Urbanization refers to the concentration of man populations into detached areas. This concentration leads to the transformation of land for residential, commercial, industrial and transportation purposes. Information technology can include densely populated centers, too equally their adjacent periurban or suburban fringes (see Figure 1).
Figure 1. Urbanization map of the U.Southward. derived from city lights data (2009 data). Urban areas are colored carmine (represented of larger cities), while peri-urban areas are colored yellow. Image created by Flashback Imaging Corporation, nether contract with NOAA and NASA [accessed 7/sixteen/09].
Urbanization can exist quantified in many different ways (Table 1). Example definitions used to allocate areas as "urban" or "developed" include:
Cadre areas with population density ≥ one,000 people per square mile, plus surrounding areas with population density ≥ 500 people per square mile (U.South. Census Bureau, for 2000 Census).
Areas characterized past ≥ 30% constructed materials, such as cobblestone, concrete and buildings (USGS National Land Cover Dataset).
Tabular array i. Common Ways of Quantifying Urbanization
Measure
Clarification
% Total urban area
Expanse in all urban land uses
% High intensity urban
Expanse above some college development threshold
% Low intensity urban
Surface area higher up some lower development threshold
% Residential
Surface area in residential-related uses
% Commercial/industrial
Area in commercial- or industrial-related uses
% Transportation
Area in transportation-related uses
% Total impervious area
Area of impervious surfaces such as roads, parking lots and roofs; also called impervious surface embrace
% Effective impervious area
Impervious area directly connected to streams via pipes; besides called % drainage connection
Road density
Road length per area
Route crossing density
# Route-stream crossings per expanse
Population density
# People per area
Household density
# Houses per surface area
Urban intensity indices
Multimetric indices combining a suite of development-related measures into one index value [e.1000., the USGS national urban intensity index (NUII), based on housing density, % developed land in bowl and road density]
Why Does It Thing?
Urban development has increased dramatically in contempo decades, and this increase is projected to continue. For case, in the U.S. developed country is projected to increment from five.two% to 9.2% of the total state base in the next 25 years (Alig et al. 2004).
On a national scale, urbanization affects relatively footling state cover, just it has a significant ecological footprint. Even modest amounts of urban development can take large effects on stream ecosystems.
Key Pathways Past Which Urbanization Alters Streams
Riparian/channel alteration – Removal of riparian vegetation reduces stream cover and organic thing inputs. Directly modification of channel alters hydrology and physical habitat.
Wastewater inputs – Homo, industrial and other wastewaters enter streams via point (eastward.m., wastewater treatment plant effluents) and non-point (e.g., leaky infrastructure) discharges.
Impervious surfaces – Impervious cover increases surface runoff, resulting in increased delivery of stormwater and associated contaminants into streams.
The Urban Stream Syndrome
Common effects of urbanization on stream ecosystems have been referred to as the"urban stream syndrome"(Walsh et al. 2005a).
Table 2 lists symptoms typically associated with the urban stream syndrome.
Tabular array 2. Symptoms Generally Associated with the Urban Stream Syndrome
Increased channel width (in non-hardened channels)
Altered pool depth
Increased scour
Decreased channel complexity
Contradistinct bedded sediment
Energy sources
Decreased organic matter retention
Altered organic matter inputs & standing stocks
Altered algal biomass
Modified from Walsh CJ et al. 2005. The urban stream syndrome: current noesis and the search for a cure. Journal of the North American Benthological Society 24(3):706-723.
As the urban stream syndrome illustrates, these streams are simultaneously affected past multiple sources, resulting in multiple, co-occurring and interacting stressors. Thus, identifying specific causes of biological damage in urban streams, or the specific stressors that should be managed to improve condition, is difficult. Some communities are budgeted this claiming by managing overall urbanization, rather than the specific stressors associated with it—for example, by establishing total maximum daily loads (TMDLs) for impervious surfaces, rather than individual pollutants.
Total Maximum Daily Loads (TMDLs)
Many characteristics of urban evolution affect how the urban stream syndrome is expressed in a given system. These characteristics include (but are not limited to):
Location and distribution of development
Catchment vs. riparian
Upstream vs. downstream
Sprawling vs. compact
Density of development
Type of evolution and infrastructure
Residential vs. commercial/transportation
Stormwater systems
Wastewater treatment systems
Age of evolution and infrastructure
Urbanization and Biotic Integrity
Numerous studies have examined relationships betwixt land use variables and stream biota. These studies accept shown that urban-related land uses can significantly alter stream assemblages.
Land utilize variables considered include % urban land (in the catchment and in riparian areas), % impervious surface area (total and effective), road density and other measures of urbanization.
