to

205 years older than the oldest stands sampled in either the

Coast Range or the Oregon Cascades, Ranges for the ageclasses in each province are: Oregon Coast Range, young, 40

to 75; mature, 80 to 120; and old-growth, 200 to 525 years

old; Oregon Cascades, young, 35 to 79; mature, 84 to 180;

and old-growth, 200 to 500 years old; and southern Washington Cascades, young, 55 to 75; mature, 80 to 195; and

old-growth, 210 to 730 years old.

286
Small Mammal Sampling

Within each province, forest-floor small mammals were

sampled with pitfall traps, snap-traps, and tracking stations

I

(see Carey and Spies, this volume). Only pitfall trapping,

however, was used in all three provinces in both years. Other

techniques were used either in only one or two provinces, or

were not equally applied in both years. To compare results

among physiographic provinces, we therefore used only data

derived from pitfall trapping.

Thirty-six pitfall traps were installed in each stand in a 6 x 6

square array with traps spaced 15 m apart. New pitfall grids

were established in each stand in the southern Washington

Cascades during the second year of the study; in the Coast

Range and Oregon Cascades, grids were retrapped in the second year. Comparisons of results between new grids and a

subset of grids retrapped the second year showed no differences in small mammal species composition or abundances

(Gilbert and Allwine, this volume a). All stands were

sampled for about 30 days each fall, and checked about once

per week. All specimens were collected for identification and

all salvageable specimens were preserved.

Pitfall traps were opened after the onset of fall rains in 1984

and again in 1985 in the southern Washington Cascades and

in the Coast Range; traps were opened a month earlier in the

Oregon Cascades. Trapping dates for each province were:

Coast Range, 1 October-7 November 1984 and 30 September1 November 1985; Oregon Cascades, 28 August-6 October

1984 and 29 August-5 October 1985; and southern Washington Cascades, 30 September-5 November 1984 and 27

September-3 November 1985.

Data Analyses

For all analyses, we included only data for species that had

>20 total captures or that occurred in >l0 percent of the

stands sampled. Species not meeting these criteria were considered to be inadequately sampled with our techniques.

Although we captured 77 Pacific jumping mice, 58 of these

captures were from the Oregon Cascades where trapping

probably occurred before the onset of hibernation for this

species (Dalquest 1948). Because of potential bias resulting

from regional differences in the influence of hibernation on

our results, and the likelihood of jumping mice escaping

from pitfall traps (Bury and Corn 1987), this species was

excluded from all analyses. The extent to which differences

in sampling dates may have influenced results for other

species is unknown.

Three species, the red tree vole, Pacific shrew, and western

red-backed vole, occur in both of the Oregon provinces, but

not in Washington. Conversely, the southern red-backed vole

and forest deer mouse occur only in the southern Washington

Cascades. One species, the montane shrew, occurs throughout the southern Washington Cascades but is found only in

the northernmost portion of the Oregon Cascades (Carey and

Spies, this volume; Ingles 1965). Stand sample sizes were

adjusted for these species.

Community analyses-Although yearly variations in population numbers may influence results in studies of wildlife

habitat relationships, 2 years of data is not sufficient to adequately evaluate temporal variation in small mammal abundances. Furthermore, without corresponding demographic

information, causative factors for observed variation cannot

be determined (Van Home 1983). For these reasons, we

pooled data from both years, and calculated abundance indices for each stand as mean captures for both years per 100

trap-nights. We examined community patterns of small mammal abundance with detrended correspondence analysis

(DCA) using DECORANA (Hill 1979a). We interpreted ordination axes by correlating ordination scores for the small

mammal communities with major physiographic and vegetative gradients by using Spearman rank correlations (Norusis

1988a). The vegetative gradients we included as variables

represented multivariate gradients derived by Spies and

Franklin (this volume) in their analyses of the vegetative

characteristics of the study stands. These variables represented gradients of stand development (primarily stand age

and abundance of western hemlocks), coarse woody debris,

stand temperature, and stand moisture.

We conducted ordination analyses at two scales: region-wide

analyses that included stands of all age- and moisture-classes

from all three provinces (144 stands), and province-specific

ordinations conducted separately for each province (Coast

Range, 45; Oregon Cascades, 54; and southern Washington

Cascades, 45 stands). We conducted region-wide ordinations

both on the 15 species that were considered to be adequately

sampled with our techniques, and on data from only the subset of nine species that occur in all areas studied. The first

approach identifies regional community patterns that include

variation resulting from differences in species composition,

and the second approach examines differences in community

composition and structure independent of the confounding

effects of differences in species’ distributions. We also conducted province-specific ordinations to examine communityscale variation among age-classes within each province. This

approach enabled us to evaluate the effects of stand age on

small mammal community patterns for each province, and

provided a means of comparing these patterns among provinces that is not affected by differences in potential community composition or by deleting a substantial portion of the

community.

287
Species analyses-To investigate the extent to which patterns of association due to differences in stand age vary regionally, we conducted all species analyses using data only

from stands within each age gradient (127 stands). We used

two-way ANOVA (Norusis 1988a) on abundance data for

species with >100 captures, and log-linear analysis (Kennedy

1983) of presence-or-absence data for species with <100

captures.

Because of the limited number of sites that could be studied

with available resources, study areas were preferentially selected to sample the full range of variation occurring in each

stratum (Carey and Spies, this volume). Because study sites

were not selected randomly, rigorous statistical inference

based on hypothesis testing is not possible with our data sets.

We used statistical tests only as a means of examining patterns of variation in the data. However, because we intentionally sampled the full range of variation within each stratum,

the variances in our data would be expected to exceed those

resulting from random sampling. Consequently, observed

differences are less likely to result from chance than if our

sites had been selected randomly (Ruggiero and others, this

volume).

To identify regional patterns in the habitat relationships of

small mammals occurring along environmental gradients, we

examined the relationships between individual species’ abundances and major physiographic and vegetative gradients

using Spearman rank correlations (Norusis 1988a). We used

only abundance data for species with >l00 captures in these

analyses.

Results

Altogether, 8661 small mammals of 29 species were captured, including 6 shrews, 3 moles, 4 squirrels, 1 gopher, 14

mice and voles, and 1 weasel. Only 15 species had >20 total

captures or occurred in >10 percent of stands studied: the

marsh, montane, Pacific, Trowbridge’s, and vagrant shrews;

the shrew-mole and coast mole; the deer mouse and forest

deer mouse; the red tree, southern and western red-backed,

and creeping voles; the northern flying squirrel; and the

ermine (table 1).

The structure and composition of the small mammal communities sampled varied somewhat among provinces (fig. 1).

The Trowbridge’s shrew was captured in every stand sampled, and it was the most abundant small mammal in all

three provinces. This species accounted for 47 percent of

total captures for all provinces combined. In all three provinces, the Trowbridge’s shrew and one or two other species

accounted for most captures. In the Coast Range, the Trowbridge‘s shrew represented 56 percent of total captures; the

western red-backed vole, 21 percent; and the Pacific shrew,

11 percent. All other species contributed less than 4 percent

each to total captures. In the Oregon Cascades, 69 percent of

total captures were of the Trowbridge‘s shrew. The western

red-backed vole accounted for 13 percent, and all other species each represented less than 5 percent of total captures. In

the southern Washington Cascades, Trowbridge’s shrews

were relatively less important, representing only 36 percent

of total captures. Two other species, the montane shrew and

southern red-backed vole, were captured in relatively high

numbers, with each representing about 23 percent of total

captures. No other species accounted for more than 6 percent

of captures.

