How Greenways Work: A Handbook on Ecology
This chapter covers a more general set of issues (not necessarily unique to greenways) of which greenway designers and managers should be aware.
By Jonathan M. Labaree, October 1997
Chapter Five: Other Design Considerations
Other functions of greenways include:
Addressing each of these topics involves both design and management. It is therefore worth considering what those two terms mean and how they are related. Although the term design has many connotations, we are most interested in design as it relates to greenway shape, composition, and context. As seen in the previous chapter, meeting ecological goals requires careful consideration of how wide the greenway is, what parts of the landscape it connects, and what kind of habitats are included. These are design considerations.
Design also has an important aesthetic component. Diana Balmori, a contributing author in Ecology of Greenways, says of design: "It solves the functions and tries to go beyond to an aesthetic dimension, attempting to make something beautiful, and tries to ground that form into something meaningful to the society that creates it."
Management picks up where design leaves off. Managers ensure that the greenway continues to meet the objectives laid out in the design process. For example, one task might be to maintain the quality of habitat through such measures as controlling exotic species, reducing human impact through trail maintenance, or replanting to native vegetation.
Designing and managing for change
Natural systems are in constant flux -- they are always changing. The Greek philosopher Heraclites said that you can never step in the same river twice. He could have said the same for a field, tundra, forest, or desert. But that is easy to forget because we rarely see change occur. Nonetheless, it is critical to allow natural change to take place in protected areas because it is an inherent aspect of nature. There are both design and management considerations which can help achieve that goal.
Designing for change
The very idea of planning for change may seem contradictory. How can we possibly plan for something if we do not know what it will be? Greenway designers can respond to uncertainty and anticipate the long-term needs of the natural community by:
Secondary habitat -- Secondary habitat refers to areas which plants and animals need occasionally. For example, a fire could destroy part of a greenway and prevent animals from using it as a movement corridor until vegetation grows back. In a landscape where that corridor is the only way for wildlife to move between important habitats, the fire could be devastating. Guarding against such an event requires including more than one movement corridor in a greenway system so if one is temporarily unusable, wildlife can use another.
Centers of diversity -- As change occurs, some new species will arrive and others will leave. We do not know which ones and therefore cannot plan for specific future conditions. We can assume, though, that a patch of land which supports a rich diversity of species now will probably continue to do so in the future. Concentrating our efforts on preserving areas in the landscape which are currently centers for high diversity will help ensure that a greenway will continue to protect the range of species adapted to contemporary conditions, whatever they might be.
Diversity of habitats -- Another way to manage for change is to include a high diversity of habitats in a greenway. Habitat may change very dramatically over a small distance. This is particularly true where physical characteristics in the landscape cause gradients in environmental conditions. An example of an environmental gradient is altitude. Temperatures drop as altitude increases; soils often become thinner; average wind velocity rises; and precipitation usually increases. In the northern hemisphere, conditions at the top of mountains resemble lower altitude habitats farther to the north. Including a mountainside or other gradient in a greenway, therefore, will add greatly to its diversity of habitats.
For a greenway to act as a conduit for plants and animals, habitats should be as continuous along its length as possible. Patterns of habitat diversity, therefore, should be oriented along the greenway, rather than across it (see Figure 14), to maintain connectivity. In the case of the mountainside, a greenway which goes up its slope rather than along its valley may not act as an effective movement corridor.
Including secondary habitat, centers of diversity, and diverse habitats in a greenway are all design considerations: they involve creating greenway boundaries to meet certain goals. Once a greenway is created, there are management strategies which also address the issue of maintaining change as part of the greenway's ecology.
Some plants and animals are adapted to events which we consider disasters, such as floods or fires. From nature's point of view, removing such events from a landscape may be the real disaster. Greenways, many of which are in or near urban centers, will exist largely in human-dominated landscapes. We have taken great pains to remove floods, fires, and other natural disturbances from our cities and towns. Thus, greenway managers may not only have to plan for unforeseen change but they may also have to ensure that changes that were once part of the natural landscape continue to occur.
An example of change is fire. Fire is an important part of many landscapes, such as western forests and midwestern prairies. Removing fire from ecosystems where it occurs naturally disrupts the processes to which plants and animals are adapted. For example, the cones of some trees open up and disperse their seeds only when subjected to the extreme heat of a forest fire. Without fire, it is more difficult for those tree species to reproduce themselves. There are also many plants and animals which are adapted to take advantage of conditions which prevail after a forest fire. One such plant, fireweed, gets its name from its dependence upon fire. In ecosystems where plants and animals are adapted to periodic fires, greenway managers may want to initiate a prescribed burning routine which mimics natural occurrence of fire.
