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22001188 SOUTHEASTERN NATURALIST 1V7o(2l.) :1372,7 N–3o4. 42
Status of Eastern Woodrats in Isolated Remnant Populations
Following Genetic Augmentation and Habitat Disturbance
Aaron C. Gooley1,* and Eric M. Schauber1
Abstract - In Illinois, Neotoma floridana (Eastern Woodrat) experienced range reductions
and population bottlenecking over the past century. During the period 2004–2005, the isolated
remnant populations along the Mississippi bluffs in southwestern Illinois were genetically
augmented with 47 Eastern Woodrats from Arkansas and Missouri, resulting in 40%
admixture within the largest population. In 2009, a strong windstorm created canopy gaps
and woody debris throughout this area, potentially improving habitat for Eastern Woodrats.
We investigated the status of Eastern Woodrat populations in southwestern Illinois by livetrapping
remnant populations and conducting sign surveys from 2011 to 2015. We captured
263 Eastern Woodrats; mean trapping success was 62.5% higher than trapping during the
1990s, and the number of individuals captured per trap-night was 3–6 times higher than
trapping events during the previous 18 years (all P less than 0.001). We also located sign of Eastern
Woodrat 8.9 km east of the remnant populations. We recommend further genetic monitoring
to evaluate whether population increases are coupled with increased admixture and
recommend forest-management actions that create habitat disturbance and resultant piles of
woody debris that increase woodrat habitat quality.
Introduction
Isolated populations at the periphery of a species’ range are often more prone
to population fluctuations (Williams et al. 2003), local extirpations, inbreeding,
and loss of genetic diversity than populations at the center of the species’ range
(Peterman et al. 2013, Westemeier et al. 1998). However, these populations can
also represent critical refugia with great importance to conservation and the species’
long-term survival (Channell and Lomolino 2000, Fraser 2000, Lomolino and
Channell 1995). Neotoma floridana illinoensis Howell (Illinois Woodrat), a subspecies
of N. floridana (Ord) (Eastern Woodrat), was once common in southern Illinois
(Howell 1910, Nawrot and Klimstra 1976), where the subspecies reaches the northern
extent of its range (Hall 1981) and is isolated by the Mississippi and Ohio rivers.
However, by 1950, populations in Illinois only persisted along the Mississippi
Bluffs in the southwestern margin of the state (Nawrot and Klimstra 1976). Periodic
trapping and investigation of these remnant populations indicated that the number
of Eastern Woodrats declined through the 1960s (Crim 1961, Krull and Bryant
1972) and 1970s (Nawrot and Klimstra 1976) to a bottleneck of approximately
15–35 individuals at Pine Hills (Union County) during the 1980s (Hoffmeister
1989, West 1986). The remnant population then increased during the early 1990s
to an estimated high of 101 in 1994 before decreasing to 60 in 1996 (Monty 1997;
1Cooperative Wildlife Research Laboratory, Department of Zoology, Southern Illinois University
Carbondale, IL 62901. *Corresponding author - acgooley@yahoo.com.
Manuscript Editor: Michael Cove
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Monty et al. 1995, 2003; Wagle 1996). During the 1990s, small satellite populations
were found at Horseshoe Bluff, Little Grand Canyon, and Fountain Bluff
(Jackson County; Monty 1997, Monty et al. 1995). As a possible consequence of
the population bottleneck during the 1980s, Eastern Woodrats in Illinois possessed
lower heterozygosity at microsatellite markers (6 loci) than Eastern Woodrats from
a more contiguous population in Missouri (Monty 1997, Monty et al. 2003).
