Effects of Prescribed Fire and Predator Exclusion on Refuge Selection by Peromyscus gossypinus Le Conte (Cotton Mouse)
Anna M. Derrick, L. Mike Conner, and Steven B. Castleberry
Southeastern Naturalist, Volume 9, Issue 4 (2010): 773–780
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2010 SOUTHEASTERN NATURALIST 9(4):773–780
Effects of Prescribed Fire and Predator Exclusion on
Refuge Selection by Peromyscus gossypinus Le Conte
Anna M. Derrick1, L. Mike Conner2, and Steven B. Castleberry1,*
Abstract - Many small mammal species experience population declines following
prescribed fire, presumably resulting from increased predation due to lack of cover.
However, Peromyscus gossypinus (Cotton Mouse) typically shows a neutral or positive
population response following fire. Because they typically spend diurnal hours in
below-ground refuges, Cotton Mice may be less susceptible to predation following fire
than other small mammals. We examined the effects of prescribed fire and exclusion
of mammalian predators on selection of daytime refuges by Cotton Mice. We located
daytime refuges of 12 radiotagged Cotton Mice in a fenced mesomammal-predator
(hereafter, mesopredator) exclosure (23 refuge locations) and 9 Cotton Mice in an adjacent
unfenced control plot (13 refuge locations) for one month prior to and one month
after a prescribed fire in winter 2007. Refuge locations included Gopherus polyphemus
(Gopher Tortoise) burrows (27.8%), other ground holes (44.4%), stump holes (25.0%),
and holes at the base of trees (2.8%). Fire had little effect on refuge selection, likely because
Cotton Mice primarily used below-ground refuges, which allowed them to avoid
the direct effects of fire and predation following fire. Structure near the refuge, including
burrows, stumps, and coarse woody debris, was important in selection of daytime
refuges and was particularly important in the presence of mesopredators.
Prescribed fire is critical to conservation and management of many North
American ecosystems, including the Pinus palustris Miller (Longleaf Pine)
ecosystem of the southeastern United States (Stoddard 1931, Wahlenberg
1946). Because prescribed fires typically result in a temporary reduction of
cover, small mammals may be more susceptible to predation immediately
following a prescribed fire event, and this mortality may result in temporary
population declines (Bock and Bock 1978, Komarek 1963, Tester 1965,
Whelan 1995). However, Peromyscus gossypinus Le Conte (Cotton Mouse)
has shown neutral or positive immediate population responses to prescribed
fire (Hatchell 1964, Jones 1992, Layne 1974, Shadowen 1963).
Most studies examining the effects of prescribed fire have concluded
that Cotton Mouse population increases are a result of individuals from
surrounding unburned areas immediately immigrating into burned areas.
Hatchell (1964) observed that Cotton Mice invaded recently burned areas in
Louisiana pine stands, noting that the reduction in understory cover following
fire had no apparent negative effect on population abundance. Similarly,
1Warnell School of Forestry and Natural Resources, University of Georgia, Athens,
GA 30602. 2Joseph W. Jones Ecological Research Center, Route 2, Box 2324, Newton,
GA 39870. *Corresponding author - email@example.com.
774 Southeastern Naturalist Vol. 9, No. 4
Layne (1974) attributed a Cotton Mouse population increase in burned
flatwoods habitat in Florida relative to pre-burn levels to immigration and
suggested that individuals from surrounding habitats invaded burned areas
to take advantage of increased food availability in the form of seeds and
insects. Cotton Mice may be adapted to a wider range of understory cover
conditions than other small mammals, such as Sigmodon hispidus Say and
Ord (Hispid Cotton Rat), which is negatively impacted by loss of cover following
fire (Arata 1959, Layne 1974, Stoddard 1931).
