Dealing with Impacts of Urbanization on the Ecology of Surrounding Landscapes
By: Bradley J.F.B UNCG
There is little doubt that with urbanization comes a cost in ecological health. The sprawling urban centers and enormous tracts of agricultural lands that typify the modern age are both a direct and indirect threat to surrounding ecosystems. They are a direct threat through habitat conversion and an indirect threat through the effects human populations typically have on their environment: habitat fragmentation, resource depletion, waste generation, and freshwater consumption (Ricketts and Imhoft, 2003). Other ecological side effects identified with agricultural land use include chemical pollution (fertilizers, pesticides), and the disruption of natural water and nutrient cycles (Ricketts and Imhoft, 2003). Such environmental degradation has typically resulted in a decrease in biodiversity and a reduction in the quality of natural services various ecosystems provide (clean water, fresh air, esthetics, recreation) (Conroy et al., 2003). Thus, the identification and assessment of environmental impacts as a result of modern urbanization have become a top priority among conservationists. As such, many recent studies have been conducted with the goal of better understanding the impacts and issues related to urbanization, and the most efficient means of tackling this critical environmental dilemma.
Monetary resources available for the purpose of conservation at any scale are always limited. With this in mind, it is critical to identify areas of priority, especially when dealing with a threat as widespread and dynamic as urbanization. A study conducted by Ricketts and Imhoft (2003) attempted to do just this. The authors' goal was to identify biogeographic ecoregions of North America in which high levels of plant diversity coincide with intense human activity. Theoretically, these “hotspots” should receive priority when allocating conservation resources. The authors identified hotspot ecoregions by comparing species distribution data (including 20,000 species in eight taxa) to maps of urbanization and agriculture across North America. The number of total species (species richness) and number of endemic species of each taxon (birds mammals, butterflies, amphibians, reptiles, land snails, tiger beetles, and vascular plants) was obtained for each ecoregion.
The results of the study indicated that a significant correlation exists between species richness and urbanization. Interestingly, as the level of urbanization increased within an ecoregion, the species richness index also increased. A similar, although not as significant, correlation was also found between agriculture and species richness. However, no significant correlation was found between endemism and level of urbanization or agriculture. In addition, 16 ecoregions were identified (from a total of
76) as “priority sets,” because they contained “both extraordinary species richness and high levels of urbanization” (Ricketts and Imhoft, 2003). These priority ecoregions were most commonly found in the southeastern United States and California.
One significant conclusion that may be drawn from these results is that people choose to live in areas that are relatively rich in species. This pattern has been observed in many parts of the world, especially developing countries (Cincotta et al., 2000, Luck et al., 2004). The reasons for this phenomenon are many, and probably include the basic human preference to settle in mild climates where ecosystems are typically more productive. Areas that are ecologically rich often foster higher agricultural yields, and sustain greater urbanization (Ricketts and Imhoff, 2003). Specifically in the United States, people have been generally moving south over the last few decades, not because of agriculture, but to simply live in warmer, “sunbelt” locations (Larsen, 1990). Thus, for a myriad of reasons, around the world there exists a trend towards greater urbanization in areas that also support high levels of biodiversity.
Once areas of high priority are identified, the next challenge is to develop an effective plan to deal with the wide variety of ecological disturbances produced by urbanization. This is by no means a simple process, as ecological problems of this nature are usually highly complex. Conroy et al. (2003) outline what they believe are the conceptual and technical steps that should be taken in order to implement successful management and conservation strategies for ecosystems threatened by urbanization. In order to demonstrate their adaptive decision-making process in a practical manner, they use the lotic ecosystems in the southern Piedmont as an example.
The southern Piedmont, which stretches across the southeastern United States between the Appalachian Mountains (west) and the Atlantic coast (east), is an area undergoing a rapid transition towards greater urbanization (Conroy et al., 2003). In fact, the general area includes several of the ecoregions found to have both a high urban population and high species richness in Ricketts and Imhoft's (2003) study.
As such, this was an ideal region to address ecological consequences of encroaching urbanization.
Conroy et al. puts forth three main components of an adaptive decision-making process that should be conducted when dealing with any complex ecological problem:
assessment of the current state of the ecological system, development of actions and alternative actions that may be taken to correct the problem (as well as a prediction of the expected impact of each action), and the monitoring and assessment of the new current state of the system. In addition, the authors content that it is crucial to take into consideration scale-dependent relationships, resilience of ecological systems to perturbation, and the uncertainty inherit in any decision made concerning the effects of human activities on ecological systems, when undergoing each of these three main stages.
Applying this general scheme to the lotic ecosystems (including rivers and streams) of the southern Piedmont, Conroy et al. begins by discussing an assessment of the current state of the system.