Once we accept that change is an inherent part of nature, we must adopt management techniques which reflect that reality. One such technique is called adaptive management. In this approach, managers monitor the greenway to understand what changes are taking place, which management policies are improving ecological functioning, and which are not. By responding to new information from
monitoring efforts, management becomes a dynamic process of adapting to ecological and cultural changes. Areas which are facing heavy recreational impact can be closed off or restricted to walkers. Scientists may discover that animals only use part of the greenway as a corridor, allowing more area to be opened up to the public.
Designing and managing a greenway to account for and accommodate change is a challenge made greater because some changes are ecologically desirable while others are not. Change that is naturally a part of the ecosystem should be encouraged but change resulting from human intervention should be avoided. Among the more detrimental changes that can occur is the invasion of exotic species.
Most exotic species depend upon people to survive -- we bring them, intentionally or accidentally, to their new destination and we create conditions which favor exotic species by altering the landscape. A healthy landscape in its natural condition can usually ward off exotic species. Where native species are not adapted to new, human-generated conditions, exotic species may be able to replace them.
Causes of invasions
Greenways, especially ones in urban or suburban landscapes, will be susceptible to invasion by non-native species. This is due largely to human disturbance either within the greenway or in the surrounding land. Heavy recreational use can create disturbance within a greenway. Surrounding land can influence a greenway primarily through edge effects. Opportunistic plants and animals thrive in disturbed lands, competing with original inhabitants for food and shelter. Exotic species often thrive in their new environment because they have no predators or are more adapted to human landscapes.
Dangers of invasions
While exotic species pose a threat to greenways, the threat to other protected areas such as nature preserves may be even more serious. If a greenway fulfills its role as a conduit, it can provide exotic species access to important natural areas. Exotic species threaten biodiversity in protected areas because they can dominate their new landscapes to the exclusion of many native species.
Based on the understanding that exotic species can be damaging to an ecosystem, one of the design and management priorities for a greenway project should be to maintain the native mix of species. Preventing invasion, or eradicating unwanted species is likely to be site-specific, depending greatly on the species involved.
The impact of roads
Although roads will provide many people access to greenways, they present a unique set of design and management problems. Due to effects of edge and fragmentation, a road affects the landscape well beyond its pavement's edge. Two major impacts that a road can have on its surrounding landscape include:
Dispersal routes for unwanted species
Cleared areas along roads usually represent disturbances in native habitat. Weedy species benefit from these conditions and may migrate along roads to colonize areas previously out of their reach. In some cases, however, highway medians and other clearings may represent remnant patches of native habitat. This is particularly true in the midwest where cultivated fields have replaced much of the native prairie. Conservationists may want to cooperate with highway departments to implement a management plan that favors native species. Many states restrict mowing in highway medians, for example, to allow native wildflowers to grow.
Dispersal barriers for native species
Roads frequently pose barriers to species which would normally move throughout the landscape. Some species -- desert rodents and even black bears -- rarely cross a road. Road kills cause huge losses in some species. American motorists daily kill more than a million vertebrates (Lalo, 1987). Proper management, however, can reduce roadkill. In an effort to decrease highway mortality of migrating mule deer, the Colorado Division of Highways built a tunnel under Interstate 70 near Vail. Fences prevent deer from crossing the interstate and channel hundreds of them into the tunnel and safely under the highway. (See Santa Monica Mountains Case Study.)
It is easy to fall into the assumption that everything we do harms the environment. Our relationship with nature is complex and it is unfair to over-emphasize our harmful impacts. Indeed, greenways, and their popularity, bespeak a very different tradition. Greenways represent our desire to foster a healthy and responsible attitude toward nature. Two ways in which greenways can help achieve that goal is through providing opportunities for recreation and public education.
Greenways are well suited for outdoor recreation. Their linear shape corresponds to trails and rivers. Since greenways use land efficiently, and do not necessarily require strict protection, they can fit into many urban and suburban landscapes where access to natural spaces is limited. Heavy recreational use, while socially beneficial, can produce short-term ecological damage in the following ways.