As part of a state recovery plan for Eastern Woodrats (Bluett 2003), 59 individuals
were removed from remnant populations along the Mississippi Bluffs for
reintroduction to other unoccupied areas in Illinois. Forty-nine Eastern Woodrats
were removed from Pine Hills and 10 from Fountain Bluff between 2003 and
2004 (Feldhamer et al. 2007). During June 2004 and February 2005, biologists
translocated 47 Eastern Woodrats from Arkansas and Missouri to the remnant
populations—41 were released across the known range of Eastern Woodrat at Pine
Hills and 6 were released at Fountain Bluff (Feldhamer et al. 2007, Novosak 2004;
Fig. 1). The goals of the translocation to the Mississippi Bluffs were to genetically
augment those populations and facilitate population growth (Bluett 2003). After the
translocation, microsatellite heterozygosity at 12 polymorphic microsatellite loci
(Nfu01, Nfu03, Nfu05, Nlep08, Nma01, Nma02, Nma06, Nma08, Nma10, Nma11,
Nma14, Nma15) in 41 Eastern Woodrats captured at Pine Hills between 2005 and
2009 were compared to the 49 removed from Pine Hills. The augmentation resulted
in a 12% increase in average microsatellite heterozygosity, as well as representation
of Arkansas and Missouri genes in 40% of Eastern Woodrats in the Pine Hills
population (A. Poole, Southern Illinois University, Carbondale, IL, unpubl. data).
However, following the augmentation, the Pine Hills population appeared to decline:
trapping success in 2006–2007 was ~⅓ the trapping-success rate during the
1990s, and trapping was less productive in the northern areas of Pine Hills even
though stick nests identified in the 1990s were still present on the outcrops (Ing
2008). Continued population declines following the genetic augmentation could
indicate outbreeding depression where dilution of local adaptation or chromosomal
incompatibilities results in reduced fitness in outcrossed individuals or their offspring
(Allendorf et al. 2013).
Beginning with their initial description, Eastern Woodrats in Illinois have been
noted for building nests on bluffs instead of in woody structures (Howell 1910)
and nearly all captures over the past century have occurred on bluffs or other rocky
structures with sparse evidence of Eastern Woodrats inhabiting the adjacent bottomland
woods (Crim 1961, Hoffmeister 1989, Howell 1910, Krull and Bryant 1972,
Layne 1958, Monty 1997, Monty et al. 2003, Nawrot 1974, Nawrot and Klimstra
1976, Swayne 1949, Wagle 1996, West 1986). However, suitable shelter for nest
construction was the limiting resource for Eastern Woodrats in southern Illinois
(Crim 1961). The apparent restriction of Eastern Woodrats to the bluffs may have
been a result of limited hollow trees, stumps, and other coarse woody debris in the
landscape. On 8 May 2009, an unusually strong derecho (straight-line windstorm;
Coniglio et al. 2011) caused widespread windthrows in the Illinois Ozark Hills
and western Shawnee Hills, with some areas experiencing over 75% canopy loss
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(Romano et al. 2013). The US Forest Service further increased the availability of
coarse woody debris on the ground by piling logs and brush along roadsides during
the subsequent cleanup. Thus, the derecho altered forested habitat of the Eastern
Woodrat that had been largely undisturbed during the previous 70 y (Hutchinson
1987, van de Gevel and Ruffner 2007) by opening the canopy and creating an
abundance of woody debris for nest construction (Figs. 2, 3). This habitat change
may have removed the primary limiting factor for remnant populations of Eastern
Woodrat in the Illinois Ozark Hills and western Shawnee Hills. Our objective was
Figure 1. Capture
and release
locations
o f Neotoma
floridana (Eastern
Woodrat)
translocated to
Illinois during
2004 and 2005
for the purpose
of genetically
a u gme n t i n g
remnant populations.
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Figure 2. An example of the of canopy loss and windthrows at Pine Hills, IL, caused by the
2009 derecho. The understory release resulted in growth of dense tangles of woody vines.
Note the limestone outcrops in the background.
Figure 3. A Neotoma floridana (Eastern Woodrat) stick nest built into a windthrow in the
bottomlands at Pine Hills, IL. Note the vine tangles and saplings in the background and use
of sticks up to 2.5 cm in diameter for nest construction.
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to investigate the status of these populations of Eastern Woodrat in southwestern
Illinois following the genetic augmentation and widespread habitat disturbance.
