As the global human population surges to over 7 billion, Earth’s resources become more and more depleted. Urban sprawl increasingly depletes habitats, squanders the planet’s water, pollutes the air, and endangers Earth’s other species. Urban impacts on wildlife habitat are virtually endless, what is more important, however, is what environmental managers, in cooperation with other agencies, can do, and are doing, to alleviate the problem.
Urban sprawl has skyrocketed in recent years, with over half the American population living in metropolitan areas (Woolliams). As the nation’s open spaces and farmland are transformed into subdivisions, shopping centers, roads, and parking lots animal biodiversity is threatened in numerous ways (Pegg).
First, habitats are lost as natural ecosystems are converted for human development and use; secondly, habitats that are not entirely destroyed tend to be fragmented by these developmental activities; finally, ecosystems bordering urban areas are increasingly polluted and the integrity of the landscape lost.
According to a report by the National Wildlife Federation, Smart Growth America, and NatureServe, “runaway sprawl will deplete wildlife habitat in many metropolitan areas…consumption of these critical areas could bring an astonishing number of species up to, or even over, the brink of extinction” (Ewing). The study points out that although the continental United States’ fastest growing cities comprise only eight percent of the land area, these essential forest, wetland, and grassland areas are home to about one-third of America’s endangered species (Heilprin). If current rates of urban sprawl continue, by 2025, cities will have converted an area the size of West Virginia from wildlife habitat to human habitat (Pegg).
Not only is habitat being lost at an ever increasing rate, but habitat fragmentation is also negatively affecting biodiversity. Habitat fragmentation is “the conversion of a contiguous area of native vegetation into remnant patches of native vegetation for human use” (Meffe). A decrease in habitat area has dire consequences on the numbers and types of species that live on the land. Ecologists suggest a direct relationship between the number of species and the area of an ecosystem, equating an increasing area with an increasing chance of habitat heterogeneity and topographic diversity (Kricher). Thus, with habitat fragmentation, decreasing heterogeneity reduces the amount of available niches and subtle topographic variations of a landscape, which reduces species richness (Meffe).
Further, many wildlife species require large areas to survive and reproduce. These area-sensitive species may be negatively impacted by fragmentation for numerous reasons. First, some species have diets that require vast areas, such as the large range of mountain lions due to their dependence on deer populations. Other species are area-sensitive because of their seasonal movements. Elk in the Rocky Mountains, for instance, spend summers at high elevations and winters in lower elevations; both ranges comprise extensive areas. Others require large areas because they have large body sizes and require a wide range of food, such as black bears (Meffe). Habitat fragmentation has obvious negative impacts on the viability of such species.
Yet another consequence of habitat fragmentation is an increase in the amount of edge habitat in an ecosystem. Urban sprawl, including roads and paved areas, increases the amount of area with sharp gradients in temperatures, humidity, solar radiation, and moisture (Meffe). Thus, species that have specific abiotic requirements found only in interior habitats undergo significant declines in populations. Other species, however, have adapted to suburbia and become edge-generalists (Kricher). Species associated with edges can exert negative effects on already declining interior species as the likelihood of interspecies contact intensifies with increased edge. Because edge-generalists tend to be more effective competitors, predators, and nest parasites, edge-sensitive species suffer from reduced populations as urban development expands. Edge-generalist brown-headed cowbirds, for example, lay their eggs in the nests of other songbirds rather than building their own, resulting in a parasitic relationship where cowbirds reproduce at the expense of other birds. The edge effect also includes human activities associated with housing developments, which may have often overlooked impacts on wildlife habitat . Lawn mowing, yard lights, birdbaths and feeders, and other habitat altering activities may extend hundreds of yards away from a house. Also, domesticated pets associated with housing developments, such as cats and dogs, dramatically increase the amount of predation and competition among wildlife (Meffe).
The division of larger habitat patches into smaller, isolated patches also decreases the likelihood of species persistence on an individual level because smaller patches cannot support large populations. This is largely due to the loss of genetic diversity in small populations. As habitats shrink and become more isolated, the number of individuals of a population and the dispersal ability of related populations subsequently shrink. Population isolation changes the natural distribution of genetic diversity, which can reduce evolutionary flexibility and decrease reproductive fitness. Because species may be constrained from dispersal, the founder effect, demographic bottleneck, genetic drift, and inbreeding result in a loss of genetic diversity. The consequence of this is a loss in allelic diversity among a species, which decreases its ability to adapt to environmental change, whether natural or man-made (Meffe). Urban sprawl thus hinders the successful reproduction of wildlife on yet another front.
Urban sprawl’s impact on wildlife goes even further as air and water pollution, associated with cities, spreads to adjacent habitats. Runoff from paved surfaces, such as gas, salt, and oil, accumulate in nearby water bodies. During development, land clearing and construction load streams with sediment, “which smothers fish eggs and bottom-dwelling invertebrates and chokes out streamside food and cover plants” (Nickens).
