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2008 SOUTHEASTERN NATURALIST 7(2):289–300
Mammalian Predator Distribution
Around a Transmission Line
Matthew B. Smith1,*, David A. Aborn1, Timothy J. Gaudin1,
and John C. Tucker1
Abstract - The effects of a transmission line right-of-way (TROW) on the distributions
of mammalian predators were investigated by placement of track plates at
specific locations. A total of 50 tracks were detected. The large-bodied carnivores
exhibited a strong preference for the TROW (χ2 = 8.652, 2 df, p = 0.013). In contrast,
the small-bodied predators were distributed more uniformly, exhibiting no
significant differences in their distributions (χ2 = 1.927, 2 df, p = 0.382). The TROW
likely facilitates the travel of the large-bodied carnivores by offering an area that is
relatively free from obstruction. The higher-than-expected occurrence of the smallbodied
predators in the TROW may have been due to temporal variations caused by
dietary enhancements available at particular times of the year.
The loss and further fragmentation of natural habitat is consistently cited
as the primary factor contributing to the loss and decline of biological diversity
(Crooks 2002, Henle et al. 2004). A substantial amount of work has been
performed on the effects of habitat fragmentation on mammalian carnivores
because of their low population densities and relatively large home ranges
(Crooks 2002, Gittleman and Gompper 2005). However, because carnivores
are composed of a diverse group that varies greatly in physical attributes,
ecology, and behavior, predicting their distributions in fragmented landscapes
is often difficult (Crooks 2002). Body size may provide a baseline upon which
managerial decisions can be made when questions arise as to the distributions
of mammalian predators in fragmented habitats. Crooks (2002) reported
that body size differences partially accounted for habitat selection among
mammalian carnivores in landscapes fragmented by urbanization, where sensitivity
increased as body mass increased. Conversely in habitat fragmented
by agriculture, Gehring and Swihart (2003) reported that sensitivity to open
spaces (e.g., crop fields) decreased as body mass increased.
Knight and Kawashima (1993) estimated that there were >0.5 million
km of transmission line right-of-ways (TROW) in the United States, altering
some 2.1 million ha of natural habitat. Because of this, TROWs could
have great effects on vertebrate communities, especially where forests are
fragmented. Even though TROWs are ubiquitous throughout our landscapes
in the South (Graham 2002), only a few studies have investigated the effects
1Department of Biological and Environmental Sciences, The University of
Tennessee at Chattanooga, Chattanooga, TN 37403. *Corresponding author -
290 Southeastern Naturalist Vol.7, No. 2
of TROWs on southern vertebrate taxa (Anderson et al. 1977, Johnson et
al. 1979, Kroodsma 1987). We found only a single study that provided information
on mammalian predators, reporting that Procyon lotor Linnaeus
(raccoon) avoided TROWs (Gates 1991).
During a preliminary study, Smith (2006) observed trends that suggested
habitat fragmented by a TROW affected larger carnivores differently than
smaller mammalian predators. Based on these observations, we used body
size to predict the distributions of mammalian predators around a TROW
that bisects an otherwise contiguous forest. For this study, we predicted
that larger mammalian carnivores would be detected more frequently in the
TROW, using it as a means of travel. On the other hand, we predicted that
smaller mammalian predators would be detected more frequently in the forest
adjacent to the TROW, where cover is readily available.
We conducted this study in the North Chickamauga Creek Gorge State
Natural Area (NCSNA), which is found on the eastern escarpment of the
Cumberland Plateau about 20 km northeast of the city of Chattanooga and
almost entirely within the political boundaries of Hamilton County, TN except
for a small portion extending into Sequatchie County, TN (Fig. 1). It
Figure 1. Aerial photograph of study site showing the political boundaries and protected
lands of the North Chickamauga Creek Gorge State Natural Area, Hamilton
County, TN, including the location of the study site in relation to the TROW.
