28 June 2012

Ecosystem Services

Definition:  Ecosystem services are the benefits that people derive from the structures and processes generated by ecological systems.  Ecosystem services are classified into four categories: 1) Supporting services that provide the ubiquitous capacities of ecosystems to function, and to provide other more immediately useful services.  Supporting services include evolution, soil formation, nutrient retention, formation of atmospheric oxygen, and biomass production.  2) Provisioning services are products obtained from ecosystems, such as fresh water, fiber, genetic resources, medicines, and protein.  3) Regulating services are those that moderate climate, natural disturbances, water flow, and some human diseases.  4) Cultural services are non-material benefits, such as aesthetic, social, and spiritual benefits.

Examples:  People receive an abundance of services from ecosystems.  Unfortunately, in the absence of economic valuation, many ecosystem services are unrecognized by society.  In urban systems, services are diverse.  Trees provide a cooler summer environment, reduce heat stress, and protect walkers from UV radiation.  Urban vegetation and streamside habitats can contribute to reduction in the height of urban flooding.  A diversity of wildlife may reduce the impact of the vectors of some human diseases, such as Lyme disease.  Urban soils can absorb pollutants, and trees can reduce violence, or promote healing.  

Figure 1.  Ecosystem services include the basic support for life on Earth, the provisions people require, regulation of environmental conditions and mitigation of hazards, and the availability of non-material experiences or values.  Adapted from the Millennium Ecosystem Assessment.

Why important: Ecosystem services are in some cases, irreplaceable.  For example the production of food is a strictly biological process.  This is a process that can become more available in cities, where many residents have poor access to fresh fruits and vegetables and rely on diets high in processed foods.  The reduction in domestic violence, crime, and sickness that are often associated with vegetation are important benefits that are hard to engineer.  The spiritual experience of a connection to the “other” provided by urban wildlife is a benefit that would otherwise only be obtainable by traveling to distant natural locations.  The benefit of reducing stormwater flow, with their normal loads of sediment and pollutants, to coastal waters or rivers that flow through cities is potentially a major contribution to water quality nationwide.  As a majority of citizens come to live in urban areas in both industrialized and developing countries, the improvement of ecosystem services in cities and suburbs is a major need.  It would be impossible in current financial and technological circumstances to replace the services provided by ecosystems with engineered or built solutions.

For more information:
  • Millennium Ecosystem Assessment. 2003. Ecosystems and human well-being: a framework for assessment. Island Press, Washington DC.
  • Kinzig, A., W.R. Burch, N.B. Grimm, P. Groffman, J.M. Grove, C.W. Martin, and R. Pouyat. 1999. Patch dynamics and ecosystem services: a cross-site comparison of Baltimore and Phoenix.
  • Daily, G.C. 1997. Introduction: what are ecosystem services? Pages 1-10 in G.C. Daily, editor. Nature's services: societal dependence on natural ecosystems. Island Press, Washington DC.

Chicago School

Definition: The Chicago School of urban ecology is in reality an approach to the sociology of cities.  The Chicago School grew out of the Department of Sociology at the University of Chicago in the first two decades of the 20th century.  It adopted its major ideas from biological ecology of the time, for which the University of Chicago and its downstate neighbor, the University of Illinois, were major seedbeds.  The growth of the city and the spatial patterns that resulted were interpreted as a kind of ecological succession, with waves of immigrant groups replacing one another, and establishing rings of structure around the central business district.  Competition among human communities was assumed to be a major driver of spatial differentiation across the city.


Figure 1.  An idealized, simple model of urban social structure based on the situation in Chicago, Illinois, in the early 20th century.  The loop is the local term for the central business district.  The shoreline of Lake Michigan is the wavy line running near the middle of the bullseye.  To the right of that line is the generalization of zonation derived from the idealized empirical experience in Chicago, shown on the left.  This model, though important for the establishment of ecological approaches to cities, has been found lacking by later generations of researchers.