Cotton Mice commonly use below-ground refuges, such as stump holes,
root boles, and Gopherus polyphemus Daudin (Gopher Tortoise) burrows
(Frank and Layne 1992, Hinkelman and Loeb 2007, Layne and Ehrhart
1970, McCay 2000). Below-ground refuges provide lower temperatures
during summer and escape cover from fire and predators (Frank and Layne
1992). Although refuges provide protection from predators, the type of
refuge selected can influence predation susceptibility. For example, Frank
and Layne (1992) found higher predation on Cotton Mice using Gopher
Tortoise burrows than other refuge types. Because refuges provide Cotton
Mice a place to seek shelter from fire, in addition to protection from predators,
the influence of fire and predators likely interact in selection of refuge
type. Therefore, our objective was to examine the effects of prescribed fire
and mesomammal-predator (hereafter mesopredator) exclusion on Cotton
Mouse selection of daytime refuges. We hypothesized that daytime refuges
would be surrounded by more vegetative cover and coarse woody debris in
areas where mammal predators were present and after fire.
Our study site was located at the Joseph W. Jones Ecological Research
Center at Ichauway, an 11,700-ha research center located in Baker County,
GA. The Jones Center is located on the Upper Coastal Plain and is dominated
by the Longleaf Pine-Aristida stricta Michaux (Wiregrass) forest type. The
Jones Center consists of a traditional management zone that is managed for
sustainable, multiple-use practices such as Colinus virginianus L. (Northern
Bobwhite) hunting, and a benchmark management zone focusing on conservation
of natural biota and restoration of pre-settlement land-use patterns.
Prescribed fire, primarily dormant season, is utilized throughout the property
on two-year intervals. Our research plots were located on a 2897-ha area of
continuous benchmark management zone.
As part of a larger study examining mesopredator exclusion, 4 mesopredator
exclosures and 4 control research plots were established in 2002.
We chose one exclosure and one control for our study based on vegetation
similarity, Cotton Mouse abundance, and their close proximity to each other.
The 36.2-ha exclosure plot was enclosed with woven-wire fencing with wire
attached to an E2000 electrical fence charger (Twin Mountain Fence Co., San
Angelo, TX) along the top, middle, and bottom to deter mammal predators
2010 A.M. Derrick, L.M. Conner, and S.B. Castleberry 775
from climbing or digging under the fence. The adjacent 36.0-ha control
plot was unfenced. Fences were monitored twice weekly for dig-unders and
electrical failure. Mesopredator monitoring was conducted seasonally in all
exclosures and controls using track counts and thermal camera surveys between
13 July 2004 and 21 March 2007. Mammal exclusion was not total, as
Lynx rufus Schreber (Bobcat), Mephitis mephitis Schreber (Striped Skunk),
and Didelphis virginiana Kerr (Virginia Opossum) were known to breach the
fence. However, we only recorded 4 mesopredator detections in all exclosures
versus 44 in controls during our surveys.
We captured Cotton Mice on trapping grids established in the exclosure
and control, and at trap locations in study plots not restricted to grids
from 16 January–15 March 2007. Sherman live traps (7.6 cm x 8.9 cm x
30.5 cm) baited with a mixture of oats and birdseed were placed at each
trap location. Cotton Mice >22 g were selected for radio-transmitter attachment.
Mice were anesthetized with isoflourane (McColl and Boonstra
1999), and 1.1-g transmitters (Advanced Telemetry Systems, Isanti, MN)
were attached to the neck using monofilament. Four males and 5 females
were tagged in the control, and 8 males and 4 females were tagged in the
exclosure. Animal capture and handling followed recommendations of
the American Society of Mammalogists (Gannon et al. 2007) and were
approved by the University of Georgia Animal Care and Use Committee
(approval no. A2006-10207-c1).
We tracked mice for one month before and one month after prescribed
fires conducted in the exclosure and control on 15 February 2007. The
goals of the prescribed fire were consistent with the overall goals of the fire
program at Ichauway, which are fuel reduction, woody vegetation control,
perpetuation of fire-dependent species, and wildlife habitat management.
