Dynamics of wildlife corridors as a result of land use changes

Amy Karpati

Landscape Ecology Fall 2003

Critical Importance of Corridors

Many woodland patches present in the modern landscape are the result of fragmentation through anthropogenic activities (National Wildlife Federation 2003). This is particularly evident in the landscape of the northeastern United States. Prior to extensive human settlement, the majority of the land was covered with dense temperate forest, fragmented only by natural elements like rivers and mountain ranges. However, with increasingly intensive human land use such as logging and clear-cutting, agriculture, and urban development much of the original forest has been reduced to patches that often must harbor their plant and animal inhabitants in isolation from other patches (American Wildlands 2003).

The dangers of fragmentation to wildlife are many. When habitat patches are fragmented, the area available for diverse wildlife dramatically decreases. The creation of more edge may favor some species, leading to a population explosion (as in deer), but can be detrimental to species whose habitat requires large buffers and intact forest interiors. By altering patch composition and configuration, fragmentation can disrupt migration patterns, species dispersion, and gene flow. (National Wildlife Federation 2003).

Although landscape fragmentation due to human land use is extensive, it is often believed that the changes in patch composition and configuration can be buffered by corridors connecting patches of suitable habitat which allow plant populations and wildlife to move between one patch and another, thus enhancing chances of survivorship (Turner et al. 2001). Facilitated movement via corridors between patches can help organisms find food, mates, and perhaps better habitat in areas outside of their present patch. Because corridors are frequently extensions of patches that branch out across the surrounding matrix, they also undergo changes due to human land use much the same way as patches do. Under different land-use regimes, corridors can take on several forms, especially at different scales. This essay will examine the dynamics of corridors (mainly those useful to animals) in relation to increasingly intensive human land use.

What are Corridors?

Corridors are narrow strips of land that differ from the surrounding matrix. Although corridors can be isolated strips, they are often connected to patches with similar vegetation characteristics (Forman and Godron 1986). Several studies have shown that corridors can help organisms move from one patch to another, since they connect suitable habitat conditions (Conservation Economy.net). Corridors can be divided into three categories: habitat corridors, facilitated movement corridors, and barrier corridors. The following descriptions of these three types are from Forman, 1985:

1. Habitat Corridor - Linear landscape element that provides for survivorship, natality, and movement, and may provide either temporary or permanent habitat. These corridors passively increase patch connectivity. An example of a habitat corridor is a strip of wet, vegetated land that can sustain salamander populations.

2. Facilitated Movement Corridor - Linear landscape element that provides for survivorship and movement between habitat patches but not necessarily natality within the corridors. Facilitated movement corridors actively increase patch connectivity. A strip of vegetated land that has edge characteristics could provide a passageway for an interior forest species but not habitat. For example, the vegetated strip lining a road can provide food resources for a vole, but not enough shelter for protection of young.

3. Barrier or Filter Corridor - Linear landscape element that prohibits (barrier) or differentially impedes (filter) the flow of energy, mineral nutrients, and/or species across it (i.e., flows perpendicular to the length of the corridor). Barrier or filter corridors actively decrease matrix connectivity. An example of a barrier corridor is a highway that prohibits animals from crossing over it.

Temporal Changes in Fragmentation

Before extensive human settlement, forested land was the most extensive and connected element of the landscape of the northeastern U.S., and would thus be considered the matrix (Figure 1). As human development increased, patches of settlement and the roadways connecting these patches began to fragment the forested landscape. Agriculture soon followed, fragmenting the forested landscape by the clearing of land for pasture and crops. Land used for agriculture became so extensive, that farmland actually became the matrix and areas that were still forested became the patches of the landscape. Recently, in the Northeast, the agricultural areas that had previously dominated the landscape have either been turning into urban and suburban areas, in which case urbanity becomes the matrix with fewer forest patches, or have been abandoned and are returning via succession to forest..

Figure 1. Changes in types of landscape fragmentation. The far left panel hows a landscape with a forested matrix (green). Small towns (grey) are shown as patches and a river and roads as corridors. The middle panel shows an agricultural matrix, with scattered patches of forest and hedgerows as corridors. The right panel shows an urban matrix with a park (green) as a patch and tree-lined streets and a railroad right-of-way as corridors.

Forested Land

In the precolonial Northeast, with forested land as the landscape matrix, patches of agricultural land and developed towns were scattered. Few roads existed, but those that did could be considered corridors between the patches of towns. The habitat of wildlife was minimally fragmented, and so most forest-dwelling organisms were able to disperse with little difficulty, and the role of corridor was allocated to river corridors (Forman 1995).