2008 M.B. Smith, D.A. Aborn, T.J. Gaudin, and J.C. Tucker 291
protects an area of approximately 2373.13 ha (TDEC 2006). The study occurred
on the portion of the natural area atop the plateau (approx. elev. 549
m) where a TROW (approx. 50 m wide) maintained by the Tennessee Valley
Authority crosses the property. This region of the natural area consists
largely of a maturing secondary growth mesic hardwood forest composed
of Quercus spp. (oak), Carya spp. (hickory), Acer spp. (maple), Sassafras
albidum Nuttall (sassafras), Pinus virginiana Miller (Virginia pine), Oxydendrum
arboreum (L.) DC. (sourwood), and Ilex opaca Aiton (American
holly) with pockets of mixed mesophytic forests composed of Tsuga canadensis
(L.) Carr. (hemlock), Liriodendron tulipifera L. (tulip poplar),
and Kalmia latifolia L. (mountain laurel). The TROW contains vegetation
consistent with early ecological succession, including various grasses, Smilax
rotundifolia L. (greenbrier), Rubus spp. (blackberry), Ligustrum spp.
(privet), A. rubrum L. (red maple), and Virginia pine.
Mammalian predator distribution
We evaluated the effects of the TROW on the distributions of mammalian
predators from October 2004 to October 2005 by analyzing visits made to
aluminum track plates in a study site that encompassed an area of 650 m
by 2000 m (Fig. 1). Within this study site, we constructed a series of 6
transects that were spaced 400 m apart and perpendicular to the TROW,
with 3 transects extending away from the TROW into the forest on the
northern side and 3 extending away from the TROW into the forest on
the southern side (Fig. 2). Each transect contained 3 track plates, for a total
of 18 plates (Fig. 2). Along each transect, we placed the first track plate in
the center of the TROW and the second and third at 100 m and 300 m from
the edge of the forest, respectively, thus allowing for a total of 6 track plates
at each specified position (Fig. 2).
Each track plate consisted of a 0.9-m2, 16-gauge aluminum plate (Blow
Pipe, Inc., Chattanooga, TN; Odell and Knight 2001) and was operated,
checked, and re-established daily for at least 2 but up to 5 consecutive days
during 6 sampling periods spanning 12 months. We sprayed each track plate
with a solution of 100% ethanol and unscented, laboratory grade talcum
powder (approximately 800 mL of ethanol to 100 mL of talcum powder;
Odell and Knight 2001). Once the solution was applied, the ethanol evaporated,
leaving a fine layer of evenly distributed talcum powder that provided
a track medium. We placed 1 fatty acid scent (FAS) scented predator survey
disk (Pocatello Supply Depot, Pocatello, ID) in the center of each track plate.
A visitation was recorded if at least 1 track of an individual species was
detected and identifiable (Linhart and Knowlton 1975). We did not attempt
to record multiple visits made by different individuals to a track plate in a
given night, but simply indicated it as 1 visit, whether or not multiple visitations
had occurred (Linhart and Knowlton 1975). We sprayed more solution
on each plate once tracks were identified (Elbroch 2003), and re-baited as
needed. On the final day of each sampling period, we removed the FAS as the
stations were checked (Linhart and Knowlton 1975).
292 Southeastern Naturalist Vol.7, No. 2
We conducted a coarse habitat analysis in the NCSNA during the
summer of 2005. Using each of the track plates as center points, we visually
estimated the percent canopy cover, percent understory cover, percent
shrub cover, and percent ground cover within a 10-m radius plot (Gehring
and Swihart 2003, Peet et al. 1998, Wilson et al. 1996). For the purposes of
this study, the canopy stratum consisted of vegetation greater than or equal
to 6 m, the understory stratum consisted of vegetation between 3 and 5 m,
and the shrub stratum consisted of vegetation between 1 and 2 m. Percent
cover was based on a 6-point cover-class scale, where 1 = 0–5%, 2 = 5–
25%, 3 = 25–50%, 4 = 50–75%, 5 = 75–95%, and 6 = 95–100% (Daubenmire
1959). In order to produce mean percentages, we averaged cover-class
midpoints (Daubenmire 1959, Peet et al. 1998) for each site according to
the spatial arrangements of the track plates (Fig. 3). For example, we averaged
the cover-class midpoints taken from the 6 track plates located in the
TROW, the 6 track plates found 100 m from the forest edge, and the 6 track
plates found 300 m from the forest edge (Figs. 2 and 3).
