Effect of Roads on Arthropods
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Arthropods make up a significant part of the biodiversity on this planet, and are important in many ways to the overall health of ecosystems and to our understanding of natural biotic systems. At the base of many food chains, arthropods are important components of the diets of invertebrates and birds, and are also an integral part of the nutrient- and energy-processing abilities of the soil (Coleman & Crossley 1996). Arthropods also tend to demonstrate opportunism and rapid response to change. By studying arthropod responses to ecological change, we can better understand the effects of human disturbance and landscape modification on terrestrial systems (Morris 2000, Major et al 1999).
The impact of roads on arthropods is considerable. Roads affect terrestrial arthropods directly by destroying their habitat, and by increasing the risk of being crushed by vehicles or trampled by pedestrian traffic. Roads also fragment arthropod habitat, exacerbate the spread of exotic and invasive species, and create pollution in the air and on the ground.
Habitat Destruction
Roads destroy arthropod habitat on the road bed itself and on road verges by altering vegetation, changing soil dynamics, and modifying microclimates. The most obvious effect is the conversion of habitat into road surfaces. Vegetation is replaced by less permeable surfaces, thereby eradicating food sources, nesting areas, and hiding places that are essential to arthropod survival (Mader 1984). Similarly, soil dynamics are modified by road construction, which flattens terrestrial niches and causes substantial soil compaction. This contributes to increased runoff and decreased soil porosity, which impede arthropod survival in the immediate area of the road (Noss 1999). Microclimate also changes as a result of road construction. Road surfaces tend to absorb solar radiation at a higher rate than unmodified surfaces, increasing soil and air temperatures. Increased wind due to the removal of vegetation around the road, as well as the reduced capacity of the soil to retain moisture due to compaction, combine with these higher temperatures to create a more arid and hotter microclimate above the road surface (Haskell 2000).
The conversion of habitat along roadsides is also significant. Often, the disturbed areas on either side of a road support entirely different vegetation from that which was present before road construction; the effects upon forest fauna may persist up to 100 meters from the road itself (Haskell 2000). This altered vegetation sustains different species with varying success, due to changes in nesting habitat, food supply, and opportunities to hide from predators. Roadsides may receive a greater influx of nutrients from passing vehicles, increased water availability, and nearby agricultural landscapes, resulting in a greater abundance of weeds on roadsides (Major et al 1999). The composition of plant and animal species on road verges will differ notably from non-roadside habitats, which intensifies competition and broadens the disturbed area (Mader 1984). Even in cases where roadside plants remain the same, the physiology and growth of these plants, and the insects they sustain, often differs completely from areas that are more distant from roads (Martell 1995, Lightfoot & Whitford 1991, Spencer et al 1988).
Frequent mowing along roads contributes to the environmental instability of roaded areas (Morris 2000). A common response of arthropods to shortened vegetation is a reduction in the abundance and diversity of most groups and species. Such unstable conditions favor a few opportunistic and robust species, often nonnative, to the detriment of those that are slower to adapt (Morris 2000, Mader 1984, Hollifield & Dimmick 1995, Haskell 2000). The combined effects of changing roadside vegetation may lead to a more uniform set of species, an eventuality that increases the chance of local extinction, especially of small populations of flightless groundforaging insects (Vermeulen 1994). As previously mentioned, roadsides also change microclimates (Major et al 1999, Mader 1984). In one study, increased aridity and temperature on tropical forest roadsides diminished insect diversity up to 40 meters from the road, which acutely affected insectivore populations in the study area (Grindal and Brigham 1998).
Roadkill and Trampling
Although there are no figures for arthropod roadkill, it is known to have a major impact on roadside arthropod populations (Oxley & Fenton 1974, Mader 1984). In one study of ORV impacts on desert biota, it was observed that arthropod tracks were found 24 times as often on sites that were closed to motorized traffic as on ORV-impacted sites (Luckenbach and Bury 1983). Treading by humans has also been detrimental to arthropods. Even moderate trampling (5 treads per month) reduced a wide range of invertebrate species by up to 82 percent over a twelve-month period (Morris 2000). Road construction also contributes to direct mortality of slowermoving, flightless arthropods, which cannot avoid being crushed by construction machinery.
