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A central goal of transportation is the delivery of safe and efficient services with minimal environmental impact. In practice, though, human mobility has flourished while nature has suffered. Awareness of the environmental impacts of roads is increasing, yet information remains scarce for those interested in studying, understanding, or minimizing the ecological effects of roads and vehicles.
Road Ecology addresses that shortcoming by elevating previously localized and fragmented knowledge into a broad and inclusive framework for understanding and developing solutions. The book brings together fourteen leading ecologists and transportation experts to articulate state-of-the-science road ecology principles, and presents specific examples that demonstrate the application of those principles. Diverse theories, concepts, and models in the new field of road ecology are integrated to establish a coherent framework for transportation policy, planning, and projects. Topics examined include:
Foundations of Road Ecology
What is the use of running when we are not on the right road?
... great technical advances occurred in the technology of pavement structures and surfacings during the nineteenth century. Almost in their entirety, these advances predated the development of the motorcar.... Communities at last saw an alternative to a life full of mud, stench, dust, and noise.
—M. G. Lay, Ways of the World, 1992
Transportation lies at the core of society. It is what links us together. Both businesses and individuals depend on safe and efficient mobility. In the past century in North America, roads and vehicles have enlarged the spiderweb of our interactions and activities. Now we routinely use vehicles on roads to visit a friend, go shopping, travel to school, or dine out.
Unfortunately, with this dependence on roads and vehicles comes deep and widespread environmental damage. As a result, environmental protection now plays a key role in transportation policy and decisions. Ever-increasing resources are devoted to minimizing the adverse impacts of roads and vehicles on species and ecological systems.
Environmental protection is viewed and approached from many perspectives. The engineer seeks technical solutions and designs technical devices to abate damages. The economist seeks the best use of societal resources and identifies actions that yield the highest return. Legislators and lawyers craft sharply defined rules to preclude certain behaviors. Ecologists emphasize that we are too human centered in our responses and seek to elevate the importance of plants and animals. They seek to maintain the diverse characteristics and services of intact, or undegraded, nature and to maintain or reestablish relatively natural ecological systems in human-imprinted areas. Meshing this goal with the economic and social activities of our busy highways remains a daunting challenge.
In market economies, prices are a primary mechanism for allocating resources and guiding behavior. Environmental impacts remain largely outside the marketplace. When we drive a car, we degrade the quality of everyone's air. But we do not pay for damage to health or vegetation. If we did, we would probably pollute less. Although conceptual models exist, no effective mechanism in society ensures a proper balance between supply and demand for clean air. The same basic problem exists for noise and water pollution, climate change, aesthetics, loss of wetlands, and loss of biological diversity. The absence of a pricing mechanism has led to regulatory approaches.
Environmental protection is complex yet more easily regulated in transportation than in most other sectors of society because transportation networks are mainly in the public domain. Governments at various levels build and maintain most roads and largely own, operate, and subsidize transit services. Governments also own and manage many ecologically important lands where public roads exist (Figure 1.1). Entwinement of transportation with the public domain means that public goals, such as environmental protection, play a more direct role in investments and institutional behavior. Public pressure can translate directly into action by elected leaders and public officials.
Environmental protection came to the forefront of public discourse in the 1960s. Such environmental disasters as London's "killer smog," which killed scores of people, and the Cuyahoga River in Ohio (USA), which caught fire, galvanized worldwide attention. The realization that newly developed and widely used chemicals could decimate ecosystems and poison humans on an extensive scale, a discovery highlighted by Rachel Carson's Silent Spring, catalyzed public action.
In the transportation sector, air pollution proved the initial and most compelling call to action, first in California and then elsewhere. Widespread pollution in the USA culminated with the federal 1970 Clean Air Act, which accelerated the process of eliminating lead from gasoline and reducing vehicular pollution. This law was followed in the mid-1970s by fuel economy rules and "gas guzzler" disincentives. Japan pursued roughly the same track in reducing emissions and fuel consumption, as did Western Europe somewhat later.
In a larger sense, many nations were becoming more environmentally conscious as the 1960s ended. Rules and laws were passed to reduce noise and decrease air and water pollution. Greater concern for aesthetics was emerging. In the USA, the National Environmental Policy Act (NEPA) became law, which required that environmental impacts be documented for new projects using federal funds. By the 1970s, environmental and aesthetic concerns were beginning to play an important, if not always well informed, role in the design, construction, and operation of roads.
But even as environmental consciousness evolved, knowledge and political will lagged. As one concern was addressed, another would emerge. As road-side aesthetics received greater emphasis, concerns about non-native and invasive species grew. A phalanx of new rules and institutions emerged to control a carefully specified set of air and water pollutants. But new threats from new pollutants kept appearing. As four-wheel-drive and other high-clearance vehicles tended to replace cars, remote natural areas became accessible to recreational vehicles, and telecommuting from rural areas gained appeal, the threat to ecologically sensitive land increased.
