How Greenways Work: A Handbook on Ecology
The two major ecological functions that greenways can fulfill for wildlife are providing habitat and acting as conduits.
By Jonathan M. Labaree, October 1997
Chapter Three: Greenways and Wildlife
A greenway's potential to serve as habitat for native plant and animal species will depend upon its size and the condition of the surrounding land. Generally, the larger the greenway, the more species and individuals it will support. All habitat within a greenway, however, may not be suitable for some species because of edge effects. Edge effects are particularly acute where a cultivated field borders a native forest or woodlot. Increased light and wind at the forest's edge leads to drier soils, more light-favoring species, and more blowdowns. Species that are adapted to the moist soils and shady environment of the forest interior are unable to survive in edge habitat.
Vegetation and wildlife react to the effects of edge differently. While changes in vegetation at edges, due largely to increased light and wind, may extend 35 to 100 feet into a temperate forest, researchers have noted adverse effects to wildlife as deep as 1,000 to 2,000 feet from a forest's edge (Wilcove, et al., 1986). This difference stems, at least in part, from predators venturing into interior habitat (see Figure 2).
Because the problem of edge effect is so acute, many greenways will be most useful for animals whose natural habitat is linear. River systems are a good example. Riparian ecosystems support high densities of vertebrates. They often contain more species and more individuals than drier surrounding land. Seventy-five percent of the vertebrates in Oregon and Washington's Blue Mountains depend upon, or prefer, riparian habitat (Thomas, et al., 1979). In arid and semi-arid lands, river corridors are extremely important. In Arizona and New Mexico, eighty percent of the vertebrate species depend upon riparian zones during at least part of their life (Johnson, 1989).
Greenways and wildlife -- conduit
A wildlife population has two basic requirements for long-term survival: enough natural space to satisfy life needs of individuals and a population which can sustain itself. Fragmentation threatens many wildlife species' ability to meet both those requirements. Properly designed and managed greenways, however, can help wildlife overcome effects of fragmentation by:increasing the effective size of protected areas; creating access to different habitats; and connecting wildlife populations.
Increasing effective size of protected areas -- Wildlife species require a certain amount of habitat to survive -- they need to roam in order to breed and to find food and nesting, den, or burrow sites. Generally, the larger the animal, the greater its home range. An animal's home range will also depend upon its trophic level -- how high on the food chain it resides. Predator species usually have larger home ranges than herbivores of equivalent size. Since a wildlife population needs to be of a certain size to remain healthy, the amount of natural land it requires is proportional to the home range of its individuals.
Creating access to different habitats -- Natural space alone is not enough to ensure wildlife conservation -- diversity of habitat types is also critical. Species such as raccoon, white-tailed deer, river otter, and gray fox use more than one kind of habitat (see Figure 6). Thus, a patch of natural land may correspond to a species' home range, but might not contain all the habitats the species needs. Patterns of development often create barriers between necessary habitats, hampering wildlife's efforts to survive. Connecting patches which support different habitats with greenways can greatly enhance an individual's opportunity to reach the diversity of habitats it needs to survive.
Demographic effects of isolation will occasionally lead wildlife populations living in small patches of forest (or prairie, wetlands, desert) to die off in that immediate area. Causes include disease, disturbance, increase in predators or competitors, and fluctuations in birth and death rates. Small wildlife populations are more likely to succumb to these random events than large ones. These localized extinctions occur naturally. If a population of white-footed mice, say, dies off in a small patch of woodland, individuals from a nearby patch will recolonize the now vacant woodlot.
By creating small and isolated patches of natural space, fragmentation can divide a once large wildlife population into smaller, more vulnerable populations. Fragmentation also makes it more difficult for members of other populations to recolonize vacated habitat. While greenways may not make demographically driven local extinctions any less likely, they can provide conduits to make recolonization by other populations easier (see Figure 8).