In order to curb urban sprawl’s ongoing destruction of wildlife habitat, land use patterns and state and federal management policies must be improved (Ewing). As discussed earlier, the greater the size of a habitat, the greater the likelihood that certain species will occur and persist in that area. Hence, rather than building more roads, which fragments habitat and encourages urban sprawl into wildlife areas, urban growth could be directed into already settled areas. For example, a 1973 state law required Portland, Oregon to draw an urban-growth boundary line around the city to discourage sprawl. They then went beyond this and developed a Metro 2040 plan to promote the development of mixed-use centers and increasingly improve their public transportation system. This smart growth planning will create readily walkable services and cut road construction by 25 percent. It is estimated that Portland will subsequently protect 100,000 acres from development while still making room for 720,000 new residents (Nickens). Cluster development is another innovative way to allow for development, while maintaining some natural land and habitat features (Barnes). As a result of smart urban planning, wildlife will be able to thrive in areas that otherwise would have been paved over with developmental sprawl. Still, if sprawl is unavoidable in the face of an endlessly expanding human population, “efforts should be made to avoid developing high-value habitats…by developing lower-value habitats, such as the undeveloped agricultural crop fields in the same area” (Barnes).
An additional landscape-level consideration that could protect biodiversity and ecosystems is considering the area, shape, and isolation of fragmented patches. The shape of protected areas can influence what species persist there because it determines how much unaffected area is contained in a patch (Meffe). “Arithmetic increases in perimeter are associated with geometric increases in area…perimeter is directly proportional to the square root of the area” (Meffe, pg. 180), thus the more a patch diverges from circular, the more edge it has. Natural resource managers should design new protected areas and alter the shapes of existing ones based on this understanding in order to enhance the area’s ability to attract and maintain wildlife. Furthermore, because larger areas can support a greater number and variety of species, closing roads that intrude into wilderness areas could alter the shape, decrease the amount of edge habitat, and minimize the impact of human activities. Another solution is to incorporate land adjacent to protected areas, or alter land use on bordering landscapes to be more sensitive to wildlife (Meffe).
Yet another effective management technique to reduce the urban impact on wildlife habitat is the use of movement corridors. Metapopulations consist of discrete source populations, where birth rates exceeds death rates, and sink populations, where death rates exceeds birth rates. Sink populations will go extinct without bolstering from source populations through dispersal (Meffe). Accordingly, it may be vital for managers to mediate and facilitate dispersal opportunities for populations on a fragmented landscape. The protection of riparian corridors is especially important because they provide habitat for hydric and mesic species and contain the vital necessities for species survival. Line corridors (i.e. roads, hedgerows, property boundaries, drainage ditches, etc.) tend to be narrow, but provide habitat for some species and a means by which others can move from one patch to another (Kricher).
Sometimes movement corridors are man-made structures. Whether natural or human-engineered; whether for daily movement, long-distance dispersal, or seasonal migration; the design features are decisive in determining what species can and will use movement corridors. Gaps (areas of poor habitat or restrictions on movement) are inevitable in corridors, and should be scaled to species’ mobility; more mobile species can withstand wider gaps. Corridor width should also be taken into consideration and scaled with length and habitat conditions. For example, the longer the corridor is, the more time a species is likely to spend in it, and the wider it needs to be in order to maintain the capacity to provide food, water, and shelter. Also, healthier, natural habitats, where water, food, and shelter are plentiful, can be narrower than corridors in degraded areas (Meffe).
As human populations continue to expand, consuming valuable wildlife habitat, it becomes more obvious that better planning must be used to protect threatened wildlife species, while making room for inevitable urban growth. I would like to believe that most of us are tired of seeing the landscape destroyed and wildlife driven away by human’s all-consuming greed. Is it progress when development defiles the land, air, and waters of wilderness areas? Or does progress come from our ability to collaborate and plan urban growth in a smart and constructive way, so as to have the least amount of negative impact on wildlife habitat?
“The bottom line is, we live where the wild things are. We need to do a better job accommodating the natural environment along with the human environment. With proper planning, it doesn’t have to be a question of us versus them or development for people versus habitat for wildlife” (Ewing).
Barnes, T. & L. Adams, A guide to urban habitat conservation planning. FOR-74, <http://www.ca.uky.edu/agc/pubs/fo/for74/for74.htm>. Accessed 11 Apr. 2005.
Ewing, R. & J. Kostyack, America’s wildlife under attack from rapid urban development. City Mayors Development, <http://www.citymayors.com/development/urbandevelopment_usa.html>. Accessed 11 Apr. 2005.
Heilprin, J., 11 Jan. 2005, Groups: sprawl threatens plants, animals. Associated Press, Boston.com, <http://www.boston.com/news/science/articles/2005/01/11/groups_sprawl_threatens_plants>. Accessed 11 Apr. 2005.
Kricher, J. & G. Morrison, 1988, Eastern forests: a field guide to birds, mammals, trees, flowers, and more. Peterson Field Guides. Houghton Mifflin Company: New York, NY. Pgs. 221-219.
Meffe, G.K., L.A. Nielsen, R.L. Knight, & D.A. Schenborn, 2002, Ecosystem management: adaptive, community-based conservation. Island Press: Washington, DC. Pgs. 120-220.
Nickens, T.E., Aug./Sep. 2001, Paved over and pushed out. National Wildlife Magazine, vol. 39 no. 4. 2005, National Wildlife Federation, <http://www.nwf.org/nationalwildlife/printerFriendly.cfm?issueID=37&articleID=386>. Accessed 11 Apr. 2005.
Pegg, J.R., 17 Jan. 2005, Suburban sprawl rolling over imperiled wildlife. People & the Planet, <http://www.peopleandplanet.net/pdoc.php?id=2407&PHPSESSID=29c3b4cfa1ed75c6e7c>. Accessed 11 Apr. 2005.
Woolliams, J., 2000, Designing cities and buildings as if they were ethical choices. Oxford University Press. Vanishing Resources. Pgs. 426-429.