In order to test the prediction that larger mammalian carnivores would
be detected more frequently in the TROW, and that smaller mammalian
predators would be detected more frequently in the adjacent forest,
Figure 2. Aerial photograph showing the spatial arrangement of transects containing
track plates relative to the TROW.
2008 M.B. Smith, D.A. Aborn, T.J. Gaudin, and J.C. Tucker 293
we separated the mammalian predators detected during the study into 2
groups: large-bodied carnivores and small-bodied predators. We defined a
large-bodied carnivore as a member of a species with a reported mean mass
of greater than or equal to 19 kg (Wilson and Ruff 1999). This designation
applied to Canis latrans Say (coyote), Lynx rufus Schreber (bobcat), and
Canis familiaris Linnaeus (domestic or feral dog). We defined a small-bodied
predator as a member of a species with a reported mean mass of less
than 19 kg (Wilson and Ruff 1999). This designation applied to raccoon,
Didelphis virginiana Kerr (Virginia opossum), Urocyon cinereoargenteus
Schreber (common gray fox), and Vulpes vulpes Linnaeus (red fox). We did
not differentiate between the tracks of the 2 species of fox found in this region
because of similarities in track size and shape.
To test for statistical differences, we performed the chi-square test using
SigmaStat (vers. 3.2) to analyze the number of visits among the 3 locations
(i.e., the center of the TROW and 100 m and 300 m from the forest edge;
Fig. 2) made by the entire guild of mammalian predators and then the number
of visits once we had separated them into their respective groups. We
used an alpha level of 0.10.
In 306 track nights, we detected a total of 50 tracks (Table 1). Fourteen
of the 50 tracks were from large-bodied carnivores, constituting 28% of all
visits (Table 1). Thirty-six of the 50 tracks were from small-bodied predators,
constituting 72% of all visits (Table 1).
Figure 3. Mean cover-class midpoints from habitat variables at each track plate location.
Locations: TROW = track plates in the transmission line right-of-way; 100 =
track plates located 100 m from the forest edge; 300 = track plates located 300 m
from the forest edge).
294 Southeastern Naturalist Vol.7, No. 2
The number of visits made by the entire guild of mammalian predators
did not differ significantly among locations, with 36% of visitations recorded
in the TROW, 28% at 100 m from the forest edge, and 36% at 300 m
from the forest edge (χ2 = 0.338, 2 df, p = 0.845; Fig. 4). When the equality
of visits was compared between large-bodied carnivores and small-bodied
Table 1. Total number of visits made by large-bodied carnivores and small-bodied predators to
track plate locations (Locations: TROW = track plates in the transmission line right-of-way;
100 = track plates located 100 m from the forest edge; 300 = track plates located 300 m from
the forest edge).
TROW 100 300 Species total
Coyote 5 1 2 8
Bobcat 5 0 0 5
Domestic or feral dog 1 0 0 1
Large-bodied total 11 1 2 14
Raccoon 6 12 14 32
Virginia opossum 0 1 2 3
Gray or red fox 1 0 0 1
Small-bodied total 7 13 16 36
Grand total 18 14 18 50
Figure 4. Frequency of visits made by the entire guild of mammalian predators to
track plate locations (χ2 = 0.338, 2 pdf, p = 0.845; locations: TROW = track plates in
the transmission line right-of-way; 100 = track plates located 100 m from the forest
edge; 300 = track plates located 300 m from the forest edge).
2008 M.B. Smith, D.A. Aborn, T.J. Gaudin, and J.C. Tucker 295
predators at each location, significant differences were detected, indicating
that distributions differed when body size was considered (χ2 = 15.357, 2 df,
p = < 0.001; Fig. 5).