Field-site Description
The Mississippi Bluffs lie along the western side of the Illinois Ozark Hills and
overlook the bottomland forest, wetlands, and agricultural fields of the Mississippi
Alluvial Plain. The bluffs of the Ozark Hills are composed of cherty limestone and
sandstone, whereas the outcrops of the Shawnee Hills to the east are composed
primarily of sandstone (Schwegman 1973). The Western Mesophytic Forest (Braun
1950) of these regions is rich in plant diversity (Mohlenbrock 1982), with a canopy
dominated by a variety of Quercus (oak) and Carya (hickory) species (Bazzaz
1968). The Pine Hills Bluffs, Horseshoe Bluff, and Little Grand Canyon are in a
north–south line along the east side of the Big Muddy River while Fountain Bluff is
isolated from other bluffs by bottomlands on the west side of the Big Muddy River.
The majority of our study area was situated within the Shawnee National Forest.
Methods
Trapping and analysis
We conducted 7 live-trapping sessions within the assumed range of the Pine
Hills population, each lasting 2–76 d, between November 2011 and August 2014
(Table 1). We generally set 30 Tomahawk live traps (16.5 cm x 16.5 cm x 48 cm)
per day along rock outcrops, woody-debris piles, and locations of sign (stick nests
or latrines), as well as in areas of dense brush or vines. During each session, we
left traps in place for 2 nights before moving them to new locations. An exception
to this pattern was during 30 September–2 October 2013, when we placed 30
traps 20 m apart on each side of a 280-m section of gravel road and trapped for
3 consecutive nights to investigate if additional Eastern Woodrats would be detected
on the 3rd night. This section of road was bordered on both sides by woodydebris
piles and dense vegetation. We also trapped at Fountain Bluff on 3–4 December
(12 traps) and Little Grand Canyon on 13–14 December 2012 (6 traps)
to confirm the continued presence of Eastern Woodrats at those sites. Previous
researchers typically recorded areas of activity (stick nests, latrines, piles of feeding
debris) along outcrops and placed traps at them. However, after the derecho,
areas of activity as well as other likely habitat (woody-debris piles, dense brush,
or vines) were not limited to the outcrops. Also, previously accessible sections of
outcrop were rendered inaccessible by blowdowns, dense vine tangles, and thick
vegetation. Consequently, unlike previous researchers, we were only able to identify
and place traps at a fraction of the areas of activity and were not able to trap
every area repeatedly.
We baited traps with Helianthus annuus L. (Sunflower) seeds, placed a ball of
polyester stuffing in the rear of each trap to provide bedding material, and covered
traps with natural debris to provide shelter. Traps were checked the morning after
deployment because Eastern Woodrats are primarily nocturnal (Wiley 1971). We
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Table 1. Results of trapping for the Neotoma floridana (Eastern Woodrat) remnant-population along the Mississippi Bluffs, IL, November 2011–August
2014. Trapping success = woodrat captures per trap night.
Individuals captured
# recaptured # recaptured
Trapping Adult Subadult Juvenile Unknown during from
Site Dates trapped (m/d/y) Trap-nights success ♂ ♀ ♂ ♀ ♂ ♀ trapping bout previous dates
Pine Hills 11/7/2011–1/21/2012A 261 0.368 31 38 5 10 0 2 1 9 n/a
8/6–17/2012 237 0.477 37 26 10 10 0 3 0 23 12
10/18–19/2012 24 0.708 4 12 0 0 0 1 0 n/a 11
1/25/2013–2/3/2013 127 0.638 25 28 1 5 0 0 0 22 19
9/30/2013–10/2/2013 90 0.756 14 14 0 4 0 0 0 36 7
2/17–18/2014 56 0.464 6 6 0 1 0 0 0 n/a 9
8/12–15/2014 54 0.444 9 10 3 1 0 1 0 0 5
Fountain Bluff 12/3–4/2012 12 0.5 2 2 0 1 1 0 0 n/a n/a
Little Grand Canyon 12/13–14/2012 8 0.25 1 0 0 1 0 0 0 n/a n/a
AComprised of 9 short bouts totaling 22 d.