Although we did not quantify vegetation cover prior to the fire, cover was
considerably lower during the one-month tracking period after the fire than
before the fire. We attempted to obtain 1 daytime refuge location/week for
each radiotagged mouse. Because some mice died or slipped collars, the
same mice were not tracked for the entire study period. Refuges for some
individual mice were located only during the pre- (1 exclosure, 5 control) or
post-fire (5 exclosure, 3 control) periods, whereas locations for others were
obtained during both periods (6 exclosure, 1 control). Although fences were
permeable to Cotton Mice, we did not document mice crossing the fence
during our study. Refuge locations were recorded with a GPS Pathfinder Pro
XRS Receiver (Trimble Navigation Limited, Sunnyvale, CA).
At daytime refuge locations, we measured habitat characteristics known
to be biologically important to Cotton Mice (Frank and Layne 1992, Loeb
1999, McCay 2000, Mengak and Guynn 2003, Wolfe and Linzey 1977).
We recorded refuge type (Gopher Tortoise burrow, stump hole, hole in tree,
or other ground hole), measured distance to nearest coarse woody debris
(CWD), and estimated percent ground cover within a 4-m radius of the
refuge. We assessed availability of structure near the refuge by recording
presence/absence of ≥1 Gopher Tortoise burrow, stump, or log within 4 m of
the refuge. Each daytime refuge was paired with a 4-m radius circular plot
776 Southeastern Naturalist Vol. 9, No. 4
randomly located 10–100 m from the refuge. Data collection at random sites
followed protocols for refuge sites.
Sexes were combined for analysis. We developed 12 predictive models
using logistic regression with refuge or random site as the binary response.
Habitat variables and their interactions with fire and mesopredator exclusion
treatments were included as explanatory variables (Table 1). Observations
were categorized as either before or after the prescribed fire and within either
the predator exclosure or control. Because we were interested in how predator
exclusion and fire affected refuge selection, fire and predator exclusion effects
were only included in interaction terms in models and not as main effects. We
evaluated models with Akaike’s Information Criterion adjusted for small sample
sizes (AICc; Burnham and Anderson 2002). We calculated AICc, ΔAICc,
and Akaike weights (ωi) for each model to identify models best supported by
our data. We considered the best model(s) as those with ΔAICc < 2. All models
with 2 < ΔAICc < 4 were considered to have marginal support. We calculated
sum of model weights (Σωi) for each variable to determine their importance in
daytime refuge selection (Burnham and Anderson 2002).
We located and characterized 36 daytime refuge locations (23 exclosure,
13 control) from 12 and 9 Cotton Mice in the exclosure and control, respectively.
Seventeen refuge locations (9 exclosure, 8 control) were obtained
pre-fire and 19 (14 exclosure, 5 control) were obtained post-fire. In the exclosure,
8 (35%) refuges were in Gopher Tortoise burrows, 13 (57%) were in
other ground holes, 1 (4%) was in a stump hole, and 1 (4%) was in a hole at the
base of a tree. In the control, 8 (62%) refuges were in stump holes, 3 (23%)
were in other ground holes, and 2 (15%) were in Gopher Tortoise burrows.
