Roads: A Big Impact on Small Mammals
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Small mammals are an important component of many ecosystems, occupying an intermediate trophic position. They are prey for many reptilian, avian, and mammalian carnivores, including several sensitive species. Small mammals consume vegetation, seeds, fungi, and insects. Grazing by small mammals can stimulate plant production and substantially alter species composition of plant communities. Small mammals impact plants through seed consumption, which often destroys seeds, and seed caching, which disperses seeds. Also, small mammals are important consumers of hypogeous fungi, which are dependent on animals for spore dispersal.
Roads impact small mammal species in several ways. Roadside habitats often contain different small mammal species than habitats further from roads. Small mammals along roads may be exposed to increased predation and vehicular pollution. Roads also can be barriers for small mammals, resulting in reduced habitat connectivity.
Alteration of Habitat along Roads
Road construction brings about a significant habitat alteration that affects many small mammal species. In addition to the direct habitat loss that is incurred when roads are constructed, road verges (the vegetation adjacent to roadways) are usually maintained as short herbaceous vegetation through the use of mowing or herbicides. Road verge vegetation is often drastically different from the adjacent unaltered habitat. Road verge habitat may not be suitable for native small mammal species that occupy habitat further from roads. Species known to avoid road verge habitat include golden mice (Ochrotomys nuttalli) in the Southeastern United States, deer mice (Peromyscus maniculatus) in the Midwest, and Western red-backed voles (Clethrionomys californicus) and dusky-footed woodrats (Neotoma fuscipes) in the Northwest (Adams and Geis 1983). Also, some small mammal species that are sensitive to edge habitats, such as southern red-backed voles (Clethrionomys gapperi), may avoid edges along road verges (Palman 1977).
Invasions of Nonnative Small Mammals
The altered habitat conditions on road verges allow nonnative species to invade new areas that were previously inaccessible to them. In Illinois, the meadow vole (Microtus pennsylvanicus) was able to expand its range along grassy highway verges. This species was then able to outcompete the closely related native prairie vole (Microtus ochrogaster) in some grasslands connected to the road verges (Getz et al 1978). Habitat conditions next to a California highway were altered such that pocket gophers (Thomomys) extended their range into the desert along the highway (Huey 1942). In some cases, road verges are habitable for native small mammals, but those species are excluded by new species that are better adapted to the road verge habitat (Goosem 1997).
Increased Predation along Roads
Generalist predators, including coyotes (Canis latrans) and dingos (Canis dingo), frequently hunt along roads (May and Norton 1996). Also, domestic dogs (Canis familiaris) and cats (Felis catus) commonly forage along edge habitats near human dwellings (Oehler and Litvaitis 1996), and along roadsides (Bennett 1990). Consequently, small mammals occupying roadside habitat may be more vulnerable to predation than small mammals inhabiting areas further from roads. Due to roadside firewood collecting and salvage logging, fewer logs near roads reduces small mammal protective cover, increasing their exposure to generalist predators.
Pollution along Roadways
Small mammals living along roads are exposed to a variety of pollutants from vehicles and road maintenance (see The Road-RIPorter 3:2). The roadside pollutant about which the most research has been done is lead. Small mammals living within 10m of roads in Illinois had significantly greater lead concentrations than those living 50m from roads. Individuals along heavily trafficked highways had higher lead levels than those living near low use roads. The insectivorous northern short-tailed shrews (Blarina brevicauda) and least shrews (Cryptotis parva) had higher lead levels than did herbivorous prairie voles (Getz et al. 1977). In another study, the bones of deer mice living along a Colorado highway had 10 times as much lead as deer mice 800m from the highway (Mierau and Favara 1975). Fortunately, the use of unleaded gases has reduced lead pollution along roadways.
Roads as a Barrier
Wide, heavily trafficked highways create a formidable obstacle to small mammal movements through the landscape (Richardson et al. 1997). Forest dwelling small mammals avoid open spaces, and they avoid venturing onto highways with a wide clearance between forest margins (Oxley et al. 1974). Even desert-dwelling small mammals, which are adapted to open country, find highways to be an obstacle. In a 1 year study of a Nevada desert highway, only 1 of 387 tagged small mammals was recorded moving across the highway (Garland and Bradley 1984).