Community relationships-Ordination analysis of the combined abundance data from all provinces for 15 small mammal species revealed community patterns that reflect differences in the species composition and diversity of small mammals occurring in each province (fig. 2). The Coast Range

cluster forms a subset of the Oregon Cascades cluster, indicating less variability in the composition and structure of

small mammal communities in the Coast Range stands compared to the Oregon Cascades stands. The Coast Range and

the Oregon Cascades are most similar in small mammal species composition (fig. l), and their ordination clusters separated out almost completely from the southern Washington

Cascades cluster.

The first ordination axis (X axis) was significantly correlated

to gradients of increasing moisture (P = 0.000), elevation

(P = 0.000), and stand age (P = 0.010), and decreasing temperature (P = 0.000). The second axis (Y axis) was more difficult to interpret, as would be expected based on the lack of

separation of clusters in this direction. This axis was correlated primarily with decreasing moisture (P = 0.009) and

increasing temperature (P = 0.034).

Ordination results using only the nine species that occur in

all three provinces (table 1) revealed a high degree of overlap in the structure and composition of the communities occurring within each province (fig. 3). Community patterns

observed in figure 2, however, are still evident. The Coast

Range cluster is again much smaller than the Oregon Cascades cluster and constitutes a subset of that cluster, and the

southern Washington Cascades cluster still contains points

unique to that cluster. Environmental gradients represented

by the ordination axes are virtually identical to those reported

for figure 2.

288
Table I-Mean, standard error, and percentage occurrence of small mammal species with SO total captures In pitfall traps by 

province and age-class (number of stands sampled is in parentbescs) 

0.016 

o.oa 

16 

-d 

0.256 

0.033 

96 

I.677 

0.111 

m 

O.CG9 

o.oa 

20 

0.101 

0.025 

68 

0.061 

0.013 

64 

0.032 

0.013 

32 

0.144 

0.020 

96 

- 

- 

- 

0.020 

0037 

32 

0.607 

0.072 

00 

- 

- 

- 

0.037 

O.CU3 

16 

0.599 0.256 

0.158 0.070 

l&l 1M) 

1.905 1.360 

0.221 0.086 

103 lrn 

0.052 0.005 

0.029 o.co5 

40 13 

0.080 0.051 

0.030 0.019 

70 63 

0.039 0.044 

0.012 0.022 

60 38 

0.018 0.017 

0.010 0.012 

30 25 

0.054 0.135 

0.017 0.085 

60 75 

- - 

o.Mx) 0.006 

o.oco o.mx 

0 13 

0.830 0.562 

Cl.150 0.118 

IW 88 
Oregon coast Range 

Oregon Cascade Range 

Southern Washington Cascade Range 

Oregon Cascades 

I I Southern 

Washington 

Southern 

Washington 

. . . . . . Cascades 

Oregon : ’ . . 

Cascades, 4- - - . \ ... / . / . / : / 

/ 

\ 0 

\ \ 

Coast I 

Range I 

/ 

\ 

\ . . 

/ 

. 

-- 

. . 

/ 

‘A / -- 

. . . 

. . .: 

290 
Coast Range Oregon Cascades So. Washington Cascades 

7 

Old.growth 

Did-growth 

/- 

\’ 

.x 

\ \ . . 

Q 

‘.Msture\ : 

.’ \ 

\ 

.:.- 

‘oung’ 

Results of province-specific ordinations showed little vxiation among ageslasses for any province (fig. 4). No distinct 

clusters resulted from these analyses, which indicates that the 

structure and composition of the forest-floor small mammal 

communities we studied is largely independent of stand age. 

As would be expected from these results, none of the ordination axes were correlated with stand age. Attempts to COTrelate otdination axes with major environmental gradients 

revealed no interpretable patterns in community structure or 

composition within provinces. 

Species relationships-Two-way ANOVA (table 2) and loglinear analysis (table 3) revealed strong stand age-effects for 

only hvo species: the shrew-mole (P = 0.070) and red wee 

vole (P = 0.067). Both species reached highest abundances 

OI occurred most frequently in old-growth stands (table 1). 

The shrew-mole showed a strong province-effect (P = 0.005). 

but the effect appeared to be largely due to varying abundance values for young and mature stands (table 1). For tbe 

red tree vole, no province-effect was found (P = 0.191), indicating that patterns of occurrence were consistent between 

provinces. Although a very strong age-effect was also found 

for the deer mouse (I’ = O.OOl), a strong interaction-effect 

between stand age and province (P = 0.058) complicates the 

interpretation of these results. Furthermore, the deer mouse 

occupies virtually all available habitats in the Pacific Northwest (Dalquest 1948; Ingles 1965: West, this volume; and 

others): it is not primarily a forest-dwelling species. As such, 

it should not be considered a species of concern to forest 

managers. The strong province-effects found for most species 

indicate that patterns of abundance are extremely variable 

among the provinces studied. 

Table Z-Results of two-way ASOVA on abundance data for 

stand age vs. prrrvince (analyses were cnnducted only on data 

from stands within the age gradient for species that occurred 

in more than one province and had alo0 total captures) 

Source of variation 

Species Stand age Pro”incc Interaction 

Montme shrew 0.268” 0.001 0.468 

Trowbridge’s shrew 0.835 0.022 0.301 

Pacific shrew 0.143 0.000 0.130 

Shrew-mole 0.070 0.005 0.830 

Deer mouse 0.001 0.000 0.058 

Western red-backed vale 0.756 o.olnl 0.288 

LIP value 

Table 3-Results of log-linear analysis on presence-or-absence 

data for stand age vs. province (analyses were conducted only 

on data from stand.! within the age gradient for species that occurred in more than one province and had <I00 total captures) 

Species Stand age’ Pmvince Interaction 

Marsh shrew 

Vagrant shrew 

Coast mole 

0.691 

0.238 

0.213 

0.047 

0.230 

0.013 

0.117 

0.811 

0.798 

Nonhem flying squirrel 0.849 

Red bee vole 0.067 

Creeping vole 0.595 

Ermine 0.691 

291 
Table 4-Significant Spearman rank correlations (P < 0.1) between small mammal abundances and major physiographic and

vegetative gradients (only species that occurred in more than one province and had >100 captures were included)

Vegetative gradients’

Species Stand age Elevation Aspect Slope STDV CWDB TEMP MOE

Montane shrew + 0.094b

- 0.000

Trowbridge’s shrew - 0.017

Pacific shrew - 0.000 + 0.000

Shrew-mole + 0.002 - 0.067 + 0.007 + 0.015 + 0.001

Deer mouse + 0.030 - 0.001 + 0.000

Western red-backed vole - 0.021 - 0.002

a

 STDV represents a stand-development gradient; CWDB, a coarse woody debris gradient; TEMP, a temperature gradient; and MOIS, a moisture gradient.

b

 P value.

Correlation analyses for the six most abundant species are

shown in table 4. These analyses support the results of

ANOVA for the shrew-mole and deer mouse; abundances for

both species were significantly correlated with increasing

stand age. The shrew-mole was also associated with old,

moist, relatively flat stands with large amounts of coarse

woody debris.