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anesthetized captured woodrats with isoflurane, attached an individually numbered
#1005-1 Monel ear tag (National Brand and Tag Co., Newport, KY) to each ear
and recorded body mass and sex prior to release. We assigned age class based on
body mass (juvenile = <150 g, subadult = 150–200 g, and adult = >200 g; adapted
from Rainey 1956). We conducted chi-square tests to determine if our number of
previously uncaptured individuals captured per trap-night differed from trapping
conducted from 1993 to 2007 (Feldhamer et al. 2007), with α = 0.05 for each test.
We considered an increase in the number of previously uncaptured individuals
captured per trap-night indicative of an increase in abundance. A shift from the
female-biased sex ratio observed during the 1990s (Monty 1997) could indicate
that demographic processes have changed within the Pine Hills population. We
conducted chi-square tests to determine if sex ratios of 1st captures (i.e., unique
individuals) and recaptures differed from 1:1, with α = 0.05 for each test. We conducted
our research according to Southern Illinois University Animal Care and Use
Protocol # 11-003.
Distribution surveys
From 2011 through 2015, we documented the distribution of Eastern Woodrats
in the western Shawnee National Forest by visually surveying for recent Eastern
Woodrat sign (latrines, fresh scat, fresh food caches, or maintained stick-nests) at
rocky outcrops located on public land. We located rocky outcrops via aerial photos,
surficial geologic maps, and topographic maps, and surveyed them at increasing
distance from the remnant populations (Monty 1997, Monty et al. 1995, Wagle
1996) until we did not find Eastern Woodrat sign. We also investigated reports
of Eastern Woodrat captures or Eastern Woodrats inhabiting human structures in
southwestern Illinois.
Results
Trapping and analysis
We captured 255 individual Eastern Woodrats, including 200 adults (101 ♂, 99
♀), 47 subadults (18 ♂, 29 ♀), 7 juveniles (♀), and 1 of unkown age class, with
153 recaptures during 7 trapping bouts (848 trap-nights) at Pine Hills (Fig. 4). After
initial capture, we recaptured 1 juvenile female and 9 subadults (3 ♂, 6 ♀) as
adults. During the first trapping bout, 3 Eastern Woodrats escaped prior to tagging
and were not included in the overall number of individuals captured. Overall trapping
success was 0.480 woodrats captured per trap night, with trapping successes of
individual bouts varying from 0.368 to 0.708 (mean = 0.551, SE = 0.056; Table 1).
We captured 0.30 previously uncaptured individuals per trap-night, which was
significantly higher than all trapping efforts during the period 1993–2007 (all P less than
0.001) (Table 2). Sex ratios did not differ significantly from 1:1 for all individuals
(χ2 = 1.00, df = 1, P = 0.32), adults (χ2 = 0.02, df = 1, P = 0.89), subadults (χ2 = 2.57,
df = 1, P = 0.11), or recaptures (χ2 = 0.16, df = 1, P = 0.69). Our bout with 3 consecutive
nights of trapping (30 September–2 October 2014) yielded 32 individual
Eastern Woodrats (included in the above), with only 8 new individuals captured on
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Figure 4. Locations of Neotoma floridana (Eastern Woodrat) captures, reported (but unconfirmed)
captures, and sign in relation to the remnant populations along the Mississippi
Bluffs and a recently introduced population at Giant City State Park, IL. Elevation model
from Illinois Statewide 30-m shaded relief image (Illinois State Geological Survey, Champaign,
IL).
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the 2nd night and no new individuals captured on the 3rd night (Table 3). The mean
body-masses of adult males and females at final capture were 297 g (SE = 5.08,
max = 419 g) and 254 g (SE = 2.79, max = 332 g), respectively. We captured 5 male
Woodrats during January–February trapping bouts with body masses of 404–419
g (record masses for Woodrats captured in Illinois), whereas the maximum bodymass
of a captured female was 350 g. We captured 6 Woodrats at Fountain Bluff
and 2 Woodrats at Little Grand Canyon, confirming continued population persistence;
however, sign and captures at Little Grand Canyon were limited to a boulder
pile beside the Big Muddy River.