Narrow roads also can hinder small mammal movements. A forest road not open to public traffic had only 2 recorded yellow-necked mouse (Apodemus flavicollis) road crossings, while movements of an equal distance through interior habitat were 14 times greater (Mader 1984). Even seemingly harmless tire tracks running through a field had a repulsive effect on prairie voles and cotton rats (Sigmodon hispidus) (Swihart and Slade 1984).
Small mammals that have low mobility, are behaviorally rigid, or have special habitat requirements, are expected to be most affected by roads. Species with greater mobility may perceive a road to be less of an environmental discontinuity than do species with limited mobility. In an Australian rainforest, the highly mobile giant white-tailed rat (Uromys caudimaculatus) crossed roads at a much greater rate than the less mobile southern bush-rat (Rattus fuscipes) (Burnett1992). Species that are behaviorally rigid or have special habitat requirements may not be able to cross roads under the best of conditions. The mountain pygmy-possum (Burramys parvus), an Australian mammal whose movements are restricted to rock substrates, suffered reduced survival when a road bisected its habitat. When a tunnel with rocks was constructed underneath the road, the mountain pygmy-possum's survival rate substantially increased (Mansergh and Scotts 1989).
Conclusions
Research has revealed many of the ways that roads impact small mammals. However, there are still many questions remaining about the ecology of small mammals in roaded environments. How do roads affect predator-prey relationships? What impact do small mammals colonizing an area via road-verges have on the native small mammals in adjacent habitat? Do wide roads reduce small mammal movements to the extent that genetic isolation results? With our incomplete understanding of the effects of roads on small mammals in ecological systems, it is evident that more research is needed. In the mean time, we should err on the side of caution in road planning to preserve the integrity of those ecological systems.
Diane Randgaard is completing her M.S. degree in Environmental Science at Western Washington University. Her thesis research examined the effects of forest roads on small mammal movements in the North Cascades of Washington state.
Bibliography
Adams, L. W., and A. D. Geis. 1983. Effects of roads on small mammals. Journal of Applied Ecology 20: 403-415.
Bennett, A. F. 1990. Land use, forest fragmentation and the mammalian fauna at Naringal, South-western Victoria. Australian Wildlife Research 17: 325-347.
Burnett, S. E. 1992. Effects of a rainforest road on movements of small mammals: mechanisms and implications. Wildlife Research 19: 95-104.
Garland, T. Jr., and W. G. Bradley. 1984. Effects of a highway on Mojave Desert rodent populations. American Midland Naturalist 111: 47-56.
Getz, L. L., F. R. Cole, and D. L. Gates. 1978. Interstate roadsides as dispersal routes for Microtus pennsylvanicus. Journal of Mammalogy 59: 208-212.
Getz, L. L., L. Verner, and M. Prather. 1977. Lead concentrations in small mammals living near highways. Environmental Pollution 13: 151-157.
Goosem, M. 1997. Internal fragmentation: the effects of roads, highways, and powerline clearings on movements and mortality of rainforest vertebrates. Pages 241-255 in Laurance, W. F., and R. O. Bierregaard, Jr., eds. Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities. University of Chicago Press, Chicago, IL. 616 p.
Huey, L. M. 1941. Mammalian invasion via the highway. Journal of Mammalogy 22: 383-385.
Mader, H. J. 1984. Animal habitat isolation by roads and agricultural fields. Biological Conservation 29: 81-96.
Mansergh, I. M., and D. J. Scotts. 1989. Habitat continuity and social organization of the mountain pygmy-possum restored by tunnel. Journal of Wildlife Management 53: 701-707.
May, S. A., and T. W. Norton. 1996. Influence of fragmentation and disturbance on the potential impact of feral predators on native fauna in Australian forest ecosystems. Wildlife Research 23: 387-400.
Mierau, G. W., and B. W. Favara. 1975. Lead poisoning in roadside populations of deer mice. Environmental Pollution 8: 55-64.
Oehler, J. D., and J. A. Litvaitis. 1996. The role of spatial scale in understanding responses of medium-sized carnivores to forest fragmentation. Canadian Journal of Zoology 74: 2070-2079.
Oxley, D. J., M. B. Fenton, and G. R. Carmody. 1974. The effects of roads on populations of small mammals. Journal of Applied Ecology 11: 51-59.
Palman, D. S. 1977. Ecological Impact of Interstate 95 on Small and Medium-sized Mammals in Northern Maine. M.S. Thesis, University of Maine at Orono.