Habitat Fragmentation
Roads constitute major barriers to arthropod dispersal (Mader et al 1990, Haskell 2000, Vermeulen 1994). Linear barriers affect the movement of ground-dwelling animals, stimulating lengthwise dispersal and inhibiting lateral movement (Mader et al 1990). This reluctance of arthropods to cross roads may be due to changes in microclimate at road edges, pollution and noise from traffic, environmental instability, changes in the composition of flora and fauna along roads, and the immediate danger to animals of being killed by oncoming traffic. Carabid beetles have been found to avoid crossing road shoulders almost entirely, and were not observed ever crossing the road itself (Mader 1984). Despite the theoretical value of roads as connective corridors for recolonizing areas, studies on arthropods have shown that they tend to travel only short distances in a year. This suggests that roads act as a mechanism for the infiltration of opportunistic species to the detriment of local populations (Vermeulen 1993, Lightfoot 1991). Fragmentation, reduction, and isolation of carabid habitat are most likely the main causes of the significant decrease in the species since the last century (Turin 1989 in Vermeulen 1994).
Spread of Exotics, Invasives, and Opportunists
Roads intensify invasion by exotic and invasive species. Humans act as vehicles for the dispersal of exotics; roads provide movement corridors, create a disturbed environment for the establishment of opportunistic species, and alter vegetation, which differentially favors some species (often pioneers or invasives) over others (Simberloff 1989, Lightfoot 1991, Noss 1999). This has been demonstrated, to the great detriment of a number of agroecosystems and agricultural economies, by crop infestation by nonlocal pests (Fye 1980, Kemp & Barrett 1989, Snodgrass & Stadelbacher 1989, Oi and Barnes 1989). For example, the balsam wooly adelgid has nearly destroyed two varieties of firs in the southern Appalachians. It has been directly linked to roads as means of dispersal (Campbell 1996).
Pollution
Pollution caused by roads includes lead and fuel additive emissions from vehicles, salt from de-icing compounds, dust, ozone, exhaust fumes (cadmium, sulfur dioxide, nitrous oxides), and noise (Mader 1984, Hopkin and Howse 1998, Lightfoot & Whitford 1991, Oxley & Fenton 1974). Lead and other heavy metals from exhaust accumulate along roads and affect wildlife. A study near Washington, D.C. found increased lead, zinc, nickel, and cadmium in earthworms near roads (Oxley and Fenton 1974). Another study suggests the possible lead contamination of honey bees along a roadway (Pratt & Sikorski 1982 in Lightfoot and Whitford 1991).
Salt from de-icing compounds is the most apparent source of stress on roadside systems, since salt accumulation often leaves bare patches in vegetation and visibly damages trees on road verges (Spencer et al 1988). Salt has been shown to decrease predator efficiency, change the suitability of host plants for arthropods, alter interactions between herbivores and their natural enemies, and to drastically affect the health of roadside vegetation upon which arthropods are dependent (Martel 1995, Spencer et a, Hopkin and Howse 1995). Salt accumulation also degrades soil quality, reducing the suitability of roadside habitat for soil-dwelling arthropods.
Increased nitrogen levels on roadsides have been shown to increase productivity of vegetation, resulting in higher insect infestation. This increase in nitrogen may be due either to nitrous oxide emissions from vehicle exhaust, or to runoff from the road surface (Lightfoot and Whitford 1991). Not all studies have confirmed an increase in vegetal productivity, however. Overall, the roads tend to increase stress to roadside ecosystems, which makes for a less stable environment that is more vulnerable to disease and pest infestation.
Conclusion
Existing road networks affect arthropod populations by destroying habitat, changing interspecies relationships, fragmenting dispersal corridors, facilitating the introduction and establishment of exotics, and polluting biotic systems. While roads generally decrease species diversity, proper restoration techniques have increased species richness by as much as 300 percent (Hollifield and Dimmick 1995). In light of the reliance of higher-level taxa upon arthropods as a food source, as well as the importance of arthropods in processing soil nutrients and energy, the impacts of roads on arthropods are important for ecosystem conservation as a whole.