Discussion

The small mammal assemblages described here represent

only subsets of the total small mammal communities in each

stand. As with any one technique for sampling an entire

vertebrate community, pitfall trapping is more effective at

sampling certain species than others (Bury and Corn 1987,

Williams and Braun 1983). Consequently, our results apply

only to the subset of species that are well-sampled with

pitfall traps; extrapolation of our results for species not

included here is unwarranted.

Community relationships-Variation in the structure and

composition of small mammal communities in unmanaged,

closed-canopy, Douglas-fir forests is influenced more by

geography and climate than by stand age. Ecologically

interpretable patterns were detected only in the region-wide

analyses (see figs. 2-4). Although differences between the

Southern Washington Cascades province and the Oregon

provinces largely disappeared when only the nine species

common to all three provinces were included (figs. 2,3),

several important patterns remain. In both analyses, small

mammal communities were more variable in the Oregon

Cascades than in the Coast Range; the Coast Range communities represent a subset of those occurring in the Oregon

Cascades. Also, a subset of the stands in the Southern Washington Cascades province separate out ‘from both of the

Oregon provinces along the first ordination axis.

The first ordination axis is correlated to many of the environmental and physiographic features that distinguish the

Southern Washington Cascades province from the Oregon

provinces: older stands, higher elevations, and cooler, wetter

forest environments (Franklin and Dymess 1973; Spies and

Franklin, this volume). The second axis, which separates the

Coast Range stands from a portion of the Oregon Cascades

stands, represents a gradient of increasingly warmer and drier

conditions. Thus, although the Oregon Cascades and Coast

Range provinces have very similar small mammal species

composition (fig. l), only the warmest and driest stands in

the Oregon Cascades contain mammal communities comparable to those occurring in the Coast Range. In terms of

both environmental conditions (T. Spies, unpubl. data; Spies

and Franklin, this volume) and the composition of small

mammal communities, the Coast Range stands are most

similar to the southernmost stands in the Oregon Cascades.

Additionally, the oldest and wettest stands in the southern

Washington Cascades contain small mammal communities

unlike any in the Oregon provinces.

In contrast, we could detect little interpretable variation in

community composition and structure among age-classes

within each province (fig. 4). This finding agrees with the

province-specific ordinations of understory floristics conducted by Spies and Franklin (this volume) and with the

province-specific small mammal studies reported elsewhere

in this volume by Corn and Bury, Gilbert and Allwine, and

West. Within these physiographic provinces, unmanaged,

closed-canopy Douglas-fir forests apparently support small

mammal communities of similar composition and structure,

regardless of stand age. Our results agree with the conclusion

of Corn and Bury (this volume a) that different age-classes

of unmanaged, closed-canopy forest appear to represent a

single habitat type for most species. Although the

communities as a whole are very similar among age-classes,

several individual mammal species do exhibit a significant

association with stand age in unmanaged Douglas-fir forests.

Species relationships-Only two forest-dwelling species,

the shrew-mole and the red tree vole, were closely associated

with older forests (tables 3,4). Abundance values for the

shrew-mole and frequencies of occurrence for the red tree

vole were highest in old growth in each of the provinces in

which they occur (table 1).

292
The results of several other studies using similar sampling

designs provide support for these results. Raphael (1984)

reported a significant positive association with stand age for

the shrew-mole in Douglas-fir forests in northern California,

although results were based on only 1 year of data. A later

report, based on 3 years of data (Raphael 1988c), showed

that shrew-moles reached highest abundances in stands older

than 50 years and were very low in abundance in clearcuts or

in brush/sapling stands (0-20 yrs). In the Oregon Coast

Range, Corn and Bury (this volume a) found shrew-moles to

be 2.0 and 1.3 times more abundant in old-growth than in

young and mature stands, respectively.

Other studies in Oregon and Washington reported here and

elsewhere, however, have revealed no clear pattern of association between shrew-mole abundances and stand age.

The shrew-mole was reported to be weakly associated with

old-growth forests in the Oregon Cascades (Gilbert and

Allwine, this volume a), or to show no age-effect at all (Corn

and others 1988). Several studies in the southern Washington

Cascades have found little evidence that shrew-moles are

associated with stand age (Corn and others 1988; West, this

volume). Results reported by Corn and others (1988),

however, are based on only 1 year of data collected in 12

stands, and West (this volume) reported the combined results

of pitfall trapping and snap-trapping as a means of evaluating

responses of the broadest possible assemblage of small mammals. As mentioned previously, however, these techniques

are not equally effective at sampling insectivores. Meaningful patterns in the data for this species may have been

obscured by the addition of snap-trap results. Based only on

pitfall data, results from the southern Washington Cascades

also showed a trend of increasing abundance of shrew-moles

with stand age (table 1). Consequently, increased sample sizes

resulting from the pooling of data revealed a strong pattern

of association that was not clearly evident at the province

scale.

Shrew-mole abundances were also highly correlated with

increasing stand age, stand development, stand moisture,

amounts of coarse woody debris, and flatter slopes (table 4).

As with any series of statistical tests, these results should

be viewed with some caution. With 48 separate correlation

analyses, we would expect five (10 percent) to be significant

by chance alone at P < 0.1. Fifteen (31 percent) of the tests

were significant, however, and all but two were significant

at P < 0.05, indicating that most of these correlations reflect

actual patterns of association.

Terry (1974, 1981) described preferred shrew-mole habitat in

west-central Washington to consist of closed forests containing few herbs or shrubs to interfere with its burrowing activities, and a high organic matter content in the soil. She also

reported that shrew-moles were significantly correlated with

the cover of dead wood.

Tevis (1956) described shrew-moles as primarily inhabitants

of unmanaged forests; he rarely found them in cutover areas.

Results from other studies provide support for this assertion.

In west-central Oregon, Hooven and Black (1976) found

shrew-moles to be most abundant in unmanaged forest.

Shrew-moles were less abundant in an area that had been cut

and not burned, leaving coarse slash on the site, and were

not present at all in an area that had been cut and broadcastburned, leaving only fine litter. Dalquest and Orcutt (1942)

reported shrew-moles to be very scarce in logged areas in

western Washington. Corn and Bury (this volume a) sampled

three clearcuts and adjacent old-growth stands in the Coast

Range, but captured shrew-moles only in the old-growth

stands. West (this volume) captured only two shrew-moles

in eight clearcuts sampled in southern Washington.

These results indicate that shrew-moles are closely associated

with moist old-growth forests containing relatively deep soils

and many logs. However, shrew-moles appear to be responding primarily to microhabitats that occur most frequently in

old-growth forests, since they are found in relatively high

numbers in younger forests with appropriate forest-floor conditions (table 1). Catastrophic wildfires or clearcut logging

practices that include removing most of the small and large

woody debris, however, would be expected to eliminate

shrew-moles from that site until canopy closure is reestablished and forest-floor conditions again become suitable.

Corn and Bury (this volume a) reported the red tree vole to

be present significantly more often in old-growth forests than

in either mature or young forests. Although they collected

relatively few data, Gilbert and Allwine (this volume a)

reported capturing red tree voles only in mature and

old-growth forests in the Oregon Cascades. Similar studies

conducted by other workers in the Oregon Cascades (Corn

and Bury 1986, Corn and others 1988) indicated that red tree

voles were associated with old-growth forests, but their data

were insufficient to generate statistically significant results.