The number of visits made by large-bodied carnivores differed signifi-
cantly among the 3 locations, with the TROW accounting for approximately
79% of the visits versus approximately 21% in the adjacent forested habitats
(χ2 = 8.652, 2 df, p = 0.013; Fig. 5). Coyotes were detected in the TROW
approximately 63% of the time, but significant differences among the 3 locations
were not evident (χ2 = 1.647, 2 df, p = 0.439; Fig. 6). Bobcats were
detected exclusively within the TROW (Fig. 6).
The number of visits made by small-bodied predators did not differ
significantly among the 3 locations, with approximately 19% of visitations
recorded in the TROW, 36% at 100 m from the forest edge, and 44% at 300
m from the forest edge (χ2 = 1.927, 2 df, p = 0.382; Fig. 5), though certain
trends were apparent. Raccoons constituted the majority of visits made by
small-bodied predators, but visits by this species did not differ significantly
among the 3 locations (χ2 = 1.859, 2 df, p = 0.395; Fig. 6).
As a collective guild, the mammalian predators detected in the NCSNA
were evenly distributed, not displaying preferences for the TROW or for either
Figure 5. Frequency of visits made by large-bodied carnivores (χ2 = 8.652, 2 df, p =
0.013) and small-bodied predators (χ2 = 1.927, 2 df, p = 0.382) to track plate locations.
Locations: TROW = track plates in the transmission line right-of-way; 100 =
track plates located 100 m from the forest edge; 300 = track plates located 300 m
from the forest edge.
296 Southeastern Naturalist Vol.7, No. 2
of the other 2 locales in the adjacent forest (Fig. 4). However, their preferences
emerged when they were separated into groups based on body size (Fig. 5).
The large-bodied carnivores (i.e., coyotes, bobcats, and domestic or
feral dogs) displayed a preference for the TROW (Fig. 5, Table 1). Coyotes
and bobcats are the dominant carnivores in the natural area, making
them less susceptible to intra-guild predation (Gittleman and Gompper 2005)
and less restricted by the need for cover, at least while traveling, though they
must still be able to use cover to conceal themselves from potential prey, during
periods of inactivity, and for rearing young. Moreover, both coyotes and
bobcats are adaptable to a variety of habitats, including those modified by
humans (Whitaker and Hamilton 1998). In this case, it seems likely that the
TROW not only facilitated the travel of these larger carnivores by offering
a more open area that is relatively free from obstruction (Fig. 3), potentially
reducing energetic demands associated with locomotion, but also provided
them with direct foraging opportunities.
Coyotes and bobcats are opportunistic predators that feed on many vertebrate
species, but especially mammals (Ewer 1973, Whitaker and Hamilton
1998). Mammal inventories in the TROW and adjacent forest edges found
Blarina brevicauda Say (northern short-tailed shrew), Scalopus aquaticus
Linnaeus (eastern mole), Sylvilagus fl oridanus Allen (eastern cottontail),
Tamias striatus Linnaeus (eastern chipmunk), Sciurus carolinensis Gmelin
(eastern gray squirrel), Peromyscus leucopus Rafinesque (white-footed
mouse), Sigmodon hispidus Say and Ord (hispid cotton rat), and Odocoileus
Figure 6. Frequency of visits made by coyotes (χ2 = 1.647, 2 df, p = 0.439), bobcats,
and raccoons (χ2 = 1.859, 2 df, p = 0.395) to track plate locations (Locations: TROW
= track plates in the transmission line right-of-way; 100 = track plates located 100 m
from the forest edge; 300 = track plates located 300 m from the forest edge).
2008 M.B. Smith, D.A. Aborn, T.J. Gaudin, and J.C. Tucker 297
virginianus Zimmermann (white-tailed deer) (Smith 2006), all potential
prey in the diets of coyotes and bobcats (Bekoff 1977, Ewer 1973, Larivière
and Walton 1997, Whitaker and Hamilton 1998). Unlike bobcats, which are
strict carnivores (Larivière and Walton 1997), coyotes will readily consume
vegetable matter, particularly fruits and berries (Bekoff 1977, Ewer 1973).
The TROW contained early successional vegetation that produced berries
(e.g., blackberries) during the summer months, and coyotes exploited these
additional resources as indicated by the contents of several scats found in the
TROW (M.B. Smith, pers. observ.).