Distribution
We located fresh sign of Eastern Woodrat (latrines and stick nests) at outcrops up
to 3.5 km northeast of the Mississippi Bluffs at Pine Hills and 7.5 km east of Horseshoe
Bluff (Fig. 4). We verified reports of Woodrat sign in an old vehicle 7.9 km
east of Horseshoe Bluff and in 2 old vehicles 8.9 km east of the Mississippi Bluffs
at Pine Hills (Fig. 5). A landowner captured an adult male Woodrat in a shed 3.5 km
east–northeast of Little Grand Canyon and brought it to us for verification and tagging.
We also received reports of an Eastern Woodrat 7.7 km east of Pine Hills (tail
severed in a Sherman trap; W. Holland, Southern Illinois University Carbondale,
IL, pers. comm.) and of an Eastern Woodrat captured in a Sherman trap deployed
for Oryzomys palustris (Harlan) (Marsh Rice Rat) 29.6 km northeast of Little Grand
Canyon (J. van der Merwe, Southern Illinois University, Carbondale, IL, pers.
comm.). We searched both locations but were unable to locate any Eastern Woodrat
Table 3. Neotoma floridana (Eastern Woodrat) captures at 2 transects along LaRue Road at Larue-Pine
Hills, IL, 30 September–2 October 2014.
Day 1 Day 2 Day 3
Captures 11 ♂, 13 ♀ 12 ♂, 12 ♀ 8 ♂, 12 ♀
Additional individuals captured N/A 3 ♂, 5 ♀ 0 ♂, 0 ♀
Daily trap success 0.80 0.80 0.67
Table 2. Comparison of Neotoma floridana (Eastern Woodrat) live-trapping results at Pine Hills, IL.
1993-2007 data from Feldhamer et al. 2007. An asterisk (*) denotes results from our study.
Comparison of our
previously uncaptured
individuals captured per
Previously uncaptured trap night to prior trapping
Years Individuals individuals captured
monitored captured Trap-nights per trap night χ2 df P
1993–1996 283 4840 0.06 480.138 1 less than 0.0001
2002–2003 29 422 0.07 85.819 1 less than 0.0001
2005 7 70 0.10 12.686 1 0.0004
2006–2007 22 410 0.05 101.087 1 less than 0.0001
2011–2014* 255* 849* 0.30*
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sign or favorable locations to set traps. We also received a report of captures of
Eastern Woodrats in live-traps set for Rattus norvegicus Berkenhout (Norway Rat)
at a Union County State Fish and Wildlife Area grain silo located 10.2 km southeast
Figure 5. Neotoma floridana (Eastern Woodrat) scat, nesting material, cached nuts, and
feeding debris in an old vehicle 8.9 km east of the Mississippi Bluffs at Pine Hills, IL.
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of Pine Hills (K. Delahunt, Illinois Department of Natural Resources, Makanda, IL;
pers. comm.). We searched outcrops 10.8 km northwest of Little Grand Canyon, 6.2
and 10.5 km east of Horseshoe Bluff, 10.6 km east, and 15.5 south southeast of Pine
Hills, but did not locate any sign of Eastern Woodrat.
Discussion
Comparing our data with results of past studies provided evidence that the
remnant Woodrats in southeastern Illinois have increased in abundance and have a
larger distribution than previously documented. At Pine Hills, our mean trapping
success (0.551) was 62.5% higher than was achieved by trapping during 1993–1996
(0.339; Monty et al. 2003), and our number of previously uncaptured individuals
captured per trap-night was 3–6 times higher than any trapping study during the
previous 18 y (Table 2; Feldhamer et al. 2007). Our trapping was highly productive
in the northern areas of Pine Hills that had been considered not worth the effort of
trapping because of low productivity immediately after the genetic augmentation
(Ing 2008). The widespread woody debris and understory release resulting from
the 2009 derecho likely facilitated these population increases because Eastern
Woodrats are limited by availability of nesting sites (Crim 1961, Winchester et al.