Biotic responses associated with these country employ variables include (simply are not limited to):
ALGAE
Increased affluence or biomass [Roy et al. 2003, Taylor et al. 2004, Busse et al. 2006]
Other changes in assemblage structure (e.thousand., diatom composition) [Winter and Duthie 2000, Sonneman et al. 2001, Newall and Walsh 2005]
BENTHIC MACROINVERTEBRATES
Decreased full abundance, richness or multifariousness [Morley and Karr 2002, Moore and Palmer 2005, Walsh et al. 2007]
Decreased EPT (Ephemeroptera, Plecoptera, Trichoptera) [Morley and Karr 2002, Roy et al. 2003, Riley et al. 2005, Walsh 2006]
Figure 2. Plot of a measure of biotic homogenization [relative abundance of Appalachian highland owned fishes – relative abundance of cosmopolitan fishes] on the showtime axis of a principal components analysis of three catchment land apply variables [1993 forest encompass, wood cover change from 1970s–1990s, and urbanization intensity (normalized catchment building + road density)]. Sites with college wood cover and lower urban intensity had more than endemic taxa (e.one thousand., fishes such as the Tennessee shiner and the mottled sculpin, higher up left), while sites with lower forest cover and college urban intensity had more broadly distributed, generalist taxa (e.k., fishes such as the redbreast sunfish and central stoneroller, above right). From Scott MC. 2006. Winners and losers amidst stream fishes in relation to land use legacies and urban development in the southeastern U.s.. Biological Conservation 127:301-309. Reprinted with permission from Elsevier. Photos courtesy of Noel Burkhead, USGS Increased abundance of tolerant taxa [Jones and Clark 1987, Walsh et al. 2007]
Other changes in assemblage structure (due east.one thousand., functional feeding groups) [Stepenuck et al. 2002, Smith and Lamp 2008]
Decreased quality of biotic index scores [Kennen 1999, Morley and Karr 2002, DeGasperi et al. 2009]
FISHES
Decreased abundance, biomass, richness or multifariousness [Wang et al. 2003a, Bilby and Mollot 2008, Stranko et al. 2008]
Other changes in assemblage construction (e.g., reproductive guilds) [Stepenuck et al. 2002, Roy et al. 2007, Helms et al. 2009]
Decreased quality of biotic index scores [Snyder et al. 2003, Miltner et al. 2004, Morgan and Cushman 2005]
Increased biotic homogenization (replacement of more than endemic, specialist fishes with more broadly distributed, generalist fishes) [Scott 2006 (Effigy 2), Walters et al. 2009]
Catchment vs. Riparian Urbanization
Where urbanization occurs in the watershed tin can bear upon its influence on stream ecosystems. Studies examining state use variables and stream characteristics typically consider state employ at i (or more than) of 3 general spatial scales:
Catchment: the entire catchment area above the site
Riparian: the entire riparian area in the catchment above the site
Reach: the riparian expanse for a relatively short distance to a higher place the site
King et al. (2005) examined whether macroinvertebrate assemblages in Coastal Patently, Maryland streams responded differently to development in the watershed versus in areas closer to the focal site (Figure 3). They plant that where development occurs can significantly influence its effects on benthic biota:
Figure iii. Scatterplots of the threshold result of adult country on macroinvertebrate assemblage composition (Bray-Curtis dissimilarity expressed equally nonmetric multidimensional calibration [nMDS] Centrality i scores), for (A) % developed land in watershed, (B) % developed land within 250-m radius buffer of site, (C) % adult land in watershed weighted by its inverse distance (IDW) to site. Dotted lines indicate the cumulative probability of an ecological response to increasing % developed land. Sites within the watershed-scale threshold zone of 21-32% developed land in (A) are highlighted in black in all panels. From King RS et al. 2005. Spatial considerations for linking watershed state encompass to ecological indicators in streams. Ecological Applications fifteen(1):137-153. Reprinted with permission.
For % developed land in the watershed (Figure 3A), at that place was an apparent threshold betwixt 21-32% where the probability of assemblage alterations increased rapidly. Once >32% of the watershed was adult, all macroinvertebrate assemblages were affected.
When % developed land in the 250-m buffer was considered (Figure 3B), this threshold shifted left and all macroinvertebrate assemblages were affected once >22% of state in the 250-one thousand buffer was developed.
A similar pattern was seen when adult land in the watershed was changed-distance weighted (i.e., development closer to the focal site was weighted more than than development farther abroad; Figure 3C). Then, the threshold for macroinvertebrate effects occurred betwixt 18-23%.
The relative importance of evolution at dissimilar scales varies beyond studies (east.g., Sponseller et al. 2001, Wang et al. 2001, Morley and Karr 2002, Roy et al. 2003, Snyder et al. 2003, Schiff and Benoit 2007). It probable depends, at to the lowest degree in role, on the stressors considered (Allan 2004).
For example, some stressors associated with urbanization (e.k., changes in flow) are highly dependent on catchment-scale processes, while other stressors (due east.thousand., changes in basal energy sources) are more affected by reach-scale processes.
![Plot of a measure of biotic homogenization [relative abundance of Appalachian highland endemic fishes – relative abundance of cosmopolitan fishes] on the first axis of a principal components analysis of three catchment land use variables](https://www.epa.gov/sites/default/files/2015-12/urban-biotic.jpg)
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