Contrary to our prediction, the small-bodied predators (i.e., raccoons,
Virginia opossums, and foxes) did not exhibit a significant preference for
the adjacent forested habitats (Fig. 5). We were unable to detect a significant
difference in the distributions of raccoons, which was the most frequently
encountered small-bodied predator (Fig. 6). However, there were certain patterns
that seemed to suggest a minor preference for the adjacent forest (Figs.
5 and 6). For example, the greatest number of visits made by small-bodied
predators was found at 300 m from the forest edge, and the least was found
in the TROW (Fig. 5, Table 1). The absence of significant trends might be the
result of an insufficient sample size and sampling effort. A stronger preference
for forested habitat among small-bodied predators might become more
apparent if sample size were greater. Nevertheless, based on these findings,
small-bodied predators appear to be less effected by, or more tolerant to,
habitat fragmented by a TROW than was expected.
We hypothesized that the small-bodied predators would avoid the
TROW because it consisted of a relatively open habitat, which may act
to increase their vulnerability to predation, causing higher occurrences in
edge habitats where resources and cover are readily available (Dijak and
Thompson 2000, Gehring and Swihart 2003, Heske 1995). The TROW in
the natural area may not have been sufficiently open to produce such a response.
However, the movements of raccoons and Virginia opossums are
not extensive (Lotze and Anderson 1979, McManus 1974, Nowak 1999),
so it is unlikely that the TROW benefited them by facilitating their travel,
as it did with the large-bodied carnivores. The habitat in the TROW differed
considerably from the 2 adjacent forested habitats (Fig. 3). The
TROW contained greater amounts of shrub and ground cover, whereas
the 2 forested habitats consisted of greater amounts of canopy coverage
(Fig. 3). Raccoons and Virginia opossums occupy a variety of habitats
because of their fairly unrestricted diets, but they seem to prefer forested
or brushy areas where den or nest sites can be readily accessed (Lotze
and Anderson 1979, McManus 1974, Nowak 1999). The TROW may have
fulfilled these habitat requirements either because it contributes sufficient
amounts of cover itself (Fig. 3) or because its narrowness (i.e., 50 m
wide) provided sufficient access to forested habitats. Small-bodied predators,
particularly raccoons, may have benefited by utilizing the TROW
298 Southeastern Naturalist Vol.7, No. 2
to gain access to particular food resources (e.g., blackberries) at certain
times of the year. During these times, the TROW may have been utilized
more frequently because of raccoons’ ability to exploit these additional
resources, thus enhancing their diet. However, during the other times of
the year, the TROW may have been utilized less frequently because the
additional resources were not available, and their dietary and habitat requirements
were fulfilled in the forested habitats. The minor trends that
were observed may have been temporal variations in the distributions of
small-bodied predators caused by the quality and availability of resources
in the TROW.
TROWs are relatively unstudied though they are common landscape
features, and as such their cumulative effects on biodiversity could be
substantial. This study sought to understand how a TROW effects the distributions
of mammalian predators, but because of the localized nature of
the research and restricted taxonomic sample, the small sample size, and
the relatively low number of detections of mammalian predators, additional
research is needed to achieve a more complete understanding of these effects
on mammalian predators. Nevertheless, body size may be a useful criterion
for managerial purposes if used as a baseline indicator for the distributions
of mammalian predators around a TROW, as it partially accounted for their
distributions around the TROW in the NCSNA.
We thank the Tennessee Department of Environment and Conservation (TDEC)
and the Tennessee Wildlife Resources Agency (TWRA) for access to the study sites
and for applicable permits. We thank Ford Mauney, Brian Yates, Sara Ray, and Stacy
Huskins for field assistance, Andy Carroll for help with GIS, and Mark Schorr for
advice with statistical procedures. Comments by two anonymous reviewers improved
an earlier version of the manuscript. The Department of Biological and Environmental
Sciences at The University of Tennessee at Chattanooga provided financial
support. This research partially fulfilled the requirements for the Master of Science
degree for M.B. Smith.
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