Richardson, J. H., R. F. Shore, J. R. Treweek. 1997. Are major roads a barrier to small mammals? Journal of Zoology 243: 846-853.
Swihart, R. K., and N. A. Slade. 1984. Road crossing in Sigmodon hispidus and Microtus ochrogaster. Journal of Mammalogy 65: 357-360.
Roads impact small mammal species in several ways. Roadside habitats often contain different small mammal species than habitats further from roads. Small mammals along roads may be exposed to increased predation and vehicular pollution. Roads also can be barriers for small mammals, resulting in reduced habitat connectivity.
Alteration of Habitat along Roads
Road construction brings about a significant habitat alteration that affects many small mammal species. In addition to the direct habitat loss that is incurred when roads are constructed, road verges (the vegetation adjacent to roadways) are usually maintained as short herbaceous vegetation through the use of mowing or herbicides. Road verge vegetation is often drastically different from the adjacent unaltered habitat. Road verge habitat may not be suitable for native small mammal species that occupy habitat further from roads. Species known to avoid road verge habitat include golden mice (Ochrotomys nuttalli) in the Southeastern United States, deer mice (Peromyscus maniculatus) in the Midwest, and Western red-backed voles (Clethrionomys californicus) and dusky-footed woodrats (Neotoma fuscipes) in the Northwest (Adams and Geis 1983). Also, some small mammal species that are sensitive to edge habitats, such as southern red-backed voles (Clethrionomys gapperi), may avoid edges along road verges (Palman 1977).
Invasions of Nonnative Small Mammals
The altered habitat conditions on road verges allow nonnative species to invade new areas that were previously inaccessible to them. In Illinois, the meadow vole (Microtus pennsylvanicus) was able to expand its range along grassy highway verges. This species was then able to outcompete the closely related native prairie vole (Microtus ochrogaster) in some grasslands connected to the road verges (Getz et al 1978). Habitat conditions next to a California highway were altered such that pocket gophers (Thomomys) extended their range into the desert along the highway (Huey 1942). In some cases, road verges are habitable for native small mammals, but those species are excluded by new species that are better adapted to the road verge habitat (Goosem 1997).
Increased Predation along Roads
Generalist predators, including coyotes (Canis latrans) and dingos (Canis dingo), frequently hunt along roads (May and Norton 1996). Also, domestic dogs (Canis familiaris) and cats (Felis catus) commonly forage along edge habitats near human dwellings (Oehler and Litvaitis 1996), and along roadsides (Bennett 1990). Consequently, small mammals occupying roadside habitat may be more vulnerable to predation than small mammals inhabiting areas further from roads. Due to roadside firewood collecting and salvage logging, fewer logs near roads reduces small mammal protective cover, increasing their exposure to generalist predators.
Pollution along Roadways
Small mammals living along roads are exposed to a variety of pollutants from vehicles and road maintenance (see The Road-RIPorter 3:2). The roadside pollutant about which the most research has been done is lead. Small mammals living within 10m of roads in Illinois had significantly greater lead concentrations than those living 50m from roads. Individuals along heavily trafficked highways had higher lead levels than those living near low use roads. The insectivorous northern short-tailed shrews (Blarina brevicauda) and least shrews (Cryptotis parva) had higher lead levels than did herbivorous prairie voles (Getz et al. 1977). In another study, the bones of deer mice living along a Colorado highway had 10 times as much lead as deer mice 800m from the highway (Mierau and Favara 1975). Fortunately, the use of unleaded gases has reduced lead pollution along roadways.
Roads as a Barrier
Wide, heavily trafficked highways create a formidable obstacle to small mammal movements through the landscape (Richardson et al. 1997). Forest dwelling small mammals avoid open spaces, and they avoid venturing onto highways with a wide clearance between forest margins (Oxley et al. 1974). Even desert-dwelling small mammals, which are adapted to open country, find highways to be an obstacle. In a 1 year study of a Nevada desert highway, only 1 of 387 tagged small mammals was recorded moving across the highway (Garland and Bradley 1984).
Narrow roads also can hinder small mammal movements. A forest road not open to public traffic had only 2 recorded yellow-necked mouse (Apodemus flavicollis) road crossings, while movements of an equal distance through interior habitat were 14 times greater (Mader 1984). Even seemingly harmless tire tracks running through a field had a repulsive effect on prairie voles and cotton rats (Sigmodon hispidus) (Swihart and Slade 1984).