¤ Leslie Hannay is the Program Associate for Wildlands CPR.
Bibliography
Campbell, F.T. 1996. The invasion of exotics. Endangered Species Bulletin. March/April 1996.
Coleman, D.C. and D.A. Crossley, Jr. 1996. Fundamentals of soil ecology. Academic Press, San Diego.
Fye, R.E. 1980. Weed sources of Lygus Bugs in the Yakima Valley and Columbia Basin in Washington. J.Economic Entomology 73(3):469-473.
Grindal, S.D. and R.M. Brigham. 1998. Short-term effects of smallscale habitat disturbance on activity by insectivorous bats. J. Wildlife Management 62(3):996-1003.
Haskell, D.G. 2000. Effects of forest roads on macroinvertebrate soil fauna of the southern Appalachian Mountains. Conservation Biology 14(1):57-63.
Hollifield, B.K. and R.W. Dimmick. 1995. Arthropod abundance relative to forest management practices benefiting ruffed grouse in the southern Appalachians. Wildlife Society Bulletin 23(4):756-764.
Hopkin, A.A.and G.M. Howse. 1998. A survey to evaluate crown condition of forest, roadside, and urban maple trees in Ontario, 1987-1995. Northern J. Applied Forestry 15(3):141-145.
Kemp, J.C. and G.W. Barrett. 1989. Spatial patterning: impact of uncultivated corridors on arthropod populations within soybean agroecosystems. Ecology 70(1):114-128.
Lightfoot, D.C. and W.G. Whitford 1991. Productivity of Creosotebush foliage and associated canopy arthropods along a desert roadside. American Midland Naturalist 125:310-322.
Luckenbach, R.A. and R.B. Bury. 1983. Effects of off-road vehicles on the biota of the Algodones Dunes, Imperial County, California. J. Applied Ecology 20:265-286.
Mader, H.-J. 1984. Animal habitat isolation by roads and agricultural fields. Biological Conservation 29:81-96. Mader, H.-J., C. Schell, P. Kornacker. 1990. Linear barriers to arthropod movements in the landscape. Biological Conservation 54:209-222.
Major, R.E., D. Smith, G. Cassis, M. Gray, and D.J. Colgan. 1999. Are roadside strips important reservoirs of invertebrate diversity? A comparison of the ant and beetle faunas of roadside strips and large remnant woodlands. Australian J. Zoology 47:611-624.
Martel, J. 1995. Performance of Eurosta solidaginis (diptera: Tephritidae) and Epiblema scudderiana (Lepidoptera: Tortricidae), Two gall-formers of Goldenrod, in roadside environments. Environmental Entomology 24(3):697-706.
Morris, M.G. 2000. The effects of structure and its dynamics on the ecology and conservation of arthropods in British grasslands. Biological Conservation 95: 129-142.
Noss, R. 1995. The ecological effects of roads, or, the road to destruction. The Road Ripperøs Handbook, Wildlands Center for Preventing Roads, Missoula, MT.
Oi, D.H. and M.M. Barnes. 1989. Predation by the western predatory mite (Acari: Phytoseiidae) on the Pacific spider mite (Acari: Tetranychidae) in the presence of road dust. Environmental Entomology 18(5):892-896.
Oxley, D.J. and M.B. Fenton. 1974. The harm our roads do to nature and wildlife. J. Applied Ecology 11:51-59
Simberloff, D. 1989. Which insect introductions succeed and which fail? in Biological Invasions: A Global Perspective. Edited by J.A. Drake, H.A. Mooney, F. diCastri, R.H. Groves, F.J. Kruger, M. Rejmanek andM.Williamson, New York: Wiley.
Snodgrass, G.L. and E.A. Stadelbacher. 1989. Effect of different grass and legume combinations on spider and ground beetle populations in roadside habitats in the Mississippi Delta. Environmental Entomology 18(4):575-581.
Spencer, H.J. and G.R. Port. 1988. Effects of roadside conditions on plants and insects. II. Soil conditions. J. Applied Ecology 25:709-715.
