Effects of Roads on Terrestrial Vertebrates

General effects of roads and road-associated factors on a wide variety of vertebrate taxa are well documented from a broad range of studies conducted in North America, Europe, and other areas (Bennett 1991, Forman and Alexander 1998, Mader 1984, Trombulak and Frissell 2000, Vestjens 1973). Effects of roads on vertebrate populations act along three lines:

1. Direct effects, such as habitat loss and fragmentation.
2. Road use effects, such as traffic causing vertebrate avoidance or roadkill.
3. Additional facilitation effects, such as overhunting or overtrapping, which can increase with road access.

These factors and their effects on vertebrates in relation to roads are summarized from Wisdom and others (2000) as follows:

• Road construction converts large areas of habitat to nonhabitat (Forman 2000, Hann and others 1997, Reed and others 1996); the resulting motorized traffic facilitates the spread of exotic plants and animals, further reducing quality of habitat for native flora and fauna (Bennett 1991, Hann and others 1997). Roads also create habitat edge (Mader 1984, Reed and others 1996); increased edge habitat favors species that use edges, and harms species that avoid edges or experience increased mortality near or along edges (Marcot and others 1994).
• Species dependent on large trees, snags, or logs, particularly cavity-using birds and mammals, are vulnerable to increased harvest of these materials along roads (Hann and others 1997). Motorized access facilitates firewood cutting, as well as commercial harvest, of these materials.
• Several large mammals are vulnerable to poaching, such as caribou, pronghorn antelope, mountain goats, bighorn sheep, wolves, and grizzly bears (Autenrieth 1978, Bruns, 1977, Chadwick 1973, Dood and others 1986, Greer 1985, Gullison and Hardner 1993, Horejsi 1989, Knight and others 1988, Lloyd and Fleck 1977, Luce and Cundy 1994, Mattson 1990, McLellan 1990, McLellan and Shackleton 1988, Mech 1970, Scott and Servheen 1985, Singer 1978, Thiel 1993, Van Ballenberghe and others 1975, Yoakum 1978). Roads facilitate this poaching (Cole and others 1997).
• Gray wolves and grizzly bears experience chronic, negative interactions with humans, and roads are a key facilitator of such interactions (Mace and others 1996, Mattson and others 1992, Thiel 1985). Repeated, negative interactions of these two species with humans increases mortality of both species and often causes high-quality habitats near roads to function as population sinks (Mattson and others 1996, Mattson and others 1996, Mech 1973).
• Carnivorous mammals such as marten (Martes americana), fisher (M. pennanti), lynx (Lynx canadensis), and wolverine (Gulo luscus) are vulnerable to overtrapping (Bailey and others 1986, Banci 1994, Coulter 1966, Fortin and Cantin 1994, Hodgman and others 1994, Hornocker and Hash 1981, Jones 1991, Parker and others 1983, Thompson 1994, Witmer and others 1998), and overtrapping can be facilitated by road access (Bailey and others 1986, Hodgman and others 1994, Terra-Berns and others 1997, Witmer and others 1998). Movement and dispersal of some of these species also is believed to be inhibited by high rates of traffic on highways (Ruediger 1996), but this has not been validated. Carnivorous mammals such as lynx also are vulnerable to increased mortality from highway encounters with motorized vehicles (as summarized by Terra-Berns and others 1997).
• Reptiles seek roads for thermal cooling and heating, and in doing so, these species experience significant, chronic mortality from motorized vehicles (Vestjens 1973). Highways and other roads with moderate to high rates of motorized traffic may function as population sinks for many species of reptiles, resulting in reduced population size and increased isolation of populations (Bennett 1991). Roads also facilitate human access into habitats for collecting and killing reptiles.
• Many species are sensitive to harassment by humans or simply to human presence. Both often are facilitated by road access. Potential reductions in productivity, increases in energy expenditures, or displacements in population distribution or habitat use can occur (Bennett 1991, Mader 1984). Examples of such road-associated effects are human disturbance of leks (sage grouse [Centrocercus urophasianus] and sharp-tailed grouse [Tympanuchus phasianellus]), ferruginous hawk nests [Buteo regalis], and fox kits [Vulpes macrotis]). Another example is elk, which avoid large areas near roads open to traffic (Lyon 1983, Rowland and others 2000). Elk avoidance of roads increases with increasing rate of traffic (Wisdom and others 2000, Johnson and others 2000).
• Bats are vulnerable to disturbance and displacement caused by human activities in caves, mines, and on rock faces (Hill and Smith 1984, Nagorsen and Brigham 1993). Cave or mine exploration and rock climbing are examples of recreation that could reduce population fitness of bats that roost in these sites (Nagorsen and Brigham 1993, Tuttle 1988). Such activities may be facilitated by human developments and road access (Hill and Smith 1984).
• Ground squirrels often are targets of recreational shooting (plinking), which is facilitated by human developments and road access (Ingles 1965). Many species of ground squirrels are local endemics; these small, isolated populations may be especially vulnerable to recreational shooting.  The potential exists for severe reductions or local extirpations of populations.
• Roads often restrict the movements of small mammals (Mader 1984, Merriam and others 1988, Swihart and Slade 1984), and consequently can function as barriers to population dispersal and movement by some species (Oxley and Fenton 1974).
• Many granivorous birds are attracted to grains and seeds along roadsides and as a result have high mortality from collisions with vehicles (Vestjens 1973). Pine siskens (Carduelis pinus) and white-winged crossbills (Loxia leucoptera), for example, are attracted to road salt; mortality from vehicle collisions can result (Ehrlich and others 1988).
• Terrestrial vertebrates in areas near roads accumulate lead and other toxins that originate from motorized vehicles; the largely undocumented effects may be lethal (Bennett 1991).

In summary, no terrestrial vertebrates seem immune to the myriad of road-associated factors that can degrade habitat or increase mortality. These multifaceted effects have strong management implications for landscapes characterized by moderate to high densities of roads. In such landscapes, habitats are likely underused by many species that are negatively affected by road-associated factors. Moderate or high densities of roads sometimes create population sinks in areas that otherwise would be excellent environments.

See also:  Effects of Roads on Roadkill