Small mammals that have low mobility, are behaviorally rigid, or have special habitat requirements, are expected to be most affected by roads. Species with greater mobility may perceive a road to be less of an environmental discontinuity than do species with limited mobility. In an Australian rainforest, the highly mobile giant white-tailed rat (Uromys caudimaculatus) crossed roads at a much greater rate than the less mobile southern bush-rat (Rattus fuscipes) (Burnett1992). Species that are behaviorally rigid or have special habitat requirements may not be able to cross roads under the best of conditions. The mountain pygmy-possum (Burramys parvus), an Australian mammal whose movements are restricted to rock substrates, suffered reduced survival when a road bisected its habitat. When a tunnel with rocks was constructed underneath the road, the mountain pygmy-possum's survival rate substantially increased (Mansergh and Scotts 1989).
Conclusions
Research has revealed many of the ways that roads impact small mammals. However, there are still many questions remaining about the ecology of small mammals in roaded environments. How do roads affect predator-prey relationships? What impact do small mammals colonizing an area via road-verges have on the native small mammals in adjacent habitat? Do wide roads reduce small mammal movements to the extent that genetic isolation results? With our incomplete understanding of the effects of roads on small mammals in ecological systems, it is evident that more research is needed. In the mean time, we should err on the side of caution in road planning to preserve the integrity of those ecological systems.
Diane Randgaard is completing her M.S. degree in Environmental Science at Western Washington University. Her thesis research examined the effects of forest roads on small mammal movements in the North Cascades of Washington state.
Bibliography
Adams, L. W., and A. D. Geis. 1983. Effects of roads on small mammals. Journal of Applied Ecology 20: 403-415.
Bennett, A. F. 1990. Land use, forest fragmentation and the mammalian fauna at Naringal, South-western Victoria. Australian Wildlife Research 17: 325-347.
Burnett, S. E. 1992. Effects of a rainforest road on movements of small mammals: mechanisms and implications. Wildlife Research 19: 95-104.
Garland, T. Jr., and W. G. Bradley. 1984. Effects of a highway on Mojave Desert rodent populations. American Midland Naturalist 111: 47-56.
Getz, L. L., F. R. Cole, and D. L. Gates. 1978. Interstate roadsides as dispersal routes for Microtus pennsylvanicus. Journal of Mammalogy 59: 208-212.
Getz, L. L., L. Verner, and M. Prather. 1977. Lead concentrations in small mammals living near highways. Environmental Pollution 13: 151-157.
Goosem, M. 1997. Internal fragmentation: the effects of roads, highways, and powerline clearings on movements and mortality of rainforest vertebrates. Pages 241-255 in Laurance, W. F., and R. O. Bierregaard, Jr., eds. Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities. University of Chicago Press, Chicago, IL. 616 p.
Huey, L. M. 1941. Mammalian invasion via the highway. Journal of Mammalogy 22: 383-385.
Mader, H. J. 1984. Animal habitat isolation by roads and agricultural fields. Biological Conservation 29: 81-96.
Mansergh, I. M., and D. J. Scotts. 1989. Habitat continuity and social organization of the mountain pygmy-possum restored by tunnel. Journal of Wildlife Management 53: 701-707.
May, S. A., and T. W. Norton. 1996. Influence of fragmentation and disturbance on the potential impact of feral predators on native fauna in Australian forest ecosystems. Wildlife Research 23: 387-400.
Mierau, G. W., and B. W. Favara. 1975. Lead poisoning in roadside populations of deer mice. Environmental Pollution 8: 55-64.
Oehler, J. D., and J. A. Litvaitis. 1996. The role of spatial scale in understanding responses of medium-sized carnivores to forest fragmentation. Canadian Journal of Zoology 74: 2070-2079.
Oxley, D. J., M. B. Fenton, and G. R. Carmody. 1974. The effects of roads on populations of small mammals. Journal of Applied Ecology 11: 51-59.
Palman, D. S. 1977. Ecological Impact of Interstate 95 on Small and Medium-sized Mammals in Northern Maine. M.S. Thesis, University of Maine at Orono.
Richardson, J. H., R. F. Shore, J. R. Treweek. 1997. Are major roads a barrier to small mammals? Journal of Zoology 243: 846-853.
Swihart, R. K., and N. A. Slade. 1984. Road crossing in Sigmodon hispidus and Microtus ochrogaster. Journal of Mammalogy 65: 357-360.