Our results (table 3) indicate that red tree voles are closely

associated with old-growth forests in both the Coast and

Cascade Ranges in Oregon. This species showed a significant age-effect, with nonsignificant province- or interactioneffects, indicating that patterns of variation were the same in

both provinces. Although this species has been reported to

nest in second-growth stands in the Coast Range (Maser

1966), recent work based on nest surveys in the Coast Range

(Gillesberg and Carey, this volume) and in northern California (Meiselman and Doyle, in press) provide support for a

close association with old-growth forests. Both studies also

indicated a strong preference for very large (>l00 cm in

d.b.h.) Douglas-fir trees for nesting and foraging. Bailey

(1936) speculated that because red tree voles move about the

forest largely within the canopy, they would be expected to

occur only in areas of extensive, or recently isolated, forest.

293
Although the data are minimal, existing information indicates

that red tree voles are largely restricted to old-growth forests;

younger forests apparently do not provide suitable habitat for

this species.

Management Implications

The similarity of species composition and structure of the

small mammal communities in the Coast Range and Oregon

Cascades provinces (figs. l-3) suggest that management

strategies intended to provide for the habitat needs of small

mammals in these provinces need not be designed specifically

for either province. If management activities are conducted

on a province, rather than a regional scale, stands in the

southern Oregon Cascades should be considered to be more

closely allied with stands in the Coast Range, rather than

those in the central and northern Oregon Cascades. Very old

(>500 yr) and wet stands in the southern Washington Cascades contain distinct small mammal communities that may

require unique management approaches.

Shrew-moles are closely associated with moist, relatively

flat, old-growth stands, Silvicultural prescriptions that remove

the forest canopy, disturb the upper soil layers, and remove

large amounts of coarse woody debris would be expected to

eliminate this species from the site. Extensive clearcutting

that creates large expanses of unsuitable habitat could result

in local population extinctions. Shrew-moles could probably

be maintained in managed landscapes, however, if old-growth

areas are preserved as survival refugia to enable the species

to recolonize younger forests when suitable microhabitats are

reestablished.

Although relatively little is known about their specific habitat

requirements, red tree voles apparently require old-growth

stands containing large Douglas-fir trees for nesting and

foraging. For this reason, and because they use the forest

canopy as travel routes (but see Corn and Bury 1986), extensive loss or fragmentation of old-growth forests could

eliminate this species from forested landscapes.

Acknowledgments

We thank Steve Corn, Bruce Bury, and Fred Gilbert for

providing data from the Oregon provinces to use in our

analyses, Marty Raphael for assistance with log-linear analyses, and Steve Corn, Marty Raphael, and an anonymous

reviewer for helpful comments on previous drafts of this

manuscript.

This paper is contribution 135 of the Wildlife Habitat

Relationships in Western Washington and Oregon Research

Project, Pacific Northwest Research Station, USDA Forest

Service.0

294
Forest Age Associations of Bats in the Southern

Washington Cascade and Oregon Coast Ranges1

Donald W. Thomas and Stephen D. West

I

.,.‘,.l * / ,b.:.$ ,,- ‘,,., ,

Authors

DONALD W. THOMAS is an associate professor, Groupe

de Recherche en Energie, Nutrition, et Ecologic, Universite

de Sherbrooke, Sherbrooke, Quebec, Canada J 1K 2Rl.

STEPHEN D. WEST is an associate professor, Wildlife

Science Group, College of Forest Resources, University of

Washington, Seattle, Washington 98195.

Abstract

We studied the use by bats of old-growth, mature, and young

Douglas-fir stands in the southern Washington Cascade and

Oregon Coast Ranges. Bat activity was measured with

ultrasonic detectors. In Washington, the Myotis species were

detected 2.7 to 5.7 times more frequently in old-growth than

in younger stands. In Oregon, the Myotis species and the

silver-haired bat were detected 2.5 to 9.8 times more frequently in old growth. Feeding rates were extremely low in

the forest stands, and most bat activity was confined to a

brief peak during the first 15 minutes of the evening, which

suggests that bats used old-growth stands for roosting. Vegetation features, such as the abundance of damaged or diseased trees and snags, appeared to be important to bat populations, but correlations of bat activity with these features

’ Portions of this paper were drawn directly from “The distribution of bats in

different ages of Douglas-fir forests” (Journal of Wildlife Management.

52: 619-624. 1988) wirh permission of The Wildlife Society.

were weak. No reproductive females were captured at sites

>300 m in elevation on the western slope of the Washington

Cascades, but they were common to the east and south. Oldgrowth forests appear to be important habitats for bats in the

Cascade and Coast Ranges, but management priority should

be given to the latter region.

Introduction

The area west of the crest of the Cascade Range and extending from mid-Washington to southern Oregon supports

12 species of insectivorous bats (Barbour and Davis 1969,

Thomas 1988). If biological importance is measured by taxonomic diversity, then the Chiroptera rank second only to

the Rode&a in this region. Yet little is known about how

bat species use the range of age and composition of forest

patches in the Pacific Northwest. Because timber harvest

and stand-management practices are rapidly restructuring

the landscape, such knowledge is of increasing importance.

Without it, no predictions can be made of the effect of vegetation changes on bat populations, and no management

decisions can be reached to alleviate any negative effects.

In this paper, we describe how bat activity varied across a

range of age- and moisture-classes of Douglas-fir forests in

the southern Washington Cascade Range and Oregon Coast

Range, bat use of forest stands in relation to the availability

of food or roost resources, and specific vegetation characteristics that may influence the use of stands by bats.

295
Methods

Study Sites

In summer 1984, our field work was concentrated on the

western slope of the southern Washington Cascades; in

summer 1985, the study area was expanded to the Oregon

Coast Range. In each region, 45 stands were selected; nine

Douglas-fir stands in each of five stand types represented

important age and soil-moisture conditions [old-growth (OG)

wet, OG mesic, OG dry, mature mesic, and young mesic].

Descriptions of stand characteristics and selection criteria are

presented elsewhere in this volume.

Capture and Sampling Methods

To provide an index of what bat species were common at the

Washington and Oregon study sites, as well as to record their

echolocation calls, we captured bats in 10-m mist nets set

over streams and ponds during June, July, and August. No

capture sites were inside any of the study plots, but most

were within 20-km distance and 100-m elevation of the

stands. We identified bats to sex, age, and species and recorded females as parous (pregnant, lactating, or postlactating) or nonparous (no evidence of reproduction in the

current year). Bats were then released and their echolocation

calls were recorded with a “divide-by-lo” ultrasonic detector

(Miller and Andersen 1984) coupled with a Panasonic RQ355 tape recorder to store the calls. See Fenton (1988) and

Thomas and West (1989) for further details on the recording

and analysis of bat echolocation calls. We also netted bats on

the eastern slope of the Washington Cascades (near Goldendale) on 5 and 6 August 1985 to provide a contrast with

captures on the western slope.

In 1984 and 1985, we used ultrasonic bat detectors to detect

and identify bats flying in the study stands. The methods

used to sample bat activity in the forest stands differed somewhat between the two years, however. In 1984, we sampled

bat activity in 41 of the 45 stands by using hand-held, divideby-10 detectors coupled with sound-activated Panasonic RQ355 tape recorders to store echolocation calls. Each stand

was visited three times between mid-July and early September. In each stand, we sampled bat activity by recording

echolocation calls for 15 minutes at each of five stations

spaced at 100-m intervals along a transect through the stand.