Why important: The principals of the Chicago School were confronted by a city that was doubling in population and bringing people from overseas and the American South together in high density and novel mixture.  The Chicago professors were troubled by the abandonment of traditional or rural social norms with immigration, and with the social pathology they associated with urban life.  The spatial approach they founded acknowledges the importance of spatial patchiness in urban systems, but it was criticized for being too focused on group activities and not enough on individual behaviors.  While the Chicago School is labeled one of urban ecology, the ecological study of the city in the sense of biogeophysical science was not a part of the Chicago School.  Contemporary critiques of the ecology behind the Chicago School do not always recognize the difference between the biophysical ecology of the early 20th century, and the conceptual and empirical developments of that field that have emerged in the last several decades.

For more information:
  • Vasishth, A.S.H.W. and D.C. Sloane. 2002. Returning to ecology: an ecosystem approach to understanding the city. Pages 347-366 in M.J. Dear, editor. From Chicago to L.A.: making sense of urban theory. Sage Publishers, Thousand Oaks.
  • Gottdiener, M. and R. Hutchison. 2000. The new urban sociology. 2nd edition. McGraw Hill, New York.
  • Park, R.E. and E.W. Burgess. 1925. The city: suggestions for investigation of human behavior in the urban environment. University of Chicago Press, Chicago.


Definition:  An area that receives rainwater or other water input, and accumulates it at its lowest point.  In urban systems the water inputs into watersheds may be from rainfall, household and landscape application, or industrial discharge.  Drainage channels deliver surface water to larger streams.  In dry climates the low point may be a basin where water is lost entirely by evaporation to the atmosphere, while in moist climates, the low point usually empties into a larger stream, lake, or coastal waters.  In urban systems, the drainage networks may be in part or entirely engineered, and include curbs and gutters, storm drain inlets and settling basins for sediments and large debris, and pipes of increasing diameters that finally empty into a creek, river, or bay. 

Examples: Baltimore City comprises four major watersheds; Gwynns Falls, Jones Falls, Herring Run, and the direct harbor drainage.  Each of these watersheds reflects modifications by curb and storm drain infrastructure.  


Figure 1.  Diagram of a forested watershed.  The red line shows the area included, in which all water that falls drains to the lowest point.  Water that falls on the outside of the red boundary of the watershed becomes part of the flow of other watersheds not shown.  This diagram isof an ideal, non-urban watershed with no engineered movement of water in or out through pipes.

Figure 2.  The non-engineered watersheds of Baltimore City and thier extensions into Baltimore County.  The flow into these watersheds has been modified by importation of water from reservoirs beyond their boundaries, by storm drain networks that cross the watershed boundaries at some places, and by the piping out of sanitary sewage and storm runoff.  In addition, leakage between the sanitary and storm sewers, and unintentional exchanges between the sewer networks and the streams also complicate the flow of water.

Why important: Watersheds are a crucial feature of natural landscapes.  They are no less important to cities and suburbs, even if they include much built structure.  Watersheds rarely correspond to political boundaries.  Hence watershed management is a political and social process that must cross municipal boundaries, and involve the interest and understanding of many human communities.  Watersheds are a kind of ecosystem, because not only do they include the streams and groundwaters that influence the streams, but they also include the surface and subsurface flow toward the streams, the pipes that channel flow in neighborhoods and commercial developments, and the biological and social elements that occupy these lands.  The watershed as a concept is spatially extensive, three dimensional, and incorporates social, biological, and physical components.

For more information:
  • Belt, K. 2000. The Baltimore Ecosystem Study water quality and urban hydrology initiatives - stream studies along an urban rural gradient in the Gwynns Falls and Baisman Run watersheds. Maryland Water Monitoring Council Programatic Coordination Newsletter.
  • Black, P.E. 1991. Watershed hydrology. Prentice Hall, Englewood Cliffs.
  • Brush, G.S. 2009. Historical land use, nitrogen, and coastal eutrophication: a paleoecological perspective. Estuaries and Coasts 32:18-28.
  • Pickett, S.T.A., K.T. Belt, M.F. Galvin, P.M. Groffman, J.M. Grove, D.C. Outen, R.V. Pouyat, W.P. Stack, and M.L. Cadenasso. 2007. Watersheds in Baltimore, Maryland: understanding and application of integrated ecological and social processes. Journal of Contemporary Water Research and Education 136:44-55.