At a regional scale, the lotic systems of the Piedmont include seven primary Atlantic slope and two Gulf Coast drainages. The mild climate of the region has produced some of the highest levels of “aquatic faunal diversity and endemism recorded in temperate freshwaters” (Conroy et al., 2003). Unfortunately, degradation at the regional scale has resulted in declines in endemic fish populations at finer scales. Most of the declines are most likely the result of widespread habitat alteration and depletion. The largely forested landscape is increasingly being converted to urban and suburban areas as human population in this area grows. In addition, the need for water both for human consumption and agricultural irrigation has lead to the impoundment and regulation of several streams and rivers. This has had large-scale affects on the lotic ecosystems of the region, which have become less resilient to human and natural disturbances due to less connectivity. Adjacent freshwater populations are no longer able to mix as freely, and re-colonization sources in particular areas following local disturbances are vastly limited (Conroy et al., 2003). This has deep implications, especially for those areas most prone to the types of local disturbances caused by urbanization. Urbanization tends to perturb natural lotic systems by increasing drainage, infiltration, and sedimentation, resulting in larger volumes of floodwater in shorter periods of time. Thus, both large-scale issues involving cumulative alterations to stream flow and water development activities, as well as finer scale local disturbances, must be dealt with in order to combat the degradation of lotic ecosystems in the southern Piedmont.
In order to combat this problem, Conroy et al. suggests that various alternative water development scenarios be evaluated through either a simulation approach, or optimum control approach. The main idea of both approaches is to find an optimal strategy for regional water development. This is done by developing predictions about various alternative actions on the function and resilience of streams and rivers in the Piedmont by looking at the underlying assumptions of system dynamics. Conroy et al. specifically considered the problem of impounded streams, and formulated three main alternative water development scenarios: development of a few large, regional reservoirs; the use of several small reservoirs located on headwater streams; and the development of several small off-channel reservoirs. Choosing which scenario is optimal demands that the impact of each on species assemblage, diversity, hydrology, water chemistry, and ecosystem function be evaluated (Conroy et al., 2003). In addition, the expected impact of each alternative action should take into account environmental, demographic, structural, and statistical sources of uncertainty.
Once an alternative action has been determined to “lead to the greatest expected gain, or lease expected loss in system return,” it is time to adopt that action as the management recommendation (Conroy et al., 2003). This action, although deemed most likely to produce positive results, should still be considered an “experiment,” due to inherit uncertainty. As such, it is crucial that data be collected and assessed during and after the alternative action, so that management can remain as adaptive as possible to changing environmental conditions. As specific as this example may seem, it outlines a decision-making process that is broad enough to be applied to a wide variety of environmental issues related to urbanization.
Luck et al. (2004) proposes that the best general way to protect areas of high diversity threatened by urbanization is to purchase these areas specifically for the purpose of conservation. Luck et al. conducted a study, similar to one completed by Ricketts and Imhoft (2003), in Australia and North American in order to determine which areas contained both significant human populations and high species diversity. However, this study was conducted somewhat differently in that the areas of consideration were much smaller: 1°-grid cells instead of entire ecoregions. Luck et al. also looked at the percentage of threatened species, and the percentage of species with restricted geographic distributions in each grid cell. The authors used data from across 110 ecoregions. Not surprisingly, their results largely mirrored that of Ricketts and Imhoft (2003), showing a strong correlation between human population density and overall species richness across taxa (with the exception of reptiles).
One important conclusion Luck et al. drew from an analysis of their findings is that “numerous range-restricted species occur in densely populated areas” (Luck et al., 2004). This suggests that it is crucial that some areas, although already heavily populated by humans, should still be protected through conservation efforts. In addition, the authors found that many of the grids containing high species diversity and low human populations were just beginning to undergo significant development. They suggest that these areas will see the greatest threats to biodiversity in the near future, and should be a conservation priority. Thus, predicting patterns of human development in areas of high biodiversity is a crucial stage in the effort to relieve the conflict between urbanization and biodiversity, while effectively utilizing limited conservation capital.
Combating the threat of urbanization on surrounding ecosystems is one the greatest challenges facing conservationists and ecologists. It demands a thorough, and often complicated, process in which areas of high priority are identified, assessed, and either conserved or restored. These are the crucial first steps in limiting the many ecological disturbances already caused by growing urbanization around the world.
Taking a more proactive stance towards conservation right now and in the near future, however, is the only way to assure that future generations – in urban and rural areas alike - will be able to enjoy the services of nature we so often take for granted.
Literature Cited
Cincotta et al. 2003. Human population in the biodiversity hotspots. Nature (London)
404(6781):990-992.
Conroy et al. 2003.Landscape change in the southern Piedmont: challenged, solutions,
and uncertainty across scales. Conservation Ecology 8(2): 3.
[online] URL: http://www.consecol.org/vol8/iss2/art3
Larsen, L.H. 1990. The urban south: a history. University of Kentucky Press, Lexington,
Kentucky.
Luck et al. 2004. Alleviating spatial conflict between people and biodiversity.
Proceedings of the National Academy of Science of the United States of America
101(1):182-186.
Ricketts, T. and M. Imhoff. 2003. Biodiversity, urban areas, and agriculture: locating
priority ecoregions for conservation. Conservation Ecology 8(2): 1
[online] URL: http://www.consecol.org/vol8/iss2/art1
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