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parative
Several studies that compared various types sf pitfalls and live-traps
(also called box- or cage-traps) in the
field (Chclkowska 1967, &x>nstra
and Krebs 1978, Peterson 1980, Boonstra and Rodd 1984, Mengak and
Guy nn 1987) found the sampling efficiency of the two methods varied
considerably (Andrzejewski and
Rajska 1972, Briese and Smith 1974,
Cockburn eh a!. 1979, Williams and
Braun 1983). Pitfall cone traps were
more effectfve than live-traps in samling srnall mammals, particularly
shrews in sou them Finland (Pankakoski '1979). In contrast, pitfalls were
less effective than live-traps ira capturing small-bodied mice in
Durango, Mexico, although more
shrews (Nofiosorex crawfardr'! were
taken in pitfalls (Peterson 1976). Fitmaterials, shapes, and
without drift fences,
'Paper presented at symposiurrs, Managomsnt of Amphibians, Reptiles, and
Snmll Mammals h North America. (Flagstaft AZ,July 19-2?, 1988.)
'Robert C. Smro is Research WIdIife &ologist, USDA Forest Service, Rocky Momtain Forest m d Range Experim~ent
Station,
Arizona State hiversify Campus, Tempe,A.Z
85287- 1XM.
Yee H. Sirr~ons,formerly a yrcduufe sfudent, A~izonaState Ciniversiiy, Department
ofZoology, Tempe, Arizona is currently a
graduate student, Graduate Group in Ecoiogy, University of Califmnia, Davis, CA
956 16.
4 , C C ~ f tC. BelfiP is WiIdlife Bidogist, Department of the Army, Wildbfe Management. Section, Fwf. Huachuca,Ak $55 13bO00. Belfif's eurrent address is P. 0.Box 336,
Fort Belvok VA 22060$5336.
Abstract.-The effectiveness of pitfalls and livetram for assessina small mammal communitv
structure was cokpared in burned and undurned
upland Sonoran Desert-and in an elevational series
of Sycamore riparian and adjacent habitats in
Ar~sna.Although, live-traps were more effective in
recapturing previously captured small mammals
and usually resulted in more total captures of new
individuals, neither method gave a complete
assessment of small mammal community structure.
have been assed for capturing small
mammals (Howard and Brock 1961,
Andrzejewski and Wrwlawek 1963,
Pucck 1969, Bcronstra and Krebs
1978, Pankakoski 1979). This lack of
standardization makes it difficult to
assess the relative effectiveness of
pitfalls versus live-traps in sampling
small mammals by comparing data
between studies. Conflicting results
from these studies argue for more
comparisons using controls for as
many extraneous factors as possible.
Small mammals respond dramatically to many environmental factors,
thus csndrpunding attempts to assess
species or community relationships.
SarnpIi~gbiases caused by climate
and differences in activity and locomotor adaptations of various species
further compound this problem. Still,
trapping remains the most practical
method for assessing small mammal
populations (Williams and Braun
3983). Because responses to trapping
methods may differ, even within the
same species (Andrzejewski and
Rajska 1972),diverse sampling
schemes might reveal population
dynamics and community structure
more completely than any single
method (Weia-terand Smith 1972,
Bmnstsa and Kaebs 1978).
We compared the effectiveness of
live-traps versus pitfalls in riparian
and desert hahi tats in Arizona to answer the fdlowing questions: (1)
Dms samp'ling method influence estimates of species composition and
abundance? (2) Are various species
captured or recaptured differentially? (3) Are individuals within a
species captured differentially? (4)
Does habitat structure influence the
effectiveness of these methods?