Although the few roads present at the time would appear to be corridors only for human travel and have little value to wildlife, road corridors in such undeveloped areas were actually utilized by several species (Forman and Alexander 1998). Although the roads themselves were not of much interest, the vegetated areas along side the roads were distinct from the surrounding forest matrix and functioned as facilitated movement corridors for some species. Carrion feeders, for instance, traveled along roads in search of food (Forman and Alexander 1998). Small mammals have been known to enter different habitat areas by way of road edges (Forman and Alexander 1998). Also, edge species like deer could have used roadsides to travel along the edges of forest stands. It has even been speculated that birds have used these roads as navigational cues during migration, as they were readily visible at higher elevations (Forman and Alexander 1998).

Agricultural Land

When land is developed to suit agricultural practices, much of the original forested landscape is cleared, leaving scattered patches of forest areas of varying sizes. However, there are often strips of forested areas left unplowed to serve as boundaries between crop fields and barriers to livestock (Dowdeswell 1987). Although these strips, called hedgerows, were originally created with the intent of cropland maintenance, they also provided habitat and movement paths for animals and plants that would otherwise not be able to live on agricultural land (Dowdeswell 1987). In this sense, crop fields can be seen as the matrix of the landscape, remaining forested areas as the patches, and hedgerows as the corridors connecting the forest patches.

Depending on the scale in which the organism perceives the landscape, it is quite possible for hedgerows to serve as habitat corridors. Hedgerows can provide valuable resources such as food, shelter, and dispersal to insects, birds, bats and small mammals when the majority of the land is agricultural (Lincolnshire Biodiversity Action Plan). This is especially true for edge species (usually generalists). Species that thrive in the interior of the forest patches would experience more difficulty in movement through or sustainment in hedgerows; the hedgerows might not function as corridors from such species' perspectives. Mammals often found in both hedgerows and local woodlands include red foxes, badgers, rabbits, weasels, hedgehogs, moles, voles, mice, and shrews (Dowdeswell 1987). Organisms that these mammals feed on such as beetles, earthworms, slugs, spiders, crickets, snails and a variety of herbs and grasses are presumably also present in hedgerows. Birds that commonly occur and nest in hedgerows include pigeons, wrens, robins, blackbirds, thrushes, finches and warblers (Dowdeswell 1987). Because the small mammals and birds listed above can reside locally in hedgerows, the hedgerows function as habitat corridors.

In a study done by Corbit et al. (1999), the potential of hedgerows to function as corridors for plant dispersal was investigated. Previous studies on the subject had shown that hedgerows provide habitat for woodland plants, but the extent to which they act as corridors was unknown. Corbit et al. found that the most frequently occurring herbaceous plants in the hedgerows were animal-dispersed, however plants representing all dispersal methods were represented. In addition, interior forest species such as, wild ginger (Asarum canadense) and early meadow rue (Thalictrum dioicum) occurred less frequently in hedgerows, but were still present. However, the greater the distance from the forest patch, the fewer the woodland species present in the hedgerow. This observation further indicates that the hedgerow can be used as a habitat and facilitated movement corridor.

A study performed by Wegner and Merriam (1979) found that chipmunks and white-footed mice use hedgerows as corridors, as they seldom travel between woodland areas and the open agricultural field matrix. Wegner and Merriam also found that birds prefer to fly over hedgerows rather than over farm fields, presumably because hedgerows contain food and shelter, resources not found in agricultural land. This study also sought to determine the effect of extirpating small mammal populations in the scattered woodland patches by trapping and removing chipmunks from selected patches. Recolonization of the empty woodland patches occurred more rapidly in patches connected by hedgerows as opposed to those that were isolated by farmland (Turner et al. 2001).

Not only can hedgerows act as habitat corridors for an array of species, they are also important factors in succession (Forman 1995). As old abandoned farmland slowly regenerates into forest, hedgerows hold important species that will be able to disperse and recolonize such areas.

Urban Land

In the past century, many acres of agricultural land have been converted to suburban and urban development. In the recent past, this conversion has been dramatic. As agricultural land is converted to urban areas, the amount of land devoted to cropland greatly decreases as the amount of land devoted to buildings and pavement greatly increases. When this happens, the developed land becomes the matrix of the landscape. Forest patches that remained during agriculture may remain as patches in the urban landscape. Scattered public parks also become patches distributed in the urban matrix. In the urban context, tree-lined streets and rights-of-way become corridors for wildlife to reach forest or park patches.

Tree-lined streets can function as habitat corridors for some species, though many species might not be able to use them as corridors at all nor find suitable habitat in urban settings. Birds are the most frequent users of tree-lined streets. Depending on their food and nesting requirements, some species of birds, such as blackbirds (Turdus merula), woodpigeons (Columba palumbus), magpies (Pica pica), and starlings (Sturnus vulgaris) can feed and reproduce within the tree-lined streets, designating them as habitat corridors (Fernandez-Juricic 2000). A study in Madrid by Fernandez-Juricic (2000) that investigated bird use of tree-lined streets revealed that out of 24 species that inhabited a local park, 14 of those species were also observed in tree-lined streets. Furthermore, the number of species found in tree-lined streets was significantly greater than those found on streets with no vegetation. Use of tree-lined streets provides a way for birds to travel from patch to patch while avoiding the inhospitable urban matrix.

