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Some forest wildlife are excluded from or are less numerous in areas adjacent to roads and highways. Woodland breeding birds and terrestrial birds were found to have reduced densities adjacent to highways (Reijnen and others 1995, Kuitunen and others 1998). Some species clearly avoided the road, while others appeared to favor road-forest edges. Birds responding to corridor/forest edges along a power line corridor could be divided into edge, deep forest, and unaffected species (Kroodsma 1982).
Road and power line corridors may vary in their effects on forest wildlife, depending on corridor width. Forest-interior, neotropical migrant birds exhibited diminished abundances along wide power line corridors (50 to 75 feet) but not along narrow forest openings (of 25 feet) along unpaved dirt roads (Rich and others 1994). Such edge effects may not be as important for birds nesting in predominantly forested landscapes. In a landscape more than 70 percent forested, worm-eating warblers in small forest patches, separated by paved two-lane roads and house lots, were found to have nesting success comparable to those nesting in large forest tracts (Gale and others 1997). However, even in heavily forested landscapes, ovenbirds showed reduced densities of breeding territories and reduced pairing success within 500 feet of forest roads (Ortega and Capen 1999). Therefore, while edges of narrow corridors may be acceptable habitat for some bird species, they may be unsuitable for others. These issues must be evaluated in terms of the conservation concerns for the species at issue in a given situation (see chapter 4 and “Effects of urbanization on forest birds: urban fragmentation and edge effects” of this chapter for discussions concerning ovenbird response to edges versus conservation status).
Forest roads were found to reduce the abundance and species richness of macroinvertebrate soil fauna (Haskell 2000). This effect extended up to 330 feet into the forest. Although wider roads and those with a more open canopy produced steeper declines, even narrow roads through forests produced marked edge effects.
Some wildlife are attracted to roadsides and power line rights-of-way because of grassland, early-successional, or edge habitat. The value of roadsides and utility corridors has been documented for grassland and habitat generalist species of small mammals (Adams and Geis 1983, Johnson and others 1979).
Corridor width and vegetative characteristics influence the attractiveness of the habitat for bird species. Road rights-of-way are important habitat for birds that nest in edges and ecotones (Warner 1992). The number of roadside nests and species increased with roadside width. Mowing schedules, diversity of vegetation, and vegetative structural complexity affected the habitat value of roadsides for nesting birds. Narrow power line corridors (40 feet wide) had a reduced diversity of birds compared to wider corridors (100 feet or more) (Anderson and others 1977). Wide corridors attracted more grassland bird species. Power line corridors with increased patchiness of shrub vegetation, showed increased fledging success of nesting birds (Chasco and Gates 1992). Fledging success decreased, however, as the habitat became more homogeneous. Many early successional and disturbance-dependent bird species can be found in roadsides and utility rights-of-way (Hunter and others 2001a, Meehan and Hass 1997), but corridors lacking shrub growth may have fewer nesting and wintering birds (Meehan and Hass 1997). Corridor nesting birds were more dense in the corridor interiors than along the edge (Kroodsma 1987).
Small forest mammals, such as eastern chipmunks, gray squirrels, and white-footed mice, were found reluctant to venture onto road surfaces when the distance between cleared road margins exceeded 65 feet (Oxley and others 1974). Four-lane highways acted as effective barriers against the movements of these small forest mammals. Medium-sized mammals, such as woodchucks, porcupines, raccoons, and striped skunks, crossed wider cleared road margins more often, but suffered higher road mortality than small mammals. Similarly, the movements of white-footed mice across roads, including narrow gravel roads, were found to be infrequent (Merriam and others 1989); and paved roads were found to be a significant barrier to the movements of woodland mice (Mader 1984). Even small forest roads not open to public traffic were seldom crossed.
The presence of roads appeared to substantially hinder the movements of forest amphibians (Gibbs 1998). In a different study, primary and secondary roads did not affect the presence and movement of forest frogs and toads (DeMaynadier and Hunter 2000). The movement of forest salamanders was significantly inhibited by primary forest roads, but the minor forest roads had little effect.