We began sampling when the green of the vegetation could

no longer be discerned (about 30 minutes after sunset, depending on canopy coverage). This light intensity corresponds roughly with the onset of bat activity.

In 1985, we used automated detector systems (Thomas and

West 1989) to detect and record bat echolocation calls in

Washington and Oregon. In each study stand, we set out

two detector systems, one to sample the subcanopy (ground)

stratum and the other to sample the mid- to upper-canopy

stratum about 30 m above the ground. Both detectors were

left in place to sample bat activity continuously over two consecutive nights. The automated detectors began sampling

about 30 minutes after sunset and continued for 9 hours each

night. The detectors entered time markers at intervals of 32

+ 1 minute after start-up. We sampled all 45 stands in each

of the two regions in June and again in July and about half

the stands in August. Many samples, however, were either

lost because of equipment malfunctions or cut short when

insect or other noise interference caused the tapes to run out.

We analyzed the tapes containing the echolocation calls in

the laboratory by using a period meter to display the echolocation frequency and time characteristics on a calibrated

Telequipment D32 oscilloscope (Fenton 1988, Simmons and

others 1979, Thomas and West 1989). Recordings from

known species, together with previously published information (Thomas and others 1987), provided the reference data

base on frequency, time, and shape characteristics of echolocation calls. These call characteristics were used to identify

the bats detected and recorded in the stands. For each stand,

bat activity was measured as the number of bat passes recorded in a given sample period; a bat pass is defined as a

sequence of two or more echolocation calls recorded as a bat

flies by the detector microphone. For the 1985 data, we standardized bat activity as passes per 100 minutes of sampling.

We recognized feeding efforts by the high repetition rate

(about 100 pulses per second) “feeding buzz” associated with

each prey capture attempt. We measured feeding rates as the

percentage of passes that contained a feeding buzz. To provide a comparison with feeding activity within the stands, we

also measured the feeding activity over three stream or pond

sites in Washington (5 sample-hours) and over two pond

sites in Oregon (3 hours).

Species Recognition and Groupings

Ultrasonic bat detectors permit species to be assigned to categories according to their echolocation characteristics. Some

categories contain single species, but others contain assemblages of species with similar call structures (see Thomas

and West 1989). In the Pacific Northwest, we were able to

identify the following seven groups: Myotis A (the little

brown and Yuma myotis, or both); Myotis B (the California,

Keen’s, long-eared, and western small-footed myotis, or

combinations): the long-legged myotis; the silver-haired bat;

the hoary bat; Townsend’s big-eared bat; and the big brown

bat or fringed myotis. Because the fringed myotis appeared

to be uncommon in our study area, we will refer to the last

group as the big brown bat only.

Prey Abundance

In August 1984, we used Bioquip ultraviolet light traps to

provide an index of the relative abundance of nocturnal,

phototactic, flying insects in the stands and over water. At

each site, we set one trap 1.5 m above the ground and

296
activated it for 90 minutes, starting at dusk. Any insects on

or inside the trap at the end of this period were killed with a

spray insecticide and sorted into three size-classes based on

total body length (small: <5 mm, medium: 5-10 mm, large:

>l0 mm). The sorted samples were dried to constant mass at

60 oC. We obtained samples from 44 stands (9 OG dry, 8

OG wet, 9 OG mesic, 9 mature, and 10 young) and three

stream sites.

Vegetation Sampling

Vegetation in the study stands was sampled in a parallel

study (see Spies and Franklin, this volume) that characterized

the stands by age- and moisture-class and provided data on

35 variables (appendix table 5) that might affect bat

abundance.

Statistical Analyses

Bat detection rates did not meet the assumptions required for

parametric statistical tests. For all species, detection rates

were highly leptokurtic (Kolmogorov-Smirnov test for normality; P < O.OOl), and the variances were unequal (Fmax

test; P < 0.001). Because no simple transformation rectified

these problems, only Mann-Whitney and Kruskal-Wallis tests

(Seigel 1956) were used to compare detection rates among

stand types. Significance levels in all tests were a = 0.05.

To test for significant correlations between bat detection rates

and vegetation characteristics, we replaced the classical discriminant function analysis, which is highly sensitive to departures from normality (Press and Wilson 1978), with the

nonparametric stepwise logistic regression (SAS computer

package; LOGIST, Frank E. Harrell, Jr., Duke University

Medical Center, Durham, NC). The logistic regression model

requires that the dependent variable (here, bat detection rates

for each species) be categorical. Depending on the species,

we regrouped detection rates into two or three equal categories. The number of independent variables that the logistic

regression can test is approximated by n/10, where n is the

sample size of the smallest category. In our data sets, categories contained 50 to 70 entries, which limited the number

of variables that could be tested to five to seven. To circumvent this limitation, we ran logistic regressions seven times

for each species, using five or six new variables each time

(total of 35 variables) to identify potentially significant

effects. We then combined the significant variables from

the preliminary runs and reran the tests. Those variables

that had significant correlation coefficients (rij.k) in the final

grouping were considered to have a significant influence on

bat detection rates.

Results

Sex and Age of Netted Bats

Important differences in the sex and age structure of the

three bat communities were shown by netting over streams

and ponds in the western and eastern Washington Cascades

and in the Oregon Coast Range (table 1). In the western

Washington Cascades, the netting effort yielded 191 individuals of 12 species; males constituted 88.8 percent of the 161

adults. None of the 18 adult females showed any signs of

reproduction in the current year. Juveniles constituted 16

percent of the captures, but they were never captured before

21 August. In the Oregon Coast Range, males comprised 72

percent of the adults (N = 49). Unlike the western Washington Cascades, 91 percent of the adult females were pregnant,

lactating, or postlactating-indicating that they would produce or had produced young in the current year. Juveniles constituted 20 percent of the catch and made their first appearance in late July. In the eastern Washington Cascades, only

56 percent of the adults caught were male (N = 61). The

majority of adult females (82 percent) were either lactating

or postlactating. Juveniles comprised 34 percent of the catch

on 5 and 6 August. These data suggest that the females present in the western Washington Cascades were not reproductively active, but those present in the Oregon Coast

Range and the eastern Washington Cascades were.

Bat Activity in 1984

In 1984, 153.8 hours of sampling yielded only 277 detections,

of which only 151 were sufficiently clear to be identified to

species group. The low number of identified detections rendered a rigorous analysis of stand use by the different bat

groups impossible. We thus regrouped the bats into two categories based on their probable roosting locations: cavityroosting bats (Myotis A, Myotis B, the long-legged myotis,

and the big brown bat) and foliage-roosting bats (the

silver-haired and hoary bats). We also grouped the three

moisture-classes of old-growth stands. Detections of both

cavity- and foliage-roosting bats were significantly different

across the three stand age-classes (cavity: X2 = 7.9, 2 df, P

< 0.05; foliage: X2 = 6.1, 2 df, P < 0.05). Young and

mature stands did not differ significantly (cavity: X = 0.4, 1

df, P > 0.05; foliage: X

2

= 1.2, 1 df, P > 0.05), indicating a

disproportionately high use of old-growth stands by both

cavity- and foliage-roosting bats.