Patch Dynamics

Definition: Patch dynamics refers to the three dimensional structure, changes, causes, and effects of spatial differentiation within ecological systems. 

Examples: Patch dynamics include a wide variety of patterns and processes in socio-ecological systems.  Biological patch dynamics can be seen in the establishment, growth, and demise of a clone of goldenrods in a field, or in the fall of a tree in a continuous forest during a storm, with the subsequent response of younger trees and herbaceous plants in the understory, now exposed to light (Fig 1).  Social patch dynamics occurs when households move through child rearing to “empty nest” status, with changing demands on neighborhood infrastructure and changing political priorities, or when a new highway interchange promotes the establishment of an exurban commercial and business center.  Disinvestment, such as a withholding of insurance rating (Fig 2) shifting tax base, or abandonment of an old neighborhood (Fig 3), also initiate patchy change in biological and social structures and processes in a specific urban locations.  Patch dynamics is sometimes a joint socio-ecological phenomenon.  An example occurs with the maturation of trees planted when a subdivision is initially developed, enhances property values or reduces summer cooling costs. 

Patch Dynamics Figure 1.  A treefall gap in a deciduous forest canopy, looking straight upwards.  Treefalls in forests are an example of patch formation which can alter the structure and the mix of species in the forest.  The light, increased rainfall, and decreased demand for nutrients can release seedlings and saplings from the suppression they had experienced in the intact forest.  Animals may seek out new sources of browse and herbiage in treefall gaps.


Patch Dynamics Figure 2.  Patches can result from policy decisions and other social processes.  This map shows the distribution of Home Owners Loan Corporation zones for the security of mortgage loans in Baltimore City in 1930.  Such policies helped alter the dynamics racial and class distributions in the city.


Patch Dynamics Figure 3. Changes in the patch structure in West Baltimore between 1938 and 2000.  This example of patch dynamics shows the impact of abandonment and demolition of rowhouses on neighborhood physical structure.  Social changes are also associated with these changes.

Why important:  Urban systems consisting of cities, suburbs, and exurbs are dynamic systems.  The contrast from place to place, often over very short distances, is a conspicuous feature of urban systems.  How these different patches affect one another, and affect the functioning of the entire urban ecosystem is crucial information.  Understanding the historical causes for the spatial structure and how it has changed is important for making plans for the future of urban systems.  In spite of people’s best efforts, combined natural and social disruptions and innovations change urban systems in spatially patchy ways.

For more information:
McGrath, B. 2000. Manhattan timeformations.   www.skyscraper.org/timeformations
McGrath, B.P., V. Marshall, M.L. Cadenasso, J.M. Grove, S.T.A. Pickett, and J. Towers, editors. 2007. Designing patch dynamics. Columbia University Graduate School of Architecture, Preservation and Planning, New York.
Shane, D.G. 2005. Recombinant urbanism: conceptual modeling in architecture, urban design, and city theory. John Wiley & Sons, Hoboken.


Definition: A catchment defined by storm drain infrastructure emptying into a common outlet. 

Examples: Defined as an analogy to the concept of natural watershed, which refers to an area draining to a single point in a stream network, sewersheds are determined by curbs, storm drains, settling basins, pipes, and outfalls to streams.  Often storm sewers or the collecting curbs and drains cross the boundaries of the watersheds that existed before urbanization.
Sewershed Figure 1.  Sewersheds as defined by storm sewer drain networks in west Baltimore City.  Dark blue indicates remaining surface streams in the Gwynns Falls watershed, whereas lighter blue indicates the storm drains, with thickness of the lines proportional to the diameter of the pipes.  The yellow patch near the center of the map is Watershed 263, a 932 acre storm drain catchment comprising part or all of 11 neighborhoods.  Green indicates parklands.  Baltimore Harbor is the large body of water to the lower right.  The boundary of the Gwynns Falls watershed is indicated by the dotted line.  Graphic provided by Brian McGrath from his Urban Design Studio at Columbia University.