Furthermore, environmental impacts have become global in nature, through the cascading accumulation of ecological stresses and altering of ecological interactions of the earth system itself. The pervasiveness of roads and their cumulative effect on the environment are now of increasing concern for habitat fragmentation, rare species, and aquatic ecosystems.
What Is Road Ecology?
In 1994, a lone ecologist slowly drove a long, winding road up a canyon in the Rocky Mountains. Front views, like an ancient movie, flashed back and forth from towering granite cliffs to precipitous forest slopes. The road, an engineering marvel, crept over old landslides, and the car sidled past avalanche tracks. The destination was a conference of the Ecological Society of America, where 2500 ecologists were packed into the canyon. Upon arrival, the driver, who sensed that road ecology might be important but had never heard of a meeting on the subject, studied the printed program of over 2000 presentations. The word "road" appeared in only one title. He talked with people, from world authorities to promising students. Everyone could speak knowledgeably, even passionately, about the unusual birds around, how to measure the vegetation, the water flows, erosion patterns, wildlife trails, and mathematical models. No one mentioned that omnipresent road running through the canyon.
The ecologist then walked up the canyon to look more closely. The serpentlike route through the heart of the valley was the organizing force for almost all human activities. Hordes of early miners had used it as access to their dreams of wealth, and tens of thousands of sheep must have been shepherded up and down the canyon every year along this solitary route. Bandits and predators lined the route.Today, hotels and tourists, ski areas, homes, and everything else human depend on the condition of that lifeline. But what about those birds and vegetation and water flows and erosion patterns and wildlife movements ? Do they affect the road? Or does the road affect them? Indeed, how does life change for plants and animals with a road and traffic nearby?
Answering these questions, and similar ones from local spots everywhere, leads inexorably to road ecology. Indeed, a handful of key concepts and terms here helps bring the big picture into focus.
A road is an open way for the passage of vehicles, and ecology is the study of interactions between organisms and the environment. Therefore, the combination describes the essence of road ecology, namely, the interaction of organisms and the environment linked to roads and vehicles. More broadly, traffic flows on an infrastructure of roads and related facilities form a road system. Thus road ecology explores and addresses the relationship between the natural environment and the road system.
Let us delve into that concept to learn more. Roads come in many varieties, from multilane highways to suburban streets, from logging roads to farmers' lanes (Figure 1.2). All are the focus of road ecology. Sometimes the term "road" or "roadway" refers to the roadbed area between roadside ditches. Other times, road or road corridor refers to a wider strip where the land surface has been altered by construction, maintenance, or management regimes. Commonly, this wider strip includes the road surface, shoulders, ditches, and outer roadsides. Where cutting through the side of a slope, the road corridor typically includes a cut surface on the uphill side and a filled area on the downhill side. Various engineered structures to control, for instance, water flow or snow accumulation may be included in this wider road strip. A highway corridor usually also includes the strip of cultural structures, as in strip development, associated with the highway.
These attributes, and many more that will be discussed later, are useful in describing a road location or site. Road ecology also focuses on a road segment, the stretch of road between two points, such as between two intersections or towns (Figure 1.2). A road segment thus slices through a heterogeneous landscape, so that the pair of adjoining local ecosystems or land uses on opposite sides of the road keeps changing along the segment. The sequence of pairs is little studied but may be ecologically important in a road segment.
In addition, road segments are linked together to form a road network, which, with its moving vehicles, we call a road system, as mentioned earlier. The road system connects nodes, or important locations such as cities, at a broad, or regional, scale or schools and clusters of shops at a fine, or local, scale. Across the network, traffic may vary from nearly zero to over 200 000 vehicles per day and change markedly through the day, week, or season.
In effect, the road system ties almost every piece of land together for society. Yet the same road system slices nature into pieces, like little polygons enclosed in the mesh of a network. This network produces major ecological effects in a landscape.
Road density, the average total road length per unit area of landscape (i.e., kilometers per square kilometer, or miles per square mile), is a handy overall measure of a road network or the amount of roads in an area. Many ecological phenomena, from wildlife to flooding to biodiversity, have been related to road density. Mesh size, the average area or diameter of the polygons enclosed by a road network, as in a fishnet, is proportional to road density but focuses on the enclosed parcels rather than the roads. Just as average mesh size for a fishnet is of limited use to a fisherman, since all fish could escape through a single large hole, average road density, or mesh size, provides only an overview. The combination of average mesh size and variability of mesh size is ecologically more informative.
However, network form, the explicit spatial arrangement of roads and intersections (linkages and nodes), is still better. Network form determines the relative sizes, shapes, and arrangement of enclosed patches. Like the fish example, plants and animals are probably less affected by averages and variability and more sensitive to size, shape, and arrangement of habitat.
