NRDC-End of the road.

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Chapter 7


Key Finding: Roads were associated with a diversity of negative effects on the biotic integrity of both terrestrial and aquatic ecosystems.

Source: Trombulak, S. C. and C. A. Frissell. In press. Review of ecological effects of roads on terrestrial and aquatic communities. Conservation Biology.

A review of the scientific literature reveals seven general effects of roads of all kinds on the ecosystem. 1) Road construction resulted in the death or injury of roadside plants or slow-moving animals, compacted soils, and affected water bodies at road crossings. 2) Roadkill affected the demography of numerous species. 3) Animal behavior changed due to roads, with avoidance of roads, modification of movement patterns or home ranges, changes in reproductive success, escape behavior, or physiological state. 4) Roads disrupted the physical environment by changing soil characteristics such as density, surface runoff, and sedimentation. They altered the hydrology of slopes and stream channels, created barriers to the movement of fish and other aquatic animals, and altered channel and shoreline development. 5) Roads affected the chemical environment by contributing pollutants such as heavy metals, salts, or nutrients to roadside plant and animal communities as well as to aquatic ecosystems through runoff. 6) Roads promoted the spread of exotic species. 7) Roads increased access by humans, and therefore increased poaching pressure, fishing, and passive harassment of animals.

Key Finding: Based on numerous studies on the ecological impact of roads, 15-20% of the United States land area was estimated to be affected by roads.

Source: Forman, R. T. T. and L. E. Alexander. 1998. Roads and their major ecological effects. Annual Reviews of Ecology and Systematics 29: 207-231.

The authors review the scientific literature available from North America, Europe, and Australia on the impacts of roads on plant and animal populations. Roads affected vegetative communities through facilitating disturbance-tolerant plant species and introducing exotic species. Road avoidance, barrier effects, and roadkill were documented as major factors affecting animal populations. The impacts of roads on aquatic systems were also reviewed, including altered runoff patterns, increased sediment input, disrupted hydrology, and delivery of pollutants such as heavy metals and deicing salts into streams. The authors report that the ecological effect of roads extended into the landscape beyond the road itself into a broader area that they called “the road-effect zone,” based on which they estimated that 15-20% of the United States land area is affected by roads.

Key Finding: The zone of ecological effects surrounding roads averaged more than 600 m wide, and for some factors extended more than 1 km from the road surface.

Source: Forman, R. T. T. In press. The ecological road-effect zone of a Massachusetts (USA) suburban highway. Conservation Biology .

The authors investigated the area to which the ecological effects of a road can extend outward, into a zone termed the “road-effect zone.” Nine components of the ecosystem were specifically examined. The authors collected data on some criteria from a 25-km stretch of highway in Massachusetts, and used information from other scientists’ research for the remaining factors.

Habitat invasion by roadside exotic plants was investigated by searches through 1 ha each of 22 adjacent woodland locations. Eleven (50%) of the woodland sites showed evidence of invasion by exotic species such as privet or Norway maple up to distances of 120 m. Research by other authors showed that migration by salamanders in the spring was prevented by roads, with populations hindered from traveling several hundred meters to vernal pools on the other side of the highway. Bird communities were negatively affected by roads for hundreds of meters. This was hypothesized to be primarily due to traffic noise and its possible interference with bird communication during the breeding period. The population density of forest interior birds was reduced as far as 650 m from a road. Grassland birds were reduced in number for up to 1-2 km from a roadside. For mammals such as deer and black bears, the highway destroyed suitable habitat and interrupted natural travel corridors.

The effect of the Massachusetts highway on local streams and wetlands was calculated. Of the 13 streams and intermittent channels crossed by the 25 km of highway, almost all had been channelized upslope or downslope for distances ranging from 30-500 m. Of the nine wetlands crossed or adjacent to the highway, all showed evidence of drainage, and five wetlands were affected at distances of 100 m or more from the road. Evidence of road salt pollution was found in a reservoir 1 km away from the highway. Information collected from natural resource specialists indicated that road salt had moved into groundwater and polluted wells at other locations.

They authors note the variability of road-effect zones, with greater effects in some locations and lesser effects in others. They conclude that the average distance of direct ecological impacts was more than 300 m, with a road-effect zone therefore of more than 600 m.

Key Finding: Roads are a major cause of forest fragmentation because they divide large landscape patches into smaller patches and convert forest interior habitat into edge habitat.

Key Finding: Clearcuts and roads affected 2.5 to 3.5 times more of the landscape than the surface area occupied by the actual clearcuts and roads themselves.

Source: Reed, R. A., J. Johnson-Barnard and W. L. Baker. 1996. Contribution of roads to forest fragmentation in the Rocky Mountains. Conservation Biology 10: 1098-1106.

Fragmentation due to roads was quantified in a 30,123-ha area of the Medicine Bow-Routt National Forest in southeastern Wyoming. A geographic information system was used to analyze landscape structure. Forest patch and edge-related landscape changes were measured using several indices: the number of patches, mean patch area, mean interior area, mean area of edge influence, mean patch perimeter, total perimeter, and mean patch shape.

Roads contributed to forest fragmentation more than clearcuts in the study area since they dissected large forest patches into smaller fragments. They also converted more forest interior habitat into edge habitat. The edge habitat due to roads was 1.54 to 1.98 times the edge habitat created by clearcuts. Taking these factors into account, the authors calculated that together, clearcuts and roads affected 2.5 to 3.5 times more of the landscape than the area occupied by the actual clearcuts and roads themselves.

Key Finding: Road networks affected stream systems, increasing the frequency and/or magnitude of peak flows, debris flows, and landslides.

Source: Jones, J. A., F. J. Swanson, B. C. Wemple and K. U. Snyder. In press. A perspective on road effects on hydrology, geomorphology, and disturbance patches in stream networks. Conservation Biology.

The authors review research on the impact of road networks on stream networks. They looked at two key processes influencing riparian vegetation and channel morphology: peak flows (floods) and debris flows. Fifty years of research on biophysical processes on watersheds in the H. J. Andrews Experimental Forest in Oregon provided evidence for the impacts of roads. The road network was found to be hydrologically connected to the stream network and increased the frequency and/or magnitude of peak flows, particularly in small basins. Roads and logging together generally had a more severe effect. Debris slides, resulting in debris flows, were also frequently associated with roads. These debris flows affected the disturbance patterns of streams and transported sediment to segments of the stream. Both peak flows and debris flows influenced stream physical features such as channels, bars, and flood plains, which in turn are closely associated with riparian vegetation and aquatic communities. The authors review studies on native aquatic organisms, such as salmonids, for instance, that had evolved with historical disturbance patterns of their stream habitat.

Key Finding: Richness of plant, bird, amphibian, and reptile communities in wetlands decreased as road density within the adjacent 2 km increased, with the full impact on biodiversity not evident for several decades.

Source: Findlay, C. S. and J. Bourdages. In press. Lagged response of wetland biodiversity to road construction on adjacent lands. Conservation Biology.

The authors studied the decline in plant, bird, and herptile (amphibian and reptile) richness of wetlands due to road construction on adjacent lands within 1 to 2 km. They were particularly interested in the lagged effects of road density on biodiversity loss. Therefore, they calculated historical road densities over four decades on lands adjacent to their sample of wetlands in Ontario. Using multiple regression models, they examined how much variation in species richness was due to past and current road densities. For plants, species loss was not detectable until several decades after the original road construction. For birds and herptiles, species loss was detectable within eight years, and increasingly evident after several decades.