297
Table 1Sex and age compositlon of bats netted in the Washington Cascades and the Oregon Coast Range In 

1984 and 1985 

Bat species 

Western WIlshington Cascades: 

Bie bmm bat 

M&S Females 

Adult JUVeIlile Nonpamus PZIXIS JUVEXlil‘SS 

2 n 

Caiifomia myotis 2; 

Western small-footed myotis 4 

Long-eared my& 5 

Little brown myotis 92 

Keen’s my& 2 

Fringed myotis 1 

Long-legged myotis 6 

Yuma myotis 2 

Silver-haired bat 0 

Hoary bat 1 

Townsend’s big-eared bat 1 

1 

0 

0 

21 

0 

0 

0 

0 

0 

0 

0 

0 

10 

0 

0 

7 

0 

0 

0 

0 

1 

0 

0 

Eastern Washington Cascades: 

Big brown bat 

Western small-footed myotis 

Long-eared my&s 

Little brown myotis 

Hoary bat 

Oregon Coast Range: 

Big brown bat 

California myotis 

Western small-footed myotis 

Long-eared myotis 

Fringed myotis 

Long-legged myotis 

Yuma myotis 

Silver-haired bat 

2 

5 

0 

10 

5 

3 

4 

1 

5 

0 

0 

Bat Activity in 1985 

In Washington, the total sample effort of 1599 hours yielded 

3029 detections of all recognizable species (table 2). The 

genus Myotis (Myof& A, Myotis B, the long-legged myotis, 

and unidentified Myotis species) accounted for 94 percent 

of these detections and the silver-haired bat for 5 percent. 

Detections of the big brown, hoary, and Townsend’s bigcared bats were toa rare to be subjected to statistical analysis. 

In Oregon, the total sample effort of 1500 hours yielded 

6211 bat passes for all species (tablz 2). The genus Myotis 

accounted for 79 percent of all detections, and the silverhaired and big bmwn bat accounted for 6 and 4 percent of 

all detections, respectively. Detections of the hoary bat and 

Townsend’s big-eared bat (2 and 1 percent of detections, 

respectively) were too me for statistical analysis. 

The detection rates for the frst and second nights of each 

2-night sample in a given stand were significantly correlated 

in both Washington and Oregon (Spearman rank carelation, 

P < 0.05 for all species). These two consecutive nights 

clearly did not represent independent samples of bat activity, 

so the two nights were pooled and the mean detection rate 

@asses per 100 sample-minute) was calculated for each 

stand visit. 

Sampling height &round vs. canopy) had no clear effect on 

detection rates for either Washington or Oregon. In Washington, detections of only the silver-haired bat differed significantly between ground and canopy layers (Mann-Whitney 

U test; P c 0.001, canopy>ground). All other species were 

298 
Table 3-Detection of bats in old-growth, young, and mature 

stands in the southern Washington Cascades and the Oregon 

Coast Range 

61d 

., __ ,_ 

Old growth/ 

growth Younger youngerb PC 

h4yory”i.r B

Long-legged 

myotis 

Silver-haired 

bat 

01egiXl 

hfyotis A 

Myotis B 

Long-legged 

myotis 

Silver-haired 

bat 

Big brown 

bat 

1 .23m.27d 

2.41i1.75 

0.34rn.08 

0.33m.09 

0.91mo.25 

0.06M.02 

0.27mo.11 0.15mo.64 

6.29k1.98 2.48+1.13 

6.54f1.81 1.48M.50 

1.46+0.50 

2.73*1.15 

0.53m.26 

0.53m.30 

0.2xm.22 

0.16rno.08 

3.73 <0.005 

2.65 <O.Ool 

5.67 <O.Ol 

- NS 

2.54 <O.ool 

4.42 <O.OOl 

2.75 <O.Ol 

both Washington and Oregon. Feeding rates ranged from 1 

percent for the silver-haired bat to 4 percent for My&s A 

in Washington. In the Oregon Coast Range, feeding rates 

ranged from 1 percent (silver-haired bat) to 4 percent (Myotis 

A). Feeding rates were dramatically higher over water in the 

same areas, ranging from 27 percent (long-legged myotis) to 

39 percent (Myotis A) in the Cascades and from 20 percent 

(long-legged myotis) to 38.5 percent (Myotis B) in the Coast 

Range. For all My&s species, feeding rates averaged 10.3 

(Cascades) and 10.1 times (Coast Range) higher over water 

than in the forest stands. Forest stands do not appear to be 

primary feeding sites for the Myotis species. 

Some evidence was found of higher feeding activity in oldgrowth stands than young and mature stands. Myotis B in the 

southern Washington Cascades and Myotis A and Myotis B 

in the Oregon Coast Range all had higher feeding rates in 

old-growth stands (2 x 2 contingency tests, P < 0.05). 

Sample sizes were too small to permit testing for other 

species. 

Detection Rates in Washington and Oregon 

Detection rates in old-growth stands were significantly higher 

in Oregon than in Washington for all species except the longlegged myofis (table 3). The Oregon Coast Range had detection rates 2.2 times higher than the Washington Cascades 

when all detections were pooled. My&s A, Myotis B, and 

the silver-haired bat were detected at rates 5.1, 2.7, and 10.1 

times higher, respectively, in Oregon old-growth stands than 

in comparable stands in Washington. 

Patterns of Prey Abundance 

Insect biomass did not differ significantly among stand types 

for any insect size-class. Stream samples had significantly 

more small and medium-sized insects than did forest samples (small: F = 1539.8, P < 0.001; medium: F = 108.7, 

P < O.Oal), but the mass of large insects did not differ between the two (F = 3.1, P > 0.05). The approximately 166- 

fold increase in small and IO-fold increase in medium-sized 

insects over water as compared with forest stands resulted in 

a major shift in the biomass and size distribution of insects. 

In forests, large insects accounted for 65 percent of the total 

insect biomass, and small insects represented only 5 percent. 

The pattern was reversed over water: large-bodied insects 

accounted for only 11 percent of the insect mass, and smallbodied insects accounted for 64 percent. 

Vegetative Variables 

The logistic regression models showed a confusing pattern 

of significant relations (table 4). In both Washington and 

Oregon, seven variables were significantly correlated with 

the detection rates for one or more bat species; however, 

they explained only a small proportion of the total variance 

in bat activity among stands. In Washington, only 6 to 8 percent and in Oregon only 4 to 17 percent of the variance was 

explained by variables used in the regression models. Also, 

these variables did not explain the significant association of 

various bat species with the old-growth age-class. Detection 

rates for the long-legged myotis were significantly higher in 

old-growth stands in Washington, but they were not COTrelated with any variables in the regression models. Stand 

age was not a significant variable for any bat species in 

Washington and was only significant for Myotis B in 

Oregon. Clearly, the significant regression variables do not 

adequately explain the old-growth associations observed for 

bats, nor do they have a strong predictive value for use in 

further studies. 

Discussion 

Detection rates cannot be directly converted into density 

estimates because individual bats may be detected more 

than once as they fly near the microphone and because 

resident and transient individuals cannot be differentiated. 

