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Mammalian Predator Distribution Around a Transmission Line
Matthew B. Smith, David A. Aborn, Timothy J. Gaudin, and John C. Tucker

Southeastern Naturalist, Volume 7, Number 2 (2008): 289–300

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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. Introduction 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 - Matthew-Smith@utc.edu. 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. Methods Study area 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 Habitat analysis 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). Statistical analysis 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. Results 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). Locations TROW 100 300 Species total Large-bodied carnivores 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 Small-bodied predators 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). Discussion 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. Acknowledgments 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. Literature Cited Anderson, S.H., K. Mann, and H.H. Shugart, Jr. 1977. 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