Sewershed Figure 2. Removal of some of the unneeded pavement in Watershed 263 aims to reduce the stormwater flow in this engineered watershed, and to provide other environmental benefits desired by the community.  This improvement in the schoolyard Franklin Park Elementary School of was carried out by the City Department of Public Works with design input from the students at the school.  Photo courtesy of Guy Hager, Parks & People Foundation.
Why important: Storm sewersheds are intended to rapidly carry off rainwater from city streets, roof gutters, and other large areas of impervious surface.  The intent is to prevent flooding of basements and streets, and to avoid standing water that might serve as breeding sites for disease carrying organisms such as mosquitoes.  Storm drainage networks, although they are referred to as sewers, are in many cases, separate from the network of pipes designed to carry the septic effluent in sanitary sewers.  Leaks or purposeful connections between storm and sanitary sewers contribute to pollution of streams, lakes, and the coastal waters into which storm sewers drain.  Storm sewersheds are increasingly being seen as targets for improved design to reduce the amount of storm water that is generated within them.  Greening, reducing impervious surface, and on-site retention and management of rain water using green roofs, rain gardens, and the like, are strategies to reduce the negative outputs from storm sewersheds.

More information:
Cadenasso, M.L., S.T.A. Pickett, P.M. Groffman, G.S. Brush, M.F. Galvin, J.M. Grove, G. Hagar, V. Marshall, B.P. McGrath, J.P.M. O'Neil-Dunne, W.P. Stack, and A.R. Troy. 2008. Exchanges across land-water-scape boundaries in urban systems: strategies for reducing nitrate pollution. Annals of the New York Academy of Sciences 1134:213-232.


Definition:  There are several ways to define sustainability.  One, attributed to the “Brundtland Commission” considers sustainability to be the ability to satisfy current needs without compromising the ability of future generations to meet their needs.  This definition rests on intergenerational equity.  The second generation focuses on equity at the current moment, and holds the needs of classes, ethnic groups, or nations who are less empowered, to have a legitimate stake in the satisfaction of needs.  A third definition can also be identified.  It focuses on the capacity of people to live within the capacities of ecosystems to  support them.  This, in a sense, is the most fundamental definition of sustainability.  It hews very close to the ordinary definition of sustainability in the sense of sustenance or support. 

Examples:  Applying the concept of sustainability to urban systems requires some caution.  From an environmental standpoint, no urban area is strictly sustainable.  Large urban areas, including suburbs, and their non-agricultural exurbs, do not supply their own food, cleansing capacities, or other environmental benefits (see Ecosystem Services).  However, city, suburban, exurban systems that contribute toward their food needs, environmental cleansing, carbon storage, and pollution reduction, for example, can be said to be more sustainable than those which do not make the same kinds of contributions.  Urban sustainability is thus a relative condition.

Sustainability has three components: environmental, social, and economic.  There is benefit to achieving sustainability in each realm, and the overlaps between each pair also has value, as noted by the grey boxes.  However, true and lasting sustainability can only result from the overlap of all three areas.  In other words, sustainability in any one area—environment, social, or economic—without attention to the other two will ultimately fail.    
Why important:  No activity that claims a benefit to sustainability can be narrowly construed.  All definitions of sustainability from the 1980s on, recognize that sustainability has three inextricably linked components – economic, social, and environmental.  Thus, people often speak of a “triple bottom line” in making any decision, so that the health of the economy, of the environment, and of social equity are all supported by decisions and actions that enhance sustainability.  None of the three aspects of sustainability trumps the others.  Given that urban sustainability is a relative condition, there are many activities and strategies cities, suburbs, and urbanized exurbs can make toward sustainability.

For more information:
Williams, D.E. 2007.  Sustainable Design: Ecology, Architecture, and Planning.  Wiley, Hoboken NJ.
Curwell, S., M. Deakin, and M. Symes, editors. 2005. Sustainable urban development, volume 1: the framework and protocols for environmental assessment. Routledge, New York.