Both factors negate any simple one-to-one correlation 

between detection rates and quantitative measures of bat 

population density (Thomas and Lava1 1988). Because any 

biases are likely to be constant across sample sites, however, 

detection rates can provide a relative index of bat abundance 

in different habitats or stand types. With the exception of the 

silver-haired bat in Washington, all the common bat species 

in the southern Washington Cascades and the Oregon Coast 
Table 4-Vegetstlve variables that were slgoiRcnotty nrsoeinted 

with bat activity in the southern Washington Cascades and the 

Oregon Coast Range 

Snag DC1 Ns 

Snag DC3 Ns 

Ranges have dramatically higher detection rates in old-growth 

than in mature or young stands. Thus, this study indicates 

that bats are 2.5 to 9.8 times more abundant in old-growth 

than in young or mature stands in both study regions. Clearly, 

old-growth stands provide important habitats for bats in the 

Pacific Northwest. 

Forest stands can provide bats two critical resources: food 

and day roosts. The short duration of activity in early evening and the low feeding rates during this time suggest that 

forest stands are not important feeding sites for bats, Early 

evening peaks in activity over ponds and streams and in 

adjacent riptian habitat have been described for several 

species, including the California, long-eared, little brown, 

Keen’s, long-legged, and Yuma myotis and big brown and 

silver-haired bats (Anthony and Kunz 1977, Bell 1980, Cross 

1976, Kunz 1973, O’Farrell and Bradley 1970). These early 

activity peaks over water generally last 45 to 180 minutes, 

much longer than the peaks observed in the forest stands 

(about 15 minutes). The length of activity peaks over water 

appears to be determined by the insect capture rate that bats 

achieve and the amount of food they require. Pregnant and 

lactating little brown myotis foraging over water capture 

only 19 percent and 28 percent of their fust meal in 20 

minutes despite capture rates as high as five to eight insects 

per minute (Anthony and Kunz 1977; Gould 1955, 1959). 

Similarly, wild European pipistrelles and captive western 

small-footed myotis achieve high capture rates when feeding 

on insect swarms (10 insects per minute), but have feeding 

periods lasting 245 minutes (Griffin and others 1960, Racey 

and Swift 1985). Because feeding rates decline witi 

decreasing insect abundance (Racey and Swift 1985), 

habitats with lower insect abundance than aquatic sites have 

should show prolonged activity peaks, which was clearly not 

true in forest stands. Although insects in the size range taken 

by Myotis species (<IO mm; Anthony and Kunz 1977, 

Belwood and Fenton 1976) were only 1 to 10 percent as 

abundant as over water, the activity peak in forests was brief. 

lasting only about 15 minutes. Thus, this peak is unlikely to 

correspond with any significant feeding activity, but rather 

originated from bats commuting from day masts to feeding 

sites located outside the stands and over water. That feeding 

rates of the Myotis species were more than 10 times higher 

over ponds and streams in Washington and Oregon lends 

further support to this conclusion. 

Our data suggested that the activity of the My&is species 

and the big brown and silver-haired bats (Oregon) was 

skewed towards old-growth forests because they offer an 

increased variety or abundance of day roosts. Although the 

highest densities of snags occur in young and mature forests. 

these snags are typically dominated by small-diameter 

Douglas-fir with low longevity (Cline and others 1980). 

The greatest species diversity, largest diameters, the most 

advanced decay-classes, and the greatest snag longevity all 

occur in old-growth compared with younger forests. These 

features likely provide a greater abundance and diversity of 

bat roosts in old growth than in younger forests. 

Although bat activity was greater in old-growth stands in 

both Washington and Oregon, important differences were 

found between the two regions. Detection rates were significantly lower in the southern Washington Cascades for all 

species except the long-legged myotis, and no evidence of reproduction was found in this region. In the western Washington Cascades, few bats were female (11 percent), none 

showed any evidence of reproductive activity, and juveniles 

were only captured after the dissolution of maternity colonies 

in late August (Humphrey and Cope 1976). This contrasted 

sharply with the Oregon Coast Range and the eastern Washington Cascades, where pregnant and lactating females were 

abundant and juveniles constituted a higher proportion of the 

captures during the late maternity period. These data suggest 

that reproductive females were absent from the 30% to 600-m 

elevation zone on the western slope. of the Cascades, but that 

they were present at similar elevations to the east and south. 

Similar skews towards nonreproductive females and males at 

high elevations and towards reproductive females at lower 

elevations have been reported in the Cascades and the Rocky 

Mountains (Fenton and others 1980, Perkins 1983). 

301 
Table 4-Vegetative variables that were significantly associated

with bat activity in the southern Washington Cascades and the

Oregon Coast Range

Variable

Washington

Broken tops

Frost scars

conks

Snag 3

Snag 4

Snag DC1

Snag Voll

Overall R

2

Oregon

Snag D.B.H.

Snag DC3

Snag Vol

Snag Vo14

Age

Elevation

Dens 50

overall R2

Myotis A”

0.15**b

-0.16**

0.13*

Ns

Ns

Ns

0.09*

0.06

0.20***

Ns

Ns

Ns

Ns

0.27***

Ns

0.12

Myotis B

a

Long-legged Silvermyotis haired bat

Ns

Ns

Ns

-0.26***

0.10**

0.14**

Ns

Ns

Ns

0.15*

0.20***

-0.09*

0.24***

Ns

Ns

0.09

Ns Ns

Ns Ns

Ns Ns

Ns Ns

Ns Ns

Ns Ns

Ns Ns

0.08 Ns

0.15*

Ns

Ns

Ns

Ns

0.20**

Ns

0.17

Ns

Ns

Ns

Ns

Ns

Ns

-0.21**

0.04

aMyotis A includes the little brown and Yuma myotis. Myotis B includes the

California, Keen’s, long-eared, and western small-footed myotis.

‘Values are the partial regression coefficients (rijk) and *, **, and *** indicate significance levels of 0.05, 0.01, and 0.001, respectively. Ns indicates

not significant.

Ranges have dramatically higher detection rates in old-growth

than in mature or young stands. Thus, this study indicates

that bats are 2.5 to 9.8 times more abundant in old-growth

than in young or mature stands in both study regions. Clearly,

old-growth stands provide important habitats for bats in the

Pacific Northwest.

Forest stands can provide bats two critical resources: food

and day roosts. The short duration of activity in early evening and the low feeding rates during this time suggest that

forest stands are not important feeding sites for bats. Early

evening peaks in activity over ponds and streams and in

adjacent riparian habitat have been described for several

species, including the California, long-eared, little brown,

Keen’s, long-legged, and Yuma myotis and big brown and

silver-haired bats (Anthony and Kunz 1977, Bell 1980, Cross

1976, Kunz 1973, O’Farrell and Bradley 1970). These early

activity peaks over water generally last 45 to 180 minutes,

much longer than the peaks observed in the forest stands

(about 15 minutes). The length of activity peaks over water

appears to be determined by the insect capture rate that bats

achieve and the amount of food they require. Pregnant and

lactating little brown myotis foraging over water capture

only 19 percent and 28 percent of their first meal in 20

minutes despite capture rates as high as five to eight insects

per minute (Anthony and Kunz 1977; Gould 1955,1959).

Similarly, wild European pipistrelles and captive western

small-footed myotis achieve high capture rates when feeding

on insect swarms (10 insects per minute), but have feeding

periods lasting > 45 minutes (Griffin and others 1960, Racey

and Swift 1985). Because feeding rates decline with

decreasing insect abundance (Racey and Swift 1985),

habitats with lower insect abundance than aquatic sites have

should show prolonged activity peaks, which was clearly not

true in forest stands. Although insects in the size range taken

by Myotis species (<l0 mm; Anthony and Kunz 1977,

Belwood and Fenton 1976) were only 1 to 10 percent as

abundant as over water, the activity peak in forests was brief,

lasting only about 15 minutes. Thus, this peak is unlikely to

correspond with any significant feeding activity, but rather

originated from bats commuting from day roosts to feeding

sites located outside the stands and over water. That feeding

rates of the Myotis species were more than 10 times higher

over ponds and streams in Washington and Oregon lends

further support to this conclusion.