Spencer, H.J., N.E. Scott, G. R. Port, and A. W. Davison. 1988. Effects of roadside conditions on plants and insects. I. Atmospheric conditions. J. Applied Ecology 25:699-707.
Vermeulen, H.J.W. 1994. Corridor function of a road verge for dispersal of stenotopic heathland ground beetles Carabidae. Biological Conservation 69:339-349.
The impact of roads on arthropods is considerable. Roads affect terrestrial arthropods directly by destroying their habitat, and by increasing the risk of being crushed by vehicles or trampled by pedestrian traffic. Roads also fragment arthropod habitat, exacerbate the spread of exotic and invasive species, and create pollution in the air and on the ground.
Habitat Destruction
Roads destroy arthropod habitat on the road bed itself and on road verges by altering vegetation, changing soil dynamics, and modifying microclimates. The most obvious effect is the conversion of habitat into road surfaces. Vegetation is replaced by less permeable surfaces, thereby eradicating food sources, nesting areas, and hiding places that are essential to arthropod survival (Mader 1984). Similarly, soil dynamics are modified by road construction, which flattens terrestrial niches and causes substantial soil compaction. This contributes to increased runoff and decreased soil porosity, which impede arthropod survival in the immediate area of the road (Noss 1999). Microclimate also changes as a result of road construction. Road surfaces tend to absorb solar radiation at a higher rate than unmodified surfaces, increasing soil and air temperatures. Increased wind due to the removal of vegetation around the road, as well as the reduced capacity of the soil to retain moisture due to compaction, combine with these higher temperatures to create a more arid and hotter microclimate above the road surface (Haskell 2000).
The conversion of habitat along roadsides is also significant. Often, the disturbed areas on either side of a road support entirely different vegetation from that which was present before road construction; the effects upon forest fauna may persist up to 100 meters from the road itself (Haskell 2000). This altered vegetation sustains different species with varying success, due to changes in nesting habitat, food supply, and opportunities to hide from predators. Roadsides may receive a greater influx of nutrients from passing vehicles, increased water availability, and nearby agricultural landscapes, resulting in a greater abundance of weeds on roadsides (Major et al 1999). The composition of plant and animal species on road verges will differ notably from non-roadside habitats, which intensifies competition and broadens the disturbed area (Mader 1984). Even in cases where roadside plants remain the same, the physiology and growth of these plants, and the insects they sustain, often differs completely from areas that are more distant from roads (Martell 1995, Lightfoot & Whitford 1991, Spencer et al 1988).
Frequent mowing along roads contributes to the environmental instability of roaded areas (Morris 2000). A common response of arthropods to shortened vegetation is a reduction in the abundance and diversity of most groups and species. Such unstable conditions favor a few opportunistic and robust species, often nonnative, to the detriment of those that are slower to adapt (Morris 2000, Mader 1984, Hollifield & Dimmick 1995, Haskell 2000). The combined effects of changing roadside vegetation may lead to a more uniform set of species, an eventuality that increases the chance of local extinction, especially of small populations of flightless groundforaging insects (Vermeulen 1994). As previously mentioned, roadsides also change microclimates (Major et al 1999, Mader 1984). In one study, increased aridity and temperature on tropical forest roadsides diminished insect diversity up to 40 meters from the road, which acutely affected insectivore populations in the study area (Grindal and Brigham 1998).
Roadkill and Trampling
Although there are no figures for arthropod roadkill, it is known to have a major impact on roadside arthropod populations (Oxley & Fenton 1974, Mader 1984). In one study of ORV impacts on desert biota, it was observed that arthropod tracks were found 24 times as often on sites that were closed to motorized traffic as on ORV-impacted sites (Luckenbach and Bury 1983). Treading by humans has also been detrimental to arthropods. Even moderate trampling (5 treads per month) reduced a wide range of invertebrate species by up to 82 percent over a twelve-month period (Morris 2000). Road construction also contributes to direct mortality of slowermoving, flightless arthropods, which cannot avoid being crushed by construction machinery.
