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How Greenways Work: A Handbook on Ecology

The role of wildlife corridors, rivers and greenways, in the landscape.

By Jonathan M. Labaree, October 1997

Chapter Two: Greenway Functions

Nature is a series of interactions among plants, animals, and even air, soil, and water. A healthy environment depends upon keeping those interactions intact. The threats described in the previous chapter result from alterations we have made in the landscape that hinder natural processes. The challenge for conservationists is to ensure that we orient future development in a way which sustains natural processes. Protecting environmental corridors through establishing and managing greenways represents one method (to be used in conjunction with other approaches) to safeguard vital ecological processes.

If we are to understand truly our impact on the landscape, we must learn to see it not as a bunch of independent pieces a woodlot here, a river there, and a grassland yonder but as intricately connected parts of a larger whole. Any given part of a landscape affects other parts. When designing a greenway, it is important to consider what impact it will have on natural processes. A greenway like their natural counterparts, environmental corridors can operate in six basic ways:

  • as habitat for plant and animal communities
  • as a conduit for plants, animals, water, sediment, and chemicals;
  • as a barrier preventing movement;
  • as a filter allowing some things to pass while inhibiting others;
  • as a source for animals or seeds which move to other parts of the landscape; and
  • as a sink for trapping sediment, toxins, or nutrients.

Since it may not be possible, or desirable, to design a greenway to fulfill all six functions, planners should identify which ones are most important to the site. For example, in a heavily developed area, a greenway can offer scarce habitat. Severely fragmented landscapes would benefit from a greenway designed to be a conduit allowing animals to reach isolated protected areas. A greenway which is along a river should be planned to filter excess nutrients from surrounding lands (in which case it also acts as a barrier preventing movement of sediments and a sink storing them). One proposed along an abandoned rail bed in the midwest can act as a source of native prairie grasses.

graphic: Greenways as habitat

Greenways as habitat

A species' habitat may include many different types of vegetation and geography, such as wetlands, upland forests, and fields. A greenway's ability to provide habitat will depend upon its size, location, and the needs of native species. A greenway that is 200 feet wide will generally contain habitat for fewer species than one in a similar location that is half a mile wide. A 200-foot-wide greenway along a river, however, that includes a variety of vegetation, may provide habitat for as many species as a wider one in a setting with less natural variation. Some species require more natural area than others. A 200-foot-wide greenway, therefore, may provide plenty of habitat for salamanders, beetles, and frogs, but very little for bears, eagles, and elk.

graphic: Greenways as conduit

Greenways as conduit

Conduits are areas in the landscape along which water, animals, plants, and people move. A river is among the most obvious examples of a conduit. Water carries sediment, nutrients, leaves, insects, bacteria, and plankton along with it. Acting as a conduit to connect otherwise isolated parts of the landscape is an important function of greenways. Such connections will allow animals to reach necessary elements of their habitat. As animals or water move along a greenway conduit, seeds do as well, thus aiding in plant dispersal.

graphic: Greenways as barrier

Greenways as barrier

While a greenway may be a conduit to some things, it presents a barrier to others. Again, a river serves as an example. Small animals, such as mice, may be unable to cross a river. Or its moist habitat may be inhospitable to creatures that prefer drier surroundings. Even very narrow corridors, such as hedgerows, can present a physical barrier of impassable habitat for some species.

graphic: Greenways as filter

Greenways as filter

A filter prevents the passage of some things but allows the passage of others. Filtering can occur in a greenway either perpendicular to its axis or along its length. Large animals, able to traverse a river, for example, can pass across a riparian greenway, but small ones may not. Similarly, some animals may be able to move along the entire length of a greenway, while others may fall victim to predators or find the habitat inhospitable. The next chapter deals extensively with a greenway's potential to filter sediments and nutrients from surface and groundwater.

graphic: Greenways as source

Greenways as source

A greenway may act as a source, providing surrounding land with a variety of things. A riparian greenway may be the only source of water in an otherwise arid landscape. In human-dominated areas, even narrow strips of relatively undisturbed land, such as hedgerows or steep slopes, may be a source of seeds of either native or non-native species.

graphic: Greenways as sink

Greenways as sink

A greenway acts as an ecological sink when something moves into it but does not travel back out into the surrounding land. Perhaps the most significant way a greenway can be a sink is by trapping sediments and nutrients carried in surface and groundwater. This function is, however, dependent upon a specific time frame because sediments may eventually wash downstream during a dramatic flood, or nutrients absorbed by vegetation will re-enter the soil and atmosphere when the plant decays.


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