Soil compaction -- The weight of walkers and runners compacts the soil along a trail. Once compacted, soil can no longer hold as much water or support as much life, such as roots and microorganisms. Generally, compaction is limited to the immediate vicinity of a trail.
Erosion -- Compaction and loss of vegetation often leads to erosion because rain which would have soaked into the ground instead runs over its surface, building velocity and carrying sediment. Erosion also often occurs where trails traverse steep slopes or meet the water's edge.
Damage to vegetation -- Trampling and mutilation of vegetation is most common at campsites and other areas of intense use such as scenic overlooks. Small saplings, twigs, and near-surface roots get damaged. People sometimes carve initials and other graffiti into mature trees.
Disruption of wildlife -- Although narrow, trails can produce effects associated with fragmentation. While not as damaging as roads or land conversion, trails can increase edge habitat, bringing edge species into contact with interior ones. Trails offer access to people, whose presence wildlife may learn to avoid, further reducing the amount of land species can utilize.
Introduction of non-native species -- Disturbed environments, such as trails and campsites, favor opportunistic exotic species over native ones. Hikers, bikers, and horses offer a mechanism for these species to reach disturbed areas.
Avoiding and controlling use -- Minimizing effects of recreation can follow two approaches: avoiding sensitive areas and controlling use of trails and other facilities. A natural resource inventory identifies sensitive areas. Wetlands, steep slopes, thin soils, and river- and streambanks are all examples of conditions which are not appropriate for heavy recreational use. Siting trails along the edge of forests reduces fragmentation and provides diverse views.
Physical constructions or signs can control use where it is unavoidable or desirable. For example, wetlands are among the most interesting aspects of the environment but sensitive to human use. They afford wonderful educational opportunities to see rare plants and animals. Wooden platforms, bridges, and causeways protect the habitat while allowing people a view. Greenway planners should also consult design manuals which describe different trail materials such as crushed native stone or pine needles.
Some types of use are more damaging than others. Given similar numbers of users, walking is the most benign use while motorized all-terrain vehicles tend to be the most destructive. Between the two extremes lie horseback riding and mountainbiking. Not all these uses are socially compatible, although this will depend on the region. Whatever combination of uses are socially and ecologically acceptable, it is important that they be restricted to planned and maintained trails. It is often tempting to cut one's own path to get a view or to walk alongside a friend.
Environmental -- Our interactions with the environment are not one-way. We may have an impact on nature, but it also has an impact on us. Greenway designers can capitalize on this by placing trails and signs to educate the public. Education extends a greenway's conservation potential beyond such direct benefits as combating fragmentation or protecting riparian functions. The attention a greenway focuses on a community's river, for example, can inspire residents to form a citizen monitoring group. Keeping a close watch on a river's water quality allows a community to pinpoint sources of pollution. Through disseminating information, organizing, and lobbying, such efforts lead to cleaner waters and a healthier environment.
Cultural -- Cultural heritage can be part of a greenway along with natural heritage. Old homesteads, locks, mills, or fishing weirs attract people's attention and interest. Emphasizing such forms of development, which often relied greatly on the natural landscape, highlights a community's past and its connection with the environment. People's natural pride in their heritage will make the greenway a fixture in the community. Signs and educational programs can play a major role in enhancing public awareness and support.
Maintaining ecological health
Bolstering the ecological health of a landscape is the major reason for designing and managing greenways with ecological functions in mind. We may have some vague notion of what ecological health means, but faced with complexities like exotic species or successional change, what appears healthy may not be. A greenway can be green, yet not support the full diversity of species native to the area. Conversely, a greenway may be dry and sparsely populated, yet represent a region's natural vegetative cover and wildlife.
The appearance of ecological health is site- and function-specific -- so how do we know what is natural and how do we ensure that natural conditions prevail? In some cases, where development has dramatically changed the environment, it may not be possible to determine what "natural" means. But, when some remnant natural space exists, it can serve as a guide to how the ecosystem should function.
Careful study of the natural sites leads to understanding of what "ecological health" means for the altered site. The study should concentrate on the number of species, what species are represented (composition should match natural balances of plant-, insect-, and meat-eating species), and the number and health of individuals. This technique measures a site's biological integrity against a similar one from its own region. It does not favor increasing diversity or populations to match popular notions of healthy ecosystems, rather it reflects natural balances.
Interactions between people and greenways
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Updated March 16, 2007