Wildlife populations can be susceptible to a number of genetic effects from isolation. Close relatives are more likely to mate as populations become isolated. Common results of breeding between relatives are increased juvenile mortality, decreased fertility, and reduced overall ability of a species to survive. Small populations are likely to become more genetically similar -- the potential for change and adaptation is lost. A species may lose a rare genetic trait which could help it survive under different circumstances.
In most species, some individuals will disperse away from their natal population to ensure genetic diversity. In fragmented landscapes, greenways can play an important role in making such dispersal possible. Given the opportunity to fulfill their natural tendencies to disperse, individuals will bring new genes to a population with declining genetic diversity. A greenway can provide that opportunity.
Adapting to long-term changes -- As the climate changes, whether due to human influence or not, plants and animals will have to disperse and find areas to which they have adapted. While a single, locally oriented greenway may help connect two populations of a wildlife species, it is unlikely to provide the substantial linkages necessary for plant and animal species to respond to long-term climatic changes. When possible, cooperating with neighboring towns or jurisdictions could ensure that greenways join up at municipal boundaries, thereby establishing broad-scale connections extending beyond the borders of a given town.
Designing greenways as effective conduits
A greenway will only act as a conduit for wildlife if individuals are willing and able to travel along it. Width and quality play major roles in determining whether animals will use a greenway for movement. A greenway must also offer a continuous link with as little disturbance from roads and other development as possible.
How wide? There is not a magic width at which a greenway will begin to act as a conduit. As a general rule, wildlife ecologists recommend that there be a one-to-one ratio between edge and interior habitat in a conduit greenway. Drawing on figures presented on page 12 relating to edge effects, a greenway would need to be 400 feet wide to balance 200 feet of edge habitat (100 feet of edge on either side) with 200 feet of interior habitat.
Many species use riparian corridors for movement. Thus, establishing a greenway in a riparian zone will offer a good opportunity to protect its conduit function. Proper width of a riparian movement greenway depends largely on the nature of the riparian zone. Such a greenway should be wide enough to include the entire riparian zone (see Figure 10, page 23) on at least one side of the stream and interior habitat in associated uplands. Ideally, this interior habitat would exist in a one-to-one ratio with edge habitat.
Species sensitive to humans will need wider greenways for movement than will more tolerant ones. Therefore, another factor determining width for a greenway is the species most likely to use it. Large predator species may require greenway widths measured in miles. Other species such as the blue jay will travel along a narrow hedgerow (although capable of flying across open lands, jays, like many birds, prefer to remain in woodlands).
Quality of the greenway is as important as width. Not all species should be able to move along a corridor. Exotic species may move along corridors creating competition for native ones. Maintaining a greenway with native plant species and minimizing the amount of edge habitat will help ensure that exotic species will not displace desirable ones.
The quality of surrounding land will also influence how effectively a greenway will function as a conduit. The greater level of development in lands adjacent to the greenway, the wider the greenway will need to be to overcome disturbance from noise, people, and pets (dogs and cats are very efficient predators).
Guidelines for designing greenways to provide habitat and act as conduits Habitat: Identify the needs of the most sensitive species for which the greenway is to provide habitat. Greenway managers should be particularly aware of species which are sensitive to edge effects because greenways will generally contain a lot of edge habitat.
For the most part, greenways will not, by themselves, provide good habitat. Therefore, do not allow greenway projects to proceed at the expense of other initiatives which will provide habitat, such as large reserves.
Concentrate on matching natural characteristics of the landscape -- both in terms of species composition and connecting patches of habitat which were originally connected but human development has left isolated.
Ensure that the greenway meets the movement needs of the species which are most sensitive to people. It may be necessary to build tunnels under roads or include specific habitat which a particular species requires.
Undertake a natural resource inventory before planning and designing a greenway. The inventory will identify the area's native mixture of species and special plants, animals, and habitats. Follow the inventory with a review of research done on the local landscape. Such a review may reveal studies concerning conservation needs of native plant and animal species or local impacts of edge effects.
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Updated March 16, 2007