Black bears in the Pisgah National Forest of North Carolina almost never crossed an interstate highway; roads with low traffic volume were crossed more frequently than those with high traffic volume (Brody and Pelton 1989). Bears also appeared to adjust their home ranges to areas with lower road densities.
The nature of the corridor edge may determine how strongly that edge serves as a boundary for wildlife. Abrupt vegetative transition from forest to mowed grass on the edge of a power line corridor was found to be a barrier to forest birds and served as a natural territorial boundary for many bird species (Chasco and Gates 1992). When the vegetative contrast of the corridor was softened by shrubby vegetation, however, there was greater overlap between mixed-habitat and forest bird species. Power line corridors with abrupt edges were also avoided by small and medium-sized mammals because of difficulties in crossing the dense grass mats (Gates 1991). Corridors with a wide shrub zone along the edge had increased use and permeability to movement.
Wildlife underpasses can be an effective way to relieve the barrier effect of roads for some wildlife species (Clevenger and Waltho 2000). Wildlife differ in their abilities to utilize underpasses. In south Florida, white-tailed deer, raccoons, bobcats, the endangered Florida panther, alligators, and black bears were all documented to use underpasses to traverse an interstate highway (Foster and Humphrey 1995). Considerations for topography, habitat quality, location, and the level of human activity in the vicinity are important in designing a successful wildlife underpass (Clevenger and Waltho 2000).
Road rights-of-way also can facilitate the movement of wildlife. Some grassland and early-successional species, such as Bachman’s sparrow, require grassy and shrub-dominated corridors to facilitate their movement to and from isolated patches of suitable habitat (Dunning and others 1995). Meadow voles greatly expanded their range in central Illinois after the establishment of continuous strips of dense, grassy vegetation along interstate highways (Getz and others 1978). In contrast, the prairie vole is not restricted in movement by interruptions in grassy habitats. This species remains dominant in grassy sites not connected to the interstate, such as pastures and county roadsides. Similarly, a shrubby power line corridor and edges served as travel lanes for red foxes and striped skunks in a fragmented landscape (Gates 1991); but mammalian nest predator abundance was found to be influenced by both local and landscape-level features (Dijak and Thompson 2000).
Black bears use roads in the Great Dismal Swamp National Wildlife Refuge as travel corridors through the dense pocosin vegetation (Hellgren and others 1991). Such road use by bears are more characteristic among “unharvested” or protected populations. Hunted bear populations generally avoid roads, especially those with unrestricted use by humans.
Wooded roadside corridors serve as travel lanes for native forest mammals, but use of corridors taper off with distance from the forest (Downes and others 1997a and 1997b). Wooded road corridors appear to be used heavily, by nonnative house mice and black rats, reducing their value as a remedy for habitat fragmentation. Males of some mammal species may utilize corridor habitats in greater numbers than females, indicating that roadside forest corridors may function as intraspecific filters.
Mortality along roads and highways has been well documented for many species of wildlife, but a number of factors influence the severity, including season, weather events, type of road, location of road, and road density. During a 14-month period along a dual lane highway, road mortalities were documented for 11 species of mammals, 12 species of birds, 5 species of reptiles, 9 species of amphibians, and insects belonging to 11 orders (and more than 249 different species) (Seibert and Conover 1991). Amphibian mortalities were higher in certain seasons and after rains. Populations of timber rattlesnakes were reduced in areas of eastern Texas having high road densities (Rudolph and Burgdorf 1997). Road-related mortality was a significant threat to raptors, especially northern saw-whet owls and eastern screech owls (Loos and Kerlinger 1993); but road kill numbers varied with season, location, road type, and species involved.
Mortality rates of small forest mammals, such as Eastern chipmunks, gray squirrels, and white-footed mice, were highest when cleared road margins were about 45 to 115 feet (Oxley and others 1974). Mortality rates for these small mammals dropped as cleared margins grew wider, mainly because they seldom attempted crossings of wider forest clearings. Mortality of medium-sized mammals, such as woodchucks, porcupines, raccoons, and striped skunks, increased with increased cleared width, reaching a peak when traffic density was high and young were emerging. Small mammal road mortalities on interstate highways was found to be greatest for species with highest densities in the right-of-way habitat, but the loss did not appear to be detrimental to populations of these species (Adams and Geis 1983). Road mortalities for white-tailed deer along interstate highways have been documented by Reilly and Green (1974) and Puglisi and others (1974). Road mortality of vertebrates were recorded in north Florida (Cristoffer 1991).