Sfudy Areas and Meth
Riparian and Adjacent
Communities
The riparian and adjacent communities (referred to in general as the riparian area) were located at Garden
Canyon, Fort Huachuca Military Reservation, Arizona; elevations ranged
from 1500 to 1630 m. Riparian communities sampled, from lowest to
highest elevation, were sycamore
(Platanus wigh tii), sycamore/ juniper
(Juniperus monospermd, and sycamore/junipcr (J. deppmna)/oak
(Quercus arizonica, Q. ernoryi, and Q.
hypoluecoides) (Szaro 1988).Plant
cornunities sampled adjacent to the
riparian corridor, from lowest to
highest elevation, were composite
(Heferofhec~spp.)/grassland (Poa
spp.), juniper (J. nronosperma) woodland, and oak (Quercus emoyf) woodland.
Six trap stations were set in each
of six habitats: composite/grassland,
sycamore riparian, juniper woodland, sycamore/ juniper riparian, oak
woodland, and sycarnore/juniper/
oak riparian forest (figs. 1-6) (36 stations in all). Trap stations consisted
of two unbaited pitfalls (18.9 L or 5
Figure 1 .-Arkma tyeamr
Canyon, Fort Huachuca M1
i
1500 rn.
Figure 3.-Adzsna s y ~ o w r e(PIafant;~
v~~gh%~i~,~aIIjgaB~r
%miper(L
x e d acak ~ Q U ~ P C S~P~'ZOG.~CCY,
S
Q. Q ~ " : Qand
~ & 42. hytudy site, Garden Canyon, Fad .buuchuc=aMilittev
i z e % ;elevatio~
~;
sa. h6!0 m.
gal.; 29 cm in diameter by 36 cm
deep) with a 7.6-m-long by 20-cmhigh drift fence between buckets.
Covers were propped 2.5-5 cm above
openings mouths. Pitfalls were open
from 16 April through 28 May and
from 20 July through 5 September
1986 (6408 trap-nights) and were
checked three times each week. Sherman live-traps (8 by 9 by 23 cm)
baited with rolled oats were set
around each pitfall station in an 8trap pattern with at least 5 m between traps and pitfalls. Live-traps
were set from 12 to 16 May and from
17 to 21 August 1984 (2304 trapnights) and were checked each morning. Most live-trap captures were released after being car-tagged. Except
for some Notiosorex, all pitfall captures were co1lectc.d. Identification of
all mammals follows Hoffmeister
(1986). Thomomys species include
pure and hybrid T , urnbrinus and T.
bottae.
diameter by 40 cm deep) buried to
the rim with a cover propped 5-10
cm over the opening. Live-traps were
set and baited with rolled oats for
woconsecutive nights on 19 occasions between 10 June 1985 and 3
Desert Community
-%w ?.-Unburned desert study area, Tonto National Forest, Maricopa County, 30 km east
' ' ; nix, Adzona; elevation ranged from 450 to 550 m.
The desert study area was in the
Tonto National Forest, Maricopa
County, 30 km east of Phoenix, Arizona. The site was rocky desert dissected by sandy washes; elevations
ranged from 450 to 550 m. Vegetation
was typical of the Arizona upland
subdivision of the Sonoran Desert
biome (Brown 1982), with mesquite
(Prosopsis juliflora) along wash banks
and palo verde (Cercidium microphyllum), bursage (Ambrosia del toides),
and cholla (Oyuntia acanthocappa) on
slopes.
Two grids were established 90 m
apart, each with 100 sampling stations placed in a 10 by 10 pattern
with 10-m intervals between stations.
Grid 1 was in mature desert and grid
2 had 50% of vegetative cover
burned on 7 June 1985, immediately
before the start of trapping (figs. 7-81.
lnterspaced between live-traps (10 by
10 by 25 crn) on each grid, but no
closer than 10-rn intervals, were 20
single pitfalls (37.9 L or 10 gal., 34 cm
Figure &.-Burned desert study area, Tonto National Forest, MQ~ICO~CI
County, 30 krn east of
Phoenix, Arizona; elevation ranged from 450 to 550 m.