2009) and security cover (Rainey 1956). We captured most Eastern Woodrats in
the bottomlands at woody-debris piles surrounded by dense understory vegetation,
which was in stark contrast to past studies in Illinois that captured Eastern Woodrats
primarily at rock outcrops (Crim 1961, Hoffmeister 1989, Howell 1910, Krull and
Bryant 1972, Layne 1958, Monty 1997, Monty et al. 2003, Nawrot 1974, Nawrot
and Klimstra 1976, Swayne 1949, West 1986). We encountered numerous active
stick nests of Eastern Woodrats built into woody-debris piles and windfalls, and
found that most roadside debris piles at Pine Hills contained a stick nest. However,
vegetative succession appears to be progressing rapidly at Pine Hills (reducing low
overhead cover) and woody-debris piles deteriorated noticeably during our study
(reducing the availability of nesting sites), especially in the moist bottomlands.
Without periodic disturbance, habitat will eventually return to a more uniform
late-successional state, with a sparse understory and little downed woody structure,
perhaps again restricting Woodrats to the bluff line and reducing their abundance.
In addition to increased woody structure and dense understory vegetation, genetic
augmentation also may have contributed to increases in abundance of Eastern
Woodrats. Although our study was not designed to determine if demographic and
distributional changes in the populations of Illinois Woodrat were caused in part
by the augmentation, other genetically augmented populations of woodrats have
experienced increases in abundance, although not to the extent of those in Illinois
(Smyser et al. 2013). Also, the Eastern Woodrats translocated from Arkansas and
Missouri were captured in forest habitat or disturbed habitat (i.e., slash piles),
instead of at outcrops (Feldhamer et al. 2007). It is possible that the genetic augmentation
increased the population’s propensity to utilize structures other than
outcroppings for stick-nest construction, thus increasing both demographic performance
in situ and habitat availability in the landscape. Alternatively, the presumed
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restriction of Illinois Woodrats to outcroppings may have simply been an artifact
of sparse downed-woody structure in the landscape or insufficient investigation of
non-outcrop habitat by past researchers. It is not unusual for Eastern Woodrats to
inhabit areas away from outcrops (Rainey 1956), where they can go unnoticed due
to a lack of easily visible sign (Knowles and Burger 2008).
The distribution of the Eastern Woodrat in southwestern Illinois apparently has
expanded eastward from remnant populations that were considered isolated in the
1990s (Monty et al. 2003); we detected latrines and stick nests almost 9 km from
previously known extant populations. Our failure to locate sign at some locations
may not indicate that Eastern Woodrats were not present (Peles and Wright 2008).
Outcrop exposure to rain, and outcrop structure likely affected our ability to locate
sign. Rain washes away and degrades scat (Wallmo et al. 1962), and outcrop
structure can limit the visibility of latrines and nests (e.g., nests or latrines hidden
within deep crevices or on inaccessible areas of bluff would not be detected).
The reports of Eastern Woodrat captures that we received suggest the species occupies
sites farther from the remnant populations than our sign-surveys indicate.
Lengthy dispersal distances in woodrats have been documented (Wright et al.
1998), and we consider the sources that reported captures of Eastern Woodrat to
us to be reliable.
The Illinois Department of Natural Resources translocated 62 Eastern Woodrats
from Pine Hills during 2013–2014 to reestablish a population in Giant City
State Park (R.D. Bluett, Illinois Department of Natural Resources, Springfield,
IL; unpubl. data) at historically occupied outcrops (Nawrot and Klimstra 1976).
This translocated population is ~12 km from the nearest occupied sites we
mapped, so if it is successful and colonizes surrounding habitat and the Mississippi
Bluffs populations continue to colonize eastward, then gene flow between the 2
areas is possible. The disjunct populations may eventually become contiguous if
the Eastern Woodrats continue to utilize forested habitat in addition to outcrops.
Increasing the number of interconnected populations, as suggested by Monty et
al. (2003), would help maintain genetic diversity and metapopulation persistence
(Wood 2008).