Habitat Fragmentation
Roads constitute major barriers to arthropod dispersal (Mader et al 1990, Haskell 2000, Vermeulen 1994). Linear barriers affect the movement of ground-dwelling animals, stimulating lengthwise dispersal and inhibiting lateral movement (Mader et al 1990). This reluctance of arthropods to cross roads may be due to changes in microclimate at road edges, pollution and noise from traffic, environmental instability, changes in the composition of flora and fauna along roads, and the immediate danger to animals of being killed by oncoming traffic. Carabid beetles have been found to avoid crossing road shoulders almost entirely, and were not observed ever crossing the road itself (Mader 1984). Despite the theoretical value of roads as connective corridors for recolonizing areas, studies on arthropods have shown that they tend to travel only short distances in a year. This suggests that roads act as a mechanism for the infiltration of opportunistic species to the detriment of local populations (Vermeulen 1993, Lightfoot 1991). Fragmentation, reduction, and isolation of carabid habitat are most likely the main causes of the significant decrease in the species since the last century (Turin 1989 in Vermeulen 1994).
Spread of Exotics, Invasives, and Opportunists
Roads intensify invasion by exotic and invasive species. Humans act as vehicles for the dispersal of exotics; roads provide movement corridors, create a disturbed environment for the establishment of opportunistic species, and alter vegetation, which differentially favors some species (often pioneers or invasives) over others (Simberloff 1989, Lightfoot 1991, Noss 1999). This has been demonstrated, to the great detriment of a number of agroecosystems and agricultural economies, by crop infestation by nonlocal pests (Fye 1980, Kemp & Barrett 1989, Snodgrass & Stadelbacher 1989, Oi and Barnes 1989). For example, the balsam wooly adelgid has nearly destroyed two varieties of firs in the southern Appalachians. It has been directly linked to roads as means of dispersal (Campbell 1996).
Pollution
Pollution caused by roads includes lead and fuel additive emissions from vehicles, salt from de-icing compounds, dust, ozone, exhaust fumes (cadmium, sulfur dioxide, nitrous oxides), and noise (Mader 1984, Hopkin and Howse 1998, Lightfoot & Whitford 1991, Oxley & Fenton 1974). Lead and other heavy metals from exhaust accumulate along roads and affect wildlife. A study near Washington, D.C. found increased lead, zinc, nickel, and cadmium in earthworms near roads (Oxley and Fenton 1974). Another study suggests the possible lead contamination of honey bees along a roadway (Pratt & Sikorski 1982 in Lightfoot and Whitford 1991).
Salt from de-icing compounds is the most apparent source of stress on roadside systems, since salt accumulation often leaves bare patches in vegetation and visibly damages trees on road verges (Spencer et al 1988). Salt has been shown to decrease predator efficiency, change the suitability of host plants for arthropods, alter interactions between herbivores and their natural enemies, and to drastically affect the health of roadside vegetation upon which arthropods are dependent (Martel 1995, Spencer et a, Hopkin and Howse 1995). Salt accumulation also degrades soil quality, reducing the suitability of roadside habitat for soil-dwelling arthropods.
Increased nitrogen levels on roadsides have been shown to increase productivity of vegetation, resulting in higher insect infestation. This increase in nitrogen may be due either to nitrous oxide emissions from vehicle exhaust, or to runoff from the road surface (Lightfoot and Whitford 1991). Not all studies have confirmed an increase in vegetal productivity, however. Overall, the roads tend to increase stress to roadside ecosystems, which makes for a less stable environment that is more vulnerable to disease and pest infestation.
Conclusion
Existing road networks affect arthropod populations by destroying habitat, changing interspecies relationships, fragmenting dispersal corridors, facilitating the introduction and establishment of exotics, and polluting biotic systems. While roads generally decrease species diversity, proper restoration techniques have increased species richness by as much as 300 percent (Hollifield and Dimmick 1995). In light of the reliance of higher-level taxa upon arthropods as a food source, as well as the importance of arthropods in processing soil nutrients and energy, the impacts of roads on arthropods are important for ecosystem conservation as a whole.
¤ Leslie Hannay is the Program Associate for Wildlands CPR.
Bibliography
Campbell, F.T. 1996. The invasion of exotics. Endangered Species Bulletin. March/April 1996.
Coleman, D.C. and D.A. Crossley, Jr. 1996. Fundamentals of soil ecology. Academic Press, San Diego.