2 6
A u p s t 1986-weekly in spring and
early summer, biweekly from middle
to late summer, and monthly in fall
and winter (Simons 1986). Unbaited
pitfalls were always open during
live-trapping and often in between
when live-trapping occurred weekly
or biweekly (March-September).All
captures except for casualties were
marked and released. Each method
was matched with an approximately
equal sampling effort (about 3800
trap-nights per grid).
Results and Discussion
Species Composition and
Abundance
Live-traps and pitfalls provided different estimates of species composition and relative abundance at both
study areas. In the riparian area we
observed no consistent pattern between trapping method and number
of species captured (table 1). Livetraps caught more species in two
habitats, pitfalls: in three habitats,
and in the sycarncre/juniper/oak
both methods captured two species.
Neither method captured all species
in a given habitat except in oak
woodland where only two species
were encountered and pitfalls captured both. However, live-trapping
was significantly more successful
than pitfalls in number of new captures per tpap-night (chi-square! P (
0.05) in all habitats except juniper
woodland, where both methods
yielded equal numbers.
In the desert, live-traps caught
more species than pitfalls (table 2).
Moreover, significantly more new
captures and total captures (chisquare, % 5 0.05) occurred in Iivetraps than in bucket-traps in both
burned and unburned plots (tablie 2).
These results differfrom those of
Williams and Braun (1983) who reported that number of species and
total number of captures were
greater in pitfalls than in the cornbined catch of snap- and live-traps.
They recorded six species in pitfalls
and four in snap- and live-traps.
Their success with pitfalls was no
doubt increased because each trap
was one-third filled with water,
drowning all captures. Trapping success for voles (Clethrionomys glareolus) was also reported to be higher in
pitfalls versus livetraps but may
v x y with social level, age, and reproductive period (Andrzejewski
and Rajska 1972, Andrzejewski and
VJrwlawek 1963, Chelkowska 1967).
New individtlals represented only
31.5% and 26.2 % of total captures in
live-traps on the burned and on the
unburned plots, respectively. In conhast, 95.8% and 92.7% of all captures
in pitfdls on the burned and unburned areas, respectively, represent
new individuals. The lack of recap
tures in pitfalls is not explained by
differential mortality between methods 'because sampling with both
methods occurred simultaneously,
and most animals were marked and
released. These differences maybe at
least partially due to increased attractiveness of live-traps with bait
and with concentrated odors from
previous captures (Boonstra and
Krebs 1978, Daly and Behrends
1984).Our results show that pitfalls
provide very different estimates of
species composition and abundance
than live-traps. We therefore question basic assumptions of the popular
methods of population estimation
that assume either equal catchability
of all members in the population
(Jolly 1965) or nearly complete c a p
ture and enumeration of a population (Krebs 1966, Hilborn et al. 1976).
Differential Trapping Effectiveness
Between Species
In the riparian area, 80 of 81 shrews
(Notiosorex crawfordi and Sorex arizom e ) and all gophers (Thomomys spp.)
were captured in pitfalls. In contrast,
only 5 of 67 captures of Peromyscus (3
species) were in pitfalls (table 1).
Peromyscus spp. were also recaptured
most frequently (57 of 64 recaptures).
Similar results were found in the Sierra Nevada where species such as
shrews (Sorex trowbn'dgii and S. monticolus) and gophers (Thomomys bottae), which tend to travel in burrows
or runways or along obstacles, were
usually captured in pitfalls (Williams
and Braun 1983). Williams and Braun
(1983) reported in their first test that
pitfalls were particularly poor for
capturing white-footed mice (Peromyscus). In a subsequent test they
implied these mice might be taken in
pitfalls after losing their caution for
strange objects. This did not happen
in our study because very few Peromyscus wfre captured in pitfalls over
an extended period even though livetrapping showed them to be common. More likely Peromyscus may
easily escape pitfalls by jumping out,
but more are recorded after drowning in water-filled pitfalls (Williams
and Braun 1983),especially when
other traps, such as snap- or livetraps, are missing.