The models Structure and Structure*Exclusion received the strongest
support from our data (ΔAICc < 2; Table 2) with greater use of structure at
Table 1. Definitions of candidate models used to compare characteristics of daytime Cotton
Mouse refuges with random locations at the Jones Ecological Research Center, Baker County,
Cover Percent vegetation cover within 4-m radius of refuge
CWD Distance from refuge to nearest coarse woody debris
Structure Presence/absence of a Gopher Tortoise burrow, stump, or fallen
log within 4 m of refuge
Cover*Exclusion Interaction between Cover and predator-exclusion treatment
Cover*Fire Interaction between Cover and fire treatment
CWD*Exclusion Interaction between CWD and predator-exclusion treatment
CWD*Fire Interaction between CWD and fire treatment
Structure*Exclusion Interaction between Structure and predator-exclusion treatment
Structure*Fire Interaction between Structure and fire treatment
Main effects model Model including Cover, CWD, and Structure (no interactions)
Global model Model including Cover, CWD, and Structure and their interactions
with fire and exclusion treatments
Null model Model containing no variables
2010 A.M. Derrick, L.M. Conner, and S.B. Castleberry 777
refuges in the control plot than exclosure. Structure*Fire, all main effects,
and CWD*Exclusion were marginally supported by the data (2 > ΔAICc
> 4). Use of structure was marginally greater following fire, and use of CWD
was marginally greater at refuges in the control plot than exclosure. Sum of
model weights that contained the variable Structure was 0.909. There was
less support for distance to nearest CWD as an important characteristic of selected
refuges (Σωi = 0.163). Vegetation cover received virtually no support
from our data. Sum of model weights for models that included vegetation
cover was 0.088, and ΔAICc for all 3 models that examined cover without
structure or CWD was >4. Models with predator-exclusion treatment interactions
(Σωi = 0.313) had greater support from the data than fire treatment
interactions (Σωi = 0.154). Evidence for a predator-exclusion effect was supported
when interacting with Structure (Structure*Exclusion) and distance
to the nearest CWD (CWD*Exclusion), whereas evidence for the fire treatment
was only supported when interacting with structure (Structure*Fire).
We found that presence of structure within 4 m of the refuge, in the form
of Gopher Tortoise burrows, stumps, or CWD, was the primary criterion in
refuge selection by Cotton Mice. Other studies have noted a similar positive
relationship between Cotton Mice and the presence of structure (Hinkelman
and Loeb 2007, Loeb 1999, McCay 2000). Additionally, strong support for
our Structure*Exclusion model suggests that structure may be especially
important in refuge selection in the presence of mesopredators. Mesopredators
had been excluded from exclosure plots for approximately 5 years prior
to our study, which was likely sufficient time for Cotton Mice to exhibit a
behavioral response to mesopredator absence.
Although we did not examine avian and reptilian predators in our study,
absence of mesopredators may cause compensatory shifts among predator
taxa (Derrick et al., in press). Because of differences in feeding strategies,
Table 2. Models, number of parameters in the model (K), Akaike’s information criterion adjusted
for small sample size (AICc), AICc difference between a model and the model with the
lowest AICc (ΔAICc), and weights (ωi) of models used to examine characteristics of Cotton
Mouse refuges at the Jones Ecological Research Center, Baker County, GA, 2007.
Model K AICc ΔAICc ωi
Structure 2 92.65 0.00 0.43
Structure*Exclusion 3 93.78 1.14 0.25
Structure*Fire 3 94.78 2.14 0.15
Main Effects 4 95.97 3.32 0.08
CWD*Exclusion 3 96.39 3.72 0.07
CWD 2 100.19 7.55 0.01
Null 1 101.87 9.23 0.00
Cover 2 102.23 9.59 0.00
CWD*Fire 3 102.25 9.61 0.00
Cover*Exclusion 3 104.35 11.70 0.00
Cover*Fire 3 104.38 11.74 0.00
Global 10 109.11 16.47 0.00
778 Southeastern Naturalist Vol. 9, No. 4
selection pressures exerted by avian and reptilian predators in the absence
of mesopredators may result in differences in refuge selection. For example,
we found less use of Gopher Tortoise burrows as refuges in the control (15%)
relative to the exclosure (35%). Cotton Mice using Gopher Tortoise burrows
as refuges may be more susceptible to mammalian predation than when using
other structure types (Frank and Layne 1992). Cotton Mice appeared to
shift refuge selection in the absence of mesopredators, suggesting an adaptive
advantage of using Gopher Tortoise burrows when birds and reptiles are the
Similar to other studies of Cotton Mouse use of daytime refuges, we
documented almost all refuges in below-ground structures. Frank and Layne
(1992) found that 94.7% of refuges in Florida were found in below-ground
structures, with 70.5% in Gopher Tortoise burrows. Although only 27.8% of
refuges observed in our study were associated with Gopher Tortoise burrow
openings, many of the other holes used as refuges were small tunnels that
likely connected to burrows (chimneys). In contrast, on a South Carolina
study site where the Gopher Tortoise was absent, stump holes created by
rotting stumps and root boles were the most important refuge locations (Hinkelman
and Loeb 2007, McCay 2000). Frank and Layne (1992) suggested
that selection of refuge type by Cotton Mice likely reflects availability of
structures. Gopher Tortoise burrows and associated holes were commonly
available structures for small mammal refuges in the Longleaf Pine forests
on our study site. Jones and Franz (1990) suggested that Gopher Tortoise
burrows provide thermal refuge and escape from fire for Podomys floridana
Chapman (Florida Mouse). Because temperature decreases at a constant
rate along the length of the burrow, they suggested that mice avoid surface
temperature extremes and select a preferred temperature within the burrow.