Fye, R.E. 1980. Weed sources of Lygus Bugs in the Yakima Valley and Columbia Basin in Washington. J.Economic Entomology 73(3):469-473.
Grindal, S.D. and R.M. Brigham. 1998. Short-term effects of smallscale habitat disturbance on activity by insectivorous bats. J. Wildlife Management 62(3):996-1003.
Haskell, D.G. 2000. Effects of forest roads on macroinvertebrate soil fauna of the southern Appalachian Mountains. Conservation Biology 14(1):57-63.
Hollifield, B.K. and R.W. Dimmick. 1995. Arthropod abundance relative to forest management practices benefiting ruffed grouse in the southern Appalachians. Wildlife Society Bulletin 23(4):756-764.
Hopkin, A.A.and G.M. Howse. 1998. A survey to evaluate crown condition of forest, roadside, and urban maple trees in Ontario, 1987-1995. Northern J. Applied Forestry 15(3):141-145.
Kemp, J.C. and G.W. Barrett. 1989. Spatial patterning: impact of uncultivated corridors on arthropod populations within soybean agroecosystems. Ecology 70(1):114-128.
Lightfoot, D.C. and W.G. Whitford 1991. Productivity of Creosotebush foliage and associated canopy arthropods along a desert roadside. American Midland Naturalist 125:310-322.
Luckenbach, R.A. and R.B. Bury. 1983. Effects of off-road vehicles on the biota of the Algodones Dunes, Imperial County, California. J. Applied Ecology 20:265-286.
Mader, H.-J. 1984. Animal habitat isolation by roads and agricultural fields. Biological Conservation 29:81-96. Mader, H.-J., C. Schell, P. Kornacker. 1990. Linear barriers to arthropod movements in the landscape. Biological Conservation 54:209-222.
Major, R.E., D. Smith, G. Cassis, M. Gray, and D.J. Colgan. 1999. Are roadside strips important reservoirs of invertebrate diversity? A comparison of the ant and beetle faunas of roadside strips and large remnant woodlands. Australian J. Zoology 47:611-624.
Martel, J. 1995. Performance of Eurosta solidaginis (diptera: Tephritidae) and Epiblema scudderiana (Lepidoptera: Tortricidae), Two gall-formers of Goldenrod, in roadside environments. Environmental Entomology 24(3):697-706.
Morris, M.G. 2000. The effects of structure and its dynamics on the ecology and conservation of arthropods in British grasslands. Biological Conservation 95: 129-142.
Noss, R. 1995. The ecological effects of roads, or, the road to destruction. The Road Ripperøs Handbook, Wildlands Center for Preventing Roads, Missoula, MT.
Oi, D.H. and M.M. Barnes. 1989. Predation by the western predatory mite (Acari: Phytoseiidae) on the Pacific spider mite (Acari: Tetranychidae) in the presence of road dust. Environmental Entomology 18(5):892-896.
Oxley, D.J. and M.B. Fenton. 1974. The harm our roads do to nature and wildlife. J. Applied Ecology 11:51-59
Simberloff, D. 1989. Which insect introductions succeed and which fail? in Biological Invasions: A Global Perspective. Edited by J.A. Drake, H.A. Mooney, F. diCastri, R.H. Groves, F.J. Kruger, M. Rejmanek andM.Williamson, New York: Wiley.
Snodgrass, G.L. and E.A. Stadelbacher. 1989. Effect of different grass and legume combinations on spider and ground beetle populations in roadside habitats in the Mississippi Delta. Environmental Entomology 18(4):575-581.
Spencer, H.J. and G.R. Port. 1988. Effects of roadside conditions on plants and insects. II. Soil conditions. J. Applied Ecology 25:709-715.
Spencer, H.J., N.E. Scott, G. R. Port, and A. W. Davison. 1988. Effects of roadside conditions on plants and insects. I. Atmospheric conditions. J. Applied Ecology 25:699-707.
Vermeulen, H.J.W. 1994. Corridor function of a road verge for dispersal of stenotopic heathland ground beetles Carabidae. Biological Conservation 69:339-349.