Mortality increased with increasing speed limits and increasing density of roadside vegetative cover.
Population impacts of road-induced mortality can be significant for some wildlife species. In south Florida, road kills are the largest source of human-induced mortality for the endangered Florida panther and the endangered Key deer (U.S. Fish and Wildlife Service 1999).
Roads and power line corridors provide habitat and mechanisms for the spread of some exotic plants and animals. All high- and low-use roads sampled in an experimental forest contained at least one exotic plant species, some had as many as 14 (Parendes and Jones 2000). Even abandoned spur roads with no traffic over the last 20 to 40 years still had numerous exotic plants. Narrow, linear forest openings associated with roads and power lines appear conducive to establishment of the red imported fire ant (Stiles and Jones 1998). See the review in Trombulak and Frissell (2000) and the information compiled by the National Resources Defense Council (2000) for more information about the spread of exotic plants and animals along roads.
Roads can provide hunters and poachers with increased access into forested areas (Natural Resources Defense Council 2000). Many large mammals are exposed to increased hunting pressure near roads, and some may have difficulties maintaining their populations near roadsides. In the Appalachian Highlands, management of black bears requires a special concern for road density (Clark and Pelton 1999). While overall black bear populations in the Southern Appalachians are considered stable to increasing at the present time, most black bear mortality is human-induced and includes hunting, poaching, and road kills. Hunting and poaching efficiencies increase along with improved vehicle access, and black bear habitat suitability is increased when the density of roads is kept low or if logging roads are closed after the timber has been harvested (Clark and Pelton 1999). Similarly, Brody and Pelton (1989) concluded that the primary effect of roads in bear habitat in western North Carolina was an increase in the vulnerability of bears to hunting.
Roads can subject wildlife to increased levels of heavy metals, salts, and organic compounds through accumulation in plants, soil, and water (see the review in Trombulak and Frissell 2000). Corridor maintenance by mowing presents a hazard for some ground-nesting birds and other wildlife species (Bolen and Robinson 1995).
For a discussion of indirect effects of roads, including promotion of further human land use changes, see the review in Trombulak and Frissell (2000).
The effects of trails appear to be better documented for plants than other taxa. Trampling by hikers and other forest recreational users has been implicated in the decline of sensitive forest understory plants (Gross and others 1998).
Research from regions outside of the South has documented shifts in forest bird composition along trails (Hickman 1990, Miller and others 1998, Van der Zande and others 1984). Such effects may depend on the intensity and timing of the recreational disturbance, however (Van der Zand and others 1984).
In other more general studies, research indicates that human intrusion can alter bird behavior and community structure. Disturbance by pedestrians and vehicles was found to reduce the number of bird species on wooded streets, as well as species persistence, guild density, and probability of occupation by individual bird species (Fernadez-Juricic 2000). Crows were found to be more vigilant in areas of high human disturbance than in areas of low human disturbance (Ward and Low 1997). Since vigilance and foraging are mutually exclusive behaviors, the level of human activity can affect the foraging success of sensitive bird species. Others have found, however, that low levels of human intrusion (one person for 1 or 2 hours per week) did not significantly affect the vertical distributions of any forest bird species in three vegetation strata above the ground (Gutzwiller and others 1998). The forest bird species studied were apparently able to tolerate low levels of human intrusion.
Black bears also are sensitive to human disturbance and may be affected by the presence of trails. Hibernating black bears were found to readily abandon their dens and cubs in response to investigator disturbance (Goodrich and Berger 1994).
As observed by Schlauch (1976), some “collectable” wildlife, such as box turtles or salamanders, disappear quickly in the vicinity of ground-level nature trails due to pet collection.
Not all wildlife are disturbed or excluded by trails. Mammalian nest predators, including raccoons, skunks, and coyotes, were observed to be common along trails (Miller and others 1998) and seem to be abundant in edge habitats (Gates and Gysel 1978).
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