Earth, Fill, and Soil
Ancient peoples lived with, though also feared and revered, the four basic elements of the universe: earth, water, air, and fire. The first two—earth and water—form the core of road engineering in action. Earth is moved and molded to create a road that will persist through the vicissitudes of both daily and heavy water flows. Highway engineers routinely deal with earth and fill, while ecologists deal with soil. This section ties the two perspectives together.
Rock, either by weathering in place or being mechanically splintered by machine, forms various smaller rocks, gravels, sands, and finer particles. This earth or earthen material may be transported and deposited as fill in road construction to form much of the roadbed beneath the road surface and shoulders, as well as to cover roadside areas (Figure 1.3). Fill areas are often covered by topsoil and then seeded. Roadside vegetation, whether originating naturally or by seeding, helps create a thin layer of soil, which contains blackish organic matter in the upper portion of the fill.
The particle sizes in fill vary widely, from boulders to clay. Gravel (2 to 75 mm diameter) is particularly useful because of the large pore spaces between particles, which permit relatively copious and rapid water flow. Sand (0.05 to 2.0 mm diameter), silt (0.002 to 0.05 mm), and clay (<0.002 mm) are the fine particles. The texture of earth or soil refers to the relative proportions of sand, silt, and clay. Sand has good (rapid) water drainage, silt is intermediate, and clay drains poorly (slowly). Puddles and mud tend to form on clay.
The roadbed supports the road surface and shoulders and typically is sandwiched between ditches. To reduce flooding problems for both the roadbed and the traffic on it, usually the roadbed is higher than the surrounding land surface. In addition, various base layers of sandy or gravelly material are commonly laid down in the roadbed to support traffic on the road surface. When water penetrates the roadbed, these porous layers facilitate water drainage into deeper levels or adjacent ditches.
In contrast, ditches may accumulate silt in their bottom or be lined with impermeable material in local spots to facilitate rapid, unimpeded runoff of surface water horizontally. In flat terrain, the outer roadside beyond the ditch is often covered by a material with mixed particle sizes, which is intermediate in porosity and water drainage. In hilly and mountainous terrain, roadsides are more variable in porosity because some natural earth surfaces are not covered by fill.
Where a road surface is more than about a meter above the surrounding land, ditches may be absent, since road water runoff simply runs down and away on the outer slopes of the roadbed. These outer slopes or surfaces of a roadbed composed of deposited earthen material are fillslopes (Figure 1.3). A fillslope tends to be highly erodible because its particles have only partially self-compacted over time.
In hilly and mountainous terrain, fillslopes may be quite long on the downhill side of a road. On the uphill side, in contrast, a cutbank surface remains where earth, rock, or both were removed for constructing the road. Normally, a ditch to catch and drain water separates the cutbank from the roadbed. A roadcut has cutbanks and ditches on both sides of the roadbed. Cutbanks range from a complete earthen bank with particles naturally compacted over time to a rock face that is often topped by a thin mantle of soil. Cutbanks are conspicuous to travelers, whereas fillslopes are rarely noticed.
Along a road segment, the roadside soil tends to be much more constant than the heterogeneous sequence of soils in the adjoining land. In road construction, rock particles originating from different nearby sites tend to be intermixed, averaging out differences, or fill is trucked in from a single site. Furthermore, the fill is deposited and contoured to a relatively smooth surface. Analogously, the chemical constituents of the earth material are averaged in the mixing process. The net effect ecologically is that microhabitat heterogeneity is sharply reduced in roadsides. Earth-moving equipment normally works at a broader scale than the natural processes of soil and plants. Consequently, the range of plant species and natural communities on roadsides is truncated.
At the same time, cutbanks, open ditches, road shoulders, and fillslopes are novel microhabitats in a natural landscape. They increase overall habitat heterogeneity in a natural landscape, whereas they may add little heterogeneity to a suburban or agricultural landscape.
Excerpted from Road Ecology by Richard T.T. Forman, Daniel Sperling, John A. Bissonette. Copyright © 2003 Island Press. Excerpted by permission of ISLAND PRESS.
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|The Metric System in North America|
|Pt. I||Roads, Vehicles, and Ecology|
|Ch. 1||Foundations of Road Ecology||3|
|Ch. 3||Vehicles and Planning||49|
|Pt. II||Vegetation and Wildlife|
|Ch. 4||Roadsides and Vegetation||75|
|Ch. 5||Wildlife Populations||113|
|Ch. 6||Mitigation for Wildlife||139|
|Pt. III||Water, Chemicals, and Atmosphere|
|Ch. 7||Water and Sediment Flows||171|
|Ch. 8||Chemicals along Roads||201|
|Ch. 9||Aquatic Ecosystems||225|
|Ch. 10||Wind and Atmospheric Effects||253|
|Pt. IV||Road Systems and Further Perspectives|
|Ch. 11||Road Systems Linked with the Land||291|
|Ch. 12||The Four Landscapes With Major Road Systems||319|
|Ch. 13||Roads and Vehicles in Natural Landscapes||351|
|Ch. 14||Further Perspectives||375|
|About the Authors||459|