Our data suggested that the activity of the Myotis species

and the big brown and silver-haired bats (Oregon) was

skewed towards old-growth forests because they offer an

increased variety or abundance of day roosts. Although the

highest densities of snags occur in young and mature forests,

these snags are typically dominated by small-diameter

Douglas-fir with low longevity (Cline and others 1980).

The greatest species diversity, largest diameters, the most

advanced decay-classes, and the greatest snag longevity all

occur in old-growth compared with younger forests. These

features likely provide a greater abundance and diversity of

bat roosts in old growth than in younger forests.

Although bat activity was greater in old-growth stands in

both Washington and Oregon, important differences were

found between the two regions. Detection rates were significantly lower in the southern Washington Cascades for all

species except the long-legged myotis, and no evidence of reproduction was found in this region. In the western Washington Cascades, few bats were female (11 percent), none

showed any evidence of reproductive activity, and juveniles

were only captured after the dissolution of maternity colonies

in late August (Humphrey and Cope 1976). This contrasted

sharply with the Oregon Coast Range and the eastern Washington Cascades, where pregnant and lactating females were

abundant and juveniles constituted a higher proportion of the

captures during the late maternity period. These data suggest

that reproductive females were absent from the 300- to 600-m

elevation zone on the western slope of the Cascades, but that

they were present at similar elevations to the east and south.

Similar skews towards nonreproductive females and males at

high elevations and towards reproductive females at lower

elevations have been reported in the Cascades and the Rocky

Mountains (Fenton and others 1980, Perkins 1983).

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Differences in the incidence of reproductive females may be

related to climate. Forests in the western Washington Cascades receive higher rainfall, more days of rain, and are more

frequently shrouded by low clouds during the pregnancy and

lactation periods (April to July) than sites to the east, to the

south, and at lower elevations (NOAA 1960-85). These factors may cause more frequent interruptions in foraging activities and make timing of food availability less predictable.

Racey (1973) and Racey and Swift (1981) showed that pregnant females that could not forage daily were incapable of

sustaining the energy demands of homeothermy. Torpor in

females delayed parturition both in the laboratory and under

natural conditions (Racey 1973, Racey and Swift 1981). Low

clouds and frequent rain in the western Washington Cascades

may make timing of insect availability too patchy for pregnant or lactating females to remain homeothermic. Females

may not be able to complete the reproductive season and

prehibernation fattening in the limited time available during

summer. If nonreproductive females and males are able to

resort to torpor without a reproductive penalty, they would

be less sensitive to temporal patchiness in prey availability

and so could make use of the more adverse, high-rainfall

sites (Kurta and Kunz 1988).

Vegetative Relationships

The correlations of bat activity to vegetative features indicate

that the abundance of damaged or diseased trees or snag size

and decay states were associated with higher bat activity.

These variables explained such a small proportion of the total

variance in bat activity that they did not identify specific vegetation features that affect bat abundance. These weak correlations are probably a result of the study design; vegetative

characteristics were measured to describe differences in stand

types rather than to quantify roosting sites for bats. Until

more precise data on the roosting requirements of bats in the

Pacific Northwest become available, the only way to manage

for bat populations will be to retain undisturbed stands of oldgrowth forest. The minimum size of these stands can only be

determined by the size in which the full range of snag sizes

and decay-classes are represented. If the abundance and diversity of snags and diseased or damaged trees influences bat

abundance in stands, then several avenues for future research

are apparent. Managed and thinned stands where damaged

trees and snags have been removed should be essentially

devoid of bats and can provide a valuable contrast with firegenerated young and mature stands. Young stands produced

from old-growth stands damaged by canopy fires should

retain the snag characteristics of the original stand for about

41 years (Cline and others 1980), and they may have an

increased diversity of roost sites because these stands have

more standing dead trees. These stands should have bat

activity equivalent to or higher than intact old-growth stands.

In contrast, young stands produced by logging operations

that remove the large snags should have low bat activity.

Because most of the young stands studied here were not a

product of logging, this contrast was not tested. Coupled

with these possible contrasts based on ultrasonic detection,

locating and characterizing suitable roosts would seem

imperative.

This study shows that old-growth forests provide important

habitats to bats in both the Cascade and Coast Ranges. We

predict that forestry practices that remove old growth and

thus reduce the overall age structure of forests without

retaining snags and damaged trees will significantly affect

bat populations. The evidence that reproductive females are

less abundant on or absent from the western Washington

Cascades suggests that protection of old growth in the Coast

Ranges and mountains to the south should receive highest

priority.

Acknowledgments

This study was made possible by the field assistance of J.

Buchanan, A. Gillesburg, A. Humphrey, R. Lundquist, K.

March, and R. Sherry. Dick Hinshaw (Electronics Workshop,

University of Washington) gave unstintingly of his time and

knowledge in developing field equipment. This study was

funded by Cooperative Agreement PNW-84-234 of the

USDA Forest Service Pacific Northwest Research Station

and by operating grants from the Natural Sciences and

Engineering Research Council to D.W. Thomas. This is

contribution 67 of the Group de Recherche en Energie,

Nutrition, et Ecologic, Universite de Sherbrooke.

This paper is contribution 117 of the Wildlife Habitat Relationships in Western Washington and Oregon Research Project, Pacific Northwest Research Station, USDA Forest

Service. n

302
Appendix

Table 5-Vegetation, elevation, and topography variables used

in logistic regressions

Variable Description

Broken tops

Dead tops

AnScar

LiScar

Frost Scar

ExCav

NatCav

conks

Snag3a

Snag4

Snag5

Snag6

SnagDCl-5b

SnagD.B.H.

SnagMass

SnagVol

SnagVol1-5

Age

Topo

Elevation

Species

TotDens

D.B.H.

D.B.H.75

Dens50

Dens100

Dens150

Live trees with broken tops

Live trees with dead tops

Trees with animal-induced scars

Trees with lightning scars

Trees with frost scars

Trees or snags with excavated cavities

Trees or snags with natural cavities

Trees with conks of primary pathogens

Snags: height = 1.5-5.0 m; d.b.h. = l0-50 cm

Snags: height >15 m; d.b.h. = l0-50 cm

Snags: height = 5-15 m; d.b.h. >50 cm

Snags: height >15 m; d.b.h. >50 cm

Snags: decay classes 1 to 5

Snags: mean d.b.h.

Snags: total biomass

Snags: total volume

Snags: volumes in decay classes 1 to 5

Chronological age in years

Topography: crest; upper, middle, lower slope,

depression

Mean stand elevation

Total number of tree species

Total tree density

Mean tree d.b.h.

D.b.h. of the 75 percent quantile

Density of trees with d.b.h. = 50-99 cm

Density of trees with d.b.h. = 100-149 cm

Density of trees with d.b.h. >149 cm

a

Snag number designators conform with those used in vegetation studies

(this volume).

b

Snag decay-classes follow those in Clime and others (1980).

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