Prior to the translocation and habitat disturbance, 58.3% of Eastern Woodrats
captured at Pine Hills were female (Monty 1997); the overall and subadult sex ratio
was significantly female-biased (Monty et al. 2003). Although not significantly
different from an equal sex ratio, 52.8% of all Woodrats and 61.7% of subadults
we captured were female. However, Monty et al. (2003) captured more subadults
and juveniles than we did, most likely because they trapped throughout the year,
whereas we trapped between August and February. The breeding season for Eastern
Woodrats in Illinois occurs February through September (Monty et al. 2003).
Had we trapped throughout the year, we might have captured more subadults and
juveniles and experienced a more pronounced unequal sex ratio. Eastern Woodrats
reportedly display equal sex ratios at birth (Birney 1973) and female-biased sex
ratios may be a result of greater site fidelity of females (Wood 2008), maternal
bias against male offspring when resources are limited (McClure 1981), generally
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higher postnatal male mortality (Moses et al. 1998), or greater detection probability
for females. On several occasions at Pine Hills, we captured 2 adult females at a
stick nest on a single night and both returned to the stick nest upon release. This
observation suggests that 2 adult females can inhabit the same stick nest, thereby
increasing the number of females that can occupy an area with limited nesting sites.
In captivity, N. f. smalli Sherman (Key Largo Woodrat) have exhibited alloparental
care, where a daughter from a previous litter assisted her mother with rearing a subsequent
litter (Wheaton et al. 2013), and the closely related N. magister Baird (Allegheny
Woodrat) displayed seasonal patterns of aggregate living (Kinsey 1977),
but our observation is to our knowldege the first evidence of wild Eastern Woodrat
adult females living communally. The possibility of communal behavior stands in
contrast to reports of strict territoriality and antagonistic behavior toward conspecifics
in wild conditions (Layne 1958, Murphy 1952, Rainey 1956) and might also
explain female-biased sex ratios from trapping studies. The 4 male woodrats with
masses over 400 g are among the largest Eastern Woodrats recorded, although a
420-g male Eastern Woodrat was captured in Missouri (Feldhamer et al. 2007) and
masses up to 425 g have been reported for male N. f. osagensis Blair (Osage Woodrat;
Goertz 1970).
Even when management goals are achieved, genetic augmentations (also
called genetic introductions) can be controversial and perceived as humans are
tampering with nature when donor populations are not taxonomically identical
or geographically similar to the recipient populations (Cowlishaw et al. 2006).
Potential negative consequences of genetic augmentation include disruption of
evolutionary processes and outbreeding depression in the recipient population
(Allendorf et al. 2013). Genetic augmentation can also affect the taxonomic status
of an imperiled population and consequently the population’s legal status because
hybrids (crosses between individuals from genetically distinct populations) are
usually not addressed in endangered-species legislation (Allendorf et al. 2001,
Haig and Allendorf 2006). Despite these potential problems, genetic augmentation
is an effective yet underutilized tool for improving the fitness of genetically
depauperate or inbred populations (Whiteley et al. 2015). The risk of outbreeding
depression may be overemphasized (Frankham 2010, Frankham et al. 2011, Mortiz
1999). The probability of a genetic augmentation causing outbreeding depression
can be estimated and used to guide management decisions when the taxonomy of
the donor and recipient populations are well understood (Frankham et al. 2011).
Unfortunately, lack of range-wide genetic studies for species, such as the Eastern
Woodrat, whose taxonomy are primarily based on minor morphometric differences
can make selection of donor populations for augmentations challenging.