Rights-of-ways also function as habitat corridors, but for more than just birds. Rights-of-way are managed vegetated areas that line railroad tracks, highways and other anthropogenic constructs. Although usually highly managed by periodic mowing, these greenways consist of herbaceous cover, shrubs, and occasional trees. Small mammals, such as rabbits, shrews and mice can feed and reproduce in these strips of land. Mammals as large as foxes can survive in rights-of-way, as can many bird species (Transportation Research Board, 2002). Organisms can travel along rights-of-way to intersections with forest patches, even though vegetation structure is different between the rights-of-way and forest patches. Since management of rights-of-way varies (ie. herbicide use vs. mowing), the continuity of vegetation throughout the length of the right-of-way can be highly inconsistent (Transportation Research Board, 2002).

Conclusion

As human land use changes, the impacts on the landscape are so dramatic that species must adapt to new movement patterns. Types of corridors available to reach habitable areas change as land use becomes more intensive. Because of these corridor dynamics, many species might not be able to adapt as well as others, placing a strain on their success. It is crucial to be aware of constraints placed on wildlife movement as a result of human development in order to minimize destructive impacts on the ecosystem.

Several plans have been proposed to increase patch connectivity across anthropogenically-altered landscapes. For example, the Lincolnshire Biodiversity Action Plan in England has been working to improve the condition of agricultural hedgerows so that they might harbor a higher diversity of animals within farming regions. They stress the importance of general hedgerow upkeep such as appropriately-timed cutting of hedgerow vegetation, abstaining from using pesticides and fertilizers close to hedgerows and replacing dead vegetation with seedlings (Lincolnshire Biodiversity Action Plan 2003). In the U.S., the National Cooperative Highway Research Program has carried out studies examining the negative effects urbanization imposes on wildlife. They recognize the importance of habitat connectivity, advocating continued research by individual states to determine how to better address conservation issues. Documentation of transportation infrastructure impacts, long-term connectivity and effectiveness studies need to be considered (Transportation Research Board, 2002).

References

American Wildlands. What are Corridors? http://www.wildlands.org/corridor/what_are_corr.html. (7 Dec. 2003).

Conservation Economy.net. Pattern: Wildlife Corridors. The Patterns of a Conservation Economy.

http://www.conservationeconomy.net/content.cfm?PatternID=21. (6 Dec. 2003).

Corbit, M., P.L. Marks, and S. Gardescu. 1999. Hedgerows as habitat corridors for forest herbs in central New York,

USA. Journal of Ecology 87: 220-232.

Dowdeswell, W.H. 1987. Hedgerows and Verges. Allen & Unwin, London, UK.

Fernandez-Juricic, E. 2000. Avifaunal use of wooded streets in an urban landscape. Conservation Biology 14: 513-521.

Forman, R.T.T. 1995. Land mosaics: the ecology of landscapes and regions. Cambridge University Press, Cambridge, Uk.

Forman, R.T.T., and L.E. Alexander. 1998. Roads and their major ecological effects. Annual Review of Ecology &

Systematics 29: 207-331.

Forman, R. T. T., and M. Godron. 1986. Landscape ecology. John Wiley & Sons, New York, USA.

Lincolnshire Biodiversity Action Plan. Habitat Action Plan: Hedgerows and Hedgerow Trees.

http://www.lincsbap.org.uk/Farm/hedge1.htm. (12 Nov. 2003).

National Wildlife Federation. Evidence of Habitat Loss Due to Sprawl. Smart Growth and Wildlife: Why Care?

http://www.nwf.org/smartgrowth/evidence.html. (6 Dec. 2003).

National Wildlife Federation. Habitat Fragmentation. Smart Growth and Wildlife: Why Care?

http://www.nwf.org/smartgrowth/fragmentation.html. (15 Nov. 2003).

Tigas, L.A., D.H. Van Vuren, and R.M. Sauvajot. 2002. Behavioral responses of bobcats and coyotes to habitat

fragmentation and corridors in an urban environment. Biological Conservation 108: 299-306.

Transportation Research Board. 2002. Interactions Between Roadways and Wildlife Ecology: A Synthesis of Highway

Practice. National Cooperative Highway Research Program, Washington, USA.

Turner, M.G., R.H. Gardner, and R.V. O’Neill. 2001. Landscape Ecology in Theory and Practice. Springer-Verlag, New

York, USA.

Wegner, J. and G. Merriam. 1979. Movements by birds and small mammals between a wood and adjoining farm habitats.

Journal of Applied Ecology 16: 349-357.