Although Cotton Mice appear to have a close association with Gopher
Tortoise burrows in sympatric areas, they exhibit considerable plasticity in
refuge selection across their range (Hinkelman and Loeb 2007).
Many small mammals use structures, such as logs, as travel routes presumably
because they provide a quieter substrate than leaf litter (Barnum et
al. 1992, Graves et al. 1988, Planz and Kirkland 1992). Our data marginally
supported the CWD*Exclusion model, providing limited evidence that Cotton
Mice select daytime refuges in close proximity to CWD when exposed to
mesopredators. Mice may select daytime refuges in habitat with more CWD
which allows them to travel more quietly and provides more cover. McCay
(2000) found that Cotton Mice in Coastal Plain pine stands were associated
with CWD, but use of CWD for travel was less than reported for Peromyscus
species in other habitats. He speculated that Cotton Mice in pine stands use
CWD less for travel because pine litter is quieter than deciduous leaf litter
and less likely to attract predators. Lower use of CWD in pine stands relative
to other habitat types may explain why the CWD*Exclusion model was only
marginally supported by our data.
Studies on other Peromyscus species have shown drastic population declines
following fire events (Tester 1965, Tevis 1956). Population declines
in these studies appeared to have occurred from a combination of direct
2010 A.M. Derrick, L.M. Conner, and S.B. Castleberry 779
fire-induced mortality, increased predation following fire, and emigration.
In our study, we observed no direct mortality from fire, and no radiotagged
mice were depredated while under study. Additionally, no radiotagged mice
emigrated to unburned areas during or after the fire. Cotton Mice in our study
used below-ground refuges, which allowed them to avoid the direct effects
of fire and predation following fire despite the reduction in vegetation cover.
Cotton Mice in the Longleaf Pine ecosystem evolved with frequent fires and
likely have evolved behavioral mechanisms (e.g., selecting below-ground
refuges) to avoid the temporary population reduction following fire reported
for other Peromyscus species (Tester 1965, Tevis 1956).
None of the 3 models that examined vegetation cover in the absence of
structure were supported by our data, suggesting that vegetation cover has
minimal importance in daytime refuge selection by Cotton Mice. Similarly,
Hinkelman and Loeb (2007) found that refuges with less vegetation cover
were used more frequently than expected. They suggested that Cotton Mice
may avoid areas of high vegetative cover surrounding the refuge because
densely vegetated areas provide ambush sites for predators. The same selection
criterion likely is exhibited by mice on our study area. We additionally
suggest that pine-litter substrate, which makes less noise and is less likely
to attract predators than deciduous leaf litter (McCay 2000), reduces dependence
on vegetation cover for concealment from predators.
We thank J. Rutledge, G. Morris, and E. Wilichowski for field assistance. We
thank G. Barrett for an earlier review of this manuscript. Funding was provided by
the Joseph W. Jones Ecological Research Center and Warnell School of Forestry and
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