The results of our study were not suggestive of outbreeding depression within
the Pine Hills population; however, the taxonomic relationship between the Eastern
Woodrats translocated into Illinois and the remnant Illinois populations into which
they were released is unclear. The Illinois Department of Natural Resources chose
these source populations because they were similar in latitude and, based on published
distribution maps (e.g., Hall 1981) and morphometric studies, contained the
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same subspecies (Poole et al. 2013), and were the nearest populations capable of
sustaining removals for translocation (R.D. Bluett, Illinois Department of Natural
Resources, Springfield, IL; pers. comm.). However, genetic studies confirming the
geographic range of the Eastern Woodrat subspecies are lacking. The 2 genetic
studies of the taxonomy of the Eastern Woodrat either did not sample the Illinois
Woodrat subspecies (Edwards and Bradley 2001), or only included 1 specimen
(Hayes and Harrison 1992). Interestingly, the single Illinois Woodrat specimen utilized
by Hayes and Harrison (1992) originated from west of the Mississippi River
(western Arkansas) and formed a clade (based on mtDNA restriction sites) with the
other subspecies found west of the Mississippi (i.e., N. f. attwateri Mearns [Attwater’s
Woodrat], N. f. baileyi Merriam [Bailey’s Woodrat], N. f. campestris Allen
[Kansas Woodrat]). The Mississippi River, as well as other large rivers, may present
substantial barriers to woodrats and other small terrestrial animals (although
smaller rivers may not be complete barriers to woodrats; e.g., Castleberry et al.
2002, Manjerovic 2004) and thus serve as a driver of allopatric speciation (Gascon
et al. 2000, Haffer 1997, Wallace 1852). Though phenotypes (i.e., morphology)
may appear contiguous across a large river due to the presence of the same selective
pressures on both sides, populations on each side may in fact be isolated from one
another and taxonomically distinct (Pyron and Burbrink 2009). A. Poole (Southern
Illinois University Carbondale, IL, unpubl. data) demonstrated that remnant
Illinois Woodrats could be reliably differentiated from the translocated Arkansas
and Missouri Woodrats using 12 microsatellite loci. However, microsatellite differences
between these groups may be a consequence of a previous bottleneck of the
Illinois population, and are not by themselves indicative of taxonomic distinctiveness.
Further genetic work is needed to clarify the taxonomic relationship between
Eastern Woodrats from west and east of the Mississippi River and to understand the
evolutionary consequences of the augmentation.
Recommendations for further study and management
We recommend further monitoring of genetic admixture at Pine Hills to evaluate
if genetic augmentation facilitated the increase in Woodrat abundance and range
or if recent habitat-changes are solely responsible. Assessing the percentage of
individuals that are admixed (have genes from both Arkansas/Missouri and Illinois
populations) both at and away from augmentation sites would help determine
the effectiveness of genetic augmentation in addition to habitat management as
population-management strategies for Woodrats and other small mammals. We also
recommend forest-management practices, such as variable-density thinning (Carey
and Curtis 1996), that result in periodic canopy disturbance, felled trees, and piles
of woody debris to increase Woodrat habitat quality (Carey et al. 1999, Innes et al.
2007). Woodrats at Pine Hills clearly responded positively to similar habitat disturbance
by increasing in abundance and establishing nests within the woody debris.
Increasing periodic disturbance in southern Illinois forests should assist Woodrat
dispersal and recolonization of historically occupied areas.
Southeastern Naturalist
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A.C. Gooley and E.M. Schauber
2018 Vol. 17, No. 2
Acknowledgments
This project was funded in part by Federal Aid in Wildlife Restoration Project W-135-R,
the US Fish and Wildlife Service, the Illinois Department of Natural Resources, and the
Department of Zoology at Southern Illinois University Carbondale. E. Frieling, K. Lewis,
and B. Rapier assisted greatly with the capture, processing, and release of Woodrats. We
thank J. Rains and T. Jones for notifying us of Woodrat sign in old vehicles, R. Tuthill for
capturing a Woodrat on his property for us, and R. Bluett, C. Deaton, S. Lietz. T. Pohlman,
and E. Shimp for their assistance. R. Bluett, M. Cove, and 1 anonymous reviewer provided
comments on the manuscript. Work was conducted under Illinois Department of Natural
Resources permit numbers 11-37s, NH11.5552, NH12.5552, NH13.5552, NH14.5552, and
NH15.5552 and with authorization from the US Forest Service. Part of this study was conducted
within the Larue Pine Hills–Otter Pond Research Natural Area.
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