In the desert habitat, a single
shrew (Notiosorex crawfordi) was
caught in a pitfall whereas two
species (Dipodomys rnerriami and
Peromyscus ermicus) were caught
only in live-traps. Only 1 of 181 captures (50 different individuals) of
Neotoma albigula was in a pitfall
whereas only 1 of 9 Onychomys torridus was not captured in a pitfall.
Onychomys was probably unable to
jump out of the buckets used in this
habitat. Noted accumulations of
Neotoma feces overnight in many pitfalls indicated these rodents had
been present but left. Apparently
larger species either avoid pitfalls or
simply jump out of them (Cockburn
et al. 1979, Williams and Braun 1983).
Differential Trapping Effectiveness
Within Species
Few significant differencesin
weights of small mammals caught
with the two methods were observed, but weights tended to be
lower in pitfalls. In the riparian area,
mean weights of Reithrodontomys fulvescens were significantly higher in
live-traps (14.3 + 0.65 (S.E.) g versus
5.1 + 0.56, t-test, P < 0.001, N = 12). In
the desert, weight differences between trap methods were not significant for animals less than about 20 g.
However, a significant difference occurred in the mean weight of Perognathus baileyi in live-traps (25.7 +
0.97 g) versus pitfalls (20.8 + 1.61 g; ttest, P = 0.014). Similarly, the mean
weight of Neotoma albigula caught in
live-traps was 109.0 + 8.93 g, whereas
the single capture in a
weighed 31.0 g.
Likewise in Canada and Poland,
voles (Microtus townsendii and Clethrionornys glareolus) captured in pitfalls were smaller than conspecifics
taken in live-traps (Andrzejewski
and Rajska 1972, Boonstra and Krebs
1978).This apparent relationship between size and susceptibility to pitfalls is likely related to jumping ability which tends to increase with age.
For some species, pregnant females
may be more susceptible to pitfalls.
Effects of Habitat on Trapping
Effectiveness
Trapping results for Onychornys torridus varied substantially between
vegetative communities. On the desert sites, 8 of 9 captures were in pitfalls whereas in composite/grass
habitat in Garden Canyon, 8 of 10
captures were in live-traps. Four captures were made with each method
in the juniper woodland. Differences
in trapability of Oncychornys may be
due to different depths of pitfalls in
desert (40 cm) versus riparian (36
cm) habitats.
Except for Perognathus spp., rodents were about equally susceptible
to pitfalls relative to live-traps in
both burned and unburned desert
habitats. Differences in total number
of individuals captured by both
methods in the desert areas may be
due to (1) difference in abundance of
species on burned and unburned
plots (Simons 1986); or (2) differences
in activity patterns related to the
drastic difference in shrub cover. Perognathus spp. typically prefer brush
or "cover" microhabitats (Price 1978)
and raised pitfall covers may have
attracted these mice more on the
burned area where natural cover was
scare than on the unburned area
where natural cover was dense (Simons 1986).Whatever the cause, the
results are similar to those found in
desertshrub and mesquite-grassland
habitats in Durango, Mexico, where
significantly more small-bodied
mammals were captured with livetraps than with pitfalls (5.4 L tin can
pitfalls with a depth of 25.4 cm) (Peterson 1980).Possibly a greater number of captures (i.e., sample size) may
be needed to fully reveal the impact
of habitat on trapping methodology.
Conclusions
Neither method alone was able to
fully assess small mammal communities in the desert-scrub and riparian
communities we investigated. We
recommend the use of both methods,
particularly when it is important to
include species such as shrews that
are not easily caught in live-traps in
investigations of small mammal com-
munity structure and habitat relationships.
Acknowledgments
We thank T. J. O'Shea, M. G. Ryan,
D. W. Uresk, and D. F. Williams for
their critical reviews of this manuscript. C. Munns helped check pitfalls at Garden Canyon. The Department of Zoology, Arizona State University provided support for L. Simons.
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